Indole-3-acetic acid biosynthesis by Azospirillum brasilense Az39 and its regulation under biotic and abiotic stress conditions

FABRICIO CASSÁN; STIJN SPAEPEN; JOS VANDERLEYDEN

Tarragona

Congreso; 20th International Conference on Plant Growth Substances; 2010

International Plant Growth Substances Association

Indole-3-acetic acid (IAA) biosynthesis is considered one of the most important physiological mechanisms for plant growth promotion in plants inoculated with Azospirillum sp. In liquid cultures, phytohormone production by Azospirillum sp. follows a kinetic similar to those of secondary metabolites. Our hypothesis states that IAA biosynthesis in Azospirillum brasilense could be modified by addition of biotic or abiotic stress effectors in exponential or stationary growth phase liquid cultures. In this way, plasmid pFAJ64 (for ipdC gene expression studies) was transferred to A. brasilense strain Az39, the most commonly used strain in Argentina for maize and wheat inoculation, by tri-parental conjugation. A. brasilense Az39 (pFAJ64) was pre-incubated in MMAB minimal medium plus 10 mg.l-1 L-tryptophane until exponential or stationary growth phase (OD595 ≈ 0.8 and 1.2), then fractionated in 10 ml tubes and individually modified by addition of abiotic (150 mM NaCl; 132 mM Na2SO4; 4.0% (w/v) PEG6000; 0.5 mM H2O2; 0.1 mM abscisic acid (ABA) and 1-aminocyclopropane 1-carboxylic acid) or biotic stress effectors (1.0 % (v/v) filtered supernatant of Pseudomonas syringae (Ps) and Fusarium oxysporum (Fo); 0.1 nM salicylic acid (SA) and methyl jasmonate (MeJA); 1.0% (w/v) chitosane (CH) and 1.0 % (w/v) TSB (source of plant protein hydrolizated)). After 30, 120 and 240 minutes incubation the following parameters were evaluated: biomass production (OD595); cellular viability (CFU.ml-1); IAA production by colorimetric method (μg.ml-1) and ipdC gene expression (Miller Units). Our results showed that IAA production in A. brasilense Az39 (pFAJ64) increased in presence of PEG6000, ABA, CH and Fo supernatant. Contrary, H2O2, NaCl, Na2SO4, MeJA, TSB and Ps supernatant decreased the hormone production and accumulation in the same experimental conditions. The ipdC gene expression increased in the treatments with PEG6000, ABA, MeJA and Ps and decreased in those treated with H2O2.