Promoting Plant Protection by Root-Associated Microbes

The rhizosphere is a narrow area around plant roots in which root-secreted secondary metabolites and microorganisms co-exist and interact. This area can also be considered as a battle field where beneficial bacteria and deleterious microorganisms compete. Therefore, this specific root area has been a topic of interest for researchers for more than a century. Among the diverse microbial population of the rhizosphere, the beneficial root-associated bacteria, also referred to as plant growth-promoting rhizobacteria (PGPR), have been used to manage plant diseases and promote plant growth and yield. Such a disease control method is referred to as biological control in contrast to chemical control. The biological control strategy was first proposed 50 years ago by Baker and Cook, who defined biological control as " the reduction of the amount of inoculum or disease producing activity of a pathogen accomplished by or through one or more organisms than man " (Baker and Cook, 1974). However, biological control of plant diseases is still in its infancy in comparison to the chemical control method based on market shares. Few biological control agents (BCAs) are currently available on the market. Meanwhile , there is a recent public demand for healthier foods with less contamination from chemical residues, and a great concern for the preservation of the environment. Therefore, a review of recent advances in biological control research including the success and failure of its implementation has been important to avoid repeating the similar mistakes made in the past. Early biological control in the 1970's – 1980's consisted of screening antagonistic microorganisms for their biological activity, and elucidating their mechanism of action before developing a biological control product. The discovery of PGPR-elicited induced systemic resistance (ISR) in the 1990s shifted the focus of research towards the development of BCAs exhibiting ISR capacity. Genome-and multi-omics-based cutting-edge technologies developed in 2000's have contributed to our understanding of the molecular mechanisms on the microbial determinants of biological control and plant responses. Taken together, I introduce the results of 11 studies that used a multidisciplinary approach to the management of plant diseases with rhizosphere microbes. The papers were organized into four different types as follows: (1) reviews on biological control and ISR, (2) biological control by antagonistic rhizobacteria, (3) biological control by ISR, and (4) induced systemic tolerance to abiotic stresses. Kwak and Weller reviewed the well-characterized and historically well-defined take-all decline (TAD) that occurred naturally in the northwest United …