The plant immune system is activated by root colonization of beneficial micro-organisms

Sergio Molinari and Paola Leonetti

The plant immune system is activated by root colonization of beneficial micro-organisms

Sergio Molinari and Paola Leonetti

Institute of Sustainable Plant Protection - National Research Council of Italy, Italy

: J Plant Physiol Pathol

Abstract

Mycorrhizal fungi, endophytic fungi and rhizobacteria play a key role in preserving soil fertility in forest agro-ecosystems. Enriching soil of potted plants with these beneficial micro-organisms lead to activation of plant innate immunity with decreased sensitivity to soil borne phytoparasitic nematodes and miner insects. Immune response in plants is regulated by phytohormones that are low molecular weight molecules which interact in a complex network to regulate many aspects of plant growth, photosynthesis, flowering, reproduction, seed production and response to environmental abiotic challenges. Energy resources supporting innate immunity in plants are the same as those involved in plant growth processes; thus, expressing constitutive defense systems occurs only at the cost of plant growth and encounters the risk of allocating resources to defense in the absence of natural pathogens and pests. An effective alternative is to fine-tune immune responses by modulating the "immunological memory" of plants, as it occurs in animals. An aspect of this modulation may be represented by the socalled "priming" by which previously attacked plants respond more quickly or more strongly to a subsequent attack. Beneficial micro-organisms stimulate both salicylic acid (SA)-mediated (Systemic Acquired Resistance, SAR) and jasmonic acid (JA) and ethylene (ET)-mediated (Induced Systemic Resistance, ISR) induced resistance. Expressions of marker genes of SAR and ISR are monitored by qRT-PCR after treatment of plants with beneficial micro-organisms and after parasite inoculation. Activation of plant immunity by beneficial micro-organisms may be repressed by pathogens and parasites that have developed sophisticated molecular mechanisms to deregulate the biosynthesis of hormones and/or to interfere with hormonal signaling pathways.