Zrenner R, Krause K P, Apel P, Sonnewald U
Institut für Pflanzengenetik und Kulturpflanzenforschung, Gatersleben, Germany.
Plant J. 1996 May;9(5):671-81. doi: 10.1046/j.1365-313x.1996.9050671.x.
Sucrose produced in source leaves is the predominant carbon source for developing sink tissues in most higher plants. Consequently the rate of sucrose synthesis is likely to be important for sink development and final crop yield. Two sucrose biosynthetic enzymes are believed to possess regulatory properties with respect to the rate of sucrose synthesis: (i) cytosolic FBPase and (ii) sucrose phosphate synthase. To study the impact of reduced photosynthetic sucrose biosynthesis on plant growth and crop yield a cDNA clone encoding cytosolic FBPase was isolated from a potato leaf cDNA library and used for antisense experiments in transgenic potato plants. The cDNA clone cy-F1, containing an open reading frame of 1020 bp highly homologous (85%) to other known sequences of plant cytosolic FBPases, was cloned in reversed orientation between the 35S CaMV promoter and the octopine synthase polyadenylation signal. Out of 75 independent transformants five transgenic lines having 9 to 55% of the wild-type FBPase activity were chosen for further analysis. A 45% reduction of the cytosolic FBPase activity did not cause any measurable change in metabolite concentrations, growth behaviour or photosynthetic parameters of the transgenic plants. Inhibition of cytosolic FBPase activity below 20% of the wild-type activity led to an accumulation of 3-PGA, triose-phosphates and fructose-1,6-biphosphate in source leaves. This resulted in a reduced light-saturated rate of assimilation measured via gas exchange and a decreased photosynthetic rate under conditions of the leaf disc electrode with saturating light and CO2. Measuring photosynthetic carbon fluxes by labelling leaf discs with 14CO2 revealed a 53-65% reduction of sucrose synthesis whereas starch synthesis decreased only by 18-24%. The flux into the anionic and cationic fraction was not altered. Despite these changes steady-state sucrose concentrations were not effected in source leaves from transgenic plants. Starch accumulated by more than a factor of 3 compared with wild-type leaves and was degraded during the night. This provides strong evidence for the hypothesis that hexoses and/or hexosephosphates are exported out of the chloroplasts, thereby circumventing the limitation of sucrose biosynthesis caused by the inhibition of cytosolic FBPase in the dark. Accordingly, plant growth and potato tuber yield remained unaltered. From these data it can be concluded that a reduced photosynthetic sucrose biosynthetic capacity can be efficiently compensated without any reduction in crop yield under greenhouse or growth chamber conditions by changing carbon export strategy. Whether the same holds true for field conditions remains to be elucidated.
在大多数高等植物中，源叶中产生的蔗糖是发育中的库组织的主要碳源。因此，蔗糖合成速率可能对库发育和最终作物产量很重要。据信有两种蔗糖生物合成酶对蔗糖合成速率具有调节特性：（i）胞质FBPase和（ii）蔗糖磷酸合酶。为了研究光合蔗糖生物合成减少对植物生长和作物产量的影响，从马铃薯叶cDNA文库中分离出一个编码胞质FBPase的cDNA克隆，并用于转基因马铃薯植株的反义实验。cDNA克隆cy-F1含有一个1020 bp的开放阅读框，与植物胞质FBPase的其他已知序列高度同源（85%），以反向方向克隆在35S CaMV启动子和章鱼碱合酶多聚腺苷酸化信号之间。在75个独立转化体中，选择了5个具有野生型FBPase活性9%至55%的转基因株系进行进一步分析。胞质FBPase活性降低45%并未导致转基因植株的代谢物浓度、生长行为或光合参数发生任何可测量的变化。将胞质FBPase活性抑制到低于野生型活性的20%会导致源叶中3-PGA、磷酸丙糖和果糖-1,6-二磷酸的积累。这导致通过气体交换测量的光饱和同化速率降低，以及在叶盘电极饱和光和CO2条件下光合速率降低。用14CO2标记叶盘测量光合碳通量显示蔗糖合成减少了53 - 65%，而淀粉合成仅减少了18 - 24%。进入阴离子和阳离子部分的通量没有改变。尽管有这些变化，转基因植物源叶中的稳态蔗糖浓度并未受到影响。与野生型叶片相比，淀粉积累增加了3倍多，并在夜间降解。这为以下假设提供了有力证据：己糖和/或己糖磷酸从叶绿体中输出，从而规避了黑暗中胞质FBPase抑制导致的蔗糖生物合成限制。因此，植物生长和马铃薯块茎产量保持不变。从这些数据可以得出结论，在温室或生长室条件下，通过改变碳输出策略，可以有效补偿光合蔗糖生物合成能力的降低，而不会降低作物产量。在田间条件下是否同样如此还有待阐明。
