BACKGROUND
soybean  merr.
), a major source of protein and oil in the human diet is an important crop world wide.
soybean mosaic virus  is the most prevalent viral disease in many soybean production areas  <cit> .
infection with smv causes severe symptoms, including mosaic symptoms , necrosis , and seed mottling, resulting in serious yield losses  <cit> .
yield losses due to smv infection range from 8% to 50% under natural field conditions  <cit> , to total crop loss during severe outbreaks  <cit> .
there are seven smv strain groups  and three resistance loci  reported in soybean  <cit> .
soybean germplasm with rsv <dig> locus are resistant to smv strain groups g1-g <dig>  but susceptible to strains g5-g <dig>  <cit> .
in contrast, lines containing rsv <dig> confer resistance to strain groups g5-g <dig>  and condition stem-tip necrosis and/or mosaic symptoms to g1-g <dig>  <cit> .
the rsv <dig> locus was reported to produce seedling resistance to most smv isolates but systemic symptoms can appear as plants mature  <cit> .
rsv <dig>   <dig>  and  <dig> loci were mapped to chromosome  <dig>   <dig> and  <dig>  respectively  <cit>  and candidate genes for loci rsv <dig> and rsv <dig> have been putatively identified.
rsv <dig> was shown to link to a cluster of six non-toll interleukin  <dig> receptor  nucleotide-binding site leucine rich repeat  genes  <cit> , and recently directly verified using virus-induced gene silencing approach  <cit> .
furthermore, rsv <dig> gene has been reported to associate with a cluster of the coiled-coil nucleotide-binding leucine-rich repeat  resistance genes  <cit> .
overall, due to the large number of rsv loci and alleles, multiple smv strains, and the rapid evolution in avirulence/effector genes under r gene selective pressure, breeding to pyramid these rsv loci is complicated.
alternative modes of resistance to pathogen resistance are also possible.
potassium , the most abundant inorganic solute in plant cells, plays many important regulatory roles in plant development and stress responses  <cit> .
high k+ status decreases the occurrence of many diseases  <cit> .
perrenoud  reviewed more than  <dig>  studies and found a correlation between k+ status and disease incidence  <cit> .
a high k+ status reduced bacterial, fungal, and viral diseases in 69%, 70%, and 41% of the studies, respectively.
in seventeen case studies on viral disease, high k+ status reduced the incidence of viral diseases in nine studies, although five studies showed the opposite effect  <cit> .
the correlation between k+ status of plants and their susceptibility to pathogens involves changes in their primary metabolite profiles and distribution and the hormonal pathways in plants with altered k+ status  <cit> .
k+ status affects the function of multiple plant enzymes, and thus it changes metabolite profiles and concentrations  <cit> .
the changes in metabolites ultimately alter the susceptibility of plants to pathogens.
furthermore, k+ status also affects plant hormonal pathways, i.e.
salicylic acid  and jasmonic acid  pathways  <cit> , that are involved in hypersensitive responses or aquired systemic resistance to pathogens.
plants absorb and transport k ions using a number of transport proteins  <cit> .
the superfamily of voltage-gated k+ channels, the shaker family, plays roles in k+ uptake and k+ loading into xylem and phloem  <cit> .
k channel proteins mediate either k+ uptake  or k+ release   <cit> .
the arabidopsis k+ weak channel akt <dig> can mediate both k+ uptake and release  <cit> .
in arabidopsis, akt <dig> is predominantly expressed in phloem tissues, guard cells, and root stellar tissues  <cit> .
akt <dig> regulates transport of k+ and other small molecules in phloem through its roles in electric cell signaling and membrane excitability  <cit> .
akt <dig> may be involved in plant stress responses by adjusting potassium gradients that are important energy sources in plant vascular tissues  <cit> .
we observed that the incidence of smv can be significantly reduced by application of k+ fertilizer.
analysis of k+ channel gene expression in smv-resistant and smv-susceptible cultivars showed that the expression of gmakt <dig>  the soybean inner k+ transporter gene, was induced in the resistant variety, but not in the susceptible cultivar.
overexpression of gmakt <dig> significantly increased smv resistance in the smv susceptible cultivar williams  <dig>  our results suggest that alterlation of the expression of the k+ transporter akt <dig> is a novel molecular approach to genetically enhance smv resistance in soybean.
