Strigolactones are chemoattractants for host tropism in Orobanchaceae parasitic plants

Abstract

Parasitic plants are worldwide threats that damage major agricultural crops. To initiate infection, parasitic plants have developed the ability to locate hosts and grow towards them. This ability, called host tropism, is critical for parasite survival, but its underlying mechanism remains mostly unresolved. To characterise host tropism, we used the model facultative root parasite Phtheirospermum japonicum, a member of the Orobanchaceae. Here, we show that strigolactones (SLs) function as host-derived chemoattractants. Chemotropism to SLs is also found in Striga hermonthica, a parasitic member of the Orobanchaceae, but not in non-parasites. Intriguingly, chemotropism to SLs in P. japonicum is attenuated in ammonium ion-rich conditions, where SLs are perceived, but the resulting asymmetrical accumulation of the auxin transporter PIN2 is diminished. P. japonicum encodes putative receptors that sense exogenous SLs, whereas expression of a dominant-negative form reduces its chemotropic ability. We propose a function for SLs as navigators for parasite roots.

Strigol induces germination of the facultative parasitic plant Phtheirospermum japonicum in the absence of nitrate ions

“…In addition, we found that exogenous strigol, an SL, enhanced the germination of P. japonicum in nitrate-deficient conditions. In combination with our previous findings demonstrating that steps for host tropism are promoted by exogenous SLs and attenuated by ambient nitrogen sources, 17 we propose a model for determining whether P. japonicum can live independently or become a parasite.…”

“…Since seed germination is often enhanced by light-induced biosynthesis of the phytohormone gibberellin, 24 we germinated P. japonicum seeds in dark conditions to exclude the effects of light. The germination percentage on agar without any supplementary nutrients was lower than that on the 1/2 MS agar with sucrose, 22 a nutrient-rich medium that has been used for P. japonicum germination 17 , 18 , 23 ( Figure 1a ). We noticed that the germination percentage of P. japonicum seeds in the dark was only around 50% even on 1/2 MS with sucrose, in contrast to around 100% germination of A. thaliana seeds in the dark.…”

“…We noticed that the germination percentage of P. japonicum seeds in the dark was only around 50% even on 1/2 MS with sucrose, in contrast to around 100% germination of A. thaliana seeds in the dark. 17 This difference suggests that the lower germination percentage might reflect the nature of P. japonicum seeds. To identify the nutrient component(s) that enhance germination, we measured germination percentages on water agar containing each of the MS macronutrients or sucrose.…”

“… 11 , 16 Germination assays using the Arabidopsis thaliana kai2 mutants revealed that KAI2 and KAI2d contribute to promoting KAR/KL-stimulated germination and SL-stimulated germination, respectively. 11 , 17 …”

“… 18 We recently discovered that at least two KAI2d proteins, designated PjKAI2d2 and PjKAI2d3.2, are important for root tropism to host-derived SLs in P. japonicum . 17 However, the relationships between exogenous SLs and germination of P. japonicum remain unclear. We wondered if as a facultative parasite P. japonicum would germinate when the ambient nutrition was sufficient to survive alone and that in the case of low nutrient conditions, such as low nitrogen sources, the parasite would not germinate until ensuring that hosts are nearby to infect.…”