No abstract

Striga is a major biotic constraint to sorghum production in semiarid tropical Africa and Asia. Genetic resistance to this parasitic weed is the most economically feasible control measure. Mutant alleles at the LGS1 (LOW GERMINATION STIMULANT 1) locus drastically reduce Striga germination stimulant activity. We provide evidence that the responsible gene at LGS1 codes for an enzyme annotated as a sulfotransferase and show that functional loss of this gene results in a change of the dominant strigolactone (SL) in root exudates from 5-deoxystrigol, a highly active Striga germination stimulant, to orobanchol, an SL with opposite stereochemistry. Orobanchol, although not previously reported in sorghum, functions in the multiple SL roles required for normal growth and environmental responsiveness but does not stimulate germination of Striga. This work describes the identification of a gene regulating Striga resistance and the underlying protective chemistry resulting from mutation.

Analysis of Sorghum bicolor bloomless (bm) mutants with altered epicuticular wax (EW) structure uncovered a mutation affeding both EW and cuticle deposition. l h e cuticle of mutant bm-22 was about 60% thinner and approximately one-fifth the weight of the wild-type parent P954035 (Wl-P954035) cuticles. Reduced cuticle deposition was associated with increased epidermal conductance to water vapor. The reduction in EW and cuticle deposition increased susceptibility to the funga1 pathogen Exserohilum turcicum. Evidence suggests that this recessive mutation occurs at a single locus with pleiotropic effects. l h e independently occurring gene mutations of bm-2, bm-6, bm-22, and bm-33 are allelic. These chemically induced mutants had essentially identical EW strudure, water loss, and cuticle deposition. Furthermore, 138 Fz plants from a bm-22 x WT-P954035 backcross showed no recombination of these traits. This unique mutation in a near-isogenic background provides a useful biological system to examine plant cuticle biosynthesis, physiology, and function.EW provides the outermost bamer between plants and their environment. Previous studies have implicated EW layers in tolerance to various kinds of biotic and abiotic environmental stress (Eglinton and Hamilton, 1967; Thomas and Barber, 1974;Blum, 1975; Webster, 1977; Bengston et al., 1978;Jordan et al., 1984; El-Otmani et al., 1989;Jefferson et al., 1989;Percy and Baker, 1990; Stoner, 1990;Bergman et al., 1991). Near-isogenic mutants provide a model system for the dissection of biochemical (Koorneef et al., 1989; Somerville and Browse, 1991) and biophysical (Blum, 1975;Jordan et al., 1984; Saneoka and Ogata, 1987) under drought stress and disease pressure in southwestem Mexico suggested that mutants bm-2, bm-6, bm-22, and bm-33, making up an individual allelic group, were more susceptible to drought and more susceptible to leaf blight. Since individuals in this allelic group had EW structures and total EW load similar to other bm mutants that were not susceptible to drought and disease, we suspected that these responses were due to alterations in the cuticle proper. To test this hypothesis, cuticle ultrastructures and depositions on the wild-type and bm mutants were analyzed and then compared with water loss rates and disease resistance. Our results suggest that allelic mutants bm-2, bm-6, bm-22, and bm-33 possessa mutation affecting cuticle deposition. To our knowledge, a cuticle mutation in plants has not been previously reported. MATERIALS AND METHODS Plant MaterialThe mutagenesis program was initiated using two droughtresistant inbred lines of Sorghum bicolor (L.) Moench. These inbred lines, designated P954035 and P898012, were produced in the Purdue Sorghum Improvement Program (Dr. Gebisa Ejeta, Department of Agronomy). Seeds (Mo) were exposed to the chemical mutagens diethyl sulfate (J. T. Baker, Phillipsburg, NJ) or ethyl methanesulfate (Eastman Kodak Co., Rochester, NY). Seeds treated with diethyl sulfate were submerged in either a 5.7, 7.7, or 11.5 mM solut...

No abstract

Summary• Breeding for resistance to Striga in maize (Zea mays), with paucity of donor source and known mechanisms of resistance, has been challenging.• Here, post-attachment development of S. hermonthica was monitored on two maize inbreds selected for field resistance and susceptibility reactions to Striga at the International Institute of Tropical Agriculture. Haustorial invasion of the parasite into roots of these inbreds was examined histologically.• Morphological differences were observed between roots of the susceptible and the resistant inbreds. The resistant maize had fewer Striga attachments, delayed parasitic development and higher mortality of attached parasites compared with the susceptible inbred. Striga on the susceptible inbred usually penetrated the xylem and showed substantial internal haustorial development. Haustorial ingress on the resistant inbred was often stopped at the endodermis. Parasites able to reach resistant host xylem vessels showed diminished haustorial development relative to those invading susceptible roots.• These results suggest that the resistant inbred expresses a developmental barrier and incompatible response against Striga parasitism.