High levels of methionine in Arabidopsis seeds result in a higher influx of nutrients from the leaves toward the developing seeds

Methionine is an essential metabolite that participates in many developmental processes in plant cells through its main catabolite, S-adenosyl-methionine, the primary methyl group donor. Methionine is regulated mainly by the expression level of its first enzyme, cystathionine-g-synthase (CGS). Expression of CGS, under the control of seed-specific promoter (phaseolin), resulted in an increased level of methionine in Arabidopsis seeds. Unexpectedly these seeds had higher levels of other amino acids (AAs) and soluble sugars than the control. The observed metabolic profile resembled a metabolic response to abiotic stress. Indeed, transcriptional analysis showed that many genes associated with abiotic stress were up-regulated in these seeds. However, only a few genes related to AAs and sugars synthesis were up-regulated. Isotope labeling experiments confirmed an increase in the metabolic flux from leaves towards the developing seeds, supporting the assumption that AAs and sugars found in seeds were derived from non-seed tissues. Interestingly, the expression level of CGS and the level of methionine were also up-regulated in the leaves. The RNAseq analysis of these leaves showed transcriptional induction of many genes related to DNA and histone methylation. We assume that the higher level of methionine in vegetative tissues provides more SAM causing changes in the methylation of DNA and histones, leading to transcriptional changes.