Haustorial formation

The apparent necessity of a second signal for haustorial initiation provides another possibility for chemical control. Riopel and coworkers (2,12-14) observed a few years ago that gum tragacanth, a foliar extract, was a very potent haustorial initiator. Two active compounds were isolated (Xenognosin A shown below, and a flavonoid with similar structure). The structural features required for haustorial formation include a meta relationship of hydroxyl and methoxyl groups and an alkyl branching ortho to the methoxy substituent. 

A number of analogs were prepared to arrive at the above relationship. However, the identity of the chemicals needed for haustorial initiation and produced by any cereal crop remains unknown. The possibility of using a chemical to suppress or confuse haustorial formation has been recognized, but only a little work has been done on this aspect of control. The germinated seed will die in a short time if haustorial initiation and attachment are not realized. 

Spore germination and appressoria formation during the pathogenesis of rust fungi is affected to a very low extent. The interference with haustorial formation is the most important effect. Colonized cells undergo a hypersensitivity reaction finally they become necrotic. Further development of the obligate parasite is therefore stopped. 

Scrophulariaceae) approach those of the host, Lespedeza sericea, soyasapogenol B (74) induces haustorial formation (Fig. 23.27) (Lynn, 1985). 

In combination, BA + GA3 additively stimulated PAL increase and BA augmented G A3 s effect in bringing about normal level of lignification in the xylem cells (Table 5). BA has been shown to stimulate the formation of xylem elements in the base of preformed haustorial mounds in Cuscuta campestris [29]. Taken together. 

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