Tissue culture of cassava (Manihot esculenta Crantz): development and use in elucidating pathogenic mechanisms of Erwinia species

Tissue culture systems were developed for cultivars of cassava grown in both southern Africa and Australia. Using meristem derived in vivo plantlets and leaves from plants grown under environmentally controlled conditions, a method was established to isolate consistently high yields of viable protoplasts and a new medium promoting protoplast division was formulated. Callus was initiated from stem tissue and both green, compact callus and pale, friable callus were obtained. The former was used in regeneration studies, whilst the latter was used to establish fast growing cell suspensions. Plant cell suspension cultures, together with protoplast division and plantlet assays, were used to effectively differentiate between a non pathogenic strain of Erwinia herbicola and two strains which cause a blight disease of cassava. The pathogenic strains appeared to produce an extracellular cytotoxic metabolite in vitro. Purification of extracellular material revealed the presence of pili and polysaccharide on the outer surface of the three bacterial strains. The role of pili in this bacterial-plant interaction is uncertain but they may play a role in recognition. The toxic effect displayed by the polysaccharide, produced by the two pathogenic strains, may be due to physical blocking of plant conductive tissue. The lower production and apparent conformational differences of the polysaccharide from the non pathogenic strain, m a y account for the lack of effect of this material in plant assays. A range of plant cell and tissue culture techniques applicable to cassava have thus been established and are available for the study of diseases of cassava.