RESULTS
the effect of k+ supply on smv incidence
to investigate whether k+ supply affected the resistance of soybean plants to smv, the susceptible soybean cultivar williams  <dig> was planted in pots containing low-k+ soil with or without the addition of k+ fertilizer.
ten-day-old seedlings with completely unrolled unifoliate leaves in both k+-sufficient and -starvation treatments were inoculated with smv-g <dig> or buffer .
at  <dig> dai, mild mottling and crinkled appearances were observed in the second unrolled trifoliate leaves in plants grown in low-k+ soil .
in contrast, only slight chlorotic spots were observed in the second unrolled trifoliate leaves of plants grown in high-k+ soil .
at  <dig> dai, all smv-infected soybean plants grown in k+-starvation soil showed typical smv-susceptible symptoms: the young trifoliate leaves were severely mosaic and curled .
however, plants grown in high-k+ soil showed late-susceptible symptoms or resistant phenotypes .
to quantify the amount of smv viral accumulation, quantitative rt-pcr  was carried out to determine the amount of smv rna in these soybean plants.
as shown in figure 1b, smv rna can be detected in all inoculated plants at  <dig> and  <dig> dai, indicating that the inoculation process was effective.
the amount of smv rna in soybean plants grown in k+-starvation soil was significantly higher than that in k+-sufficient soil at both  <dig> and  <dig> dai , suggesting that sufficient supply of k+ could reduce smv accumulation.
leaf k+ concentrations from the first to the seventh trifoliate leaves of 45-day-old plants grown in high- or low-k+ soil were measured.
as expected, k+ concentrations in older leaves were significantly lower than in younger leaves.
k+ concentrations in the leaves of plants with sufficient k+ supplies were significantly higher than in the corresponding leaves of those in k+-starvation soil .
differential expression of gmakt <dig> in smv-resistant and -susceptible genotypes in response to smv infection
to determine whether k+ transporter genes were involved in soybean responses to smv infection, the transcript levels of the soybean k+ channel genes gmakt <dig> and gmakt <dig> in response to smv infection were investigated.
genotypes used for the assay included rsmv <dig>  williams  <dig>  and ssmv <dig>  which are highly-resistant, susceptible, and highly susceptible to smv, respectively .
the amounts of smv rna accumulation detected by qrt-pcr matched to the symptoms of viral infection .
while the inoculation of smv g <dig> significantly increased the accumulation of smv rna in williams  <dig> and ssmv <dig> plants at  <dig> and  <dig> dai, but not in rsmv <dig> plants.
compared to williams  <dig>  ssmv <dig> plant was more susceptible to smv infection .
to determine whether smv infection will affect the expression of k+ transporters, qrt-pcr was performed to examine gmakt <dig> and gmakt <dig> expression in first trifoliate leaves at  <dig> dai.
gmakt <dig> transcript levels were significantly induced by smv infection in rsmv <dig>  but not in ssmv <dig> .
in williams  <dig>  gmakt <dig> transcript levels were induced to a lower extent than in rsmv <dig> .
in contrast, gmakt <dig> expression was not affected by smv inoculation in all three genotypes .
gmakt <dig> was preferentially expressed in aerial tissues and induced by k+ starvation
to analyze gmakt <dig> expression in various soybean tissues, qrt-pcr was performed on rna samples extracted from 6-week-old williams  <dig> seedlings grown hydroponically.
gmakt <dig> was preferentially expressed in aerial tissues, especially leaves .
to determine whether the expression of gmakt <dig> was affected by the status of nutrient supplies, 10-day-old soybean seedlings were transferred to solution cultures lacking nitrogen, phosphate, or potassium.
