GSE192383
Azospirillum brasilense alters root system architecture through both auxin-dependent and -independent pathways
Azospirillum is a plant growth promoting rhizobacteria (PGPR) with ability to produce several phytohormones such as auxins, mainly indole-3-acetic acid (IAA).  The positive interaction of Azospirillum with plants has been simplified and explained through the bacterial capacity to produce IAA. Typical changes on root architecture by promoting the number of lateral roots and hair formation, and reducing the primary root length were established in inoculated plants. These changes increase the root surface improving the water and nutrients acquisition, and thus the growth of the whole plant. The mechanisms by which Azospirillum induces such changes fails to be explained only by the bacterial capacity to produce IAA. In this work, we have evaluated the root architecture and gene expression changes occurred in Arabidopsis thaliana inoculated with A. brasilense Az39 and the IAA-deficient mutant (Az39 ipdC-), or treated with exogenous IAA solution to confirm both, the IAA-dependent and IAA-independent Azospirillum´s pathways to promote the root growth. 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. Jasmonates, ethylene and salicylic acid were increased in the IAA-deficient bacterial treatments, as the ipdC mutant significantly up-regulated transcription of genes enriched of these phytohormones signaling after 7 days. Further, the physical presence of the inactive bacteria (Az39f) seems to mediate the development of root hairs, a mechanism common to other non-PGPR as E. coli DH5a. Our results suggest that Az39 inoculation induces morphological changes in root architecture through both IAA-dependent and independent 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.   Overall design: 2 strains of Azospirillum rhizobacteria (Az39 and -ipdc mutant) and an NaCl control were applied to the roots of Arabidopsis thaliana seedlings for transcriptomic analysis at 24 hours and 7 days post treatment; 2 replicates each, of at least 50 root systems per rep