Effects of indole-3-acetic acid on Sinorhizobium meliloti survival and on symbiotic nitrogen fixation and stem dry weight production.

We evaluated the effects of the main auxin phytohormone, indole-3-acetic acid (IAA), on the central metabolism of Sinorhizobium meliloti strain 1021. We either treated the Sinorhizobium meliloti 1021 strain with 0.5 mM IAA (1021+) or use a derivative, RD64, of the same strain harbouring a pathway for IAA biosynthesis converting tryptophan into IAA via indoleacetamide. We assayed the activity of key enzymes in the major energy-yielding pathways (Entner-Doudoroff, Embden-Meyerhof-Parnas, pentose phosphate, glyoxylate bypass and tricarboxylic acid cycle). We found that activity of two main regulative tricarboxylic acid (TCA) cycle enzymes was increased. Citrate synthase (CS) activity, as compared to the wild type strain, was enhanced in 1021+ and RD64 by 38% and 67%, respectively. α-Ketoglutarate dehydrogenase (α-KGD) activity was induced, as compared to the wild type strain, in 1021+ and RD64, by 100% and 86% respectively. We show here that intracellular acetyl-CoA was 30% increased in 1021+ and RD64 as compared to the wild-type strain and that the activities of the key enzymes which utilize acetyl-CoA for poly-β-hydroxybutyrate (PHB) biosynthesis, were enhanced in RD64 and 1021+ strains. Electron microscopy analysis showed that RD64 cells accumulated more PHB granules than the wild type ones. Moreover, RD64 strain improves cell survival ability reaching 166% at 4 weeks over the parental strain. Medicago truncatula plants nodulated by RD64 increase both acetylene reduction activity and stem dry weight production. RD64 induced nodules shows an increased starch granule deposition than nodules derived from the wild type strain.