gmakt <dig> expression was specifically induced by k+ deficiency .
generation of transgenic soybean overexpressing gmakt2
the gmakt <dig> full-length cdna was amplified from rsmv <dig>  williams  <dig>  and ssmv <dig>  respectively.
sequence analysis showed that there is no variation on the sequence of gmakt <dig> cdna .
to assay the effect of gmakt <dig> on smv resistance, the obtained gmakt <dig> cdna driven by a modified camv 35s promoter  was introduced into williams  <dig> via agrobacterium-mediated transformation.
four independent t <dig> transgenic lines overexpressing gmakt <dig>  were verified by southern blot analysis using the phosphinothricin acetyl transferase gene  as the probe.
oe <dig>   <dig>   <dig>  and  <dig> were four independent transgenic lines each containing a single copy  or two copies  of the transgene .
qrt-pcr analysis of the t <dig> transgenic soybean plants showed that the transgene gmakt <dig> was highly expressed in all transgenic lines .
the two transgenic events, oe <dig> and oe <dig>  showing the highest levels of gmakt <dig> gene expression, were selected for further experiments.
the growth of transgenic soybean plants was compared with the growth of recipient soybean plants.
no significant differences were observed between wild type and transgenic plants in the agronomic traits investigated, including plant height, stem diameter, branch number, node number, pod number, seed number, seed yield, and 100-seed weights .
gmakt2
-overexpressing transgenic  plants in field condition
data collected from a field experiment in anhui province,  <dig>  data are given as means ± sd.
different letters in a row indicate significant differences .
gmakt <dig> overexpression altered k distribution in soybean leaves
k+ concentrations in the first to seventh trifoliate leaves of 6-week-old transgenic and wt plants grown in k+-sufficient or -starvation soil were analyzed.
compared to wt plants, overexpression of gmakt <dig> significantly increased k+ concentrations in young leaves  of plants grown in k+-sufficient or -starvation conditions .
for instance, k+ concentrations in the seventh leaves of the gmakt2-overexpression lines were 27-40 % higher than that of the wt plants .
in contrast, k+ concentrations in the old leaves  of the gmakt2-oe plants were significantly lower than those in the wt plants .
these results indicated that gmakt <dig> acted as a k+ transporter and affected the distribution of k+ in soybean plants.
gmakt <dig> overexpression increased smv resistance in transgenic soybean
to determine the effect of gmakt <dig> overexpression on smv resistance, gmakt2-oe and wt plants were grown in pots with soil without the addition of k fertilizer in greenhouses.
at least five plants were inoculated with smv g <dig> or buffer in their unrolled unifoliate leaves at the vc stage  <cit>  when the unifoliate leaves were fully expanded.
starting from  <dig> dai, while severe mosaic symptoms were observed on the leaves of the wt plants, the gmakt2-overexpression lines oe <dig> and oe <dig> showed no symptom of smv infection .
to confirm the results, qrt-pcr was used to determine smv virus accumulation in these soybean plants.
compared to those in the mock treatment, smv inoculation significantly increased the amount of smv virus in both wt and gmakt2-oe plants.
at  <dig> dai, the level of smv rna in wt plants were about  <dig> and  <dig> times greater than that in the gmakt2- oe <dig> and oe <dig> lines, respectively .
with the increase of the infection process, at  <dig> dai, the amount of smv rna in the wt was further increased.
it was about  <dig> times as much as that of the average level of smv rna in gmakt2-overexpression lines.
to verify whether the smv resistance conferred by gmakt <dig> overexpression is also effective on the other smv stain, we tested the resistance of the transgenic plants to smv strain g <dig>  results showed that the transgenic lines gmakt2-oe <dig> and oe <dig> were also resistant to the infection of g <dig>  overexpression of gmakt <dig> significantly reduced the amount of g <dig> rna in plants .
the increased virus resistance in the transgenic lines gmakt2-oe <dig> and oe <dig> significantly increased their growth performance and yield when the plants inoculated smv strain g <dig>  as shown in table  <dig>  the smv infection significantly reduced the plant height, numbers of node, pod, and seed, and the seed weight per plant of the wt soybean.
