INDUCED ACCESSIBILITY AND ENHANCED INACCESSIBILITY AT THE CELLULAR-LEVEL IN BARLEY COLEOPTILES - XV - INTERFERENCE OF AOA AND AOPP WITH THEESTABLISHMENT OF ACCESSIBILITY

Several researchers have reported that the level of phenylalanine ammo nia-lyase (PXL) is elevated in barley and wheat leaves within 3-4 h af ter inoculation with either a compatible or an incompatible race of Er ysiphe graminis, and that the susceptibility of oat leaves to the powd ery mildew fungus increases when leaves are treated with the PAL inhib itors, alpha-aminooxyacetic acid (AOA) or alpha-aminooxy-beta-phenylpr opionic acid (AOPP). In this paper we consider induction of accessibil ity in barley cells by inoculation with E. graminis f. sp. hordei, a f ungal pathogen of barley, in relation to PAL activity in coleoptile ce lls. We studied events at the single spore - single host cell level, a nd examined influences exerted by the fungus al the prepenetration sta ge of development using AOA and AOPP to inhibit PAL activity in coleop tile cells. The degree of accessibility induced in cells successfully penetrated by E. graminis (first inoculum) was evaluated in terms of t he penetration efficiency into those cells that was achieved by a subs equent challenge inoculum of either E. graminis or a non-pathogen of b arley E. pisi. Treatment of lower surfaces of coleoptiles with an appr opriate concentration of either PAL inhibitor did not affect the penet ration capability of either fungus inoculated onto the upper surfaces of the same coleoptiles. Sequential H2O-AOA or H2O-AOPP treatment of i noculated coleoptiles revealed that the accessibility induced by the f irst inoculum was suppressed when AOA or AOPP treatment was initialed before 6 h after inoculation. The first inoculum attempted penetration from appressoria at 9-10 h after inoculation. The present results sug gest that components released from E. graminis germlings could affect the physiological condition of coleoptile cells, possibly through phen ylpropanoid metabolism, which ultimately affects accessibility. (C) 19 97 Academic Press Limited..