Plants have been evolving for millions of years, creating new and improved ways to protect themselves from attack. Plants protect themselves from pathogen attack through activation of innate immune systems, and plant immunity is a highly complex phenomenon with its own unique features that have emerged as a result of the evolutionary arms race between plants and pathogens.
The main immune response is called Induced Resistance, and it can be over-simplified to contain two general pathways: salicylic acid, which enables Systemic Acquired Resistance (SAR), and jasmonic acid, which enables Induced Systemic Resistance (ISR). SAR can be generally thought of as being effective against biotrophic pathogens (those that can invade feed off of living plant cells without killing them), whereas ISR can be thought of as being effective against necrotrophic pathogens (those that kill cells to derive their nutrition) and insects.
ISR can be enhanced by “priming,” which works a little like inoculations in humans. Priming enhances the plant immune system’s sensitivity to attack, which makes it more effective in activating defense mechanisms against pathogens. Beneficial rhizobacteria can prime a host of cellular defense responses such as oxidative burst, cell-wall reinforcement, defense-related enzymes accumulation, and secondary metabolites production.
The signaling of defense responses relies on the production of two plant hormones, salicylic acid and jasmonic acid. These two signaling pathways influence each other through a complex network of synergistic and antagonistic interactions. Probably, cross talks between the pathways help the plant to minimize fitness costs and create a flexible signaling network that allow the plant to regulate its defense responses against invaders. Indeed, there are some data showing that salicylic acid can also enhance or is synergistic with the ISR response.
Rhizobacteria belonging to the genera Pseudomonas and Bacillus are well known for their antagonistic effects and their ability to trigger ISR. These rhizobacteria might be useful in formulating new inoculants with combinations of different mechanisms of action, leading to more efficient biocontrol strategies utilizing natural defensive responses. Maybe by better understanding and eventually harnessing the signaling pathways that are triggered by salicylic and jasmonic acids, we can do a better job of helping our crops help themselves.
