|Baldridge, Gerald - UNIVERSITY OF MINNESOTA|
Submitted to: Plant Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 15, 1998
Publication Date: N/A
Interpretive Summary: Diseases and nematodes (microscopic round worms) are major barriers to the successful cultivation of alfalfa. They contribute to poor seedling growth, reduce the productive life of a field, and lead to major yield losses. One type of nematode, the root-lesion nematode, occurs throughout temperate regions of the world and feeds on roots of many important crop species. The best means of controlling this pathogen is through use of plants naturally resistant to the nematode. Plant breeding programs have identified plants with resistance to the root lesion nematode, but little is known about how resistance works. We studied the differences between resistant and susceptible alfalfa plants before nematode infection. Before infection, resistant plant roots had a higher capacity for producing some defense related compounds than susceptible roots. After infecting roots with nematodes, the resistant roots increased their capacity for making other defense related proteins. Susceptible roots increased their capacit for making defensive chemical compounds only after nematode infection and their response was slower than the response of resistant plants. We found that one of these defense related compounds is able to paralyze root lesion nematodes and prevent them from feeding on susceptible roots. Resistant plants were found to have higher levels of this compound than susceptible plants. These results indicate that resistant plants have two types of resistance mechanisms, one that is preformed and present before infection, and one that occurs only after infection. A clear understanding of the basis of resistance to root lesion nematodes will aid in the development of more highly disease resistant types of alfalfa and other crops.
Technical Abstract: Alfalfa (Medicago sativa) varieties with antibiosis-based resistance to the root-lesion nematode (Pratylenchus penetrans), a migratory endoparasite of many crops, have been developed by recurrent selection. Individual plants from these varieties that support significantly lower nematode reproduction were identified for molecular and biochemical characterization of defense responses. Before nematode infection, RNA blot analysis revealed 1.3-1.8 fold higher defense-response gene mRNA levels in roots of three resistant plants as compared to three susceptible alfalfa plants. The mRNAs encoded phenylpropanoid pathway enzymes including: phenylalanine ammonia lyase, chalcone synthase, isoflavone reductase, and caffeic acid O-methyl transferase. Following nematode infection, these mRNAs declined over 48 hr in resistant roots. In susceptible plants, phenylpropanoid mRNAs rose during the first 12 hr after nematode infection and then declined. Histone eH3.2 mRNA levels, initially 1.3-fold higher in resistant roots, declined within 6 hr to levels found in susceptible roots and remained stable in both root types thereafter. Acidic beta-1,3-glucanase mRNA levels were initially similar in both root types but accumulated more rapidly in resistant than in susceptible roots after nematode infection. Levels of a class I chitinase mRNA were similar in both root types. HPLC analysis of isoflavonoid phytoalexin products of the phenylpropanoid pathway revealed similar total constitutive levels, but varying relative proportions, of the isoflavonoids in roots of the resistant and susceptible plant. Nematode infection had no effect on isoflavonoid levels. Constitutive levels of medicarpin were highest in roots of the two most resistant plants. Medicarpin inhibited motility of P. penetrans in vitro.