The relationship between pathogen-induced systemic resistance (ISR) and multigenic (horizontal) resistance in plants

Plants have developed mechanisms to successfully co-exist in the presence o f pathogenic organisms. Some interactions between plants and pathogens are based on recognition of specific elicitor molecules from avirulent pathogen races (avr gene products), which is described in the gene-for-gene resista nce theory. Another type of resistance, multigenic (horizontal) resistance, is a less well-studied phenomenon that depends upon multiple genes in the plant host. All plants possess resistance mechamisms which can be induced u pon pre-treatment of plants with a variety of organisms or compounds. This general phenomenon is known as induced systemic resistance (ISR). At least in some plant species, ISR depends on the timely accumulation of multiple g ene products, such as hydrolytic enzymes, peroxidases or other gene product s related to plant defences. The pre-treatment of plants with an inducing o rganism or compound appears to incite the plant to mount an effective defen se response upon subsequent encounters with pathogens, converting what woul d have been a compatible interaction to an incompatible one. Our studies in three plant-pathogen systems clearly document that multigenic-resistant pl ants constitutively express specific isozymes of hydrolytic enzymes that re lease cell wall elicitors, which in turn may activate other defense mechani sms. ISR induces constitutive accumulation of these and other gene products prior to challenge. ISR is known to function against multiple organisms, a nd there is no specificity observed in the accumulation patterns of defense -related gene products when ISR is induced. It is therefore hypothesized th at the constitutive accumulation of specific isozymes of hydrolytic enzymes , or other defense related gene products, is an integral part of both multi genic resistance and the phenomenon of ISR. Further, plants in which ISR ha s been activated appear to move from a latent resistance state to one in wh ich a multigenic, non-specific form of resistance is active.