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Summary: In plants, L‐serine (Ser) biosynthesis occurs through various pathways and is highly dependent on the atmospheric CO2 concentration, especially in C3 species, due to the association of the Glycolate Pathway of Ser Biosynthesis (GPSB) with photorespiration. Characterization of a second plant Ser pathway, the Phosphorylated Pathway of Ser Biosynthesis (PPSB), revealed that it is at the crossroads of carbon, nitrogen, and sulphur metabolism. The PPSB comprises three sequential reactions catalysed by 3‐phosphoglycerate dehydrogenase (PGDH), 3‐phosphoSer aminotransferase (PSAT) and 3‐phosphoSer phosphatase (PSP). PPSB was overexpressed in plants exhibiting two different modes of photosynthesis: Arabidopsis (C3 metabolism), and maize (C4 metabolism), under ambient (aCO2) and elevated (eCO2) CO2 growth conditions. Overexpression in Arabidopsis of the PGDH1 gene alone or PGDH1, PSAT1 and PSP1 in combination increased the Ser levels but also the essential amino acids threonine (aCO2), isoleucine, leucine, lysine, phenylalanine, threonine and methionine (eCO2) compared to the wild‐type. These increases translated into higher protein levels. Likewise, starch levels were also increased in the PPSB‐overexpressing lines. In maize, PPSB‐deficient lines were obtained by targeting PSP1 using Cas9 endonuclease. We concluded that the expression of PPSB in maize male gametophyte is required for viable pollen development. Maize lines overexpressing the AtPGDH1 gene only displayed higher protein levels but not starch at both aCO2 and eCO2 conditions, this translated into a significant rise in the nitrogen/carbon ratio. These results suggest that metabolic engineering of PPSB in crops could enhance nitrogen content, particularly under upcoming eCO2 conditions where the activity of GPSB is limited. [ABSTRACT FROM AUTHOR] more...

Light/dark (LD) cycles are responsible for oscillations in gene expression, which modulate several aspects of plant physiology. Those oscillations can persist under constant conditions due to regulation by the circadian oscillator. The response of the transcriptome to light regimes is dynamic and allows plants to adapt rapidly to changing environmental conditions. We compared the transcriptome of Arabidopsis under LD and constant light (LL) for 3 days and identified different gene co‐expression networks in the two light regimes. Our studies yielded unforeseen insights into circadian regulation. Intuitively, we anticipated that gene clusters regulated by the circadian oscillator would display oscillations under LD cycles. However, we found transcripts encoding components of the flavonoid metabolism pathway that were rhythmic in LL but not in LD. We also discovered that the expressions of many stress‐related genes were significantly increased during the dark period in LD relative to the subjective night in LL, whereas the expression of these genes in the light period was similar. The nocturnal pattern of these stress‐related gene expressions suggested a form of "skotoprotection." The transcriptomics data were made available in a web application named Cyclath, which we believe will be a useful tool to contribute to a better understanding of the impact of light regimes on plants. [ABSTRACT FROM AUTHOR] more...