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.

Understanding the fruit developmental process is critical for fruit quality improvement. Here, we report a comprehensive proteomic analysis of apple fruit development over five growth stages, from young fruit to maturity, coupled with metabolomic profiling. A tandem mass tag (TMT)-based comparative proteomics approach led to the identification and quantification of 7098 and 6247 proteins, respectively. This large-scale proteomic dataset presents a global view of the critical pathways involved in fruit development and metabolism. When linked with metabolomics data, these results provide new insights into the modulation of fruit development, the metabolism and storage of sugars and organic acids (mainly malate), and events within the energy-related pathways for respiration and glycolysis. We suggest that the key steps identified here (e.g. those involving the FK2, TST, EDR6, SPS, mtME and mtMDH switches), can be further targeted to confirm their roles in accumulation and balance of fructose, sucrose and malate. Moreover, our findings imply that the primary reason for decreases in amino acid concentrations during fruit development is related to a reduction in substrate flux via glycolysis, which is mainly regulated by fructose-bisphosphate aldolase and bisphosphoglycerate mutase.