Sensory physiology of dodder (Cuscuta spp) seedlings prior to host attachment

Abstract

Dodder is an obligate stem parasite. Characteristic of seedlings of holoparasitic higher plants, early detection of potential hosts by young seedlings is essential for survival. An array of sequential environmental cues mediate dodder development and modify its growth from a non-parasitic higher plant to a parasitic growth mode. Exposure of the apical hook of etiolated seedling to red light stimulates hook opening. Only de-etiolated higher plant seedlings have the potential to transform to the parasitic mode of growth. In white light dodder begins nutating 'in search' of a host. Neighboring plants (i.e., potential hosts) are detected by dodder initially by changes in light quality. Within the sphere of influence of a neighboring plant, an environment enriched in B and FR light potentiates a change in the state of the thread-like organ of dodder to that of its parasitic growth mode. When a dodder seedling contacts a potential host, its growth slows. Potentiation for change in growth has now become actuated. The seedling forms tight coils around the host stem. At this stage, dodder seedlings become susceptible to mechanical stimulation (thigmomorphogenesis). Soon prehaustoria develop (an early stage infection structure) on the concave side of coils. Auxin/cytokinin ratios and other chemicals· influence development of these structures. Ethylene has no effect at this stage. De-etiolation of dodder seedlings (hook opening) is controlled by phytochrome. Phytochrome maintains dodder as a non-parasitic higher plant seedling. Parasitic growth of dodder (twining and prehaustoria development) is controlled by two photoreceptors: phytochrome and a specific UV-A/B light photoreceptor (cryptochrome). The involvement of phytochrome was demonstrated by light pulses and R/FR reversibility experiments. Based on chemical and light studies, our results indicate that phytochrome (type I and II) alone cannot mediate parasitic growth of dodder under B light. Blue light acts via a specific UV-A/B photoreceptor, possibly flavin. However, the state of phytochrome is the principal limiting factor that affects B light action. The FR-absorbing form of type II phytochrome (Pfr II) opposes the action of cryptochrome; Pfr II opposes the action of an excited state of cryptochrome on coiling and prehaustoria formation. The data indicate that cryptochrome action is dependent on the state of phytochrome (ϕ). A model of interaction between phytochrome and cryptochrome have been proposed. Both photoreceptors are competing for the same intermediate, X; or same intermediate state, X' in mediating dodder parasitic growth.