Azospirillum brasilense modifies Arabidopsis root architecture through indoleacetic acid-dependent and independent mechanisms

MORA VERONICA; LOPEZ GASTON; MOLINA, ROMINA; CONIGLIO ANAHÍ; DONADIO, FLORENCIA; NIEVAS SOFIA; TORRES DANIELA; CASSAN, FABRICIO

Otro; Relar PGPR; 2021

Azospirillum is one of the most studied genus of plant growth promoting rhizobacteria (PGPR). The positive effects of A. brasilense on plants have been mostly, but not exclusively, explained by the bacterial production of the phytohormone indole-3-acetic acid (IAA). The main objective of our study was to evaluate the role of the bacterial IAA as co-responsible of changes observed in morphology and architecture roots of Arabidopsis thaliana and to evaluate the nature of a putative IAA-independent mechanism. The root architecture of A. thaliana was evaluated after inoculation with A. brasilense Az39, other bacteria or the treatment with exogenous IAA. Seeds of A. thaliana Col-0 or its mutant tir 1.1 were germinated vertically in Petri dishes containing MS medium for 7 d (photoperiod 16/8h, 22 ºC). Seedlings were then aseptically transferred to MS plates containing 108 cfu·ml-1 of Az39; the heat inactivated cells of this strain (Az39φ); the IAA-deficient mutant (Az39∆ipdC); or E. coli DH5α, as negative control. After 5 d post-transplantation, the root architecture was evaluated using an image analysis system. Our results demonstrate the ability of Az39 to modify the primary root development through IAA biosynthesis, while other IAA-independent mechanisms were related to an increase in the lateral roots development and the root hairs number. The physical presence of the inactive bacteria (Az39φ) seems to mediate the development of root hairs, a mechanism common to other non-PGPR as E. coli DH5α. Our results suggest that Az39 inoculation induces morphological changes in root architecture through both IAA-independent and dependent mechanism. The IAA biosynthesis by Az39 reduces the primary root length; while the cells contact with the roots increases the root hairs production. Both the synthesis of active IAA and the presence of metabolically active Az39 cells increase the growth and development of lateral roots.