Interaction of cowpea with Phytophthora vignae: inheritance of resistance and production of phenylalanine ammonia-lyase as a resistance response

The interactions between Phytophthora vignae and two cowpea cultivars are described. The cultivars are near-isogenic but differ in their resistance to P. vignae race 2. Cultivar Caloona is resistant, cv. Poona is susceptible. Resistance of cv. Caloona to race 2 is controlled by a single dominant gene and is expressed throughout the life of the plant in all tissues at temperatures up to 35°C. Inoculation of cut cowpea epicotyls with a plug of agar containing race 2 hyphae provides a reliable method of obtaining host tissue which responds in a predictable way to the pathogen. Phenylalanine ammonia-lyase (PAL) activity increases rapidly in invaded epicotyl tissue of cv. Caloona in response to inoculation with race 2. Increased PAL activity in advance of the hyphae is not detected. Low levels of PAL are produced by cv. Poona and cv. Caloona in response to inoculation with a race virulent on both cultivars (race 3). Application of l -2-aminooxy-3-phenylpropionic acid (1 m m ) or aminooxy acetic acid (10 m m ) through the cut bases of hypocotyls of cv. Caloona seedlings results in this cultivar, normally resistant to P. vignae race 2, becoming susceptible.