The response of aminopeptidases of Phaseolus vulgaris to drought depends on the developmental stage of the leaves

Abstract

Aminopeptidases, together with other proteases, execute and regulate the total and specifically limited protein breakdown involved in plant physiology, raising the possibility of their involvement in response to drought. We have identified, in leaves of Phaseolus vulgaris L., five aminopeptidases (E.C.3.4.11) whose levels of activity changed when three week old plants were subjected to drought. First, second and third trifoliate leaves were investigated separately. The aminopeptidases were first identified then isolated using ion exchange chromatography of leaf extracts. Three, named PvAP1, PvAP2 and PvAP4, are metallo aminopeptidases with broad substrate specificity, active against substrates conjugated to alanine and lysine. Two others, PvAP3 and PvAP5, are apparently serine aminopeptidases, the former active against substrates conjugated to phenylalanine and leucine, and the latter characterised by narrow specificity against substrates conjugated to phenylalanine. Their apparent molecular weights range from ∼37 kDa to ∼80 kDa. Levels of activity of individual aminopeptidases in both watered and drought stressed plants are shown to depend on the age of leaves. In watered plants they were generally highest in young, and very low in older, trifoliate leaves, the latter with the exception of PvAP5. Drought initiated an almost general increase of their activities, although to different extents, with the exception of PvAP4 and PvAP5 in young trifoliate leaves. Thus, in such studies it is necessary to investigate the effects of drought separately in leaves of different ages in order to elucidate the different complex and probably specific roles of aminopeptidases in the response of common bean to drought.

Keywords: Aminopeptidases; Common bean; Drought stress; Leaf age; Phaseolus vulgaris; Proteolytic activities.