Characterization of Strigolactone Antagonists as Inhibitor of Striga hermonthica Seed Germination and the Discovery of a Nitric Oxide Responsive Protein in Arabidopsis thaliana

Abstract

Plants have evolved different communication mechanisms that convey information encoded in chemical signals, both internally and to surrounding organisms. Two such signals are strigolactones (SLs) and nitric oxide (NO). SLs are plant hormones that shape plant architecture according to nutrition availability and mediate interactions with beneficial arbuscular mycorrhizal fungi. For this second purpose, plant roots release SLs into the soil where they also trigger seed germination in root parasitic weeds, such as Striga hermonthica. Attachment of Striga causes severe damage to crops yield, particularly in sub-Saharan Africa. One way to control this threat to food security in infested African regions is to develop SL antagonists, which can inhibit Striga germination. Recently, we have shown that Triton X-100 can bind to the Striga SL-receptor, HYPOSENSITIVE to LIGHT 7 (ShHTL7). In addition, triazole urea compounds have been shown to specifically bind to Oryza sativa SL-receptor DWARF-14 (OsD14), blocking SL signalling. Here we used the structures of Triton X-100 and triazole urea to design two isomers and putative ShHTL7 inhibitors: KK023-N1 and KK023-N2. We demonstrate that these compounds antagonize SL signalling in S. hermonthica via specific binding to ShHTL7, and that application of KK023-N1 results in a 38% reduction in Striga germination under greenhouse conditions. Furthermore, we discovered a histidine residue (H51) in ShHTL7, which may be involved in SL perception in addition to known residues. Substitution of H51 to asparagine (N) led to a significant reduction in ShHTL7 hydrolysis activity, indicating the importance of this H residue. Our work provides a starting point for designing new series of SLs inhibitors to combat Striga, and improve food security worldwide.

NO is a gaseous signaling molecule involved in regulating plant development and adaptive responses to biotic and abiotic stresses. In this work, we characterized AtLRB3, a Light Response Bric-a-Brac/Tramtrack/Broad Complex (BTB) family protein, and showed that it contains a Heme Nitric Oxide/Oxygen (H-NOX) domain that can sense NO, providing an evidence of the existence of NO binding proteins in planta.