Plant responses to plant growth-promoting rhizobacteria

Abstract

Non-pathogenic soilborne microorganisms can promote plant growth, as well as suppress diseases. Plant growth promotion is taken to result from improved nutrient acquisition or hormonal stimulation. Disease suppression can occur through microbial antagonism or induction of resistance in the plant. Several rhizobacterial strains have been shown to act as plant growth-promoting bacteria through both stimulation of growth and induced systemic resistance (ISR), but it is not clear in how far both mechanisms are connected. Induced resistance is manifested as a reduction of the number of diseased plants or in disease severity upon subsequent infection by a pathogen. Such reduced disease susceptibility can be local or systemic, result from developmental or environmental factors and depend on multiple mechanisms. The spectrum of diseases to which PGPRelicited ISR confers enhanced resistance overlaps partly with that of pathogen-induced systemic acquired resistance (SAR). Both ISR and SAR represent a state of enhanced basal resistance of the plant that depends on the signalling compounds jasmonic acid and salicylic acid, respectively, and pathogens are differentially sensitive to the resistances activated by each of these signalling pathways. Root-colonizing Pseudomonas bacteria have been shown to alter plant gene expression in roots and leaves to different extents, indicative of recognition of one or more bacterial determinants by specific plant receptors. Conversely, plants can alter root exudation and secrete compounds that interfere with quorum sensing (QS) regulation in the bacteria. Such two-way signalling resembles the interaction of root-nodulating Rhizobia with legumes and between mycorrhizal fungi and roots of the majority of plant species. Although ISR-eliciting rhizobacteria can induce typical early defence-related responses in cell suspensions, in plants they do not necessarily activate defence-related gene expression. Instead, they appear to act through priming of effective resistance mechanisms, as reflected by earlier and stronger defence reactions once infection occurs.

Rhizobacteria isolated from a metal-polluted area enhance plant growth in zinc and cadmium-contaminated soil

“…The indirect promotion occurs when the rhizobacteria decrease or prevent the deleterious effects of pathogenic microorganisms (Penrose and Glick 2003). This process is accomplished by the production of antagonistic substances such as antibiotics; antifungal metabolites (e.g., hydrogen cyanide-HCN); and extracellular enzymes (Glick 2001;van Loon 2007).…”

Characterization of Bacteria Isolated from the Saffron (Crocus sativus L.) Rhizosphere

“…In addition to nitrogen fixation and modification of the uptake of macro-and micronutrients by plants, these species were characterized as synthesizing and secreting some plant growth regulators and various amino acids, antibiotic, and so on (Mayak et al 2004;van Loon 2007); and in this way, they have been able to stimulate the growth and development of saffron roots and shoots.…”

Organic hydroponics induces systemic resistance against the air-borne pathogen,Botrytis cinerea(gray mould)

“…For instance, rhizosphere microbes (Ramamoorthy et al 2001) and the biochemical they produce (Pal & Gardener 2006;Thakur & Sohal 2013) are possible elicitors that initiate responses on the root surface that activate signal transduction pathways and stimulate positive regulators of plant resistance (Thakur & Sohal 2013). The reduction of disease symptoms in above-ground plant parts due to the presence of rhizosphere microbes on the plant roots is known as induced systemic resistance (ISR) (van Loon et al 1998;Ramamoorthy et al 2001;Pieterse et al 2003;van Loon 2007;Bogino et al 2013). ISR was previously described by van Peer et al (1991) in carnation and Wei et al (1996) in cucumber.…”

“…In ISR, rhizosphere microbes trigger the JA/ET pathway by accumulating JA and ET. Moreover, pathogens and other challenges up-regulate SA, eventually leading to systemic acquired resistance, a defense response synonymous to ISR (Ramamoorthy et al 2001;Pieterse et al 2003;Verhagen et al 2006;van Loon 2007;Haggag 2010;Saharan & Nehra 2011). SA also conjugates with β-glucoside to form salicylic acid β-glucoside (SAG), which accumulates around disease lesions as an early response to pathogen infection (Conrath et al 1995).…”