THE CHARACTERIZATION OF DEFENSE RESPONSES TO FUNGAL INFECTION IN ALFALFA

Authors: Saunders, James; Oneill, Nichole

Submitted to: BioControl
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2004
Publication Date: 6/1/2004
Citation: Saunders, J.A., Oneill, N.R. 2004. The characterization of defense responses to fungal infection in alfalfa. Biocontrol. 49:715-728.

Interpretive Summary: Many plants have natural defense responses to attack by fungal organisms. These defensive systems often utilize a metabolic pathway known as phenlypropanoid biosynthesis. This important group of compounds has been shown in this study to be very important in making alfalfa plants resistant to serious fungal infection by first being exposed to a non-lethal fungus. This paper describes the genetic and biosynthesis processes that enable alfalfa plants to withstand infection by normally lethal fungal infections. Understanding this process is key to providing alfalfa farmers with tools that will help them grow a healthy crop which is resistant to an important fungal disease known as anthracnose.

Technical Abstract: The enzyme activity and transcript level of key flavonoid precursor enzymes, phenylalanine ammonia-lyase (PAL), cinnamic acid 4-hydroxylase (CA4H), and isoflavone reductase (IFR), were monitored alfalfa seedlings that had been challenged with the fungal pathogen Colletotrichum trifolii. Inoculation of alfalfa with avirulent fungal conidia (race 1, incompatible interaction) induced protection from subsequent challenge inoculation with virulent conidia (race 2, compatible interaction). A defensive plant protection response was accompanied by increases in transcript levels of PAL, CA4H and IFR gene expression, by increases in PAL enzyme activity and by production of the end product phytoalexins, medicarpin and sativan. The expression 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. The phytoalexins medicarpin and sativan increased rapidly and reached maximal 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. Negligible amounts of these phytoalexins were detected in uninoculated control plants and plants inoculated with race 2 alone. 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 flavonoid biosynthesis.