Development of Acaulospora rehmii spore and hyphal swellings under root-organ culture

Abstract

A strain of Acaulospora rehmii was, for the first time, successfully grown in vitro on Ri T-DNA transformed carrot roots allowing the in situ observation of Acaulospora spore development and of extraradical thin-walled hyphal swellings. The sporogenous hypha developed intercalarly along thin-walled coenocytic hyphae. The distal part of the sporogenous hypha swelled slightly as a sporiferous saccule primordium followed by the differentiation of a lateral spore primordium along the neck of the sporogenous hypha. Both structures matured simultaneously, and the sporiferous saccule began to collapse after spore maturation and complete differentiation of the spore wall. Several of the in situ observations on in vitro differentiated A. rehmii spores are concordant with previous ontogenic studies done on other Acaulospora species obtained from in vivo cultures. New and original observations on the early developmental stages of sporiferous saccules and spores and on the occurrence of small diameter intercalary hyphal swellings provide additional elements in the study of Acaulospora sporulation process and life cycle.

• Acaulosporaceae
• Glomerales
• hyphal swellings
• spore ontogenesis

This work was supported by a grant from EC: INCO-DC ERBIC18*CT970208. S.D. gratefully acknowledges the financial support from the Belgian Federal Office for Scientific, Technical, and Cultural affairs (OSTC, contract BCCM C2/10/007). Thanks are addressed to R. Fogain (CARBAP Cameroon) for providing A. rehmii strain, to S. Seguin for technical assistance, to manuscript reviewers for their fruitful comments, and to the director of MUCL for the facilities provided and for continuous encouragement.