Comparison of Leptosphaeria biglobosa-induced and chemically induced systemic resistance to L-maculans in Brassica napus

Abstract

Brassica napus (cv. Madrigal) seedlings pre-treated with ascospores of Leptosphaeria biglobosa or foliar sprays of either acibenzolar-S-methyl (ASM) or menadione sodium bisulphite (MSB) were challenge inoculated with L. maculans ascospores and assessed for phoma leaf spot development and tissue morphology and gene expression responses to infection. Rates of increase in phoma leaf spot area 8-21 d after challenge inoculation were significantly greater on water pre-treated plants than on plants pre-treated with L. biglobosa, ASM or MSB on both pre-treated leaves (local effect) and younger leaves without pre-treatment (systemic effect). Ninety-six h after challenge inoculation, the invasive hyphae of L. maculans were encircled by rings of necrotic mesophyll cells on leaves pre-treated with L. biglobosa, ASM or MSB but not those pre-treated with water. Quantification of transcript levels of genes commonly used as markers of the major defence signalling pathways (PDF1.2, PR-1, NPR1, APX, CHB4) 0-96 h after L. maculans challenge inoculation showed expression patterns indicating preferential activation of the jasmonate/ethylene pathway and involved induction of NPR1 locally and systemically in leaves of plants pre-treated with L. biglobosa ascospores.