Vegetative/Parasitic Transition: Control and Plasticity in Striga Development

Abstract

Striga asiatica (Scrophulariaceae), an obligate parasite of grasses including many of the world's major grain crops, switches from vegetative to parasitic development by the differentiation of the root meristem into the host attachment organ, the haustorium. This change was induced in culture by the exposure to a single, low molecular weight signal molecule, 2,6-dimethoxy-p-benzoquinone. A concentration of 10-6 molar quinone and an exposure time of >6 hours were required before the developmental process could be completed. With shorter exposure times, haustorial development was prematurely aborted and meristematic elongation was reestablished. The new meristem was capable of developing a second haustorium if reexposed to the signal molecule. These results are discussed in terms of the transition to the parasitic phase and the general control of plant cellular development.The initial gene expression events involved in the induction of both parasitic (7, 24) and symbiotic (14,19,20) relation-ships have been shown to be controlled in many cases by specific, structurally simple, signal molecules. These compounds must convey critical information required for host selection since, in most cases, the shift from a vegetative to a parasitic mode is both sudden and dramatic, requiring a significant recommitment of resources. The magnitude of this change is particularly striking in the parasitic angiosperms that are strictly dependent on the host for continued survival ( 15). In general, these plant holoparasites are dormant as seeds and apparently break dormancy only when exposed to a specific host-derived signal (10, 13). The change from the vegetative to a parasitic mode involved formation ofa primary haustorium, an organ specialized for host plant attachment.Several theories have been proposed for the mechanism of haustorial induction in parasitic plants, each generally involving haustorial inducing signals (2, 3, 8, 9, 12,17,26 suggests an active screening process on the part ofthe parasite and ensures intimate host-parasite association before the induction of haustoria.During primary haustorial induction, cellular expansion at the root meristem is redirected from longitudinal to radial dimensions followed closely by the formation of specialized haustorial hairs just distal to the swollen tip. The process is one of the most rapid differentiation processes known. Agalinis purpurea (Scrophulariaceae) has been shown to attach within 24 h following induction (22), and Striga attachment occurs at least as rapidly (8). Therefore, haustorial development is dependent to a large extent on cellular resources already present in the meristem. Nevertheless, the cost of this meristematic recommitment cannot be minimized. The redirection of meristematic growth results in a cessation of elongation. Therefore, host contact must be established for successful attachment (8, 9), and premature commitment to haustorial formation would certainly restrict the possibility of such contact. The importance of this differentiation event ...

A Single-Electron Reducing Quinone Oxidoreductase Is Necessary to Induce Haustorium Development in the Root Parasitic Plant Triphysaria

“…Often, only a few haustorial hairs were observed without noticeable swelling ( Figures 2F and 2G). These partially formed haustoria appear similar to haustoria obtained with suboptimal HIF concentrations or shortened incubation times (Smith et al, 1990).…”

Haustorial Hairs Are Specialized Root Hairs That Support Parasitism in the Facultative Parasitic Plant Phtheirospermum japonicum

“…Firm attachment to hosts may also enhance the recognition of host signals for haustorium maturation. It was shown in Striga asiatica that a short exposure of the root tips to DMBQ induced immature and smaller haustoria than those formed by longer exposure (Smith et al, 1990), suggesting that the transduction of host signals must be maintained in order to continue haustorium expansion after the initiation of haustorium formation. As haustorial hairs are able to dramatically increase the surface areas of epidermal cells, it is also possible that haustorial hairs are important for an efficient response to host signals to maintain haustorium development.…”

Do inorganic solutes limit growth of the facultative hemiparasite Rhinanthus minor L in the absence of a host?

“…O/«xMy//a«f/?0, parasites may also be governed by the supply ot prior to the development of functional haustoria solutes. Positive linear correlations between nitrogen (Pate Kuo & Davidson, 1990;Smith, Dudley & supply from the host and light saturated rates of Lynn 1990) Some hemiparasitic genera are truly photosynthesis of root hemiparasites have been facult'ative and can complete tbeir life-cycle in the reported (Cechin & Press, 1993;Press et al, 1993), absence of a bost. Such occurrences are compara-and in the mistletoes there are data to suggest that tively rare in nature, but do exist, as seen in a study water use efficiency (WUE) may be controlled by of the Mediterranean hemiparasitic annual Bartsia host nitrogen supply (Schulze & Ehlennger, 1984)t rixago .…”