EARLIER PAPILLA FORMATION AND RESISTANCE TO BARLEY POWDERY MILDEW INDUCED BY A PAPILLA-REGULATING EXTRACT

Timing of papilla formation has been suggested to be an important fact or for papilla-mediated resistance. Formation of oversize papillae and resistance to penetration attempts by the powdery mildew fungus have been reported to be induced in inoculated epidermal cells of susceptib le barley coleoptiles by a partially purified, papilla-regulating extr act (PRE) from healthy barley leaves. A time-course study was conducte d to determine whether or not early papilla formation may be an import ant component of this PRE-induced resistance. Inoculated, susceptible barley coleoptiles were incubated on the PRE in a 10 mM Ca(NO3)(2) sol ution. The timing of initiation and termination of cytoplasmic aggrega tes, initiation of papillae, initiation of fungal penetration pegs and initiation of haustoria were determined in vivo using differential in terference contrast optics. On average, papilla formation was initiate d 23-27 min earlier in the PRE treatment than in controls incubated wi thout the PRE. An individual site-based analysis showed that PRE treat ment of coleoptiles increased, from 23% to 68%, the frequency of earli er-formed papillae that fungal penetration pegs encountered when they were first detected. Thus, early initiation of papilla formation is a component of the PRE-mediated resistance to barley powdery mildew. How ever, since papillae in the PRE-treated coleoptiles were highly effect ive in preventing penetration regardless of their time of initiation, it appears that papilla content may be more important than earliness i n determining their efficacy.