POLYMORPHISM OF TOX(O) LEPTOSPHAERIA-MACULANS ISOLATES AS REVEALED BY SOLUBLE-PROTEIN AND ISOZYME ELECTROPHORESIS

Single-ascospore isolates of Leptosphaeria maculans were characterized according to their sirodesmin PL production in vitro and separated as Tox(+) and Tox(0) isolates. They were further assessed for (i) their pathogenicity on various crucifers using a cotyledon-inoculation test; (ii) their soluble protein patterns obtained by isoelectric focusing; and (iii) their profiles following polyacrylamide electrophoresis and visualization of eight isozyme systems. As compared to Tox(+) isolates, Tox(0) isolates did not show any host specificity since they were able to infect cotyledons of all the analysed crucifer species. In most isozyme studies, a few or no common bands were observed between the profiles of Tox(0) and Tox(+) isolates. Tox(+) isolates usually displayed low, or no polymorphism whereas Tox(0) isolates displayed a high polymorphism as all the analyses separated them into three groups corresponding to the NA1, NA2 and NA3 subgroups previously described using RFLP techniques (Koch el al., 1991). AU the European Tox(0) isolates analysed shared similarities with an isolate of the NAI subgroup. Three isozymes, i.e., acid phosphatase (ACP), malate dehydrogenase (MDH) and phosphoglucose isomerase (PGI) did not reveal any polymorphism within the NA1 subgroup. In contrast, glutamate oxaloacetate transaminase (GOT), esterase (EST), glucose-6-phosphate dehydrogenase (G6PDH) and shikimate dehydrogenase (SKD) separated the NA1 isolates into two to three subgroups. Finally, a high degree of polymorphism between isolates of the NA1 subgroup was shown following alkaline phosphatase (ALP) analysis. The possible prevalence of isolates from the NA1 subgroup within the European Tox(0) population is discussed.