Genotypic Variation for Low Striga Germination Stimulation in Sorghum “Sorghum bicolor (L.) Moench” Landraces from Eritrea

Sorghum (Sorghum bicolour (L.) Moench), the second most important staple crop in Sub-Saharan Africa (SSA) after maize, is well adapted to marginal environments of drought stress and high temperatures. But besides drought stress, the obligate root-parasitic flowering plant Striga hermonthica is an equally economically important biotic stress in agro-ecological zones where soils are marginal. Notwithstanding widespread and intense Striga infestation, genetic variations in defence mechanisms against the parasite have been reported. Sorghum variants, producing low levels of chemical stimulants such as sorgolactones that deter the advance of Striga seed germination and are therefore deemed resistant to the parasite, have been also reported in a few studies. But the existence of sorghum genetic variation for this resistance especially among farmers’ landraces is yet to be demonstrated. The objective of this study was therefore to determine the levels of Striga germination stimulants in response to each of the 111 collected sorghum landraces and their progenies from Eritrea. The ability of a sorghum genotype to cause germination of a Striga seed as a measure of the amount of the germination stimulant produced was used to assess the resistance of these accessions. The data were recorded as Striga germination percentage by counting the number of germinated Striga seeds. Landraces EG47, EG1261, EG830, EG1076, EG54 and EG746 with 14.68%, 15.32%, 11.85%, 13.05%, 15.74% and 16.5% germination percentages respectively were found to stimulate low levels of Striga germination percentage compared to commercial checks, IS9830, SRN39, Framida, with 22.46%, 22.67%, 23.27% germination respectively. While these variants did not show complete resistance against Striga seed germination, the low level production of stimulant indicated their high level of resistance to Striga . These results implied that these accessions are likely potential sources of resistance against Striga infestation in SSA sorghum breeding programs.

Chemicsi stimulation nd control of barsyardgrass seed germination in rice

This experiment was conducted during1989-1990 at the Experiment Station of CNRRI.Each pot contained 3 kg soil of silt loam with pH6.2 and 3.7% Organic matter.Chemical treatmentsand rates are shown in Table.The experiment wasdesigned in a randomized complete block witheight replications.Barnyardgrass(BYG)[Echinochloa crus-galli(L.)Beauv]seeds for thetreatment were conccted from a late season ricefield near the station in Oct 1989.Two hundred

Confirming Stereochemical Structures of Strigolactones Produced by Rice and Tobacco

Major strigolactones （SLs） produced by rice （Oryza sativa L. cv. Nipponbare） and tobacco （Nicotiana tabacum L. cv. Michinoku No. 1） were purified and their stereochemical structures were determined by comparing with optically pure synthetic standards for their NMR and CD data and retention times and mass fragmentations in ESI-LC/MS and GC-MS. SLs purified from root exudates of rice plants were orobanchol, orobanchyl acetate, and ent-2＂-epi-5-deoxystr- igol. In addition to these SLs, 7-oxoorobanchyl acetate and the putative three methoxy-5~deoxystrigol isomers were detected by LC-MS/MS. The production of 7-oxoorobanchyl acetate seemed to occur in the early growth stage, as it was detected only in the root exudates collected during the first week of incubation. The root exudates of tobacco contained at least 11 SLs, including solanacol, solanacyl acetate, orobanchol, ent-2＂-epi-orobanchol, orobanchyl acetate, ent-2＇- epi-orobanchyl acetate, 5-deoxystrigol, ent-2＂-epi-5-deoxystrigol, and three isomers of putative didehydro-orobanchol whose structures remain to be clarified. Furthermore, two sorgolactone isomers but not sorgolactone were detected as minor SLs by LC-MS/MS analysis. It is intriguing to note that rice plants produced only orobanchol-type SLs, derived from ent-2＂-epi-5-deoxystrigol, but both orobanchol-type and strigol-type SLs, derived from 5-deoxystrigol were detected in tobacco plants.

Structure and Activity of Strigolactones： New Plant Hormones with a Rich Future

Strigolactones （SLs） constitute a new class of plant hormones which are active as germination stimulants for seeds of parasitic weeds of Striga, Orobanche, and Pelipanchi spp, in hyphal branching of arbuscular mycorrhizal （AM） fungi and as inhibitors of shoot branching. In this review, the focus is on molecular features of these SLs. The occurrence of SLs in root exudates of host plants is described. The naming protocol for SL according to the International Union of Pure and Applied Chemistry （IUPAC） rules and the ＇at a glance＇ method is explained. The total synthesis of some natural SLs is described with details for all eight stereoisomers of strigol. The problems encountered with assign- ing the correct structure of natural SLs are analyzed for orobanchol, alectrol, and solanacol. The structure-activity relationship of SLs as germination stimulants leads to the identification of the bioactiphore of SLs. Together with a tentative mechanism for the mode of action, a model has been derived that can be used to design and prepare active SL analogs. This working model has been used for the preparation of a series of new SL analogs such as Nijmegen-1, and analogs derived from simple ketones, keto enols, and saccharine. The serendipitous finding of SL mimics which are derived from the D-ring in SLs （appropriately substituted butenolides） is reported. For SL mimics, a mode of action is proposed as well. Recent new results support this proposal. The stability of SLs and SL analogs towards hydrolysis is described and some details of the mechanism of hydrolysis are discussed as well. The attempted isolation of the protein receptor for germination and the current status concerning the biosynthesis of natural SLs are briefly discussed. Some non-SLs as germinating agents are mentioned. The structure-activity relationship for SLs in hyphal branching of AM fungi and in repression of shoot branching is also analyzed. For each of the principle functions, a working model for the design of new active SL analogs is described and its applicability and implications are discussed. It is shown that the three principal functions use a distinct perception system. The importance of stereochemistry for bioactivity has been described for the various functions.

A Phelipanche ramosa KAI2 protein perceives strigolactones and isothiocyanates enzymatically

Phelipanche ramosa is an obligate root-parasitic weed that threatens major crops in central Europe.In order to germinate,it must perceive various structurally divergent host-exuded signals,including isothiocyanates(ITCs)and strigolactones(SLs).However,the receptors involved are still uncharacterized.Here,we identify five putative SL receptors in P.ramosa and show that PrKAI2d3 is involved in the stimulation of seed germination.We demonstrate the high plasticity of PrKAI2d3,which allows it to interact with different chemicals,including ITCs.The SL perception mechanism of PrKAI2d3 is similar to that of endogenous SLs in non-parasitic plants.We provide evidence that PrKAI2d3 enzymatic activity confers hypersensitivity to SLs.Additionally,we demonstrate that methylbutenolide-OH binds PrKAI2d3 and stimulates P.ramosa germination with bioactivity comparable to that of ITCs.This study demonstrates that P.ramosa has extended its signal perception system during evolution,a fact that should be considered for the development of specific and efficient biocontrol methods.