<i>Azospirillum</i> Growth Promotion Is Related to Changes in Ferulate and Dehydrodiferulate Contents in Cell Wall of Inoculated Cucumber Seedlings Hypocotyls

Abstract

Azospirillum brasilense is a well known plant growth-promoting rhizobacterium. Plant cell growth is constrained by the primary cell wall (CW) which contains polysaccharide-bound hydroxycinammic acids (HCAs). They derive from phenylpropanoid metabolism, which first step is the reaction catalyzed by phenylalanine ammonia-lyase (PAL; EC 4.3.1.24). Also CW peroxidases (FAPOD; EC 1.11.1.7) play a key role in the stiffening of the CW, and in the cessation of cell elongation. Knowledge of the biochemical effects the bacteria could elicit into plant CW and how these responses could change the hypocotyl physiology still remains scarce. The objective of this work was to unravel the effects of A. brasilense Sp245 inoculation on HCAs of the primary CW in apical and basal segments of cucumber seedlings hypocotyls. Azospirillum inoculation increased hypocotyls' length. Trans-ferulate and p-coumarate were the major HCAs. Dimmers were detected only in the basal region of 13-and 15-d-old hypocotyls. The ratio t-ferulate/dimmer (an inverse index of CW crosslinking) was five times higher in inoculated seedlings. Enzyme activities determined were not directly involved in HCAs content changes. It was previously reported that the CW was a target for A. brasilense growth promotion. This work corroborates that the phenolics, FA, DFA, and p-coumarate bound to the CW are also involved in Azospirillum plant growth promoting strategies.