Plant Innate Immunity Multicomponent Model

Abstract

Our understanding of plant-pathogen interactions is making rapid advances in order to address issues of global importance such as improving agricultural productivity and sustainable food security. Innate immunity has evolved in plants, resulting in a wide diversity of defense mechanisms adapted to specific threats. The postulated PTI/ETI model describes two perception layers of plant innate immune system, which belong to a first immunity component of defense response activation. To better describe the sophisticated defense system of plants, we propose a new model of plant immunity. This model considers the plant's ability to distinguish the feeding behavior of their many foes, such as a second component that modulates innate immunity. This hypothesis provides a new viewpoint highlighting the relevance of hormone crosstalk and primary metabolism in regulating plant defense against the different behaviors of pathogens with the intention to stimulate further interest in this research area.

Keywords: IAC; IMC; biotrophic; necrotrophic; phytohormone; plant defense; primary metabolism.

FIGURE 1

Two different resistance directions are supposed to be activated in a multi-trophic interaction. Once a plant containing a R-gene (different green shades) comes in contact with biotrophic or necrotrophic pathogens, only an incompatible interaction will activate the plant resistance. In this scheme, the interaction spaces of plant resistance (green triangle) and of biotrophic and necrotrophic pathogens (blue and red rectangles, respectively), are indicated. The intersections of interaction spaces identify three plant–pathogen interaction areas: two are pathogen lifestyle-related (small black triangles) and one is common (violet circle). The synergic effect of immunity activation and of pathogen lifestyle-dependent components result in plant immunity to biotrophic or necrotrophic pathogens.

FIGURE 2

The circular model. The model schematically shows the key points of activation and modulation of plant immunity. Plant resistance mechanism of an incompatible interaction might be divided into three phases: (1) interaction, (2) activation/modulation, and (3) effective resistance (immunity). During the interaction stage, two principal effects are detected: (A) modifications of virulence factor targets and (B) specific alterations of primary plant metabolism. In the activation stage: the modifications of virulence factor targets induce the Nibblers Triggered Signaling (NTS) or PPRs Triggered Signaling (PTS), mediated by R-genes activation. These metabolic alterations induce a feedback regulation of primary metabolic pathways resulting in a Hormone Tempered Resistance (HTR). In the effective resistance stage, the NTS/PTS, and the HTR converge to confer a resistance specific to the lifestyle of pathogen (Pathogen lifestyle-Specific Resistance, PSR).