Stress-related compounds in xylem fluid of blight-diseased citrus containingFusarium solaninaphthazarin toxins and their effects on the host

Abstract

Naphthazarin toxins of Fusarium solani were detected and quantified by competitive enzyme-linked immunosorbent assay (ELISA) in xylem fluid of scaffold roots from blight-diseased trees. These toxins alter plant metabolic activity; this study examined their effects on xylem health by measuring physiological components in xylem fluid. Protein concentration in fluid was positively correlated with increases in toxin concentration. In fluid containing about 100 μg∙L−1toxin, total amino acids reached levels 2.5 to 3.0 times greater than those in fluid containing no detectable toxin; asparagine, glutamic acid, proline, glycine, and arginine were the most abundant. Levels of phenylalanine ammonia-lyase, polyphenol oxidase, chlorogenic acid oxidase, and superoxide dismutase activity did not increase in xylem fluid containing toxin, which may be a reason why vascular discoloration did not occur. Xylem fluid containing about 20 μg∙L−1toxin was associated with a 9-fold increase in total phenolics and a 15-fold increase in peroxidase. Peroxidases were predominantly anionic and may function in defense. Some of these peroxidases may function as lignases, releasing phenolic and other constituents from cells and cell walls. These toxins are known to enhance membrane permeability, which may be the main reason for the accumulation of these stress metabolites in xylem fluid. These data explain the disruption of hydraulic conductivity in blight tree roots and the eventual physiological breakdown of roots on diseased trees.Key words: phytotoxins, isomarticin, ELISA, fungi, roots.