PATHOGENESIS-RELATED FUNCTIONS OF PLANT BETA-1,3-GLUCANASES INVESTIGATED BY ANTISENSE TRANSFORMATION - A REVIEW

Plant beta-1,3-glucanases (beta Glu) have been implicated in several p hysiological and developmental processes, e.g., cell division, microsp orogenesis, pollen germination, fertilization and seed germination. Th ese enzymes, particularly the antifungal class-I vacuolar isoforms, ar e also believed to be part of the defences of plants against fungal in fection. The function of beta Glu in tobacco and Nicotiana sylvestris has been investigated by antisense transformation. Transformation with GLA, the gene encoding the A isoform of tobacco class-I beta Glu, in reverse orientation regulated by the strong cauliflower mosaic virus 3 5S RNA promoter effectively and specifically blocked the induction of class-I beta Glu. This induction was in response to ethylene treatment and following infection with the pathogenic fungus, Cercospora nicoti anae, tobacco mosaic virus (TMV) and tobacco necrosis virus (TNV). Nev ertheless, the plants compensated for this deficiency by producing a f unctionally equivalent (i.e., 'ersatz') enzyme or enzymes. The fact th at compensation occurred specifically in response to infection suggest s that beta Glu activity has an important role in pathogenesis. Antise nse transformation substantially reduced lesion size and number in vir us-infected local-lesion hosts. These results suggest novel antisense- based strategies for protecting plants against virus infection. They a lso raise the intriguing possibility that viruses use a defence mechan ism of the host, production of antifungal beta Glu, to promote their o wn replication and spread.