in contrast, the infection of smv virus slightly affected those agronomic traits.
the difference in those traits between the infected plants and mock was not significant.
gmakt2
-overexpressing transgenic  plants with or without smv inoculation
data collected from a field experiment in anhui province,  <dig>  data are given as means ± sd.
different letters in a row indicate significant differences .
discussion
arabidopsis akt2/ <dig> is a rectifying k+ channel that regulates k+ loading in phloem in source tissues and the unloading in sink tissues  <cit> .
in this study, the soybean atakt <dig> orthologous gene, gmakt <dig> was overexpressed in soybean.
the amino acid sequence similarity between atakt <dig> and gmakt <dig> is 63% and they are phylogenetically classified in the same subcluster .
it is proposed that gmakt <dig> functions similarly as atakt2/ <dig> to regulate the k+ loading in phloem.
indeed, we observed that overexpression of gmakt <dig> enhanced k+ reallocation from the source ) to the sink , which led to a significant increase in k+ concentrations in the latter .
in addition to the enhancement of k+ transport, overexpression of gmakt <dig> on a smv-susceptible genotype williams  <dig> significantly retarded the development of smv and resulted in higher resistance to smv .
plant resistance to viral disease can be achieved by inhibiting viral long-distance movement.
in a typical virus infection process, virus moves from the source leaves to the sink leaves via the phloem transport system  <cit> , similar to the direction of k+ movement.
a mutation in atakt <dig> was found to inhibit sugar loading into the phloem of arabidopsis  <cit> .
it is possible that overexpression of the soybean akt <dig> resulted in changes in phloem environment to be unfavorable to virus loading, movement and unloading.
identification of the factor to link the k+ transportation and virus movement will help elucidate the mechanism of how the gmakt <dig> overexpression in soybean results in greater resistance to smv.
plant resistance to virus disease could also be achieved by inhibition of the viral multiplication.
as expected, plant nutritional status affects plant health and, consequently, influences plant resistance to pathogen infection.
in this study, the application of k+ fertilizer significantly reduced the smv accumulation in the smv-susceptible cultivar williams  <dig>  thus, the sufficient k+ concentrations in soybean plant cells should be important for suppression of smv spread.
on the other hand, in the smv-resistant gmakt <dig> overexpression plants, the increase of k+ concentrations only occurred in younger leaves, but not in the older leaves.
the results suggest that maintenance sufficient k+ content in young leaves should be important for soybean resistance to virus infection.
CONCLUSIONS
the soybean potassium channel gene, gmakt <dig> was significantly induced by smv inoculation in the smv highly-resistant genotype rsmv <dig>  but not in the highly-susceptible genotype ssmv <dig>  the role of gmakt <dig> on smv resistance was evaluated in the study.
results showed that overexpression of gmakt <dig> on a smv-susceptible genotype williams  <dig> enhanced the reallocation of k+ in the soybean plants and increased k+ concentrations in young leaves.
furthermore, smv development was significantly retarded in gmakt2-overexpressing transgenic soybean plants following smv inoculation.
these data provide a novel route for initiating molecular breeding to improve smv resistance in soybean, an important crop world wide.

