INDUCED SUSCEPTIBILITY AND ACTIVITY OF PHENYLALANINE AMMONIA-LYASE IN MAIZE MESOCOTYLS WOUNDED OR INOCULATED WITH FUNGAL PATHOGENS
Abstract
The problem of whether resistance expression blocks susceptibility or whether susceptibility blocks resistance in maize was addressed. Anthocyanin accumulation in mesocotyls of the maize hybrid B73(,Ht) x Va26(,Ht) was used as a quantitative measure of the plant response to infection. Mesocotyls were inoculated with Helminthosporium carbonum race 1 (resistant combination) followed by inoculation with Helminthosporium maydis race 0 (susceptible combination) at 0, 15, 30, 45, and 60 h. H. maydis blocked the accumulation of anthocyanins at each inoculation interval except at 60 h after inoculation with H. carbonum. Development of typical susceptible-type lesions by H. maydis occurred for the 0, 15, 30, and 45 h challenge inoculations but not for the 60 h challenge. Microscopic observations demonstrated after H. carbonum established substantially more hyphal growth on resistant-maize mesocotyls when the mesocotyls were subsequently inoculated with H. maydis. These observations suggest that H. maydis actively alters the host plant to make it susceptible to H. carbonum. The results are discussed with reference to induced susceptibility. Activity of phenylalanine ammonia-lyase (PAL), a key enzyme in phenylpropanoid metabolism, was determined in inoculated mesocotyls. The maize hybrid B73(,Ht) x Va26(,Ht) and the inbreds B73(,Ht) (resistant to H. carbonum and susceptible to H. maydis) and B73(,Ht,rhm) (resistant to both fungi) were used. No significant differences in PAL activity were observed for mesocotyls from any maize line inoculated with either H. carbonum or H. maydis and maintained in a photoperiod. The role of PAL in these host-parasite interactions is discussed. PAL activity and anthocyanin accumulation was also determined in wounded mesocotyls. Mesocotyls were wounded by rubbing with Al(,2)O(,3) and were maintained in a photoperiod or in the dark. An increase in extractable PAL occurred in response to wounding in the photoperiod but not in the dark. Anthocyanin accumulation in mesocotyls maintained in the photoperiod decreased in response to wounding. The data suggest that PAL activity following infection is different from that induced by wounding.
Degree
Ph.D.
Subject Area
Plant pathology
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