BACKGROUND
the ripening of fleshy fruits involves a number of physiological processes including the production of aromatic compounds, nutrients, pigmentation, and softening of flesh to an edible texture
<cit> .
these processes have direct impacts not only on fruit firmness, color, flavor and nutritional content, but also on shelf life, consumer acceptability, processing qualities, in addition to pre- and postharvest disease resistance
<cit> .
excessive fruit softening is the main factor contributing to damage during shipping, storage and post-harvest handling
<cit> .
fruit firmness and texture also affect the integrity of chopped and diced fruit used for canning and fruit products
<cit> .
because postharvest losses of fresh fruits due to excessive softening can account for as much as 30~40% of total production, considerable research had focused on mechanisms of fruit softening, often using tomato  as a model system
<cit> .
fruit softening during the ripening process results in part from disassembly of the cell walls, leading to a reduction in intercellular adhesion, depolymerization and solubilization of pectins, depolymerization of hemicelluloses, and loss of pectic galactose side chains
<cit> .
generally, the decline in fruit firmness due to softening is accompanied by elevated expression of numerous cell metabolism enzymes, including polygalacturonase
<cit> , pectin methylesterase
<cit> , β-galactosidase
<cit> , as well as cell wall loosening proteins such as expansin
<cit> .
suppression of single genes encoding fruit pg
<cit>  or pme
<cit>  in transgenic tomato plants had limited impact on fruit softening during ripening, but conferred longer shelf life resulting from reduced susceptibility to postharvest pathogens.
these results suggest that suppression of certain enzymes acting on cellulose, hemicellulose or pectin alone are not sufficient to prevent softening, likely due to functional redundancy of enzymes involved in what is likely a complex metabolic process
<cit> .
nevertheless, a recent study has shown that down-regulation of genes encoding the n-glycan processing enzymes α-mannosidase and β-d-n-acetylhexosaminidase significantly increased fruit shelf life, which was attributed to decreased softening during ripening
<cit> .
these enzymes have been shown to break glycosidic bonds between carbohydrates, or between carbohydrates and noncarbohydrate structural molecules
<cit> .
expansins are cell wall-localized proteins faciliating wall loosening.
they are involved in many aspects of cell wall modification during development through disruption of non-covalent bonds between matrix glycans and cellulose microfibrils
<cit> .
transgenic silencing of the tomato expansin gene leexp <dig> resulted in increased fruit firmness throughout ripening and improved fruit integrity during storage
<cit> .
molecular and genetic investigations have identified additional regulators of cell wall biosynthesis and regulation of cell expansion.
one such activity is encoded by the cobra gene previously reported in arabidopsis, rice and maize
<cit> .
the cobra gene encodes a plant-specific glycosylphosphatidylinositol -anchored protein with a ω-attachment site at the c terminus, a hydrophilic central region, a ccvs domain, a potential n-glycosylation site, an n-terminal secretion signal sequence, and a predicted cellulose binding site
<cit> .
it has been reported that cobra localizes at the external plasma membrane leaflet through a glycosylphosphatidylinositol  moiety
<cit> .
genetic impairment of the cobra gene results in reduced levels and improper orientation of crystalline cellulose microfibrils in arabidopsis and rice
<cit> .
despite the many studies of the cobra gene in several plant species, little has been learned concerning cobra ortholog in tomatoes, the model system for fleshy fruit development and ripening.
here we report functional characterization of a tomato cobra gene .
we specifically demonstrate its role in early fruit development and the potential for enhanced fruit firmness and shelf life by manipulating its expression in maturing transgenic tomato fruits.
RESULTS
cobra gene family members in tomato
in order to identify tomato orthologs of the cobra gene, we ran a blast search in the sol genomics network  using the arabidopsis cobra gene sequence  as query.
<dig> unigene tomato cdnas homologous to atcob were found in the sgn database and were designated as slcobls .
all of the corresponding predicted amino acid sequences contain the characteristic interpro ipr <dig> or ipr <dig> omain of the plant cobra-like gene family.
these slcobl members, together with other plant cobra-like proteins from arabidopsis, rice and maize, can be grouped in two clades of a phylogenetic tree.
<dig> slcobl members consisting of 5~ <dig> exons were clustered into clade i and the other  <dig> members  were grouped in clade ii .
we focused further analyses on slcobra-like as it appeared to be constitutively expressed in many tomato tissues suggesting an essential role in tomato biology.
a total of  <dig> slcobra-like ests were found in the sgn est database, which were derived from various tissues including leaf, flower and developing fruits.
a the number of exons.
bamino acid residue number in each predicted slcobl protein.
predictions of n-terminal signal peptide and potential c-terminal gpi-modification sites  were performed with signalp version  <dig>   <cit>  and big-pi
<cit> , respectively.
expression of slcobra-like in tomato
the full-length slcobra-like cdna was isolated from tomato seedlings by rt-pcr using gene-specific primers .
the deduced slcobra-like protein contains a central cysteine-rich domain , a n-terminal secretion signal sequence for targeting to the endoplasmic reticulum, a highly hydrophobic c terminus and the ω-site required for processing the c-terminal
<cit>  .
moreover, several potential n-glycosylation sites frequently associated with gpi-anchored proteins and extracellular proteins, as well as one hmm-predicted putative cellulose binding domain ii  were observed in the slcobra-like sequence
<cit> .
basic local alignment search tool  analysis showed that slcobra-like shares 63~80% similarity with other cobra proteins from arabidopsis, oryza sativa, and zea mays <cit> .