 soybean merr.), a major source of protein and oil in the human diet is an important crop world wide. soybean mosaic virus is the most prevalent viral disease in many soybean production areas. infection with smv causes severe symptoms, including mosaic symptoms, necrosis, and seed mottling, resulting in serious yield losses. yield losses due to smv infection range from 8% to 50% under natural field conditions, to total crop loss during severe outbreaks. there are seven smv strain groups and three resistance loci reported in soybean. soybean germplasm with rsv locus are resistant to smv strain groups g1-g but susceptible to strains g5-g. in contrast, lines containing rsv confer resistance to strain groups g5-g and condition stem-tip necrosis and/or mosaic symptoms to g1-g. the rsv locus was reported to produce seedling resistance to most smv isolates but systemic symptoms can appear as plants mature. rsv and loci were mapped to chromosome and respectively and candidate genes for loci rsv and rsv have been putatively identified. rsv was shown to link to a cluster of six non-toll interleukin receptor nucleotide-binding site leucine rich repeat genes, and recently directly verified using virus-induced gene silencing approach. furthermore, rsv gene has been reported to associate with a cluster of the coiled-coil nucleotide-binding leucine-rich repeat resistance genes. overall, due to the large number of rsv loci and alleles, multiple smv strains, and the rapid evolution in avirulence/effector genes under r gene selective pressure, breeding to pyramid these rsv loci is complicated. alternative modes of resistance to pathogen resistance are also possible. potassium, the most abundant inorganic solute in plant cells, plays many important regulatory roles in plant development and stress responses. high k+ status decreases the occurrence of many diseases. perrenoud reviewed more than studies and found a correlation between k+ status and disease incidence. a high k+ status reduced bacterial, fungal, and viral diseases in 69%, 70%, and 41% of the studies, respectively. in seventeen case studies on viral disease, high k+ status reduced the incidence of viral diseases in nine studies, although five studies showed the opposite effect. the correlation between k+ status of plants and their susceptibility to pathogens involves changes in their primary metabolite profiles and distribution and the hormonal pathways in plants with altered k+ status. k+ status affects the function of multiple plant enzymes, and thus it changes metabolite profiles and concentrations. the changes in metabolites ultimately alter the susceptibility of plants to pathogens. furthermore, k+ status also affects plant hormonal pathways, i.e. salicylic acid and jasmonic acid pathways, that are involved in hypersensitive responses or aquired systemic resistance to pathogens. plants absorb and transport k ions using a number of transport proteins. the superfamily of voltage-gated k+ channels, the shaker family, plays roles in k+ uptake and k+ loading into xylem and phloem. k channel proteins mediate either k+ uptake or k+ release. the arabidopsis k+ weak channel akt can mediate both k+ uptake and release. in arabidopsis, akt is predominantly expressed in phloem tissues, guard cells, and root stellar tissues. akt regulates transport of k+ and other small molecules in phloem through its roles in electric cell signaling and membrane excitability. akt may be involved in plant stress responses by adjusting potassium gradients that are important energy sources in plant vascular tissues. we observed that the incidence of smv can be significantly reduced by application of k+ fertilizer. analysis of k+ channel gene expression in smv-resistant and smv-susceptible cultivars showed that the expression of gmakt the soybean inner k+ transporter gene, was induced in the resistant variety, but not in the susceptible cultivar. overexpression of gmakt significantly increased smv resistance in the smv susceptible cultivar williams our results suggest that alterlation of the expression of the k+ transporter akt is a novel molecular approach to genetically enhance smv resistance in soybean.  the effect of k+ supply on smv incidence to investigate whether k+ supply affected the resistance of soybean plants to smv, the susceptible soybean cultivar williams was planted in pots containing low-k+ soil with or without the addition of k+ fertilizer. ten-day-old seedlings with completely unrolled unifoliate leaves in both k+-sufficient and -starvation treatments were inoculated with smv-g or buffer. at dai, mild mottling and crinkled appearances were observed in the second unrolled trifoliate leaves in plants grown in low-k+ soil. in contrast, only slight chlorotic spots were observed in the second unrolled trifoliate leaves of plants grown in high-k+ soil. at dai, all smv-infected soybean plants grown in k+-starvation soil showed typical smv-susceptible symptoms: the young trifoliate leaves were severely mosaic and curled. however, plants grown in high-k+ soil showed late-susceptible