system: [SYSTEM]

user: 3. 常见花色表型变异的遗传机制 花色表型多样性不仅体现在花色参数上，多样的着色模式(图2)更加丰富了花色的多态。这些变异可 图2.已报道的花色着色模式类型 能有利于增加植物的环境适应性。花色的变异具有一定的遗传基础，花青素代谢途径上结构或调控基因的突变(包括编码区和调控区)都或多或少会影响基因的功能或表达量而影响花色，基因的组织特异性表达或功能分化也会影响花色表型。 3.1. 花色的获得 花青素代谢为类黄酮的一个代谢分支，根据类黄酮类物质和代谢途径基因类似片段的存在与否，人们推断类黄酮代谢途径应该起源于植物登陆 [ 45 ] 。一些基因功能的获得可能源于基因重复后的功能分化，如CHI被推测起源于脂肪酸合酶基因功能分化 [ 46 ] 。从产物类别来看，在原始植物角苔类(liverworts)、苔藓(mosses)和石松类(club mosses)中发现有黄烷酮、黄酮和黄酮醇物质的出现 [ 47 ] 。在蕨类(ferns)植物里出现原花青素 [ 47 ] ，而花青素则从在松柏类裸子植物中出现到广泛存在于被子植物中 [ 48 ] [ 49 ] 。在调控基因方面，由于MYB较强的功能和组织特异性，花色素从无到有或从少到多的获得转换有可能通过cis或trans的方式使激活型MYB高表达 [ 50 ] [ 51 ] [ 52 ] [ 53 ] [ 54 ] 或抑制型MYB低表达 [ 55 ] 来实现(表1)。 3.2. 从蓝/紫色到红/粉色的花色变异 花色从蓝/紫色到红/粉色的变异通常是由于花青素从飞燕草素或矢车菊素到天竺葵素的转变，即花青素羟基数目的减少，因而常伴随羟基化酶基因F3’H或F3’5’H的功能失活或表达量降低(表2)。牵牛花仅含有矢车菊素和天竺葵素 [ 56 ] [ 57 ] [ 58 ] ，对于本身就具有蓝/紫色花的物种如圆叶牵牛(Ipomoea purpurea)、 Table 1 Summary of MYB mutations for gain of pigmen 物种名 基因名 突变形式 表型变异 参考文献 Mimulus aurantiacus ssp. puniceus MaMYB2 Higher expression (CIS) Yellow to red [ 54 ] M. luteus ssp. cupreus PLA1 (R2R3-MYB) ? Yellow to orange red [ 52 ] M. luteus ssp. variegatus PLA2 (R2R3-MYB) ? Yellow to purple [ 52 ] M. cardinalis ROI1 (R3-MYB) Lower expression (CIS or Trans) Light pink to dark red [ 55 ] Phlox drummondii R2R3-MYB Higher expression (CIS) Light blue to dark red [ 53 ] Antirrhinum majus ssp. Pseudomajus ROSEA el Gain of an activator (?) Light yellow to dark red [ 51 ] A. majus ssp. Majus ROSEA el Gain of an activator (?) Light yellow to dark red [ 50 ] 表1. 花色获得的基因突变形式 COD代表编码区的突变；CIS代表cis调控区的突变；Trans代表反式作用转录因子的改变；“?”代表原因不详。 Table 2 Summary of mutations causing flower color changes from blue/purple to red/pin 物种名 基因名 突变形式 表型变异 参考文献 Ipomoea purpurea F3'H 550-bp Tip201 insertion in exon3 (COD) Blue to pink [ 59 ] I. purpurea F3'H 402-bp DNA sequences insertion in exon3 (COD) Blue to pink [ 60 ] I. nil F3'H C to T mutation in exon3 (COD) Blue to pink [ 59 ] I. tricolor F3'H T insertion in exon2 (COD) Blue to pink [ 59 ] I. quamoclit F3'H Lower expression in flower (CIS and tissue specific regulatory) Blue to red [ 75 ] [ 76 ] [ 77 ] DFR-B 15，3，3，6-bp in-frame and 98-bp DNA sequences insertion in 3'-flanking; Substrate specificity change (COD) I. horsfalliae F3'H Lower expression in flower (CIS and tissue specific regulatory) Blue to red [ 76 ] I. coccinea F3'H Lower expression in flower (CIS and tissue specific regulatory) Blue to red [ 77 ] Iochrominae.gesnerioides