Molecular genetics of plant hormone signaling

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Parasitic weeds threaten approximately 100 million subsistence framers in Africa. The most pernicious of these weeds, Striga hermonthica (Striga), infests 40% of the sub-Saharan farmland and is the biggest impediment to poverty alleviation on the continent. Presently, the most common method of Striga control is hand weeding, which is time consuming, labour-intensive and disproportionately carried out by women and children. Unfortunately, since much of the crop damage inflicted by Striga occurs before it surfaces, hand weeding is ineffective. Further compounding the situation is Striga seeds remain viable in the soil for up to 20 years, thus purging fields is extremely difficult with severe infestations often resulting in abandonment of the field. Breeding approaches are presently the most economically sought method of parasitic weed control, however, long-term resistance to Striga has not been attained. These challenges necessitates innovative approaches to eradicate Striga that exploit weaknesses in its lifecycle. To this end, Striga, requires a small molecule, Strigolactone (SL), to germinate. SLs emanate from host plant roots causing Striga to germinate and infest the host. Because Striga is an obligate parasite, researchers have tried to develop cheap stable SL analogs that stimulate Striga "suicide" seed germination in the absence of the host. The development of compounds that perturb Striga germination is predicated on mechanistically understanding SL receptors in this species. Over the past grant cycle, my group identified the key SL receptors in Striga and showed one, designated ShHTL7, is essential to Striga germination. In this grant cycle, we will use a combination of genetic and structural information to understand how ShHTL7 perceives SL. Our previous work shows this receptor is highly sensitive to picomolar levels of SLs in the plant (in vivo) but this sensitivity is not reflected in the test tube (in vitro). The nature of the ShHTL7 hypersensitivity is biochemically intriguing because ShHTL7 has a large ligand binding domain (LBD) that should, in principle, decrease not increase its affinity to SL. To reconcile in vivo with in vitro observations we have developed three approaches. First, we will use a "reverse genetics" approach to identify key amino acids in the LBD that contribute to SL sensitivity. Second, we will use an unbiased "forward genetics" approach to also identify key amino acids in the ShHTL7 LBD. This second approach will also identify any ShHTL7 partner proteins that may contribute to SL sensitivity. Finally, we will develop a yeast based high throughput system to assay Striga SL receptors. The basic information gleaned will not only enlighten our understanding of this uniquely hypersensitive parasitic plant SL receptor but will also direct the development of new lead compounds that perturb Striga germination. This in turn, may have long term utility in curbing Striga infestations in Africa.