Harnessing the genetic potential for arbuscular mycorrhizal symbiosis in rice

Arbuscular mycorrhizal fungi (AMF) have long been recognised to form a mutually beneficial symbiosis with over 80% of land plants present today. The symbiosis provides an integral link to mineral nutrients in the soil (Bago et al., 2000; Jakobsen & Rosendahl, 1990; Smith & Read, 2008) and this has led to increasing interest in the potential for use of AMF as natural biofertilizers in sustainable agroecosystems (Berruti et al., 2015). Despite this, such potential is currently limited by the variable nature of plant host response, particularly mycorrhizal growth responsiveness (MGR) (Hetrick et al., 1992; Lehnert et al., 2018; Taylor et al., 2015; Thirkell et al., 2022; Xavier & Germida, 1998). Here, in collaboration with the International Rice Research Institute (IRRI), Philippines, we use a glasshouse and field approach to address the genetic potential for AM symbiosis in rice, *Oryza sativa*, a staple food crop for over half of the world’s population (Childs & LeBeau, 2023). First, use of foliar abundance of blumenol C glycosides (Mindt et al., 2019; M. Wang et al., 2018) as a novel, high throughput (HTP) marker of AM colonisation was newly evaluated in rice. I offer the first interrogation of blumenol phenotypes in rice mutants with aberrant AMF life cycle stages and report that extraradical colonisation is not sufficient to promote accumulation. In addition, I newly report that blumenol accumulation is independent to SL signalling, the phosphate starvation response network and downstream D14L/SMAX1 signalling, which are integral parts of AM symbiosis regulation, and provide further evidence for a model of blumenol biosynthesis in rice. I further describe evidence supporting recommendation of abundance of 11-carboxyblumenol C glucosides as a foliar marker of AM colonisation in model and diverse cultivars of rice. The marker was successfully used to probe natural variation in AM colonisation and host response in a panel of *Indica* rice from the 3K-RG (Z. Li et al., 2014; W. Wang et al., 2018), a germplasm containing large amounts of diversity which has not yet been interrogated for AM phenotypes. I described natural variation in AM colonisation and host responsiveness, identified exciting candidate genes and provided data for further interrogation of genetic determinants of MGR and host response. This will have exciting applications for future studies interrogating AM symbiosis in diverse rice cultivars. Importantly, in addition to use of glasshouse experiments to probe phenotypes without interference of other factors, understanding of the potential of AMF as natural biofertilizers of rice requires placement of the interaction in a field context. First-look assessments in the field at IRRI, Philippines identified the potential for AMF in dry, direct seeded (DSR), sustainable agroecosystems of rice. Furthermore, a field experiment additionally identified natural variation in host response to AMF-inoculation compared to native AMF, corroborating glasshouse results and further evidencing genetic variation in host response to AM symbiosis in rice. The trial valuably defined field conditions for future, complementary analyses of genetic determinants of host responsiveness in the field at IRRI. Overall, the project advocates for use of blumenols as a novel tool for large scale studies of natural variation in AM colonisation and response in rice. I successfully identified candidate genetic determinants of blumenol abundance and colonisation, providing further evidence for the genetic potential for AM symbiosis in rice. In addition, the collaborative study with IRRI, Philippines offers groundwork and recommendations for further studies assessing the potential for AMF as biofertilizers driving beneficial host response in the rice field.

Biotechnology and Biological Sciences Research Council (2118594); Darwin College, University of Cambridge; Department of Plant Sciences, University of Cambridge