F3'H Lower expression in flower (CIS and tissue specific regulatory) Blue to red [ 78 ] [ 79 ] F3'5'H A premature stop condon in exon1 and a 136-bp out-of-frame deletion in exon2 (COD) DFR-B Substrate specificity change (COD) Penstemon barbatus F3'H Lower expression in flower (Tissue specific regulatory) Blue to red [ 80 ] [ 81 ] F3'5'H A premature stop condon at amino acid 407; 11 nonsynonymous mutations (COD) P. labrosus F3'5'H Low activity on DHK and very low activity on DHQ Blue to red [ 81 ] P. subulatus F3'5'H No activity on DHK and very low activity on DHQ Blue to red [ 81 ] Penstemon (10 species) F3'5'H Two or more premature stop codons， large deletions， or frameshifting indels (COD) Blue to red [ 81 ] Phlox drummondii F3'5'H Lower expression (CIS) Blue to red [ 53 ] Petunia hybrida F3'5'H A Spm-like transposon (Psl) insertion in exon2 (COD) Blue to red [ 82 ] P. hybrida F3'5'H Transposon insertion in exon3 (COD) Blue to red [ 83 ] Gentiana scabra F3'5'H Two independent transposons insertion in exon1 (COD) Blue to pink [ 84 ] Antirrhinum majus F3'5'H Lose of function (COD) Blue to red [ 85 ] Glycine max FLS Single base deletion in coding region (COD) Purple to magenta [ 86 ] 表2. 花色由蓝/紫色到红/粉色变化的基因突变形式 COD代表编码区的突变；CIS代表cis调控区的突变；Trans代表反式作用转录因子的改变；“?”代表原因不详。 裂叶牵牛(I. nil)、三色牵牛(I. tricolor)而言，其红/粉色变异通常是由于F3’H的编码区突变造成 [ 59 ] [ 60 ] 。对于同属内蓝/紫色到红/粉色的转变，若蓝/紫色由飞燕草素决定，通常会伴随F3’5’H的功能失活 [ 78 ] [ 80 ] - [ 85 ] [ 87 ] 或表达量下降 [ 53 ] ，有时也伴随F3’H在花组织中表达量的下降 [ 78 ] [ 80 ] [ 81 ] ；若蓝/紫色由矢车菊素决定，通常红/粉花中F3’H的启动子序列(cis调控)与蓝/紫花相比会发生变异而导致其表达量下调，且组织特异性表达调控使其仅降低花组织而不影响营养组织中矢车菊素的含量 [ 75 ] [ 76 ] [ 77 ] [ 87 ] [ 88 ] 。这是因为矢车菊素除了贡献于花色外，在紫外防御、抵御病原菌侵染或动物啃食、生长素极性运输等方面还具有重要的作用，所以F3’H的功能失活可能会影响整个植株的生长，组织特异性表达调控可以降低其有害多效性(deleterious pleiotropy)。统计数据表明植物营养组织中的花青素主要为矢车菊素，很少有植物会缺少有功能的F3’H拷贝，而F3’5’H会在许多科属如蔷薇科(Rosaceae)、菊科(Asteraceae)、拟南芥属(Arabidopsis)、番薯属(Ipomoea)、紫罗兰属(Matthiola)、郁金香属(Tulipa)植物中缺失 [ 45 ] [ 87 ] 。 由于F3’H或F3’5’H分支被阻断，花青素代谢通路只能走F3H-DFR的路径，所以花色从蓝/紫色到红/粉色的变异有时还伴随着DFR底物特异性的改变，即对底物DHK亲和性的提高，对DHQ和DHM亲和性的降低 [ 77 ] [ 78 ] [ 79 ] 。FLS表达量下降导致的黄酮醇含量下降也会使花由蓝紫色变为红色 [ 86 ] 。 3.3. 从蓝/紫/红色到白/黄色的花色变异 花色从蓝/紫/红色到白/黄色的变异通常被认为是花青素从有到无的变异，是质的变化。这些变化可能来自花青素代谢途径一个或多个步骤的阻断(表3)。代谢通路上结构基因的转座子插入 [ 62 ] [ 63 ] [ 89 ] [ 90 ] [ 91 ] 或移码突变 [ 92 ] 导致的功能失活或cis突变导致的表达量下降 [ 95 ] [ 96 ] 均会使植物产生白花。但结构基因的突变通常会使整个植物都无法产生花青素，最上游基因的突变可以让植物连黄酮、黄酮醇、黄烷酮等类黄酮物质也无法产生，会因而产生较多的有害多效性，降低植物适合度(e.g. [ 93 ] [ 94 ] )。