The potential role of apoptosis in the host-parasite interaction of Leptosphaeria maculans with Brassica napus

The fungus Leptosphaeria maculans (Phoma or blackleg disease) causes lesions on leaf, stem and crown tissue of oilseed rape (Brassica napus). It is similar to many other necrotrophic fungi that rely on plant cell death for growth and reproduction. Recent published evidence supports the theory that some, if not all, necrotrophic bacteria and fungi stimulate apoptosis as a prerequisite for infection of host tissue. Apoptosis in animals is characterized by the activation of an enzyme pathway (the caspase cascade) that leads to ordered proteolytic inactivation of specific cellular repair and structure proteins. This in turn leads to nuclear disintegration and cellular collapse. Caspases belong to a conserved family of cysteine proteases with aspartate specificity – one of the most specific protease groups known in animals. Sequence searches in plants have not identified close homologues, although functional assays suggest caspase-like proteases occur in plants. We examined the effect of an animal caspase inhibitor on development of leaf lesions caused by L. maculans in susceptible B. napus ‘Westar’. Leaves were co-infiltrated with a conidial suspension of L. maculans and the tetrapetide caspase inhibitor acetyl-asp-glu-val-aspaldehyde (Ac-DEVD-CHO) in 10 μL containing an average of 100 or 1000 conidia. Lesions increased in severity from 10-14 days after inoculation, and from 100 to 1000 conidia per inoculation site. In every case, the inhibitor reduced lesion severity and area in paired inoculations across a range of inhibitor concentrations from 50 to 500 μM. The inhibitory effect of Ac-DEVD-CHO, which is highly specific to caspase enzymes, occurred at low concentrations that are known to affect caspases in animal systems. The inhibitor had no affect on conidial germination or growth in vitro. By understanding the role of apoptosis in susceptibility, plant breeders will be armed with molecular genetic information to select novel genes for resistance to these diseases.