TEMPORAL PATTERNS OF MRNA AND PHYTOALEXIN ACCUMULATION FROM ISOFLAVONOID METABOLISM DURING COMPATIBLE AND INCOMPATIBLE INTERACTIONS BETWEEN MEDICAGOSATIVA AND COLLETOTRICHUM TRIFOLII.

Authors: Oneill, Nichole; Saunders, James; CHENG, JIANPING - MISC

Submitted to: Phytochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/4/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Foliar, crown, and stem pathogens cause serious diseases and significant yield losses in alfalfa. Anthracnose, caused by Colletotrichum trifolii, is the most important of these diseases. Induced resistance is a phenomenon by which alfalfa and some other crops can be rendered immune to attack by virulent fungi by prior inoculation with non-virulent strains of a fungus. We discovered a new and unique component of induced resistance. Plants in the acquired immune state responded to challenge by highly virulent pathogens by producing dramatically elevated levels of fungitoxic compounds. This second component, a response to challenge, renders the plant immune to infection and disease. This enhanced defense expression is important because it may increase disease resistance under field conditions where plants are subject to attack by numerous foliar pathogens. We determined that the origin of the increase in fungitoxic compounds is due to increased expression of genes of isoflavonoid biosynthesis, with coordinate increase in enzyme activity. It may be possible to alter the expression of these genes by genetically engineered alfalfa plants so that the initial priming is expressed constitutively in the whole plant. This will enable a enhanced defense response under field conditions when plants are challenged by pathogenic fungi. This information is important for scientists who are working to increase resistance to fungal pathogens.

Technical Abstract: The transcriptional activity of genes encoding PAL, CA4H, and IFR in isoflavonoid metabolism were monitored in compatible, incompatible and induced resistance interactions between seedlings of alfalfa (Medicago sativa) cultivar Arc and the fungal pathogen C. trifolii. Inoculation of alfalfa seedlings with race 1 conidia (avirulent, incompatible interaction) induced protection from subsequent challenge inoculation with conidia of race 2 (virulent, compatible interaction). Protection was accompanied by a coordinate increase in PAL, IFR, and CA4H gene expression, PAL activity, and production of phytoalexins medicarpin and sativan. The temporal expression patterns of defense genes and PAL enzyme activity were significantly greater in induced resistant seedlings responding to challenge inoculation with race 2, compared to plants inoculated with race 1 alone. Inoculations with race 2 alone produced lower amounts of the gene transcripts. Gene transcripts reached their peaks after 48 hours, then declined. Coordinate with patterns of gene transcription, phytoalexins medicarpin and sativan increased rapidly and reached highest levels 97 hours after inoculation with race 1. The challenge inoculation with race 2 was followed by significantly greater accumulation of medicarpin than in any other treatment. These results suggest that the increased medicarpin accumulation produced in induced resistant tissues, following challenge inoculation with race 2, is attributable to increased expression of genes of isoflavonoid biosynthesis with coordinate increase in PAL enzyme activity.