R2R3-MYB [ 50 ] [ 51 ] [ 96 ] - [ 103 ] 、bHLH [ 104 ] [ 105 ] 和WDR [ 99 ] 的突变均可以造成花青素代谢途径上结构基因的表达下调而阻断花青素的合成，但由于R2R3-MYB具有较高的组织表达特异性和功能分化 [ 33 ] [ 34 ] [ 39 ] [ 50 ] [ 103 ] [ 106 ] [ 107 ] ，所以其突变具有较低的有害多效性。有观点认为自然选择应该偏爱R2R3-MYB的突变 [ 88 ] 。这与R2R3-MYB较bHLH和WDR及结构基因具有较高的异义和同义突变比率(ω)是一致的 [ 98 ] [ 108 ] [ 109 ] [ 110 ] [ 111 ] 。 花青素量的变化可能与花青素代谢途径基因的表达量变化有关，但具体机制还不清楚。在亚洲百合(Asiatic hybrid lilies Lilium spp.)中，MYB12的表达量与花瓣中花青素的积累量呈正比 [ 112 ] 。R2R3-MYB的表达量可解释小天蓝绣球(P. drummondii) (深红)与其近缘种P. cuspidate (浅蓝色)杂交F2代群体中花色的深浅 [ 53 ] [ 113 ] 。 3.4. 花着色模式的变异 花的着色式样主要有断色、彩斑、缘环、星状、紫外模式、斑点、脉色和芽红等(图2)。目前已知的花色着色模式成因主要有四种：病毒感染、转座子的插入及部分回复突变、siRNA介导的基因转录后沉默、基因特别是调控基因的时空特异性表达(表4)。黄水仙花(Narcissus pseudonarcissus)由于水仙花叶病毒的感染会产生断色现象 [ 61 ] 。通常转座子在基因内部的插入和部分回复突变会使花色呈彩斑状 [ 62 ] [ 63 ] [ 64 ] [ 89 ] [ 90 ] [ 102 ] 。双色花(包括缘环和星状)的形成通常是由于白色区域CHS的表达下调(主要由siRNA介导的其在转录后mRNA的降解造成) [ 65 ] [ 67 ] [ 114 ] [ 115 ] [ 116 ] ，或有色区域FLS的表达上调(原因未知) [ 65 ] 。UV模式、斑点、脉色和芽红等表型通常与花青素代谢途径的结构或调控基因的时空表达模式和不同拷贝数有关。在圆叶牵牛中，CHS-D主要在花檐中表达而控制整个花檐的着色，而CHS-E主要在花筒、花脉和花药中表达，可能与花筒的脉色和花脉上有色斑点的形成有关 [ 117 ] [ 118 ] [ 119 ] 。黑心金光菊(Rudbeckia hirta)在花瓣基部有F3H、F3’H、FLS、F6H、F7GT的高量表达使UV吸收类的黄酮醇类物质 Table 3 Summary of known mutations for flower color changes from blue/purple/red to white/yello 物种名 基因名 突变形式 表型变异 参考文献 Ipomoea purpurea CHS-D One copy of 3.9-kb Tip100 insertion in intron (COD) Purple to white， flaked [ 62 ] I. purpurea CHS-D Two copies of Tip100 insertion in intron (COD) Purple to stable white [ 62 ] I. purpurea CHS-D Rearrangement of DNA sequences between exon1 and an adjacent Tip100 (COD) Purple to stable white [ 90 ] I. purpurea CHS-D One copy of Tip100 insertion in 5'-flanking (COD) Purple to white， flaked [ 90 ] I. purpurea MYB1 6-bp and19-bp deletions (COD) Purple to white flowers with colored rays [ 98 ] I. purpurea bHLH IpMu1 insertion in exon2 and IpMu2 insertion in intron5 (COD) Bright blue to pale blue [ 105 ] I. nil CHS-D 5.57-kb Tpn3 insertion in intron (COD) Purple to white flowers with colored tubes [ 91 ] I. nil CHI Tpn2 insertion in intron2 and a speckled-activator (COD) Purple to white， speckled [ 63 ] I. nil DFR-B Tpn1 insertion in intron2 (COD) Purple to white， flecked [ 89 ] I. nil MYB1 AG insertion in exon2 (COD) Blue to white [ 99 ] I. nil bHLH 583-bp tandem duplication derived from exons 5 and 6 (COD) Bright blue to pale blue [ 104 ] I. nil WDR1 7-bp (GGAGTAC/TCCGTAC) insertion (COD) Blue to white [ 99 ] Torenia fournieri F3H A