The role of microbial signals in plant growth and development

Abstract

Plant growth and development involves a tight coordination of the spatial and temporal organization of cell division, cell expansion and cell differentiation. Orchestration of these events requires the exchange of signaling molecules between the root and shoot, which can be affected by both biotic and abiotic factors. The interactions that occur between plants and their associated microorganisms have long been of interest, as knowledge of these processes could lead to the development of novel agricultural applications. Plants produce a wide range of organic compounds including sugars, organic acids and vitamins, which can be used as nutrients or signals by microbial populations. On the other hand, microorganisms release phytohormones, small molecules or volatile compounds, which may act directly or indirectly to activate plant immunity or regulate plant growth and morphogenesis. In this review, we focus on recent developments in the identification of signals from free-living bacteria and fungi that interact with plants in a beneficial way. Evidence has accumulated indicating that classic plant signals such as auxins and cytokinins can be produced by microorganisms to efficiently colonize the root and modulate root system architecture. Other classes of signals, including N-acyl-L-homoserine lactones, which are used by bacteria for cell-to-cell communication, can be perceived by plants to modulate gene expression, metabolism and growth. Finally, we discuss the role played by volatile organic compounds released by certain plant growth-promoting rhizobacteria in plant immunity and developmental processes. The picture that emerges is one in which plants and microbes communicate themselves through transkingdom signaling systems involving classic and novel signals.

Figure 1

Rhizosphere modification improves plant productivity. Inoculation with plant beneficial microorganisms offers many advantages to crops, including enhanced rooting, activation of immunity and improved plant yield.

Figure 2

Use of Arabidopsis thaliana for research in plant-microbe interactions. Representative photographs showing uninoculated Arabidopsis seedlings grown in a 0.2× Murashige and Skoog medium (A), inoculated with Trichoderma atroviride (B), or with Bacillus megaterium (C). Note the elicitation of shoot growth by both the fungus and bacterium and the formation of branched root systems.

Figure 3

Communication between plants and bacteria mediated by AHLs and plant-produced signals. (A) N-octanoyl-homoserine lactone (C8-HL), (B) N-decanoyl-homoserine lactone (C10-HL), (C) N-3-oxodecanoyl-homoserine lactone, (D) N-dodecanoyl-homoserine lactone (C12-HL), (E) N-isobutil decanamide, (F) N-isobutyl-2E,6Z,8E-decatrienamide (Affinin), (G) N-ethanol decanamide (NAE 10:0). The bacteria produce AHLs, which modulate root system architecture and the plant produces NAEs and alkamides, which may interfere with bacterial quorum-sensing.

Figure 4

Mechanisms involved in volatile organic compound modulation of plant growth. Microorganisms produce VOCs, which can be sensed by plants to alter morphogenesis or activate defense and stress-related responses.