The influence of cotyledons, roots and leaves on the susceptibility of hypocotyls of bean ( Phaseolus vulgaris) to compatible races of Colletotrichum lindemuthianum

Abstract

Hypocotyl segments which were excised from bean seedlings supported extensive biotrophic colonization by compatible races of Colletotrichum lindemuthianum. Subsequent death of infected tissues was followed by production of small amounts of phytoalexins, water-soaked spreading lesions and eventual complete rotting of the hypocotyls. When cotyledons or, to a lesser extent, leaves, were retained the hypocotyls did not rot. Superficial flecking occurred on segments which retained only their cotyledons, whilst large restricted lesions occurred when roots and cotyledons or only leaves were retained. Greatest concentrations of the phytoalexins, phaseollin, phaseollidin and phaseollinisoflavan, occurred in hypocotyls bearing cotyledons. Light also enhanced the production of these compounds. However, in contrast to previous results, phaseollinisoflavan was obtained from both Colletotrichum and tobacco necrosis virus-infected hypocotyls maintained in total darkness. It is suggested that symptom development is controlled by complex interactions between the extent of biotrophic colonization, which appears to be enhanced by roots but reduced by cotyledons, by the ability of necrotrophic hyphae to kill tissues quickly and hence reduce the production of phytoalexins, and by the ability of the healthy tissues which surround lesions to produce large quantities of phytoalexins, a process enhanced by both cotyledons and leaves.