Plant growth-promoting rhizobacteria and root system functioning

Abstract

The rhizosphere supports the development and activity of a huge and diversified microbial community, including microorganisms capable to promote plant growth. Among the latter, plant growth-promoting rhizobacteria (PGPR) colonize roots of monocots and dicots, and enhance plant growth by direct and indirect mechanisms. Modification of root system architecture by PGPR implicates the production of phytohormones and other signals that lead, mostly, to enhanced lateral root branching and development of root hairs. PGPR also modify root functioning, improve plant nutrition and influence the physiology of the whole plant. Recent results provided first clues as to how PGPR signals could trigger these plant responses. Whether local and/or systemic, the plant molecular pathways involved remain often unknown. From an ecological point of view, it emerged that PGPR form coherent functional groups, whose rhizosphere ecology is influenced by a myriad of abiotic and biotic factors in natural and agricultural soils, and these factors can in turn modulate PGPR effects on roots. In this paper, we address novel knowledge and gaps on PGPR modes of action and signals, and highlight recent progress on the links between plant morphological and physiological effects induced by PGPR. We also show the importance of taking into account the size, diversity, and gene expression patterns of PGPR assemblages in the rhizosphere to better understand their impact on plant growth and functioning. Integrating mechanistic and ecological knowledge on PGPR populations in soil will be a prerequisite to develop novel management strategies for sustainable agriculture.

Keywords: ISR; functional group; phytohormone; plant nutrition; plant-PGPR cooperation; rhizo-microbiome; rhizosphere.

FIGURE 1

Impact of phytostimulating PGPR on RSA, nutrient acquisition and root functioning. PGPR can modulate root development and growth through the production of phytohormones, secondary metabolites and enzymes. The most commonly observed effects are a reduction of the growth rate of primary root, and an increase of the number and length of lateral roots and root hairs. PGPR also influence plant nutrition via nitrogen fixation, solubilization of phosphorus, or siderophore production, and modify root physiology by changing gene transcription and metabolite biosynthesis in plant cells.

FIGURE 2

PGPR-mediated changes in RSA may relate to modifications of auxin content in roots. Six-day-old Arabidopsis plantlets expressing the GFP gene under the control of the auxin-sensitive DR5 artificial promoter were inoculated (C, D) or not (A, B) with the PGPR Phyllobacterium brassicacearum STM196. Six days later, root tips were observed under normal light (A, C) or UV light (B, D) with a microscope (Z16APO, Leica, Bensheim, Germany). Scale bars represent 200 μm. Inoculation by STM196 modified root traits such as root hair elongation and primary root growth, which coincided with an increase in GFP signal in the root tip in inoculated (D) compared with control plants (B). These observations confirm previous results with a different Arabidopsis DR5 reporter line (Contesto et al., 2010).

FIGURE 3

Implementation of plant-growth promoting traits in PGPR functional groups. Selected PGPR functional groups are represented by different colored circles. The resulting effect of all PGPR functional groups on the plant is symbolized by the gray circle. Abiotic and biotic factors may influence the activity of each functional group. Solid arrows represent potential interactions (inhibition, signaling, etc.) between members of the functional groups, which may impact on the size, diversity and activity of these groups.