Performance of Cd-resistant rhizobacteria to improve Brassica napus growth under cadmium stress

Cadmium is one of the most toxic heavy metal for plants, inhibiting root and shoot growth and affecting nutrient uptake and homeostasis. Plant growth promoting rhizobacteria (PGPR) can improve plant nutrition and growth, competitiveness and responses to external stress factors. The plant growth promoting activity of four cadmium-resistant rhizobacteria (Pseudomonas tolaasii ACC23, Pseudomonas fluorescens ACC9, Alcaligenes sp. ZN4 and Mycobacterium sp. ACC14) with multiple PGP traits was assessed on canola (Brassica napus) in presence and in absence of Cd. The considered PGP traits were the production of indole acetic acid (IAA) and siderophores and the capability of consuming 1-aminocyclopropane-1-carboxylate (ACC), the immediate precursor of ethylene. All the studied strains had in vitro ACC deaminase (ACCD) activity. The strains produced IAA and siderophores more actively under Cd-stress and the ACCD activity of only two strains was reduced by the metal. Root elongation assay conducted on B. napus under gnotobiotic conditions demonstrated significant increases in root elongation of inoculated canola seedlings grown in the presence or in the absence of cadmium (from 21 to 104% and from 34 to 97%, respectively ) compared to the control plants. The pot culture experiment with treated (15 g Cd2+ g-1 dw) and un-treated soil clearly demonstrated the beneficial effects of the inoculations with P. tolaasii ACC23, P. fluorescens ACC9 and Mycobacterium sp. ACC14. The PGPR protected plants from growth inhibition caused by the metal, as demonstrated by dry-weight biomass data, without influencing the specific accumulation of cadmium in root and shoot systems. However, total Cd uptake per plant significantly increased, thus resulting in a higher metal removal. In particular, the maximum growth and Cd uptake were obtained with P. tolaasii ACC23 and P. fluorescens ACC9. The isolated bacteria c as inoculants to improve B. napus growth in the presence of toxic concentration of cadmium. Field experiment will confirm the PREP activity of the strains on plant growth in order to stabilize, revegetate and remediate metal-polluted soils.