LTR-type retrotransposon (TORE1) insertion in the promoter (CIS) Violet to white [ 96 ] T. fournieri MYB1 A En/Spm-like transposon (Ttf1) insertion in intron2 (COD) Purple to white， flecked [ 102 ] Petunia axillaris AN2 Frameshift and nonsense mutations (COD) Red to white [ 97 ] Parrya nudicaulis CHS Lower expression (CIS) Purple to white [ 95 ] Mimulus lewisii DFR A frameshift in exon2 (COD) Pink to white [ 92 ] Antirrhinum majus ssp. striatum ROSEA el Lose of an activator (?) Dark red to light yellow [ 51 ] A. majus ssp. Latifolium ROSEA el Lose of an activator (?) Dark red to light yellow [ 50 ] A. majus ssp. Meonanthemum ROSEA el Lose of an activator (?) Dark red to light yellow [ 50 ] Antirrhinum (3 species) ROSEA Lose of function (COD and CIS) Red to pale or yellow [ 50 ] Aquilegia spp. R2R3-MYB? ? Purple to white or yellow [ 100 ] M. cardinalis SM PELAN Gene deletion (?) Red to yellow [ 103 ] M. cardinalis CI PELAN Lower expression (?) Red to yellow [ 103 ] M. guttatus PELAN No expression (?) Red to yellow [ 103 ] Oriental Lilium spp. MYB12 Two amino acid substitutions (COD) Pink to white [ 74 ] [ 101 ] 表3. 花色由蓝/紫/红色到白/黄色变化的基因突变形式 COD代表编码区的突变；CIS代表cis调控区的突变；Trans代表反式作用转录因子的改变；“?”代表原因不详。 Table 4 Summary of known mutations that cause pigmentation pattern 物种名 基因名 突变形式 表型变异 参考 文献 Narcissus pseudonarcissus Narcissus mosaic virus Viral infection Color break (figure2a) [ 61 ] Ipomoea purpurea CHS-D One copy of 3.9 kb Tip100 insertion in intron (somatic excision of transposon) Variegation (white， flaked) (figure 2b) [ 62 ] I. purpurea CHS-D One copy of Tip100 insertion in 5’-flanking (somatic excision of transposon) Variegation (white， flaked) [ 90 ] I. nil CHI Tpn2 insertion in intron2 and a speckled-activator (somatic excision of transposon) Variegation (white， speckled) (figure 2c) [ 63 ] I. nil DFR-B Tpn1 insertion in intron2 (somatic excision of transposon) Variegation (white， flecked) (figure 2d， e) [ 89 ] Dahlia variabilis bHLH A 5.4-kb CACTA transposon (Tdv1) insertion in intron4 (somatic excision of transposon) Yellow ray florets with orange variegation (figure 2f) [ 64 ] Torenia fournieri MYB1 A En/Spm-like transposon (Ttf1) insertion in intron2 (somatic excision of transposon) Variegation (white， flecked) [ 102 ] Petunia hybrida CHS-A Lower expression in white margin (siRNA directed silencing in exon2) White marginal picotee (figure 2g， h) [ 65 ] [ 116 ] P. hybrida FLS Higher expression in white center (?) Colored marginal picotee (figure 2i， j) [ 65 ] Dahlia variabilis CHS Lower expression in white regions (siRNA directed silencing in exon2) Red-white bicolor (figure 2k) [ 66 ] Camellia japonica “Tamanoura” CHS Lower expression in white regions (?) White Marginal Picotee [ 114 ] P. hybrida CHS-A Lower expression in white stripe (siRNA directed silencing in exon2) A star-type red and white bicolor pattern (figure 2l) [ 67 ] [ 115 ] [ 116 ] Rudbeckia hirta F3H， F3’H， FLS， F6H， F7GT Higher expression in the basal part (?) UV/bull’s eye pattern (figure 2m， n) [ 68 ] I. purpurea CHS-D Tissue specific expression Flower limb pigmentation [ 117 ] [ 118 ] [ 119 ] CHS-E Tissue specific expression Flower tube， ray， and anthers pigmentation Clarkia gracilis F3’H1， F3’5’H1， DFR1， DFR2 Early expression of F3’H1-A throughout the petal and DFR2-A/B in spots; later expression of F3’5’H1-A/B and DFR1-A/B in the petal background (?) A single red-purple spot against a pink background (figure 2t) [ 72 ] [ 120 ] Paeonia suffruticosa cultivar “Jinrong” CHS， F3’H， DFR， ANS Higher expression in the purple spot (?) Purple spot at the base of white petal [ 121 ] Pansy (Viola × wittrockiana Gams.) F3H， F3’5’H， DFR， ANS Higher expression in the cyanic blotches (?) Cyanic blotches on the yellow petal [ 122 ] Antirrhinum ssp. VENOSA Tissue specific expression Petal lobe and tube venation pigmentation (figure 2o) [ 50 ] [ 124 ] Antirrhinum ssp. ROSEA el Tissue specific expression Petal lobe and tube pigmentation [ 50 ] P. hybrida DPL Tissue specific expression Tube venation pigmentation (figure 2p) [ 106 ] AN2 Tissue specific expression Flower limb pigmentation (figure 2p) AN4 Tissue specific expression Flower tube and anthers pigmentation PHZ Tissue specific expression (light induced) Vegetative tissue pigmentation and bud blushing Mimulus lewisii PELAN Tissue specific expression Petal lobe pigmentation [ 103 ] M. lewisii， M. guttatus NEGAN Tissue specific expression Spot formation in the nectar guide [ 103 ] Phalaenopsis spp. MYB2 Tissue specific expression Full-red pigmentation in tepals [ 73 ] MYB11 Tissue specific expression Red spots in sepals， petals and lip (figure 2v) MYB12 Tissue specific expression Venation pattern in the sepals， petals and the full pigmentation pattern in the central lobe of the lip Lilium regale MYB15 Tissue specific