EARLY EVENTS IN RESISTANT AND SUSCEPTIBLE INTERACTIONS BETWEEN BRASSICA NAPUS AND Leptosphaeria maculans

摘要

Despite the agricultural importance of blackleg disease caused by Leptosphaeria maculans, little is known about the early cellullar events leading to resistance or susceptibility. We now show that compatible pathogenicity group PG3 germinates within 3 days postinoculation, with extensive hyphal proliferation by 9 days. In contrast, incompatible isolate PG2 shows little evidence of germination until 5 days postinoculation, with only limited growth apparent by 9 days. To probe the events prior to spore germination, we have cloned two cDNAs encoding basic PR1 proteins from the interaction of Brassica napus cv. Glacier with incompatible isolate PG2. With incompatible isolate PG2, PR1 gene expression is apparent within 8 hours postinoculation, and remains strong for at least 96 hours, while expression in response to compatible isolate PG3 declines after 12 hr. A similar induction pattern was observed for -1, 3-glucanase. Surprisingly, no induction of defensin or PR5 was detected, suggesting that these genes are regulated through a different pathway than PR1 and -1, 3-glucanase. Heat-killed pycnidiospores elicit only weak PR1 expression. When different pathogenicity groups were compared, strongest PR1 expression was found in single gene resistance to PG2 in Glacier and PG3 in Quinta, compared to compatible interactions. Weaker PR1 expression was seen in non-host resistance to non-agressive isolate PG1. Salicylic acid, but not wounding or heat shock, can also induce PR1 expression in B. napus. Negligible expression was seen in healthy cotyledons, young leaves, mature leaves, flowers, and young siliques. These results show B. napus respounds differentially to compatible and incompatible isolates within 12 hours after inoculation, long before fungal germination occurs.