phylogenetic analysis revealed slcobra-like shares the highest amino acid identity with atcobl <dig>  thus localizing within in the same clade .
we also conducted real time rt-pcr analysis and found that the slcobra-like gene is predominantly expressed in roots, stems, leaves, flowers and early development fruits.
however, the expression level in fruits declined dramatically at the breaker and later ripening stages , implying a possible role in early fruit development.
in addition, we isolated total rnas from exocarp, mesocarp, columella or locular fruit tissue from immature fruit  <dig> days post anthesis  for rt-pcr analysis.
the results showed that the slcobra-like expression level is relatively high in locular tissue as compared to other tissues at this specific developmental stage .
transgenic tomatoes whose endogenous slcobra-like gene was repressed displayed abnormal fruit phenotypes
to determine the role of slcobra-like in fruit development, transgenic tomato plants over-expressing the slcobra-like cdna in fruits were generated.
the slcobra-like transgene was driven by a fruit-specific tfm <dig> promoter
<cit> .
twenty independent transgenic lines were generated.
as expected, several transgenic lines  overexpressing slcobra-like  were confirmed by rt-pcr analyses .
none of these oe lines exhibited altered fruit phenotypes .
however, three of the twenty transgenic lines exhibited extensive non-uniform cracking on the surface of their immature green fruits .
rt-pcr analyses revealed that in cracked transgenic fruits , the endogenous slcobra-like expression was substantially co-suppressed  as compared to non-transformed wild type  control plants .
we then generated transgenic tomato plants with rna interference -based repression of the slcobra-like gene to confirm this repression effect.
unfortunately, none of our rnai transgenic lines displayed significant repression of the slcobra-like gene .
one possibility is that more complete suppression of the slcobra-like gene by rnai in culture tissues than resulting from cs may have caused lethality in early development.
because there are  <dig> cobra members in tomato, it was necessary to verify the specificity of suppression of slcobra-like in the  <dig> available cs lines.
we examined the expression of slcobl <dig>   <dig> and  <dig> in cs fruits.
there were two reasons we selected these  <dig> slcobls: firstly they represent high , medium , and low  similarity to slcobra-like, respectively; secondly the fruit-derived ests of these  <dig> slcobls were found in the sgn est database, suggesting they are expressed in fruits, the tissues where phenotypes were most apparent.
as shown in figure
2b, the real-time rt-pcr assay indicated the expression of slcobl <dig>   <dig> and  <dig> was not affected in cs lines, suggesting that fruit cracking was caused specifically by repression of slcobra-like expression.
anatomical alterations of fruit pericarps in transgenic plants with altered slcobra-like expression
since the exocarp  plays important roles in determining the rate of expansion and mechanical support for the entire fruit
<cit> , light microscopy analysis was conducted to examine possible changes in this tissue of transgenic fruits.
<dig> dpa fruit exocarp sections were mounted in 15% hcl and photographed using a leica ldm  <dig> microscope.
as shown in figures
3a-c, the epidermal cell layer of fruit exocarp displayed no significant differences  in epidermal cell size  between wt  and tfm7-oe .
however, the epidermal cell size of tfm7-cs exocarp tissue was  <dig>  ±  <dig>  10- <dig> mm <dig>  far exceeding that of the wt or tfm7-oe.
in addition, the epidermal cells of tfm7-cs pericarp expanded radially to a far greater extent than those of the wt or tfm7-oe lines.
in contrast, the epidermal cell separation  was significantly altered in both tfm7-oe  and tfm7-cs  fruits, as compared with that in wt , suggesting thicker epidermal cell walls in tfm7-oe fruits but thinner epidermal cell walls in the tfm7-cs fruits, respectively.
to further compare the pericarp structure of fruits in wt and transgenic lines, detailed cytological analysis was performed.
anatomical paraffin sectioning and calcofluor staining of pericarp tissue  revealed an increased number of collenchymatous cells beneath the epidermis in the tfm7-oe fruits compared to wt .
in contrast, the tfm7-cs fruits displayed a "waviness" of the surface with an apparent lack of cuticle and an abnormal shape and size-distribution of the cells, particularly in the thin layer of elongated epidermal cells at the fruit surface.
the small cells beneath the epidermis were almost absent in the tfm7-cs fruits and some collapsed parenchymatous cells in the mesocarp were observed in tfm7-cs .
taken together, these results suggest that the slcobra-like gene plays important roles in pericarp cell wall development of immature fruit.
analysis of cell wall macromolecules by fourier transform infrared  spectroscopy
to further investigate the effect of altered slcobra-like expression on cell wall macromolecules, a rapid assay for macromolecule composition in the wt and transgenic pericarp cell walls was performed using ftir spectroscopy.
as shown in figure
4a, the characteristic peaks of functional cellulose groups
<cit>  in the spectra were higher in the tfm7-oe fruits, but lower in the tfm7-cs fruits, as compared to wt.
these results indicate that, compared with the wt, tfm7-oe cell walls contain more crystalline cellulose, whereas tfm7-cs has less.
in addition, exploratory principal component analysis , a statistical method usually used for discriminant analysis of spectroscopic data
<cit> , was performed for the complete set of  <dig> ftir spectra collected from the wt and transgenic pericarp tissues .
three principal component loadings  were extracted, among which pc <dig> accounted for  <dig> % of the total variability, and with characteristic cellulose peaks .
