The APL4 Subunit of ADP-glucose Pyrophosphorylase (Starch Synthesis) as Potential Target for Improvement of Photosynthesis and Source-sink Relationships

Abstract There is currently a major challenge to improve photosynthetic yield for increased sustainable crop production and meeting mankind's nutritional requirements. Among other factors, suboptimal carbon partitioning and suboptimal photoprotection may contribute to limitations of photosynthetic efficiency (1). Herbicide and oxidative stress tolerance can involve mechanisms that are related to photosynthesis efficiency and carbon dynamics (2-4). In this context, an Arabidopsis thaliana T-DNA insertional mutant was identified and characterized for enhanced tolerance to the singlet-oxygen-generating herbicide atrazine in comparison to wild-type (5). This enhanced atrazine tolerance mutant was affected in the promoter structure and gene expression regulation of the APL4 subunit of ADP-glucose pyrophosphorylase, a key enzyme of starch biosynthesis (6), thus resulting in decrease of APL4 mRNA levels. The impact of this regulatory mutation was confirmed by analysis of an independent T-DNA insertional mutant also affected in the APL4 gene promoter. The resulting tissue-specific modifications of carbon partitioning and the effects on plant growth and stress tolerance revealed specific and non-redundant roles of APL4, primarily in root tissues, with important consequences for carbon dynamics, shoot-root relationships and sink regulations of photosynthesis. The apl4 phenotype of increased carbohydrate accumulation, not only in roots, but also in shoots, associated with enhanced root and shoot biomass, pointed out to enhanced photosynthesis and carbon fixation. On the other hand, given the effects of exogenous sugar treatments and of endogenous sugar levels on atrazine tolerance in wild-type Arabidopsis plantlets (2,7), atrazine tolerance of this apl4 mutant could be ascribed to perception of carbon status and to investment of sugar allocation in xenobiotic and oxidative stress responses (8,9). Such relationships between carbon partitioning, carbon/nitrogen balance, shoot-root development and stress responses are likely to be important for plant breeding in the context of global change and of rising demands for food and bioenergy. The characteristics of these apl4 mutants indicate that novel regulations of root:shoot ratios remain to be characterized and that the regulatory subunits of ADP-glucose pyrophosphorylase and the mutations and polymorphisms affecting their regulation could be important targets of plant breeding for improving jointly yield potential and abiotic stress tolerance.