PECTATE LYASE IN TRANSGENIC POTATOES CONFERS PREACTIVATION OF DEFENSEAGAINST ERWINIA-CAROTOVORA

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 iso enzyme PL3 in planta and its effect on the soft-rot resistance of tran sgenic potatoes was investigated. The DNA sequence encoding the mature enzyme was fused to the promoter of the potato patatin B33 gene and t o the 35S promoter of the cauliflower mosaic virus. The expression cas settes were inserted into the binary vector pBin19. The plasmids pB33- PL3 and p35S-PL3 were used for Agrobacterium-mediated transformation o f potato, cv. Desiree. Enzyme production in plant lines transformed wi th plasmid pB33-PL3 was confined to tuber tissue, yielding a maximum o i 340 mU ml(-1) extract. Plants harbouring the 35S-PL3 chimeric gene e xhibited constitutive expression of PL in various tissues, with activi ties up to 280 mU ml(-1) in leaf extracts and 50 mU ml(-1) in tuber ti ssue. After wounding the tuber tissue, the liberated PL3 caused a decr ease of cell viability. Compared with nontransformed plants, the trans genic lines which synthesized PL3 were more resistant to tissue macera tion by E. carotovora or its enzymes. Wounding induced the transcripti on of the plant defence related gene encoding phenylalanine ammonia-ly ase to a high level in tubers of a PL3-expressing transgenic line. The possible implications of PLs and their enzymatic products for protect ion against invading microbes are discussed. (C) 1996 Academic Press L imited.