The effect of ethylene on phenylalanine ammonia lyase (PAL) induction by a fungal elicitor in Phaseolus vulgaris

Abstract

The effects of ethylene on the induction of phenylalanine ammonia-lyase (PAL) by cell wall elicitors of Colletotrichum lindemuthianum was investigated in leaves of susceptible and resistant cultivars of Phaseolus vulgaris (cultivars Pinto and Kievit respectively). Induction of PAL by exogenously supplied ethylene and associated test substances alone, without elicitor, was negligible. With elicitor alone, cv. Kievit responded to lower concentrations than cv Pinto. Exogenously supplied ethylene augmented the response of cv. Kievit only in the presence of limiting concentrations of elicitor. When concentrations of elicitor giving maximum PAL induction were used, ethylene was without effect in cv. Kievit also. Aminoethoxyvinylglycine (AVG) partially inhibited the induction of PAL in cv. Kievit and this inhibition could be reversed by aminocyclopropane-1-carboxylic acid (ACC) and ethylene. However, since cv. Pinto did not respond to ethylene, the application of AVG in the presence of elicitor was, as expected, without effect. Unless introduced within 2 h of elicitor application, ethylene was without effect on PAL production in cv. Kievit also; likewise, the inhibitory effect of AVG was minimal if not injected within the same time period following elicitor. Elicitor rapidly induced the synthesis of ACC during this 2 h period leading to the production of stress ethylene and this was completely inhibited by AVG and completely reversed by ACC. The effect of the elicitor, after injection into the leaf of cv. Kievit, was to change both the distribution and relative concentrations of the soluble proteins. Ethylene enhanced this effect and the 40 % increase in PAL activity observed in the presence of the hormone can largely be accounted for by the increase in the concentration of the enzyme. Changes brought about by the elicitor were eliminated by inhibitors of protein and nucleic acid synthesis, the effects of which could not be reversed by ethylene. These results are discussed in relation to other studies. It has been proposed that elicitors induce the biosynthesis of ethylene and PAL independently. Ethylene is involved in the activation of the induced response leading to the synthesis of PAL, and thereby phytoalexins, particularly when the response to elicitor is sub-optimal.