Abstract
Plant growth-promoting rhizobacteria (PGPR) induce positive effects in plants, such as increased growth or reduced stress susceptibility. The mechanisms behind PGPR/plant interaction are poorly understood, as most studies have described short-term responses on plants and only a few studies have analyzed plant molecular responses under PGPR colonization. Here, we studied the effects of the PGPR bacterial model Burkholderia phytofirmans PsJN on the whole life cycle of Arabidopsis thaliana plants. We reported that at different plant developmental points, strain PsJN can be found in the rhizosphere and also colonizing their internal tissues. In early ontogeny, strain PsJN increased several growth parameters and accelerated growth rate of the plants. Also, an Arabidopsis transcriptome analysis revealed that 408 genes showed differential expression in PsJN-inoculated plants; some of these genes are involved in stress response and hormone pathways. Specifically, genes implicated in auxin and gibberellin pathways were induced. Quantitative transcriptional analyses of selected genes in different developmental stages revealed that the beginning of these changes could be evidenced early in development, especially among the down-regulated genes. The inoculation with heat-killed bacteria provoked a more severe transcriptional response in plants, but was not able to induce plant growth-promotion. Later in ontogeny, the growth rates of inoculated plants decreased with respect to the non-inoculated group and, interestingly, the inoculation accelerated the flowering time and the appearance of senescence signs in plants; these modifications correlate with the early up-regulation of flowering control genes. Then, we show that a single inoculation with a PGPR could affect the whole life cycle of a plant, accelerating its growth rate and shortening its vegetative period, both effects relevant for most crops. Thus, these findings provide novel and interesting aspects of these relevant biological interactions.
Highlights
In the rhizosphere, plants and microorganisms are permanently interacting in a continuum ranging from deleterious to beneficial [1]
The plant growth outcome at day 14 depended on the population of bacteria that was initially associated to plant
Among genes involved in stress or defense response we found genes involved in salicylic acid (SA) pathway, like WRYK60 (At2G25000); WRKY70 (At3G56400) and WAK1 (At1G21250); and genes involved in jasmonic acid (JA) and ethylene pathways (e.g. LOX2, At3G45140 and PDF1.2, At5G44420), see Table S1, S2 and S3
Summary
Plants and microorganisms are permanently interacting in a continuum ranging from deleterious (pathogens) to beneficial (symbionts) [1]. Among the beneficial interactions are those with plant growth-promoting rhizobacteria (PGPR) which colonize the rhizosphere or internal tissues of many plant species, inducing positive effects such as increased plant growth, reduced susceptibility to diseases (caused by fungi, bacteria, viruses and nematodes) and improved tolerance to abiotic stresses [1,3,4,5,6]. Different mechanisms of rhizobacterial growth promotion have been proposed. The ability to fix atmospheric nitrogen [7]; solubilization of inorganic nutrients that are rate-limiting for plant growth [8]; stimulation of nutrient delivery and uptake by plant roots; and the modulation of plant regulatory mechanisms through the production of hormones such as auxin, gibberellins and cytokinins [9,10,11,12], the reduction of plant ethylene levels [13,14,15] or the production of other compounds that influence plant development [16,17,18]
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