Mycorrhizal stimulation of plant parasitism

Successful shoot parasitism by dodder Cuscuta pentagona on plants in a field experiment occurred almost exclusively when the plant roots were colonized by mycorrhizal fungi. Under controlled conditions, life expectancy of dodder was significantly greater on mycorrhizal Abutilon theophrasti plants than on nonmycorrhizal plants. Furthermore, colonization of roots by mycorrhizal fungi increased the growth rate of dodder to 3.4 times the rate on nonmycorrhizal plants. The mycorrhizal effect on dodder growth occurred before the haustoria of dodder had succeeded in penetrating the host. Thus, colonization by mycorrhizal fungi had systemic effects on their hosts, which altered either the nature of prepenetration dodder signals or the level of nutrients contained in host stem exudates. -from Authors