On the Inside

Met is a nutritionally essential sulfurcontaining amino acid found at low levels in plants and in their seeds. It often limits the nutritional value of crop plants as a source of dietary protein for humans and animals. In plants, Met plays key roles in protein synthesis and mRNA translation, and regulates indirectly a variety of cellular processes through its main catabolic product S-adenosyl-Met (SAM). SAM serves as the precursor for the synthesis of the plant hormone ethylene, polyamines, and biotin, and donates a primary methyl group that is essential for methylation reactions involved in a variety of developmental processes in plant cells. Genetic and biochemical studies suggest that in seeds, Met can be synthesized de novo as in vegetative tissues via the classical Asp family pathway by the activity of its main regulatory enzyme,CYSTATHIONINE g-SYNTHASE (CGS). However, isotope-labeling experiments suggest that Met can be synthesized in seeds through an alternative pathway by which Met produced in vegetative tissues is converted to S-methyl-Met (SMM) that is then transported via the phloem into the reproductive tissues, where it is converted toMet. Cohen et al. (pp. 1322–1333) have previously produced transgenic Arabidopsis (Arabidopsis thaliana) RNAi seeds with lower transcript expression of AtCGS and found unexpectedly that the seeds accumulated significantly higher levels of Met compared with controls. Using radiolabels, the authors now show that SMM synthesized in the rosette leaves of the RNAi plants significantly contributed to the accumulation of Met in their seeds at late stages of development. Seed-specific repression of AtCGS in RNAi seeds triggered the induction of genes operating in the SMM cycle of rosette leaves, leading to elevated transport of SMM toward the seeds, where higher reconversion rates of SMM to Met were detected. The results of this study suggest new strategies for improving Met contents in seeds. ABA Is Required for Cuticle Formation Independent of Water Stress