Amino acid metabolism at the maternal-filial boundary of young barley seeds: a microdissection-based study.

The nucellar projection (NP)/endosperm transfer cell (ETC) complex represents the link between maternal and filial seed tissues in barley and mediates nutrient transfer into the endosperm. Cells of NP function as metabolic interface to precondition amino acid supply of the endosperm. The organ displays a top-down gradient of differentiation, with mitotically active, differentiating/elongating as well as disintegrating cells, characterized by proteolysis and nitrogen remobilization. To understand metabolism, interconversion and transfer of amino acids at the maternal-filial boundary, we applied a combined transcriptome and metabolite approach based on laser-assisted microdissection. Results suggest that amino acid degradation observed in NP largely occurs within mitochondria, consistent with their role in controlling amino acid homeostasis and metabolism. Differentially expressed genes and free amino acid levels associated with glutamate and glutamine metabolism indicate concerted action of glutamine dehydrogenase, glutamine synthetase and alanine:glyoxylate aminotransferase 2 within a hypothetical cycle for glutamine and alanine degradation and re-synthesis of the preferred transport form glutamine. Stimulation of gene expression involved in methionine metabolism in NP suggests a pathway of regulated synthesis of S-methylmethionine and a possible mechanism for the transfer of reduced sulphur from maternal tissues into the endosperm. Thus, the established micromethods revealed strategies in NP of young barley grains for mobilization and metabolism of transient N and S reserves and transfer into the endosperm.