Downregulation and delayed induction of photosynthesis by coordinated
transcriptomic changes in response to sink-source imbalance
Abstract
Sink-source imbalance causes accumulation of non-structural
carbohydrates (NSCs) and photosynthetic downregulation. Despite numerous
studies, however, it remains unclear whether NSCs 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 NSCs 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 physiological processes of
photosynthesis and NSCs accumulation. NSCs accumulation decreased
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 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 provided an overall picture
of the photosynthetic downregulation and NSCs accumulation in G.
max, demonstrating that the photosynthetic downregulation is
triggered by NSCs accumulation and cannot be explained simply by N
deficiency.