Pectate lyase in transgenic potatoes confers pre-activation of defence againstErwinia carotovora

Abstract Erwinia carotovora bacteria cause blackleg and soft-rot of potatoes by enzymic maceration of host tissue. Pectate lyases (PLs), the main E. carotovora enzymes, depolymerize pectic substances of plant cell walls. The degradation products, unsaturated oligogalacturonates, have been proposed to induce plant defence reactions. The expression of the isoenzyme PL3 in planta and its effect on the soft-rot resistance of transgenic potatoes was investigated. The DNA sequence encoding the mature enzyme was fused to the promoter of the potato patatin B33 gene and to the 35S promoter of the cauliflower mosaic virus. The expression cassettes were inserted into the binary vector pBin19. The plasmids pB33-PL3 and p35S-PL3 were used for Agrobacterium -mediated transformation of potato, cv. Desiree. Enzyme production in plant lines transformed with plasmid pB33-PL3 was confined to tuber tissue, yielding a maximum of 340 mU ml −1 extract. Plants harbouring the 35S-PL3 chimeric gene exhibited constitutive expression of PL in various tissues, with activities up to 280 mU ml −1 in leaf extracts and 50 mU ml −1 in tuber tissue. After wounding the tuber tissue, the liberated PL3 caused a decrease of cell viability. Compared with non-transformed plants, the transgenic lines which synthesized PL3 were more resistant to tissue maceration by E. carotovora or its enzymes. Wounding induced the transcription of the plant defence related gene encoding phenylalanine ammonia-lyase to a high level in tubers of a PL3-expressing transgenic line. The possible implications of PLs and their enzymatic products for protection against invading microbes are discussed.