Abstract
Striga hermonthica is a root parasitic plant that infests cereals, decimating yields, particularly in sub‐Saharan Africa. For germination, Striga seeds require host‐released strigolactones that are perceived by the family of HYPOSENSITIVE to LIGHT (ShHTL) receptors. Inhibiting seed germination would thus be a promising approach for combating Striga. However, there are currently no strigolactone antagonists that specifically block ShHTLs and do not bind to DWARF14, the homologous strigolactone receptor of the host. Here, we show that the octyl phenol ethoxylate Triton X‐100 inhibits S. hermonthica seed germination without affecting host plants. High‐resolution X‐ray structures reveal that Triton X‐100 specifically plugs the catalytic pocket of ShHTL7. ShHTL7‐specific inhibition by Triton X‐100 demonstrates the dominant role of this particular ShHTL receptor for Striga germination. Our structural analysis provides a rationale for the broad specificity and high sensitivity of ShHTL7, and reveals that strigolactones trigger structural changes in ShHTL7 that are required for downstream signaling. Our findings identify Triton and the related 2‐[4‐(2,4,4‐trimethylpentan‐2‐yl)phenoxy]acetic acid as promising lead compounds for the rational design of efficient Striga‐specific herbicides.
Highlights
The witchweed Striga hermonthica (Orobanchaceae) is a root parasitic plant that infests cereals, including rice, maize, sorghum, and pearl millet [1]
In all crystals, ShHTL7 associated with the 4-(1,1,3,3-tetramethylbutyl)-phenyl hydrocarbon moiety of Triton X-100 that we used during protein purification (Figs 1A and B and EV1A)
Our analysis suggests that the broad specificity of ShHTL7 toward SLs is explained by the large volume of the SL binding pocket (1,174.3 A 3 compared to 894.5 and 710.1 A 3 in ShHTL5 and rice D14, respectively) that can accommodate diverse SLs without steric clashes (Figs 5C and 7D)
Summary
The witchweed Striga hermonthica (Orobanchaceae) is a root parasitic plant that infests cereals, including rice, maize, sorghum, and pearl millet [1]. Hydrolysis of SLs by D14 is mediated by a conserved Ser-His-Asp catalytic triad and results in covalent linking of the D-ring to the receptor and the release of the second moiety [9,10,11,12,13] This covalent modification promotes the interaction of D14 with the downstream effector MORE AXILLARY GROWTH 2 (MAX2), which requires substantial restructuring of the SL binding pocket of D14 to fit the MAX2 binding surface [12,13,14] and leads to the proteasome-mediated degradation of presumed repressors of SL-inducible genes, such as the rice DWARF53 [15,16,17]. The protein KARRIKIN-INSENSITIVE2 (KAI2)/ HYPOSENSITIVE to LIGHT (HTL)/D14-like is a close homolog to D14, which, binds the smoke-derived karrikins [17,18] that induce seed germination in many land plants, but not in root parasitic plants [17]
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