symptoms or resistant phenotypes. to quantify the amount of smv viral accumulation, quantitative rt-pcr was carried out to determine the amount of smv rna in these soybean plants. as shown in figure 1b, smv rna can be detected in all inoculated plants at and dai, indicating that the inoculation process was effective. the amount of smv rna in soybean plants grown in k+-starvation soil was significantly higher than that in k+-sufficient soil at both and dai, suggesting that sufficient supply of k+ could reduce smv accumulation. leaf k+ concentrations from the first to the seventh trifoliate leaves of 45-day-old plants grown in high- or low-k+ soil were measured. as expected, k+ concentrations in older leaves were significantly lower than in younger leaves. k+ concentrations in the leaves of plants with sufficient k+ supplies were significantly higher than in the corresponding leaves of those in k+-starvation soil. differential expression of gmakt in smv-resistant and -susceptible genotypes in response to smv infection to determine whether k+ transporter genes were involved in soybean responses to smv infection, the transcript levels of the soybean k+ channel genes gmakt and gmakt in response to smv infection were investigated. genotypes used for the assay included rsmv williams and ssmv which are highly-resistant, susceptible, and highly susceptible to smv, respectively. the amounts of smv rna accumulation detected by qrt-pcr matched to the symptoms of viral infection. while the inoculation of smv g significantly increased the accumulation of smv rna in williams and ssmv plants at and dai, but not in rsmv plants. compared to williams ssmv plant was more susceptible to smv infection. to determine whether smv infection will affect the expression of k+ transporters, qrt-pcr was performed to examine gmakt and gmakt expression in first trifoliate leaves at dai. gmakt transcript levels were significantly induced by smv infection in rsmv but not in ssmv. in williams gmakt transcript levels were induced to a lower extent than in rsmv. in contrast, gmakt expression was not affected by smv inoculation in all three genotypes. gmakt was preferentially expressed in aerial tissues and induced by k+ starvation to analyze gmakt expression in various soybean tissues, qrt-pcr was performed on rna samples extracted from 6-week-old williams seedlings grown hydroponically. gmakt was preferentially expressed in aerial tissues, especially leaves. to determine whether the expression of gmakt was affected by the status of nutrient supplies, 10-day-old soybean seedlings were transferred to solution cultures lacking nitrogen, phosphate, or potassium. gmakt expression was specifically induced by k+ deficiency. generation of transgenic soybean overexpressing gmakt2 the gmakt full-length cdna was amplified from rsmv williams and ssmv respectively. sequence analysis showed that there is no variation on the sequence of gmakt cdna. to assay the effect of gmakt on smv resistance, the obtained gmakt cdna driven by a modified camv 35s promoter was introduced into williams via agrobacterium-mediated transformation. four independent t transgenic lines overexpressing gmakt were verified by southern blot analysis using the phosphinothricin acetyl transferase gene as the probe. oe and were four independent transgenic lines each containing a single copy or two copies of the transgene. qrt-pcr analysis of the t transgenic soybean plants showed that the transgene gmakt was highly expressed in all transgenic lines. the two transgenic events, oe and oe showing the highest levels of gmakt gene expression, were selected for further experiments. the growth of transgenic soybean plants was compared with the growth of recipient soybean plants. no significant differences were observed between wild type and transgenic plants in the agronomic traits investigated, including plant height, stem diameter, branch number, node number, pod number, seed number, seed yield, and 100-seed weights. gmakt2 -overexpressing transgenic plants in field condition data collected from a field experiment in anhui province, data are given as means ± sd. different letters in a row indicate significant differences. gmakt overexpression altered k distribution in soybean leaves k+ concentrations in the first to seventh trifoliate leaves of 6-week-old transgenic and wt plants grown in k+-sufficient or -starvation soil were analyzed. compared to wt plants, overexpression of gmakt significantly increased k+ concentrations in young leaves of plants grown in k+-sufficient or -starvation conditions. for instance, k+ concentrations in the seventh leaves of the gmakt2-overexpression lines were 27-40 % higher than that of the wt plants. in contrast, k+ concentrations in the old leaves of the gmakt2-oe plants were significantly lower than those in the wt plants. these results indicated that gmakt acted as a k+ transporter and affected the distribution of k+ in soybean plants. gmakt overexpression increased smv resistance in transgenic soybean to determine the effect of gmakt overexpression on smv