Primary Haustorial Development of Striga asiatica

Hood, M. E., Condon, J. M., Timko, M. P., and Riopel, J. L. 1998. Primary haustorial development of Striga asiatica on host and nonhost species. Phytopathology 88:70-75. the host (sorghum) roots and advance into the cortex occurred within 24 to 48 h of inoculation. Penetration of the endodermis by the developing endophyte was delayed for 72 to 96 h after initial contact. However, upon penetration vascular continuity was established between parasite and host. In contrast, interactions with nonhosts provided evidence of active resistance mechanisms. Penetration of lettuce, marigold, and cowpea roots by S. asiatica was most frequently arrested in the cortex, and endophytic cells were necrotic 72 h after inoculation. Some species-specific differences were observed in the reactions of nonhosts to penetration, although in their general pature the interactions with S. asiatica were similar. Initial interactions of Striga asiatica with a susceptible host and nonhost plants were examined by histological methods. Haustorial development was initiated when radicles of S. asiatica were placed in contact with host or nonhost roots. Reorganization of the S. asiatica root apical meristem was rapid and involved the formation of a distal group of cells that penetrated the host or nonhost root. Penetration of the epidermis of Striga asiatica (L.) Kuntze (Scrophulariaceae), witchweed, is an obligate root parasite of most of the world's agronomically important cereal grasses. Striga parasitism causes severe chlorosis, wilting, and stunting of susceptible hosts, resulting in yield losses that range from slight to 100% (5,7). Primarily found in Africa and Asia, S. asiatica was first reported to be present in the southeastern United States in 1956 (8). Despite efforts to eradicate this pest through the use of improved cultural practices, chemical controls, and resistant or tolerant cultivars, S. asiatica persists in the United States and continues to be a major limitation on agricultural production in regions of heavy infestation (7). Since the subsistence farmers who populate the most threatened regions are unable to afford expensive chemical treatments for control of the pathogen and often find it difficult to adopt new cultural practices, the development of high-yielding host cultivars with durable resistance is of utmost importance for reducing the agricultural and social impact of S. asiatica (7). Despite decades of effort by breeders, cultivars with complete resistance have yet to be obtained for the major grain crops, including maize, sorghum, millet, and rice, although some promising cultivars have been identified (7). The possible sources and mechanisms of host resistance to parasitism by Striga spp. were recently reviewed by Ejeta and Butler (7). Among the various resistance mechanisms operating in hosts of S. asiatica, low production of germination stimulants affecting S. asiatica and direct inhibition of infection processes by the development of chemical or physical barriers appear to be most important. While penetration of host cultivars by S. asiatica has been described (6,16), the nature of interactions between'Striga and nonhosts and the possible basis for nonhost resistance are not well understood. Heath (11) suggested that nonhost resistance is important by virtue of its prevalence and durability. Characterization of defenses against the parasite and identification of developmental stages at which the parasite is vulnerable to such defenses are of particular relevance. Information of this kind may be gained from histological comparison of incompatible interactions between Striga and nonhosts and successful penetration of hosts by the parasite. Like other parasitic angiosperms, S. asiatica infects host plants by first forming a haustorium (27). This infection structure attaches to host roots and penetrates and establishes vascular connections with them. We use the term haustorium to refer to this infection structure at all developmental stages, from initiation through the establishment of vascular connections. Haustoria may form at the radicle or root apex and at lateral positions on the mature root upon induction by exogenous signals (13). In any case, haustorial development begins with the enlargement of cells in the protoderm or epidermis and underlying ground tissue and the initiation of haustorial hairs (23). Cells in the apex of the developing haustorium become specialized for penetration (2,3,20,21), and growth through the host epidermis and cortex is rapid. Haustorial maturation is completed when the host stele is penetrated and vascular connections are constructed between the haustorium and the host (24). The Striga seedling becomes largely autotrophic upon emergence from the soil but continues to procure water and nutrients from the host root system. In this study we used sorghum as the susceptible host, since it has been suggested that sorghum is likely to be the species upon which the cereal-parasitizing Stfiga spp. evolved (25). Species normally resistant to parasitism by S. asiatica are considered to be nonhosts. In general, they include most nongrarnineous plants. Some broadleaf dicots (i.e., leguminous species, tobacco, sweet potato), however, are susceptible to parasitism by other Striga species, notably S. gesnerioides (Willd.) Vatke (7,25). Plants expressing nonhost resistance might lack the chemical signals or nutritional components necessary for initiating or sustaining the development of S. asiatica, or they may possess constitutive or induced general resistance that prevents parasitism. Reports describing the interacuon of Striga with nonhosts are limited, and they differ in their descriptions of the extent to which penetration progressed. While penetration of soybean halted in the nonhost cortex and only rarely reached the endodermis (24), CorresDonding author: J. L. RioDel: E-mail: [email protected] Publication no. P-1997-1120-02R @ 1998 The American Phytopathological Society