Differential Activity of Striga hermonthica Seed Germination Stimulants and Gigaspora rosea Hyphal Branching Factors in Rice and Their Contribution to Underground Communication

Abstract

Strigolactones (SLs) trigger germination of parasitic plant seeds and hyphal branching of symbiotic arbuscular mycorrhizal (AM) fungi. There is extensive structural variation in SLs and plants usually produce blends of different SLs. The structural variation among natural SLs has been shown to impact their biological activity as hyphal branching and parasitic plant seed germination stimulants. In this study, rice root exudates were fractioned by HPLC. The resulting fractions were analyzed by MRM-LC-MS to investigate the presence of SLs and tested using bioassays to assess their Striga hermonthica seed germination and Gigaspora rosea hyphal branching stimulatory activities. A substantial number of active fractions were revealed often with very different effect on seed germination and hyphal branching. Fractions containing (−)−orobanchol and ent-2'-epi-5-deoxystrigol contributed little to the induction of S. hermonthica seed germination but strongly stimulated AM fungal hyphal branching. Three SLs in one fraction, putative methoxy-5-deoxystrigol isomers, had moderate seed germination and hyphal branching inducing activity. Two fractions contained strong germination stimulants but displayed only modest hyphal branching activity. We provide evidence that these stimulants are likely SLs although no SL-representative masses could be detected using MRM-LC-MS. Our results show that seed germination and hyphal branching are induced to very different extents by the various SLs (or other stimulants) present in rice root exudates. We propose that the development of rice varieties with different SL composition is a promising strategy to reduce parasitic plant infestation while maintaining symbiosis with AM fungi.

Genetic variation in Sorghum bicolor strigolactones and their role in resistance against Striga hermonthica

“…Likewise, we showed that at a concentration of 0.02 µM ent -2′- epi -5-deoxystrigol, ent -2′- epi -strigol, or GR24 displayed a moderate yet potent activity (>30% germination), while orobanchol showed negligible activity (<10% germination). The low germination-inducing activity of orobanchol in Striga is consistent with previous reports on the low sensitivity of Striga (sorghum strain) to the orobanchol-type strigolactones (Nomura et al , 2013; Cardoso et al , 2014a). The low sensitivity of Striga to orobanchol suggests that selection for high orobanchol producers may be an effective strategy to obtain sorghum genotypes with good arbuscular mycorrhizal fungi recruitment, but lower Striga germination-inducing activity.…”

“…In the germination bioassay, 5-deoxystrigol, sorgomol, sorgolactone, and strigol exhibited relatively high germination-inducing activity in Striga seeds compared with the other strigolactones, consistent with previous reports (Hauck et al , 1992; Yasuda et al , 2003; Xie et al , 2008; Cardoso et al , 2014a). Likewise, we showed that at a concentration of 0.02 µM ent -2′- epi -5-deoxystrigol, ent -2′- epi -strigol, or GR24 displayed a moderate yet potent activity (>30% germination), while orobanchol showed negligible activity (<10% germination).…”

Genomic signatures of host-specific selection in a parasitic plant

“…GR24 is a synthetic strigolactone analog commonly used in laboratory germination studies of parasitic plants as a positive control. 5-deoxystrigol is one of the major SLs produced by compatible grass hosts (Awad et al, 2006) and is a potent stimulator of parasite germination whereas orobanchol is a more dominant SL among dicot hosts (Yoneyama et al, 2008) and is a less potent stimulator of S. hermonthica germination for certain genotypes (Cardoso et al, 2014). Specifically, S. hermonthica tested populations from Mali and Niger germinate poorly in response to orobanchol (Haussmann et al, 2004; Bellis et al, 2020) whereas Kenyan S. hermonthica show a greater ability to germinate in response to exudate from sorghum hosts carrying loss-of-function mutations at the sorghum resistance locus LOW GERMINATION STIMULANT 1 (LGS1), which results in high amounts of orobanchol rather than 5-deoxystrigol in root exudates (Gobena et al, 2017).…”

Strigolactones, super hormones in the fight against Striga

“…The SL reduction strategy has shown promise for some species, such as faba bean and rice [59,60], but negative impacts on agronomic traits may limit its utility for maize and tomato [14,61]. Modifying SL composition rather than abundance may prove a more viable alternative [62,63]. A highly sensitive biological reporter of SLs has been developed based on germination of Arabidopsis carrying a ShHTL7 transgene.…”