Chitosan/starch beads as bioinoculants carrier: long-term survival of bacteria and plant growth promotion.

Immobilization of microorganisms in biodegradable polymeric matrices constitutes a promising technology for plant growth promoting to overcome the challenging conditions of the rhizosphere. Previously, we demonstrated that beads prepared from blends of chitosan/starch of analytical grades ionically cross-linked are useful carriers for Azospirillum brasilense and Pseudomonas fluorescens. The aims of this work were to study A. brasilense Az39 and P. fluorescens ZME4 immobilization in industrial quality beads produced with a blend of chitosan/starch, to assess bacterial survival during long-term storage and biofilm distribution in the beads. We also proposed to analyze the consortia root colonization and its performance as plant growth-promoting bioinoculants compared to liquid counterpart. Our results revealed that A. brasilense Az39 and P. fluorescens ZME4 can coexist in industrial grade chitosan/starch beads, and this mixed immobilization benefits the survival rates of both species, even for more than a year under shelf storage. Confocal laser scanning microscopy with fluorescent dyed strains showed that both species remain mainly in different locations inside and over the beads. Additionally, maize seed treatment with beads-loaded bacteria resulted in growth promotion of roots in a similar manner than traditional liquid-based inoculation. The evidence collected here demonstrate that low-cost chitosan/starch beads are a suitable carrier for bacteria consortia and could be a reliable alternative to liquid inoculation in agronomic practices with additional benefits for industrial management. KEY POINTS: • Mixed immobilization increases bacterial survival in chitosan/starch industrial beads • Beads increase competence of bacteria in rhizosphere of maize • Inoculation mediated by beads promotes plant growth of maize.

Aerobic nitric oxide production by Azospirillum brasilense Sp245 and its influence on root architecture in tomato.

The major feature of the plant-growth-promoting bacteria Azospirillum brasilense is its ability to modify plant root architecture. In plants, nitric oxide (NO) mediates indole-3-acetic acid (IAA)-signaling pathways leading to both lateral (LR) and adventitious (AR) root formation. Here, we analyzed aerobic NO production by A. brasilense Sp245 wild type (wt) and its mutants Faj009 (IAA-attenuated) and Faj164 (periplasmic nitrate reductase negative), and its correlation with tomato root-growth-promoting effects. The wt and Faj009 strains produced 120 nmol NO per gram of bacteria in aerated nitrate-containing medium. In contrast, Faj164 produced 5.6 nmol NO per gram of bacteria, indicating that aerobic denitrification could be considered an important source of NO. Inoculation of tomato (Solanum lycopersicum Mill.) seedlings with both wt and Faj009 induced LR and AR development. In contrast, Faj164 mutant was not able to promote LR or AR when seedlings grew in nitrate. When NO was removed with the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), both LR and AR formation were inhibited, providing evidence that NO mediated Azospirillum-induced root branching. These results show that aerobic NO synthesis in A. brasilense could be achieved by different pathways and give evidence for an NO-dependent promoting activity on tomato root branching regardless of bacterial capacity for IAA synthesis.