ABSTRACT
Little is known about the role of phytohormones in the formation of arbuscular mycorrhizas (AM). Although the involvement of ethylene in AM formation is unclear, it is considered very important for several aspects of plant growth and development. The effect of a suspected inhibitory level of ethylene was investigated to help elucidate its role in regulating the formation of AM. In particular, the morphology of AM fungal structures at various stages of the colonization process was documented. Exogenous application of 5.5 ppm ethylene to the substrate resulted in typical morphological changes to Pisum sativum and a significant reduction in the colonization of roots by the AM fungus Glomus aggregatum. Elevated substrate-ethylene did not affect the number of appressoria formed; however, it did result in the formation of abnormal appressoria, which appeared swollen and highly branched. Deformation of appressoria was correlated with a reduction of AM fungal entry into the root tissue, resulting in less colonization by intraradical hyphae and arbuscules. Colonization generally proceeded normally provided the fungal hyphae breached the epidermis, although the extension of colonization units was restricted.
