Possible Role of Cytokinins in Nonspecific Recognition of a Host and in Early Growth of Haustoria in the Parasitic Plant, Cuscuta campestris

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Abstract
The direct cause of haustorial initiation in C. campestris is the irreversible curving of its shoot. The haustorial primordium, formed beneath the cortical and epidermal outgrowth called a prehaustorium, requires a supply of cytokinin in order to grow. If the hormone does not become available, the haustorial apex differentiates but does not emerge from the prehaustorium. The tissue of such nonfunctional haustoria remains capable of responding to exogenous cytokinins with tracheid formation. When cytokinin is applied early in haustorial development, the haustorium grows, penetrates the prehaustorium and emerges from it. Apart from controlling early haustorial growth, cytokinins cause the formation of papillate outgrowths around the periphery of the Cuscuta shoots, a morphological feature which characterizes the parasitizing state in vivo. Effects of other hormones on early haustorial growth are marginal.