Early events in the pathogenicity of Mycoleptodiscus terrestris on the submersed aquatic plant Hydrilla verticillata
Abstract
Increased use of biological agents such as fungi for the control of aquatic weeds has precipitated the need for a basic understanding of the mechanisms employed by aquatic plants to defend against attempted pathogen ingress. Phenolic compound biosynthesis is a well known plant defense system that can be induced by tissue scarring, by microbes attempting to gain entry into the plant, and by isolated microbial elicitor molecules. The present study explores the role of phenolic compounds in affecting the early stages of fungal infection. High Performance Liquid Chromatography (HPLC) analysis of phenylpropanoids revealed 23 compounds in 5-day old unchallenged hydrilla tissue. The most abundant compound, a dicaffoyl ester of caffeic acid (RT 22.4 min.), amounted to a concentration of 0.08 $\mu$g/mg fr wt. Algal-free dioecious hydrilla was inoculated with macerated mycelia of the putative pathogen Mycoleptodiscus terrestris (M.t.) and allowed to incubate for 72 hours. Samples of fungal challenged and unchallenged leaf tissue were taken at seven time intervals for HPLC analysis of hydrilla leachates for the three major phenolic entities: RT 22.4 min., RT 23.5 min. and RT 24.1 min. In unchallenged tissue, the amount of RT 22.4 min. fluctuated on a 36-hour cycle and RT 23.1 min. fluctuated with photoperiod. The amount of RT 22.4 min. significantly increased in challenged hydrilla tissue at 60 hours postinoculation; RT 23.5 min. increased at 36 and 60 hours postinoculation. RT 24.1 min. showed no significant difference between challenged and unchallenged tissue. RT 22.4 min. was partially purified and evaluated for toxicity. The compound failed to significantly inhibit M.t. mycelial radial growth. Microscopic examination of inoculated tissues indicated intracellular penetration at 72 hours and complete disintegration of tissues at 144 hours. The presence and increased synthesis of phenylpropanoids does not appear to significantly inhibit M.t. infection of hydrilla. Procedures designed specifically for the isolation of genomic DNA, with a minimum of small contaminating fragments, and total RNA from hydrilla are described.
Degree
Ph.D.
Advisors
Lembi, Purdue University.
Subject Area
Plant pathology|Botany|Ecology|Freshwater ecology
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