Phenylalanine ammonia-lyase and susceptibility of the maize mesocotyl to Helminthosporium maydis

Extractable levels of phenylalanine ammonia-lyase (E.C. 4.3.1.5) (PAL) increased in a susceptible combination of the maize mesocotyl with Helminthosporium maydis, but not in resistant combinations with either H. maydis or H. carbonum. The increase in extractable PAL was detected only in tissue incubated in the dark and was prevented or significantly reduced by cycloheximide and cinnamic acid. The in vivo effect of H. maydis on the mesocotyl was compared with that of the PAL inhibitor a-aminooxyacetate (AOA). AOA caused an increase in the level of extractable PAL and prevented the expression of resistance of the mesocotyl to H. carbonum. H. maydis also altered the expression of resistance to H. carbonum, in that it allowed a significant increase in post-penetration growth of H. carbonum. We suggest that susceptibility of maize to H. maydis is due to the fungus interfering with the plant's resistance response.