the pca scores of pc1-pc <dig> or pc1-pc <dig> revealing the difference between the wt and transgenic pericarp tissues were pronounced : in this scatter plot, the cluster of tfm7-cs samples showed a clear separation from clusters of the wt or tfm7-oe samples, despite the fact that only subtle differences between the wt and tfm7-oe samples were observed, and in which slightly more positive scores were detected in tfm7-oe samples and more negative scores in wt samples .
altered cell wall composition of transgenic fruits with altered slcobra-like expression at the red ripe  stage
we next investigated the impact of altered slcobra-like expression on fruit cell wall composition in transgenic fruits.
cellulose quantification analysis of the pericarp cell wall
<cit>  was performed on wt and transgenic rr fruits.
cellulose levels were significantly higher  in tfm7-oe pericarp than in wt, but they decreased about 40% in tfm7-cs pericarps .
since pectin modification is important for textural changes during fruit ripening, pectin fractions were extracted using a sequential series of solvents
<cit>  and measured as the amount of galacturonic acid .
as shown in figure
5b, the amount of covalently bound pectin  in tfm7-oe fruits was higher than that in wt or tfm7-cs fruits.
moreover, the soluble cell wall fraction was significantly lower in the tfm7-oe fruits than in wt or tfm7-cs .
the ratio of bound pectin  to soluble pectin was significantly higher in tfm7-oe fruits  than in wt  or tfm7-cs .
to quantify sugar composition in fruit cell walls, the trifluoroacetic acid -soluble  wall fraction was converted to alditol acetates and analyzed by gas chromatography
<cit> .
as shown in figure
5c, the tfm7-oe fruit cell wall was found to possess more noncellulosic glucose  and cell wall galactosyl  residues.
in contrast, the tfm7-cs fruit cell wall had less arabinose , glc or gal, but more xylose .
altered fruit texture and shelf life in transgenic fruits
we conducted penetration and compression analyses  to determine the texture parameters of wt, tfm7-oe or tfm7-cs fruits at different developmental stages.
for all genotypes the skin mechanical strength and fruit firmness, represented by penetration mass and compression mass respectively, dramatically decreased from mg to rr, while fruit skin elasticity  did not significantly change between the mg and rr stages .
however, skin puncture strength was greater in tfm7-oe fruits, but lower in tfm7-cs, than in wt, and enhanced skin elasticity was also observed in tfm7-oe fruits.
in addition, despite a small reduction in firmness at the mg stage, tfm7-oe fruits were  <dig>  to  <dig> -fold firmer than wt at the rr stage.
in contrast, the compression mass of tfm7-cs fruits was about 33% that of wt fruits at the same stage .
thus, these results suggest slcobra-like gene activity manifests in fruit texture phenotypes.
texture of fresh intact fruits for wild-type  and three independent transgenic overexpressing  and cosuppressing  lines was analyzed using ta-xt plus .
a indicated that penetration analysis of fresh fruits with a 2mm cylinder probe ; three measurements for each fruit  were taken at each stage.
value was showed as mean ±se .
b indicated that compression analysis with a  <dig> mm compression platen .
value was showed as mean ±se .
the significance was calculated by the student’s t test.
asterisks indicated statistical differences to wt .
the enhanced mechanical strength and fruit firmness of tfm7-oe prompted us to assess whether overexpression of slcobra-like can extend fruit shelf life.
wt and tfm7-oe fruits were harvested at the rr stage  and stored at the same condition until they reached complete deterioration.
we found that during storage, tfm7-oe fruits lost less weight  than wt fruits .
in fact, wt fruits started to shrink  <dig> days after storage, with effusion of juice contents and loss of texture and integrity, whereas the tfm7-oe fruits did not display such signs of deterioration until  <dig> days later .
suppression of slcobra-like results in up-regulation of cell wall-degradation and cell wall-based signalling genes
the morphological alterations in tfm7-cs transgenic fruits prompted us to investigate whether slcobra-like impacts genes encoding proteins involved in cell wall-degradation.
since the slcobra-like mrna level is normally low at ripening, immature cs transgenic fruits at  <dig> dpa were selected for analysis.
as shown in figure
<dig>  elevated mrna accumulation of polygalacturonase , β-galactosidase  and expansin  was detected in tfm7-cs fruits.
notably, leexp <dig> transcripts increased 50~ <dig> fold.
in addition, several genes encoding receptor-like kinases  were also up-regulated in tfm7-cs fruits, including wall-associated kinase , theseus <dig> , fei and lectin receptor kinase  , suggesting that some of the relevant signalling pathways in the extracellular matrix that control the structure and integrity of cell wall were influecned by loss of slcobra-like expression homeostasis.
it is notable that there was considerable variation of the induction levels of these genes among the  <dig> tfm7-cs fruits.
we speculate this could be due to the different degrees of suppression of slcobra-like in the  <dig> tfm7-cs transgenic fruits .
discussion
cobra belongs to a multigene family consisting of  <dig> members in arabidopsis,  <dig> in rice, and  <dig> in maize
<cit> .
arabidopsis cobra, atcobl <dig>  maize zmbk <dig> as well as rice osbc <dig> have been shown to be required for cellulose synthesis
<cit> , while arabidopsis atcobl <dig> and maize zmbk2l <dig> impact tip-directed growth during root hair development
<cit> .
the newly released tomato genome sequence suggests there are at least  <dig> cobra family members in tomato .
however, the role of cobra in fruit development and texture has until now remained elusive.
