Department of Biology, ETH Zurich, 8092 Zurich, Switzerland.
J Exp Bot. 2021 May 4;72(10):3739-3755. doi: 10.1093/jxb/erab099.
Plastid metabolism is critical in both photoautotrophic and heterotrophic plant cells. In chloroplasts, fructose-1,6-bisphosphate aldolase (FBA) catalyses the formation of both fructose 1,6-bisphosphate and sedoheptulose 1,7-bisphosphate within the Calvin-Benson cycle. Three Arabidopsis genes, AtFBA1-AtFBA3, encode plastidial isoforms of FBA, but the contribution of each isoform is unknown. Phylogenetic analysis indicates that FBA1 and FBA2 derive from a recently duplicated gene, while FBA3 is a more ancient paralog. fba1 mutants are phenotypically indistinguishable from the wild type, while both fba2 and fba3 have reduced growth. We show that FBA2 is the major isoform in leaves, contributing most of the measurable activity. Partial redundancy with FBA1 allows both single mutants to survive, but combining both mutations is lethal, indicating a block of photoautotrophy. In contrast, FBA3 is expressed predominantly in heterotrophic tissues, especially the leaf and root vasculature, but not in the leaf mesophyll. We show that the loss of FBA3 affects plastidial glycolytic metabolism of the root, potentially limiting the biosynthesis of essential compounds such as amino acids. However, grafting experiments suggest that fba3 is dysfunctional in leaf phloem transport, and we suggest that a block in photoassimilate export from leaves causes the buildup of high carbohydrate concentrations and retarded growth.
质体代谢在光合自养型和异养型植物细胞中都至关重要。在叶绿体中，果糖-1,6-二磷酸醛缩酶（FBA）催化卡尔文-本森循环中果糖 1,6-二磷酸和景天庚酮糖 1,7-二磷酸的形成。三个拟南芥基因，AtFBA1-AtFBA3，编码质体 FBA 的同工型，但每种同工型的贡献尚不清楚。系统发育分析表明，FBA1 和 FBA2 来自最近复制的基因，而 FBA3 是更古老的同源基因。fba1 突变体的表型与野生型无法区分，而 fba2 和 fba3 的生长都受到抑制。我们表明 FBA2 是叶片中的主要同工型，贡献了大部分可测量的活性。与 FBA1 的部分冗余使两个单突变体都能存活，但同时突变两者是致命的，表明光合作用受阻。相比之下，FBA3 在异养组织中表达更为突出，特别是叶片和根部脉管系统，但不在叶片叶肉中表达。我们表明 FBA3 的缺失会影响根部质体糖酵解代谢，可能限制了如氨基酸等必需化合物的生物合成。然而，嫁接实验表明，fba3 在叶片韧皮部运输中功能失调，我们认为，叶片中光合作用产物输出受阻会导致高碳水化合物浓度的积累和生长迟缓。