resistance, gmakt2-oe and wt plants were grown in pots with soil without the addition of k fertilizer in greenhouses. at least five plants were inoculated with smv g or buffer in their unrolled unifoliate leaves at the vc stage when the unifoliate leaves were fully expanded. starting from dai, while severe mosaic symptoms were observed on the leaves of the wt plants, the gmakt2-overexpression lines oe and oe showed no symptom of smv infection. to confirm the results, qrt-pcr was used to determine smv virus accumulation in these soybean plants. compared to those in the mock treatment, smv inoculation significantly increased the amount of smv virus in both wt and gmakt2-oe plants. at dai, the level of smv rna in wt plants were about and times greater than that in the gmakt2- oe and oe lines, respectively. with the increase of the infection process, at dai, the amount of smv rna in the wt was further increased. it was about times as much as that of the average level of smv rna in gmakt2-overexpression lines. to verify whether the smv resistance conferred by gmakt overexpression is also effective on the other smv stain, we tested the resistance of the transgenic plants to smv strain g results showed that the transgenic lines gmakt2-oe and oe were also resistant to the infection of g overexpression of gmakt significantly reduced the amount of g rna in plants. the increased virus resistance in the transgenic lines gmakt2-oe and oe significantly increased their growth performance and yield when the plants inoculated smv strain g as shown in table  the smv infection significantly reduced the plant height, numbers of node, pod, and seed, and the seed weight per plant of the wt soybean. in contrast, the infection of smv virus slightly affected those agronomic traits. the difference in those traits between the infected plants and mock was not significant. gmakt2 -overexpressing transgenic plants with or without smv inoculation data collected from a field experiment in anhui province, data are given as means ± sd. different letters in a row indicate significant differences. discussion arabidopsis akt2/ is a rectifying k+ channel that regulates k+ loading in phloem in source tissues and the unloading in sink tissues. in this study, the soybean atakt orthologous gene, gmakt was overexpressed in soybean. the amino acid sequence similarity between atakt and gmakt is 63% and they are phylogenetically classified in the same subcluster. it is proposed that gmakt functions similarly as atakt2/ to regulate the k+ loading in phloem. indeed, we observed that overexpression of gmakt enhanced k+ reallocation from the source ) to the sink, which led to a significant increase in k+ concentrations in the latter. in addition to the enhancement of k+ transport, overexpression of gmakt on a smv-susceptible genotype williams significantly retarded the development of smv and resulted in higher resistance to smv. plant resistance to viral disease can be achieved by inhibiting viral long-distance movement. in a typical virus infection process, virus moves from the source leaves to the sink leaves via the phloem transport system, similar to the direction of k+ movement. a mutation in atakt was found to inhibit sugar loading into the phloem of arabidopsis. it is possible that overexpression of the soybean akt resulted in changes in phloem environment to be unfavorable to virus loading, movement and unloading. identification of the factor to link the k+ transportation and virus movement will help elucidate the mechanism of how the gmakt overexpression in soybean results in greater resistance to smv. plant resistance to virus disease could also be achieved by inhibition of the viral multiplication. as expected, plant nutritional status affects plant health and, consequently, influences plant resistance to pathogen infection. in this study, the application of k+ fertilizer significantly reduced the smv accumulation in the smv-susceptible cultivar williams thus, the sufficient k+ concentrations in soybean plant cells should be important for suppression of smv spread. on the other hand, in the smv-resistant gmakt overexpression plants, the increase of k+ concentrations only occurred in younger leaves, but not in the older leaves. the results suggest that maintenance sufficient k+ content in young leaves should be important for soybean resistance to virus infection.  the soybean potassium channel gene, gmakt was significantly induced by smv inoculation in the smv highly-resistant genotype rsmv but not in the highly-susceptible genotype ssmv the role of gmakt on smv resistance was evaluated in the study. results showed that overexpression of gmakt on a smv-susceptible genotype williams enhanced the reallocation of k+ in the soybean plants and increased k+ concentrations in young leaves. furthermore, smv development was significantly retarded in gmakt2-overexpressing transgenic soybean plants following smv inoculation. these data provide a novel route for initiating molecular breeding to improve smv resistance in soybean, an important crop world wide. 