our study addresses this question and provides a potential strategy for manipulation of fruit firmness and shelf life of tomatoes via modification of slcobra-like expression.
the role of the slcobra-like gene in fruit development
the slcobra-like gene was highly expressed during the early stages of fruit development, but the expression level dramatically declined after the breaker stage , indicating a role in early fruit development.
this was further supported by phenotypic analysis of fruit–specific suppression of the slcobra-like gene in the tfm7-cs transgenic tomato plants, which exhibited anatomical changes of fruits during early development.
it is worth noting that there are  <dig> cobra members in tomato, therefore it was necessary to verify the specificity of suppression of slcobra-like in the  <dig> tfm7-cs lines to allow accurate interpretation of our results.
rt-pcr showed that  <dig> slcobls, representing the highest, medium and low similarity to slcobra-like, were not repressed in the tfm7-cs fruits , suggesting suppression of slcobra-like is likely to be specific.
however, it is possible that other slcobl members also play important roles in fruit development.
further characterization of other slcobl members, particularly slcobl1/4/9/ <dig> that also contain all characteristic domains of cobra  and are expressed in fruit, will help to address this question.
effect of the slcobra-like gene on fruit cell wall biosynthesis and integrity
overexpression of the slcobra-like gene in transgenic fruits resulted in a significant increase of cellulose content .
on the other hand, more cell wall-bound na2co3-soluble pectin and cell wall galactosyl residues were found in the tfm7-oe rr fruit cell wall , indicating that overexpression of slcobra-like is responsible for less cell wall macromolecule solubilization/depolymerization during fruit ripening.
these results, together with the cracking phenotype of the tfm7-cs immature fruits, suggest that the slcobra-like protein is involved in not only regulating cellulose synthesis but also maintaining integrity of cell walls during processes of extension and assembly.
previous studies have also demonstrated the complexity of cell wall integrity.
for example, the brittle phenotype, observed in rice bc <dig> <cit> , arabidopsis cobl <dig> <cit>  and maize bk <dig> <cit> , is not necessarily a result of cellulose deficiency because arabidopsis cellulose synthases  mutants with a reduction in cellulose content did not display the brittle phenotype
<cit> .
slcobra-like may also play an important role in fine-tuning the expression of several genes encoding enzymes involved in cell wall degradation and cell wall-based signalling.
pg, tbg <dig> and leexp <dig> mrnas were elevated in immature tfm7-cs fruits .
these genes encode cell wall-degrading proteins and are normally induced at ripening  and throughout later ripening
<cit> .
in contrast, little if any change in expression was detected in pme and tbg <dig> , whose expression usually declines rapidly as fruit begin to ripen
<cit> .
it is also notable that the down-regulation of slcobra-like led to elevated expression of genes encoding several receptor-like kinases , which can relay a signal to the cytoplasm via the cytoplasmic kinase domain
<cit> .
similar to cobra proteins, arabinogalactan-proteins  usually have an n-terminal gpi anchor site
<cit>  and play important roles in cell expansion, proliferation and differentiation
<cit> , and signal transmission between the cell wall and cytoplasm
<cit> .
interestingly, repression of slcobra-like in tfm7-cs immature fruits repressed the mrna accumulation of tomato leagp-s <dig> encoding the s <dig> subunit of agp , thus suggesting genetic interaction between cobra and agp in fruit cell wall development.
however, the mechanism underlying this interaction remains to be elucidated.
overexpression of slcobra-like enhances fruit firmness and shelf life
the plant cell wall is a highly organized fibrillar network providing mechanical support for cells, tissues, organs and the entire plant body
<cit> .
it was suggested that over  <dig> annotated proteins are localized in the cell wall  and more than  <dig>  genes in the genome are implicated in cell wall biogenesis and modification
<cit> .
moreover, cell wall modifications have been implicated to be the major determinant of fruit softening, although changes in turgor pressure, anatomical characteristics, and cell wall integrity are also likely to play significant roles
<cit> .
in fact, transgenic manipulation of the activities of single cell wall-modifying enzymes in transgenic tomatoes had little impact on fruit softening during ripening
<cit> .
here we show that although slcobra-like is primarily expressed in early fruit development, it is required for normal fruit softening during ripening specifically through its reduced repression.
enhanced expression of slcobra-like in transgenic tfm7-oe fruits conferred increased fruit firmness and extended postharvest shelf life .
increased firmness might be due to both an increase in cellulose content also in addition to changes in pericarp anatomical structure, especially in the form of increased numbers of sub-epidermal collenchymatous cells.
it has been shown that changes in pericarp architecture can have a profound impact on fruit firmness.
guillon and co-workers reported that suppression of the tomato dr <dig> gene  caused unusual pericarp cell division and a higher proportion of sub-epidermal collenchymatous cells, resulting in pleiotropic phenotypes including enhanced fruit firmness similar to what we report here for slcobra-like overexpression
<cit> .
CONCLUSIONS
we present data demonstrating that the slcobra-like gene plays an important role in regulation of cell wall architecture during fleshy fruit development.
transgenic plants overexpressing slcobra-like exhibited enhanced fruit firmness and prolonged shelf life.
while aspects of regulation of slcobra-like expression and cell wall modification in tomato fruit development remain open to further investigation, our study provides a potential strategy for genetic manipulation of improved fleshy fruit quality and shelf-life via altered cobra expression.

