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

Although sugar regulate 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 days 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). Contrastingly, HL strongly induced sugar accumulation without repressing photosynthesis. LL more profoundly impacted leaf transcriptome, including photosynthetic genes. LL and HL contrastingly altered the expression of HXK and 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 to C3 source leave

Comparative analysis of thylakoid protein complexes in the mesophyll and bundle sheath cells from C3, C4 and C3-C4 Paniceae grasses

To better understand the coordination between dark and light reactions during the transition from C3 to C4 photosynthesis, we optimized a method for separating thylakoids from mesophyll (MC) and bundle sheath cells (BSC) across different plant species. We grew six Paniceae grasses including representatives from the C3, C3‐C4 and C4photosynthetic types and all three C4 biochemical subtypes (NADP‐ME, NAD‐ME and PEPCK) in addition to Zea mays under control conditions (1000 μmol quanta m‐2 s‐1 and 400 ppm of CO2). Proteomics analysis of thylakoids under native conditions, using BN‐PAGE followed by LC/MS, demonstrated the presence of subunits of all light‐reaction related complexes in all species and cell types. C4 NADP‐ME species showed a higher PSI/PSII ratio and a clear accumulation of NDH complexes in BSCs, while Cytb6f was more abundant in BSCs of C4 NAD‐ME species. The C4 PEPCK species showed no clear differences between cell types. Our study presents, for the first time, a good separation between BSC and MC for a C3‐C4 intermediate grass which did not show noticeable differences in the distribution of the thylakoid complexes. For the NADP‐ME species P. antidotale, growth at glacial CO2 (180 ppm of CO2) had no effect on the distribution of the light‐reaction complexes, while growth at low light (200 μmol quanta m‐2 s‐1) promoted the accumulation of light‐harvesting proteins in both cell types. These results add to our understanding of thylakoid distribution across photosynthetic types and subtypes, and introduce thylakoid distribution between the MC and BSC of a C3‐C4 intermediate species

Effect of reduced irradiance on ¹³C uptake, gene expression and protein activity of the seagrass Zostera muelleri

© 2019 Elsevier Ltd Photosynthesis in the seagrass Zostera muelleri remains poorly understood. We investigated the effect of reduced irradiance on the incorporation of 13C, gene expression of photosynthetic, photorespiratory and intermediates recycling genes as well as the enzymatic content and activity of Rubisco and PEPC within Z. muelleri. Following 48 h of reduced irradiance, we found that i) there was a ∼7 fold reduction in 13C incorporation in above ground tissue, ii) a significant down regulation of photosynthetic, photorespiratory and intermediates recycling genes and iii) no significant difference in enzyme activity and content. We propose that Z. muelleri is able to alter its physiology in order to reduce the amount of C lost through photorespiration to compensate for the reduced carbon assimilation as a result of reduced irradiance. In addition, the first estimated rate constant (Kcat) and maximum rates of carboxylation (Vcmax) of Rubisco is reported for the first time for Z. muelleri

Comparative analysis of thylakoid protein complexes in the mesophyll and bundle sheath cells from C 3

To better understand the coordination between dark and light reactions during the transition from C3 to C4 photosynthesis, we optimized a method for separating thylakoids from mesophyll (MC) and bundle sheath cells (BSC) across different plant species. We grew six Paniceae grasses including representatives from the C3, C3‐C4 and C4photosynthetic types and all three C4 biochemical subtypes (NADP‐ME, NAD‐ME and PEPCK) in addition to Zea mays under control conditions (1000 μmol quanta m‐2 s‐1 and 400 ppm of CO2). Proteomics analysis of thylakoids under native conditions, using BN‐PAGE followed by LC/MS, demonstrated the presence of subunits of all light‐reaction related complexes in all species and cell types. C4 NADP‐ME species showed a higher PSI/PSII ratio and a clear accumulation of NDH complexes in BSCs, while Cytb6f was more abundant in BSCs of C4 NAD‐ME species. The C4 PEPCK species showed no clear differences between cell types. Our study presents, for the first time, a good separation between BSC and MC for a C3‐C4 intermediate grass which did not show noticeable differences in the distribution of the thylakoid complexes. For the NADP‐ME species P. antidotale, growth at glacial CO2 (180 ppm of CO2) had no effect on the distribution of the light‐reaction complexes, while growth at low light (200 μmol quanta m‐2 s‐1) promoted the accumulation of light‐harvesting proteins in both cell types. These results add to our understanding of thylakoid distribution across photosynthetic types and subtypes, and introduce thylakoid distribution between the MC and BSC of a C3‐C4 intermediate species