Analysis of the physiological role of Yang-cycle enzymes

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During analyses of the Plantago major (common plantain) source-leaf vascular transcriptom we identified an unexpectedly high number of transcripts proteins involved in the biosynthesis of polyamines. Even more surprising was the identification of the vascular-specific expression of the gene for a methylthioribose kinase, PmMTK1, a protein of the Yang-cycle. As MTK1 is a unigene in Arabidopsis, and as we could not identify a second MTK gene in Plantago, we speculated that the vascular-specificity of one step of the Yang-cycle indicates vascular-specificity for all other reactions of the cycle as well. We could confirm this hypothesis and identified all but one of the yet unidentified Yang-cycle genes in higher plants, MTI1 and DEP1. Nevertheless, the physiological role of the Yang-cycle in Arabidopsis remains not well understood. Under normal growth conditions, Yang-cycle mutants grow like WT plants; developmental defects are observed only under un-physiological growth conditions with 5-methylthioadenosine (MTA; a cytotoxic Yang-cycle intermediate) as sole sulfur source. Here we propose experiments that (A) address the question after the reason for the vascular-specificity of the Yang-cycle and that (B) focus on physiological growth conditions that challenge Yang-cycle activity. We will also (C) perform a search for the final un-identified Yang-cycle enzyme and for a putative MTA transporter. With the proposed work we aim to elucidate the physiological role(s) of the Yang-cycle and to characterize its specific importance for polyamine and nicotianamine biosynthesis.