Insect-induced synthesis of phytoecdysteroids in spinach, Spinacia oleracea

Spinach (Spinacia oleracea) foliage is known to synthesize and accumulate i nsect molting hormones, predominantly in the form of 20-hydroxyecdysone (20 E). We previously demonstrated that root 20E accumulation is increased foll owing root damage. We designed two further experiments to address root resp onses to both mechanical and insect damage. In plants grown hydroponically, removal of 35% or less of the root mass did not result in changes in root 20E levels. However, removal of 70% of the root mass stimulated 6.0- and 1. 5-fold increases in the root and shoot 20E concentrations, respectively. Th e effects of insect damage on soil-grown plants were investigated by infest ing plant roots with black vine weevil (BVW: Otiorhynchus sulcatus) larvae and allowing them to feed for seven days. Decreases in root mass occurred i n young plants; however, no changes were detected in mature plants. In all cases, root herbivory resulted in at least a 3.0-fold increase in root 20E concentrations. Our previous experiments implicated jasmonic acid and the a nalog methyl jasmonate (MJ) in signaling the damage-induced accumulation of root 20E levels. We investigated the activity of other phytohormones and g rowth regulators (GRs) on the 20E accumulation patterns of young plants as a means of examining the significance of jasmonates in the induction respon se. Hydroponic additions of MJ (0.5 mu M) and the synthetic auxin, l-naphth aleneacetic acid (NAA; 0.5 mu M), resulted in significant increases in root 20E levels. At the concentrations tested, indole-3-acetic acid (IAA), gibb erellic acid (GA(3)), abscisic acid (ABA), and trans-zeatin (Z) had no effe cts on root 20E concentrations. However, both NAA (0.5-5.0 mu M) and Z (5.0 mu M) treatments caused increases in the root/sheet dry mass ratios, indic ating shifts in resource allocation to the roots. Treatments involving ABA (5.0 mu M) and Z (0.5-5.0 mu M) caused significant increases in shoot 20E c oncentrations. No other hormone treatments altered shoot accumulation patte rns. The mechanisms underlying the root 20E induction phenomena were invest igated through the incorporation of [2-C-14]mevalonic acid ([C-14]MVA). Wit hin one day, excised roots readily incorporated radioactivity into 20E from [C-14]MVA. In intact plants, [C-14]MVA absorbed by the roots was rapidly i ncorporated into root 20E pools following damage and MJ treatments. This im plies that the wound-induced root 20E accumulation is the result of increas ed de novo 20E synthesis in the root.