 the ripening of fleshy fruits involves a number of physiological processes including the production of aromatic compounds, nutrients, pigmentation, and softening of flesh to an edible texture. these processes have direct impacts not only on fruit firmness, color, flavor and nutritional content, but also on shelf life, consumer acceptability, processing qualities, in addition to pre- and postharvest disease resistance. excessive fruit softening is the main factor contributing to damage during shipping, storage and post-harvest handling. fruit firmness and texture also affect the integrity of chopped and diced fruit used for canning and fruit products. because postharvest losses of fresh fruits due to excessive softening can account for as much as 30~40% of total production, considerable research had focused on mechanisms of fruit softening, often using tomato as a model system. fruit softening during the ripening process results in part from disassembly of the cell walls, leading to a reduction in intercellular adhesion, depolymerization and solubilization of pectins, depolymerization of hemicelluloses, and loss of pectic galactose side chains. generally, the decline in fruit firmness due to softening is accompanied by elevated expression of numerous cell metabolism enzymes, including polygalacturonase, pectin methylesterase, β-galactosidase, as well as cell wall loosening proteins such as expansin. suppression of single genes encoding fruit pg or pme in transgenic tomato plants had limited impact on fruit softening during ripening, but conferred longer shelf life resulting from reduced susceptibility to postharvest pathogens. these results suggest that suppression of certain enzymes acting on cellulose, hemicellulose or pectin alone are not sufficient to prevent softening, likely due to functional redundancy of enzymes involved in what is likely a complex metabolic process. nevertheless, a recent study has shown that down-regulation of genes encoding the n-glycan processing enzymes α-mannosidase and β-d-n-acetylhexosaminidase significantly increased fruit shelf life, which was attributed to decreased softening during ripening. these enzymes have been shown to break glycosidic bonds between carbohydrates, or between carbohydrates and noncarbohydrate structural molecules. expansins are cell wall-localized proteins faciliating wall loosening. they are involved in many aspects of cell wall modification during development through disruption of non-covalent bonds between matrix glycans and cellulose microfibrils. transgenic silencing of the tomato expansin gene leexp resulted in increased fruit firmness throughout ripening and improved fruit integrity during storage. molecular and genetic investigations have identified additional regulators of cell wall biosynthesis and regulation of cell expansion. one such activity is encoded by the cobra gene previously reported in arabidopsis, rice and maize. the cobra gene encodes a plant-specific glycosylphosphatidylinositol -anchored protein with a ω-attachment site at the c terminus, a hydrophilic central region, a ccvs domain, a potential n-glycosylation site, an n-terminal secretion signal sequence, and a predicted cellulose binding site. it has been reported that cobra localizes at the external plasma membrane leaflet through a glycosylphosphatidylinositol moiety. genetic impairment of the cobra gene results in reduced levels and improper orientation of crystalline cellulose microfibrils in arabidopsis and rice. despite the many studies of the cobra gene in several plant species, little has been learned concerning cobra ortholog in tomatoes, the model system for fleshy fruit development and ripening. here we report functional characterization of a tomato cobra gene. we specifically demonstrate its role in early fruit development and the potential for enhanced fruit firmness and shelf life by manipulating its expression in maturing transgenic tomato fruits.  cobra gene family members in tomato in order to identify tomato orthologs of the cobra gene, we ran a blast search in the sol genomics network using the arabidopsis cobra gene sequence as query. unigene tomato cdnas homologous to atcob were found in the sgn database and were designated as slcobls. all of the corresponding predicted amino acid sequences contain the characteristic interpro ipr or ipr omain of the plant cobra-like gene family. these slcobl members, together with other plant cobra-like proteins from arabidopsis, rice and maize, can be grouped in two clades of a phylogenetic tree. slcobl members consisting of 5~ exons were clustered into clade i and the other members were grouped in clade ii. we focused further analyses on slcobra-like as it appeared to be constitutively expressed in many tomato tissues suggesting an essential role in tomato biology. a total of slcobra-like ests were found in the sgn est database, which were derived from various tissues including leaf, flower and developing fruits. a the number of exons. bamino acid residue number in each predicted slcobl protein. predictions of n-terminal signal peptide and potential c-terminal gpi-modification sites were performed with signalp version and big-pi, respectively. expression of slcobra-like in tomato the full-length slcobra-like cdna was isolated from tomato seedlings by rt-pcr using gene-specific primers. the deduced slcobra-like protein contains a central cysteine-rich domain, a n-terminal secretion signal sequence for targeting to the endoplasmic reticulum, a highly hydrophobic c terminus and the ω-site required for processing the c-terminal. moreover, several potential n-glycosylation sites frequently associated with gpi-anchored proteins and extracellular proteins, as well as one hmm-predicted putative cellulose binding domain ii were observed in the slcobra-like sequence. basic local alignment search tool analysis showed that slcobra-like shares 63~80% similarity with other cobra proteins from arabidopsis, oryza sativa, and zea mays. phylogenetic analysis revealed slcobra-like shares the highest amino acid identity with atcobl thus localizing within in the same clade. we also conducted real time rt-pcr analysis and found that the slcobra-like gene is predominantly expressed in roots, stems, leaves, flowers and early development fruits. however, the expression level in fruits declined dramatically at the breaker and later ripening stages, implying a possible role in early fruit development. in addition, we isolated total rnas from exocarp, mesocarp, columella or locular fruit tissue