DAMAGE-INDUCED ACCUMULATION OF PHYTOECDYSTEROIDS IN SPINACH - A RAPIDROOT RESPONSE INVOLVING THE OCTADECANOIC ACID PATHWAY

Some plant defenses are known to be rapidly induced following attack b y phytophagous insects. Plant-produced insect molting hormones, termed phytoecdysteroids, are believed to aid plant resistance; however, the ir dynamics are poorly understood. Using spinach (Spinacia oleracea) a s a model system, we examined the inducibility of phytoecdysteroids, p rimarily 20-hydroxyecdysone (20E), in an effort to characterize potent ial interactions with herbivorous insects. Rapid phytochemical inducti on was investigated using damage treatments and applications of defens e-related plant-signal analogs, specifically methyl jasmonate (MJ) and methyl salicylate (MSA). Within two days, mechanically damaged roots exhibited two to three fold increases in phytoecdysteroid concentratio ns. Four days after root damage, small increases in shoot levels were also detectable. Unlike roots, foliar 20E concentrations were unaltere d over a range of shoot treatments including insect herbivory (Spodopt era exigua), mechanical damage, and MJ applications. Additions of MJ ( 12.5-50 mu g/liter) to the root systems of hydroponically grown plants stimulated accumulations of root phytoecdysteroids in a dose-dependen t manner, similar in magnitude to the response induced by root damage. Under identical conditions, MSA did not affect the accumulation of 20 E when added to the hydroponic solutions of undamaged plants. Moreover , MSA inhibited the induction of 20E in wounded roots, but did not int erfere with the action of applied MJ. In contrast to mechanical damage , roots did not induce 20E levels when challenged with two different f ungal pathogens (Pythium aphanidermatum and Phytophthora capsici). We propose that wound-induced accumulations of 20E are generated in the r oots, signaled via endogenous jasmonates, and may confer enhanced resi stance against subterranean herbivorous insects.