The rice terpene synthase gene OsTPS19 functions as an (S)-limonene synthase in planta, and its overexpression leads to enhanced resistance to the blast fungus Magnaporthe oryzae.

Xujun Chen,Tobias G Köllner,Zejian Guo,Xinlu Chen,Andreas Nebenführ,Xiaofeng Zhuang,Yufen Guo,Jianyu Fu,Yuying Chen,Feng Chen,Hao Chen,Yong‐Jun Zhang,Joshua S Yuan

doi:10.1111/pbi.12914

Abstract

SummaryRice blast disease, caused by the fungus Magnaporthe oryzae, is the most devastating disease of rice. In our ongoing characterization of the defence mechanisms of rice plants against M. oryzae, a terpene synthase gene OsTPS19 was identified as a candidate defence gene. Here, we report the functional characterization of OsTPS19, which is up‐regulated by M. oryzae infection. Overexpression of OsTPS19 in rice plants enhanced resistance against M. oryzae, while OsTPS19 RNAi lines were more susceptible to the pathogen. Metabolic analysis revealed that the production of a monoterpene (S)‐limonene was increased and decreased in OsTPS19 overexpression and RNAi lines, respectively, suggesting that OsTPS19 functions as a limonene synthase in planta. This notion was further supported by in vitro enzyme assays with recombinant OsTPS19, in which OsTPS19 had both sesquiterpene activity and monoterpene synthase activity, with limonene as a major product. Furthermore, in a subcellular localization experiment, OsTPS19 was localized in plastids. OsTPS19 has a highly homologous paralog, OsTPS20, which likely resulted from a recent gene duplication event. We found that the variation in OsTPS19 and OsTPS20 enzyme activities was determined by a single amino acid in the active site cavity. The expression of OsTPS20 was not affected by M. oryzae infection. This indicates functional divergence of OsTPS19 and OsTPS20. Lastly, (S)‐limonene inhibited the germination of M. oryzae spores in vitro. OsTPS19 was determined to function as an (S)‐limonene synthase in rice and plays a role in defence against M. oryzae, at least partly, by inhibiting spore germination.

Highlights

Rice blast disease caused by Magnaporthe oryzae, one of the top 10 fungal pathogens (Dean et al, 2012), is the most devastating rice disease, causing severe loss of production
Induction of OsTPS19 and OsTPS20 was analysed in threeweek-old rice seedlings inoculated with M. oryzae, a causal agent of rice blast fungus
As there was a significant induction of OsTPS19 in the mock treatment (Figure 1b), we speculate that expression of OsTPS19 was related to diurnal rhythms because some volatile terpenoids are released in a fluctuating pattern (Cheng et al, 2007; Yazaki et al, 2017)

Summary

Introduction

Rice blast disease caused by Magnaporthe oryzae, one of the top 10 fungal pathogens (Dean et al, 2012), is the most devastating rice disease, causing severe loss of production. As a model system for molecular studies, rice and M. oryzae have been investigated for pathogenicity, host resistance and their interactions. The basal resistance triggered by pathogenassociated molecular patterns and specific effector-triggered immunity is associated with a similar network and activate innate immune responses, including global transcriptional reprogramming (Zhang and Zhou, 2010). Rice defences, which are activated by blast resistance (called Pi) genes, often break down in practical applications because the pathogen effectors evolve rapidly to evade recognition by the corresponding Pi genes. A natural allele of a C2H2-type transcription factor in the rice cultivar Digu was found to confer nonracespecific resistance against blast fungi (Li et al, 2017). In addition to innate immune responses, plants established an inducible immune response system, including systemic acquired resistance (SAR). It was recently reported that volatile monoterpenes, pinenes, promote SAR within and between Arabidopsis plants (Riedlmeier et al, 2017)

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Discussion

Conclusion