from immature fruit days post anthesis for rt-pcr analysis. the results showed that the slcobra-like expression level is relatively high in locular tissue as compared to other tissues at this specific developmental stage. transgenic tomatoes whose endogenous slcobra-like gene was repressed displayed abnormal fruit phenotypes to determine the role of slcobra-like in fruit development, transgenic tomato plants over-expressing the slcobra-like cdna in fruits were generated. the slcobra-like transgene was driven by a fruit-specific tfm promoter. twenty independent transgenic lines were generated. as expected, several transgenic lines overexpressing slcobra-like were confirmed by rt-pcr analyses. none of these oe lines exhibited altered fruit phenotypes. however, three of the twenty transgenic lines exhibited extensive non-uniform cracking on the surface of their immature green fruits. rt-pcr analyses revealed that in cracked transgenic fruits, the endogenous slcobra-like expression was substantially co-suppressed as compared to non-transformed wild type control plants. we then generated transgenic tomato plants with rna interference -based repression of the slcobra-like gene to confirm this repression effect. unfortunately, none of our rnai transgenic lines displayed significant repression of the slcobra-like gene. one possibility is that more complete suppression of the slcobra-like gene by rnai in culture tissues than resulting from cs may have caused lethality in early development. because there are cobra members in tomato, it was necessary to verify the specificity of suppression of slcobra-like in the available cs lines. we examined the expression of slcobl and in cs fruits. there were two reasons we selected these slcobls: firstly they represent high, medium, and low similarity to slcobra-like, respectively; secondly the fruit-derived ests of these slcobls were found in the sgn est database, suggesting they are expressed in fruits, the tissues where phenotypes were most apparent. as shown in figure 2b, the real-time rt-pcr assay indicated the expression of slcobl and was not affected in cs lines, suggesting that fruit cracking was caused specifically by repression of slcobra-like expression. anatomical alterations of fruit pericarps in transgenic plants with altered slcobra-like expression since the exocarp plays important roles in determining the rate of expansion and mechanical support for the entire fruit, light microscopy analysis was conducted to examine possible changes in this tissue of transgenic fruits. dpa fruit exocarp sections were mounted in 15% hcl and photographed using a leica ldm microscope. as shown in figures 3a-c, the epidermal cell layer of fruit exocarp displayed no significant differences in epidermal cell size between wt and tfm7-oe. however, the epidermal cell size of tfm7-cs exocarp tissue was ± 10- mm far exceeding that of the wt or tfm7-oe. in addition, the epidermal cells of tfm7-cs pericarp expanded radially to a far greater extent than those of the wt or tfm7-oe lines. in contrast, the epidermal cell separation was significantly altered in both tfm7-oe and tfm7-cs fruits, as compared with that in wt, suggesting thicker epidermal cell walls in tfm7-oe fruits but thinner epidermal cell walls in the tfm7-cs fruits, respectively. to further compare the pericarp structure of fruits in wt and transgenic lines, detailed cytological analysis was performed. anatomical paraffin sectioning and calcofluor staining of pericarp tissue revealed an increased number of collenchymatous cells beneath the epidermis in the tfm7-oe fruits compared to wt. in contrast, the tfm7-cs fruits displayed a "waviness" of the surface with an apparent lack of cuticle and an abnormal shape and size-distribution of the cells, particularly in the thin layer of elongated epidermal cells at the fruit surface. the small cells beneath the epidermis were almost absent in the tfm7-cs fruits and some collapsed parenchymatous cells in the mesocarp were observed in tfm7-cs. taken together, these results suggest that the slcobra-like gene plays important roles in pericarp cell wall development of immature fruit. analysis of cell wall macromolecules by fourier transform infrared spectroscopy to further investigate the effect of altered slcobra-like expression on cell wall macromolecules, a rapid assay for macromolecule composition in the wt and transgenic pericarp cell walls was performed using ftir spectroscopy. as shown in figure 4a, the characteristic peaks of functional cellulose groups in the spectra were higher in the tfm7-oe fruits, but lower in the tfm7-cs fruits, as compared to wt. these results indicate that, compared with the wt, tfm7-oe cell walls contain more crystalline cellulose, whereas tfm7-cs has less. in addition, exploratory principal component analysis, a statistical method usually used for discriminant analysis of spectroscopic data, was performed for the complete set of ftir spectra collected from the wt and transgenic pericarp tissues. three principal component loadings were extracted, among which pc accounted for % of the total variability, and with characteristic cellulose peaks. the pca scores of pc1-pc or pc1-pc revealing the difference between the wt and transgenic pericarp tissues were pronounced: in this scatter plot, the cluster of tfm7-cs samples showed a clear separation from clusters of the wt or tfm7-oe samples, despite the fact that only subtle differences between the wt and tfm7-oe samples were observed, and in which slightly more positive scores were detected in tfm7-oe samples and more negative scores in wt samples. altered cell wall composition of transgenic fruits with altered slcobra-like expression at the red ripe stage we next investigated the impact of altered slcobra-like expression on fruit cell wall composition in transgenic fruits. cellulose quantification analysis of the pericarp cell wall was performed on wt and transgenic rr fruits. cellulose levels were significantly higher in tfm7-oe pericarp than in wt, but they decreased about 40% in tfm7-cs pericarps. since pectin modification is important for textural changes during fruit ripening, pectin fractions were extracted using a sequential series of solvents and measured as the amount of galacturonic acid. as shown in figure 5b, the amount of covalently bound pectin in tfm7-oe fruits was higher than that in wt or tfm7-cs fruits. moreover, the soluble cell wall fraction was significantly lower in the tfm7-oe fruits than in wt or tfm7-cs. the ratio of bound pectin to soluble