Sugar sensing responses to low and high light in leaves of the C₄ model grass Setaria viridis

Although sugar regulates photosynthesis, the signalling pathways underlying this process remain elusive, especially for C₄ crops. To address this knowledge gap and identify potential candidate genes, we treated Setaria viridis (C₄ model) plants acclimated to medium light intensity (ML, 500 μmol m^-2 s^-1) with low (LL, 50 μmol m^-2 s^-1) or high (HL, 1000 μmol m^-2 s^-1) light for 4 d and observed the consequences on carbon metabolism and the transcriptome of source leaves. LL impaired photosynthesis and reduced leaf content of signalling sugars (glucose, sucrose, and trehalose-6-phosphate). In contrast, HL strongly induced sugar accumulation without repressing photosynthesis. LL more profoundly impacted the leaf transcriptome, including photosynthetic genes. LL and HL contrastingly altered the expression of hexokinase (HXK) and sucrose-non-fermenting 1 (Snf1)-related protein kinase 1 (SnRK1) sugar sensors and trehalose pathway genes. The expression of key target genes of HXK and SnRK1 were affected by LL and sugar depletion, while surprisingly HL and strong sugar accumulation only slightly repressed the SnRK1 signalling pathway. In conclusion, we demonstrate that LL profoundly impacted photosynthesis and the transcriptome of S. viridis source leaves, while HL altered sugar levels more than LL. We also present the first evidence that sugar signalling pathways in C₄ source leaves may respond to light intensity and sugar accumulation differently from C₃ source leaves.