Sink-source imbalance triggers delayed photosynthetic induction: Transcriptomic and physiological evidence

Sink-source imbalance causes accumulation of nonstructural carbohydrates (NSCs) and photosynthetic downregulation. However, despite numerous studies, it remains unclear whether NSC accumulation or N deficiency more directly decreases steady-state maximum photosynthesis and photosynthetic induction, as well as underlying gene expression profiles. We evaluated the relationship between photosynthetic capacity and NSC accumulation induced by cold girdling, sucrose feeding, and low nitrogen treatment in Glycine max and Phaseolus vulgaris. In G. max, changes in transcriptome profiles were further investigated, focusing on the physiological processes of photosynthesis and NSC accumulation. NSC accumulation decreased the maximum photosynthetic capacity and delayed photosynthetic induction in both species. In G. max, such photosynthetic downregulation was explained by coordinated downregulation of photosynthetic genes involved in the Calvin cycle, Rubisco activase, photochemical reactions, and stomatal opening. Furthermore, sink-source imbalance may have triggered a change in the balance of sugar-phosphate translocators in chloroplast membranes, which may have promoted starch accumulation in chloroplasts. Our findings provide an overall picture of photosynthetic downregulation and NSC accumulation in G. max, demonstrating that photosynthetic downregulation is triggered by NSC accumulation and cannot be explained solely by N deficiency.

Automated summary

Sink-source imbalance leads to the accumulation of nonstructural carbohydrates (NSCs) and downregulation of photosynthesis. However, it is still unclear whether NSC accumulation or nitrogen deficiency has a more direct effect on the decrease in steady-state maximum photosynthesis and photosynthetic induction, as well as the underlying gene expression profiles. In this study, we investigated the relationship between photosynthetic capacity and NSC accumulation induced by different treatments in soybean and common bean. The findings indicate that NSC accumulation decreases the maximum photosynthetic capacity and delays photosynthetic induction in both species. The downregulation of photosynthesis in soybean can be attributed to the coordinated downregulation of photosynthetic genes involved in various processes, including the Calvin cycle, Rubisco activase, photochemical reactions, and stomatal opening. Additionally, the sink-source imbalance may have affected the balance of sugar-phosphate translocators in chloroplast membranes, leading to starch accumulation. These results provide valuable insights into the mechanisms of photosynthetic downregulation and NSC accumulation, emphasizing the role of NSC accumulation in triggering downregulation rather than nitrogen deficiency.