pectin was significantly higher in tfm7-oe fruits than in wt or tfm7-cs. to quantify sugar composition in fruit cell walls, the trifluoroacetic acid -soluble wall fraction was converted to alditol acetates and analyzed by gas chromatography. as shown in figure 5c, the tfm7-oe fruit cell wall was found to possess more noncellulosic glucose and cell wall galactosyl residues. in contrast, the tfm7-cs fruit cell wall had less arabinose, glc or gal, but more xylose. altered fruit texture and shelf life in transgenic fruits we conducted penetration and compression analyses to determine the texture parameters of wt, tfm7-oe or tfm7-cs fruits at different developmental stages. for all genotypes the skin mechanical strength and fruit firmness, represented by penetration mass and compression mass respectively, dramatically decreased from mg to rr, while fruit skin elasticity did not significantly change between the mg and rr stages. however, skin puncture strength was greater in tfm7-oe fruits, but lower in tfm7-cs, than in wt, and enhanced skin elasticity was also observed in tfm7-oe fruits. in addition, despite a small reduction in firmness at the mg stage, tfm7-oe fruits were to -fold firmer than wt at the rr stage. in contrast, the compression mass of tfm7-cs fruits was about 33% that of wt fruits at the same stage. thus, these results suggest slcobra-like gene activity manifests in fruit texture phenotypes. texture of fresh intact fruits for wild-type and three independent transgenic overexpressing and cosuppressing lines was analyzed using ta-xt plus. a indicated that penetration analysis of fresh fruits with a 2mm cylinder probe; three measurements for each fruit were taken at each stage. value was showed as mean ±se. b indicated that compression analysis with a mm compression platen. value was showed as mean ±se. the significance was calculated by the student’s t test. asterisks indicated statistical differences to wt. the enhanced mechanical strength and fruit firmness of tfm7-oe prompted us to assess whether overexpression of slcobra-like can extend fruit shelf life. wt and tfm7-oe fruits were harvested at the rr stage and stored at the same condition until they reached complete deterioration. we found that during storage, tfm7-oe fruits lost less weight than wt fruits. in fact, wt fruits started to shrink days after storage, with effusion of juice contents and loss of texture and integrity, whereas the tfm7-oe fruits did not display such signs of deterioration until days later. suppression of slcobra-like results in up-regulation of cell wall-degradation and cell wall-based signalling genes the morphological alterations in tfm7-cs transgenic fruits prompted us to investigate whether slcobra-like impacts genes encoding proteins involved in cell wall-degradation. since the slcobra-like mrna level is normally low at ripening, immature cs transgenic fruits at dpa were selected for analysis. as shown in figure elevated mrna accumulation of polygalacturonase, β-galactosidase and expansin was detected in tfm7-cs fruits. notably, leexp transcripts increased 50~ fold. in addition, several genes encoding receptor-like kinases were also up-regulated in tfm7-cs fruits, including wall-associated kinase, theseus, fei and lectin receptor kinase, suggesting that some of the relevant signalling pathways in the extracellular matrix that control the structure and integrity of cell wall were influecned by loss of slcobra-like expression homeostasis. it is notable that there was considerable variation of the induction levels of these genes among the tfm7-cs fruits. we speculate this could be due to the different degrees of suppression of slcobra-like in the tfm7-cs transgenic fruits. discussion cobra belongs to a multigene family consisting of members in arabidopsis, in rice, and in maize. arabidopsis cobra, atcobl maize zmbk as well as rice osbc have been shown to be required for cellulose synthesis, while arabidopsis atcobl and maize zmbk2l impact tip-directed growth during root hair development. the newly released tomato genome sequence suggests there are at least cobra family members in tomato. however, the role of cobra in fruit development and texture has until now remained elusive. our study addresses this question and provides a potential strategy for manipulation of fruit firmness and shelf life of tomatoes via modification of slcobra-like expression. the role of the slcobra-like gene in fruit development the slcobra-like gene was highly expressed during the early stages of fruit development, but the expression level dramatically declined after the breaker stage, indicating a role in early fruit development. this was further supported by phenotypic analysis of fruit–specific suppression of the slcobra-like gene in the tfm7-cs transgenic tomato plants, which exhibited anatomical changes of fruits during early development. it is worth noting that there are cobra members in tomato, therefore it was necessary to verify the specificity of suppression of slcobra-like in the tfm7-cs lines to allow accurate interpretation of our results. rt-pcr showed that slcobls, representing the highest, medium and low similarity to slcobra-like, were not repressed in the tfm7-cs fruits, suggesting suppression of slcobra-like is likely to be specific. however, it is possible that other slcobl members also play important roles in fruit development. further characterization of other slcobl members, particularly slcobl1/4/9/ that also contain all characteristic domains of cobra and are expressed in fruit, will help to address this question. effect of the slcobra-like gene on fruit cell wall biosynthesis and integrity overexpression of the slcobra-like gene in transgenic fruits resulted in a significant increase of cellulose content. on the other hand, more cell wall-bound na2co3-soluble pectin and cell wall galactosyl residues were found in the tfm7-oe rr fruit cell wall, indicating that overexpression of slcobra-like is responsible for less cell wall macromolecule solubilization/depolymerization during fruit ripening. these results, together with the cracking phenotype of the tfm7-cs immature fruits, suggest that the slcobra-like protein is involved in not only regulating cellulose synthesis but also maintaining integrity of cell walls during processes of extension and assembly. previous studies have also demonstrated the complexity of cell wall integrity. for example, the brittle phenotype, observed in rice bc, arabidopsis cobl and maize bk, is not necessarily a result of cellulose deficiency