Burkholderia sp. Strain AU4i Promotes Arabidopsis Growth and Increases Plant Defence Response to Necrotrophic Pathogens

Burkholderia sp. AU4i is a strain previously isolated from pea, which can act as plant growth-promoting rhizobacteria (PGPR). Here, we demonstrate that AU4i is able to promote Arabidopsis growth and increase the number of lateral roots as well as shoot and root dry weight. The inoculation of Arabidopsis leaves with AU4i significantly reduced the lesion’s areas provoked by the necrotrophic pathogens Botrytis cinerea and Sclerotinia sclerotiorum . Bacterial inoculation induced the expression of the jasmonate responsive marker gene AtPDF1.2 , but not the expression of salicylic acid marker gene AtPR-1 . Notwithstanding, similar AU4i protective effect was observed in Arabidopsis mutants coi1 and sid2-2 (defective in jasmonate perception and salicylic acid synthesis, respectively) compared with Col-0 plants. Thus, the independence of the AU4i protective effect and plant defence signalling pathways is suggested. AU4i inoculation was able to induced anthocyanins and phenolic compounds accumulation, as well as phenylalanine ammonia-lyase activity. Moreover, the effect of AU4i inoculation on the metabolism of plant cell wall was evaluated. As a result, B. cinerea and S. sclerotiorum mycelial growth was reduced in culture media containing cell wall polysaccharides from inoculated Arabidopsis leaves. Additionally, leaves colonized by AU4i showed the downregulation of genes involved in hemicelluloses and pectins degradation ( AtExp8 and AtAra1 , respectively) and the upregulation of AtPME3 , a gene encoding a putative pectin methylesterase involved in pectin main chain de-esterification. Taken together, our results suggest Burkholderia sp . AU4i not only as potential biocontrol agent, but also as a good candidate for better understanding the mechanisms used by PGPRs to protect plants from fungal attacks.