expression (light induced) Vegetative tissue pigmentation and bud blushing (figure 2s) [ 71 ] Asiatic Lilium spp. MYB12 Mon/Ren/Lan Tissue specific expression Full-pink pigmentation in tepals [ 74 ] [ 112 ] [ 123 ] Asiatic Lilium spp. MYB6 Tissue specific expression Vegetative tissue pigmentation and raised spot in tepals [ 123 ] Asiatic Lilium spp. cv “Latvia” MYB12-lat Tissue specific expression Splatter spot in tepals (figure 2x) [ 126 ] Oriental Lilium spp. MYB12 Tissue specific expression Full-pink and raised spot pigmentation in tepals (figure 2w) [ 74 ] [ 101 ] Apple “Honeycrisp” MYB10 Lower expression in green stripes (higher methylation in promoter of green stripes) Red and green stripes on fruit skin [ 125 ] MYB10 Higher expression in red regions (light induced) Blushed fruit skin Trifolium spp. RED LEAF Tissue specific expression Red leaf， red midrib， and red fleck [ 107 ] RED V Tissue specific expression V-broken yellow and red leaflet RED LEAF DIFFUSE Tissue specific expression Diffuse red leaf Tulipa gesneriana cv. Murasakizuisho Vit1 (coding a vacuolar iron transporter) Tissue specific expression at the bottom of purple perianth Blue spot at the bottom of the inner purple perianth (figure 2u) [ 26 ] [ 27 ] [ 129 ] FER1 (coding an Fe storage protein) Tissue specific expression at the upper of purple perianth 表4. 已报道的花色着色模式相关变异 COD代表编码区的突变；CIS代表cis调控区的突变；Trans代表反式作用转录因子的改变；“?”代表原因不详。 高积累而形成类似蜜导的UV模式 [ 68 ] ；F3’H1-A在整个花瓣和DFR2-A/B在斑点的早表达及F3’5’H1-A/B和DFR1-A/B在花瓣中的晚表达可能是形成细长山字草(Clarkia gracilis)粉色带红色斑点表型的主要原因，但决定斑点位置的因子还有待确定 [ 72 ] [ 120 ] 。同样，花青素代谢途径结构基因在牡丹(Paeonia suffruticosa Cultivar “Jinrong”) [ 121 ] 和三色堇(Viola × wittrockiana Gams.) [ 122 ] 斑点位置出现高表达，这对它们花瓣上形成斑点具有很重要的作用。因为在斑点的形成过程中通常涉及多个结构基因的协同表达，且结构基因的表达受调控基因的影响，所以结构基因的时空特异性表达很大程度上可能是由调控基因的时空表达特异性决定的。许多研究表明R2R3-MYB的时空特异性表达对不同部位的着色具有非常重要的影响(表4) [ 50 ] [ 69 ] [ 71 ] [ 73 ] [ 74 ] [ 101 ] [ 103 ] [ 106 ] [ 107 ] [ 112 ] [ 123 ] [ 124 ] [ 125 ] [ 126 ] 。周围有光环的斑点表型(图2z)可能与斑点周围激活型因子(WDR)的移除或抑制型因子(R3-MYB)的流入有关 [ 68 ] 。R3-MYB和WD40均具有细胞间可移动性 [ 69 ] [ 106 ] 。在斑点处MBW复合物可促进R3-MYB的产生 [ 127 ] [ 128 ] ，斑点周围区域R3-MYB的流入与WDR的流出使其花青素合成较少或无，故呈光环型斑点的表型。另外，细胞形状 [ 15 ] [ 17 ] [ 69 ] 和Fe 3+ 转运和存储蛋白的组织特异性表达 [ 26 ] [ 27 ] [ 129 ] 也可能影响花色斑点的形成。 目前MYB的时空特异性表达机制并不清楚。影响营养组织和芽红着色的R2R3-MYB通常会受光的诱导而高表达，且抑制型的R3-MYB会被激活参与该过程以避免过激反应 [ 106 ] [ 127 ] ，但环境特别是光诱导花青素合成的分子机制并不清楚。R2R3-MYB启动子区甲基化程度的不同会决定其表达与否而影响花青素合成部位，但DNA甲基化的组织特异性机制也未知 [ 125 ] 。