because arabidopsis cellulose synthases mutants with a reduction in cellulose content did not display the brittle phenotype. slcobra-like may also play an important role in fine-tuning the expression of several genes encoding enzymes involved in cell wall degradation and cell wall-based signalling. pg, tbg and leexp mrnas were elevated in immature tfm7-cs fruits. these genes encode cell wall-degrading proteins and are normally induced at ripening and throughout later ripening. in contrast, little if any change in expression was detected in pme and tbg, whose expression usually declines rapidly as fruit begin to ripen. it is also notable that the down-regulation of slcobra-like led to elevated expression of genes encoding several receptor-like kinases, which can relay a signal to the cytoplasm via the cytoplasmic kinase domain. similar to cobra proteins, arabinogalactan-proteins usually have an n-terminal gpi anchor site and play important roles in cell expansion, proliferation and differentiation, and signal transmission between the cell wall and cytoplasm. interestingly, repression of slcobra-like in tfm7-cs immature fruits repressed the mrna accumulation of tomato leagp-s encoding the s subunit of agp, thus suggesting genetic interaction between cobra and agp in fruit cell wall development. however, the mechanism underlying this interaction remains to be elucidated. overexpression of slcobra-like enhances fruit firmness and shelf life the plant cell wall is a highly organized fibrillar network providing mechanical support for cells, tissues, organs and the entire plant body. it was suggested that over annotated proteins are localized in the cell wall and more than genes in the genome are implicated in cell wall biogenesis and modification. moreover, cell wall modifications have been implicated to be the major determinant of fruit softening, although changes in turgor pressure, anatomical characteristics, and cell wall integrity are also likely to play significant roles. in fact, transgenic manipulation of the activities of single cell wall-modifying enzymes in transgenic tomatoes had little impact on fruit softening during ripening. here we show that although slcobra-like is primarily expressed in early fruit development, it is required for normal fruit softening during ripening specifically through its reduced repression. enhanced expression of slcobra-like in transgenic tfm7-oe fruits conferred increased fruit firmness and extended postharvest shelf life. increased firmness might be due to both an increase in cellulose content also in addition to changes in pericarp anatomical structure, especially in the form of increased numbers of sub-epidermal collenchymatous cells. it has been shown that changes in pericarp architecture can have a profound impact on fruit firmness. guillon and co-workers reported that suppression of the tomato dr gene caused unusual pericarp cell division and a higher proportion of sub-epidermal collenchymatous cells, resulting in pleiotropic phenotypes including enhanced fruit firmness similar to what we report here for slcobra-like overexpression.  we present data demonstrating that the slcobra-like gene plays an important role in regulation of cell wall architecture during fleshy fruit development. transgenic plants overexpressing slcobra-like exhibited enhanced fruit firmness and prolonged shelf life. while aspects of regulation of slcobra-like expression and cell wall modification in tomato fruit development remain open to further investigation, our study provides a potential strategy for genetic manipulation of improved fleshy fruit quality and shelf-life via altered cobra expression. 
our results suggest that slcobra-like plays an important role in fruit cell wall architecture and provides a potential genetic tool for extending the shelf life of tomato and potentially additional fruits. fruit-specific suppression of slcobra-like resulted in impaired cell wall integrity and up-regulation of genes encoding proteins involved in cell wall degradation during early fruit development. the slcobra-like gene is highly expressed in vegetative organs and in early fruit development, but its expression in fruit declines dramatically during ripening stages, implying a primary role in early fruit development. extensive studies have demonstrated that the cobra gene is critical for biosynthesis of cell wall constituents comprising structural tissues of roots, stalks, leaves and other vegetative organs, however, its role in fruit development and ripening remains largely unknown. moreover, transgenic tomato fruits overexpressing slcobra-like exhibited desirable early development phenotypes including enhanced firmness and a prolonged shelf life. in contrast, fruit-specific overexpression of slcobra-like resulted in increased wall thickness of fruit epidermal cells, more collenchymatous cells beneath the epidermis, elevated levels of cellulose and reduced pectin solubilization in the pericarp cells of red ripe fruits. we identified a tomato gene homologous to arabidopsis cobra, and determined its role in fleshy fruit biology.
 extensive studies have demonstrated that the cobra gene is critical for biosynthesis of cell wall constituents comprising structural tissues of roots, stalks, leaves and other vegetative organs, however, its role in fruit development and ripening remains largely unknown.  we identified a tomato gene homologous to arabidopsis cobra, and determined its role in fleshy fruit biology. the slcobra-like gene is highly expressed in vegetative organs and in early fruit development, but its expression in fruit declines dramatically during ripening stages, implying a primary role in early fruit development. fruit-specific suppression of slcobra-like resulted in impaired cell wall integrity and up-regulation of genes encoding proteins involved in cell wall degradation during early fruit development. in contrast, fruit-specific overexpression of slcobra-like resulted in increased wall thickness of fruit epidermal cells, more collenchymatous cells beneath the epidermis, elevated levels of cellulose and reduced pectin solubilization in the pericarp cells of red ripe fruits. moreover, transgenic tomato fruits overexpressing slcobra-like exhibited desirable early development phenotypes including enhanced firmness and a prolonged shelf life.  our results suggest that slcobra-like plays an important role in fruit cell wall architecture and provides a potential genetic tool for extending the shelf life of tomato and potentially additional fruits. 