Plant-Microbe Interactions: Current Perspectives of Mechanisms Behind Symbiotic and Pathogenic Associations

Abstract

The phyllosphere and rhizosphere of plants have been a reservoir of microorganisms of both symbiotic and pathogenic nature. The interplay between plants and associated microbes involves complex and dynamic mechanisms, many of which are unexplored. The unraveling of these mechanisms is a big challenge for plant biologists. The consequence of such interactions may be beneficial, detrimental, or neutral for the hosts. There are many known mechanisms through which microorganisms especially bacteria support plant growth, i.e., fixation of atmospheric nitrogen, solubilization of inorganic phosphate, modulated phytohormones synthesis, production of stress-responsive enzymes like 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, and biocontrol of many plant diseases. Both above- and underground plant organs are frequently exposed to a plethora of microorganisms, including viruses, bacteria, oomycetes, fungi, and eukaryotic protozoans. Phytopathogens defend their habitat and infect plants by a variety of compounds (toxins) that are broad spectrum in their activity. In response, plants initiate defensive mechanisms that resist pathogen penetration and subsequent infection. Thus, various events of molecular crosstalk take place between plants, and both friendly and hostile microbes trigger a series of highly dynamic plant cellular responses. Such mechanisms are very crucial for pathogen recognition and induction of adequate defense signal transduction cascades in the plant. More research insights are required to unravel the molecular basis behind these mechanisms. Also, to support the plant life, many complex mechanisms initiated after the association of symbiotic or pathogenic microorganisms need to be explored.