in contrast, k+ concentrations in the old leaves of the gmakt2-oe plants were significantly lower than those in wt plants. at and dai, the amount of smv rna in wt plants increased 200- and 260- fold relative to gmakt2-oe plants at each time point. these results indicated that gmakt acted as a k+ transporter and affected the distribution of k+ in soybean plants. thus k+ transporters are putative candidates to target for soybean virus resistance. thus, alteration of k+ transporter expression is a novel molecular approach for enhancing smv resistance in soybean. analysis of k+ channel gene expression indicated that gmakt the ortholog of arabidopsis k+ weak channel encoding gene akt was significantly induced by smv inoculation in the smv highly-resistant genotype rsmv but not in the susceptible genotype ssmv transgenic soybean plants overexpressing gmakt were produced and verified by southern blot and rt-pcr analysis. thus, smv development was significantly retarded in gmakt2-overexpressing transgenic soybean plants. overexpression of gmakt significantly enhanced smv resistance in transgenic soybean. due to a large number of smv resistant loci and alleles, smv strains and the rapid evolution in avirulence/effector genes, traditional breeding for smv resistance is complex. potassium is the most abundant inorganic solute in plant cells, and is involved in plant responses to abiotic and biotic stresses. studies have shown that altering the level of k+ status can reduce the spread of the viral diseases. the addition of k+ fertilizer significantly reduced smv incidence. genetic engineering is an effective alternative method for improving smv resistance in soybean. analysis of k+ concentrations on different leaves of both the transgenic and the wildtype plants revealed that overexpression of gmakt significantly increased k+ concentrations in young leaves of plants. in contrast, the gmakt2-oe plants showed no symptom of smv infection. soybean mosaic virus is the most prevalent viral disease in many soybean production areas. starting from 14 days after inoculation of smv g severe mosaic symptoms were observed on the wt leaves.
 soybean mosaic virus is the most prevalent viral disease in many soybean production areas. due to a large number of smv resistant loci and alleles, smv strains and the rapid evolution in avirulence/effector genes, traditional breeding for smv resistance is complex. genetic engineering is an effective alternative method for improving smv resistance in soybean. potassium is the most abundant inorganic solute in plant cells, and is involved in plant responses to abiotic and biotic stresses. studies have shown that altering the level of k+ status can reduce the spread of the viral diseases. thus k+ transporters are putative candidates to target for soybean virus resistance.  the addition of k+ fertilizer significantly reduced smv incidence. analysis of k+ channel gene expression indicated that gmakt the ortholog of arabidopsis k+ weak channel encoding gene akt was significantly induced by smv inoculation in the smv highly-resistant genotype rsmv but not in the susceptible genotype ssmv transgenic soybean plants overexpressing gmakt were produced and verified by southern blot and rt-pcr analysis. analysis of k+ concentrations on different leaves of both the transgenic and the wildtype plants revealed that overexpression of gmakt significantly increased k+ concentrations in young leaves of plants. in contrast, k+ concentrations in the old leaves of the gmakt2-oe plants were significantly lower than those in wt plants. these results indicated that gmakt acted as a k+ transporter and affected the distribution of k+ in soybean plants. starting from 14 days after inoculation of smv g severe mosaic symptoms were observed on the wt leaves. in contrast, the gmakt2-oe plants showed no symptom of smv infection. at and dai, the amount of smv rna in wt plants increased 200- and 260- fold relative to gmakt2-oe plants at each time point. thus, smv development was significantly retarded in gmakt2-overexpressing transgenic soybean plants.  overexpression of gmakt significantly enhanced smv resistance in transgenic soybean. thus, alteration of k+ transporter expression is a novel molecular approach for enhancing smv resistance in soybean. soybean mosaic virusresistancepotassium channelgmakt2 