In planta functions of cytochrome P450 monooxygenase genes in the phytocassane biosynthetic gene cluster on rice chromosome 2

Enhancing Crop Resilience: Insights from Labdane-Related Diterpenoid Phytoalexin Research in Rice (Oryza sativa L.)

Rice (Oryza sativa L.), as one of the most significant food crops worldwide, holds paramount importance for global food security. Throughout its extensive evolutionary journey, rice has evolved a diverse array of defense mechanisms to fend off pest and disease infestations. Notably, labdane-related diterpenoid phytoalexins play a crucial role in aiding rice in its response to both biotic and abiotic stresses. This article provides a comprehensive review of the research advancements pertaining to the chemical structures, biological activities, and biosynthetic pathways, as well as the molecular regulatory mechanisms, underlying labdane-related diterpenoid phytoalexins discovered in rice. This insight into the molecular regulation of labdane-related diterpenoid phytoalexin biosynthesis offers valuable perspectives for future research aimed at improving crop resilience and productivity.

Rice Phytoalexins: Half a Century of Amazing Discoveries; Part I: Distribution, Biosynthesis, Chemical Synthesis, and Biological Activities

Cultivated rice is a staple food for more than half of the world's population, providing approximately 20% of the world's food energy needs. A broad spectrum of pathogenic microorganisms causes rice diseases leading to huge yield losses worldwide. Wild and cultivated rice species are known to possess a wide variety of antimicrobial secondary metabolites, known as phytoalexins, which are part of their active defense mechanisms. These compounds are biosynthesized transiently by rice in response to pathogens and certain abiotic stresses. Rice phytoalexins have been intensively studied for over half a century, both for their biological role and their potential application in agronomic and pharmaceutical fields. In recent decades, the growing interest of the research community, combined with advances in chemical, biological, and biomolecular investigation methods, has led to a notable acceleration in the growth of knowledge on rice phytoalexins. This review provides an overview of the knowledge gained in recent decades on the diversity, distribution, biosynthesis, chemical synthesis, and bioactivity of rice phytoalexins, with particular attention to the most recent advances in this research field.

Deletion of Diterpenoid Biosynthetic Genes CYP76M7 and CYP76M8 Induces Cell Death and Enhances Bacterial Blight Resistance in Indica Rice ‘9311’

Lesion mimic mutants (LMMs) are ideal materials for studying cell death and resistance mechanisms. Here, we identified and mapped a novel rice LMM, g380. The g380 exhibits a spontaneous hypersensitive response-like cell death phenotype accompanied by excessive accumulation of reactive oxygen species (ROS) and upregulated expression of pathogenesis-related genes, as well as enhanced resistance to Xanthomonas oryzae pv. oryzae (Xoo). Using a map-based cloning strategy, a 184,916 bp deletion on chromosome 2 that overlaps with the diterpenoid biosynthetic gene cluster was identified in g380. Accordingly, the content of diterpenoids decreased in g380. In addition, lignin, one of the physical lines of plant defense, was increased in g380. RNA-seq analysis showed 590 significantly differentially expressed genes (DEG) between the wild-type 9311 and g380, 585 of which were upregulated in g380. Upregulated genes in g380 were mainly enriched in the monolignol biosynthesis branches of the phenylpropanoid biosynthesis pathway, the plant–pathogen interaction pathway and the phytoalexin-specialized diterpenoid biosynthesis pathway. Taken together, our results indicate that the diterpenoid biosynthetic gene cluster on chromosome 2 is involved in immune reprogramming, which in turn regulates cell death in rice.

Dissecting the labdane-related diterpenoid biosynthetic gene clusters in rice reveals directional cross-cluster phytotoxicity

Rice (Oryza sativa) is a staple food crop and serves as a model cereal plant. It contains two biosynthetic gene clusters (BGCs) for the production of labdane-related diterpenoids (LRDs), which serve important roles in combating biotic and abiotic stress. While plant BGCs have been subject to genetic analyses, these analyses have been largely confined to the investigation of single genes. CRISPR/Cas9-mediated genome editing was used to precisely remove each of these BGCs, as well as simultaneously knock out both BGCs. Deletion of the BGC from chromosome 2 (c2BGC), which is associated with phytocassane biosynthesis, but not that from chromosome 4 (c4BGC), which is associated with momilactone biosynthesis, led to a lesion mimic phenotype. This phenotype is dependent on two closely related genes encoding cytochrome P450 (CYP) mono-oxygenases, CYP76M7 and CYP76M8, from the c2BGC. However, rather than being redundant, CYP76M7 has been associated with the production of phytocassanes, whereas CYP76M8 is associated with momilactone biosynthesis. Intriguingly, the lesion mimic phenotype is not present in a line with both BGCs deleted. These results reveal directional cross-cluster phytotoxicity, presumably arising from the accumulation of LRD intermediates dependent on the c4BGC in the absence of CYP76M7 and CYP76M8, further highlighting their interdependent evolution and the selective pressures driving BGC assembly.

A review: biosynthesis of plant-derived labdane-related diterpenoids

Plant-derived labdane-related diterpenoids (LRDs) represent a large group of terpenoids. LRDs possess either a labdane-type bicyclic core structure or more complex ring systems derived from labdane-type skeletons, such as abietane, pimarane, kaurane, etc. Due to their various pharmaceutical activities and unique properties, many of LRDs have been widely used in pharmaceutical, food and perfume industries. Biosynthesis of various LRDs has been extensively studied, leading to characterization of a large number of new biosynthetic enzymes. The biosynthetic pathways of important LRDs and the relevant enzymes (especially diterpene synthases and cytochrome P450 enzymes) were summarized in this review.

Interdependent evolution of biosynthetic gene clusters for momilactone production in rice

Plants can contain biosynthetic gene clusters (BGCs) that nominally resemble those found in microbes. However, while horizontal gene transmission is often observed in microbes, plants are limited to vertical gene transmission, implying that their BGCs may exhibit distinct inheritance patterns. Rice (Oryza sativa) contains two unlinked BGCs involved in diterpenoid phytoalexin metabolism, with one clearly required for momilactone biosynthesis, while the other is associated with production of phytocassanes. Here, in the process of elucidating momilactone biosynthesis, genetic evidence was found demonstrating a role for a cytochrome P450 (CYP) from the other "phytocassane" BGC. This CYP76M8 acts after the CYP99A2/3 from the "momilactone" BGC, producing a hemiacetal intermediate that is oxidized to the eponymous lactone by a short-chain alcohol dehydrogenase also from this BGC. Thus, the "momilactone" BGC is not only incomplete, but also fractured by the need for CYP76M8 to act in between steps catalyzed by enzymes from this BGC. Moreover, as supported by similar activity observed with orthologs from the momilactone-producing wild-rice species Oryza punctata, the presence of CYP76M8 in the other "phytocassane" BGC indicates interdependent evolution of these two BGCs, highlighting the distinct nature of BGC assembly in plants.

A genome-wide survey of interaction between rice and Magnaporthe oryzae via microarray analysis

The main aim of the work is to study the regulation of gene expression in the interaction between rice and Magnaporthe oryzae by gene chip technology. In this study, we mainly focused on changes of gene expression at 24, 48, and 72 hours post-inoculation (hpi), through which we could conduct a more comprehensive analysis of rice blast-related genes in the process of infection. The results showed that the experimental groups contained 460, 1227, and 3937 significant differentially expressed genes at 24, 48, and 72 hpi, respectively. Furthermore, 115 significantly differentially expressed genes were identified in response to rice blast infection at all three time points. By annotating these 115 genes, they were divided into three categories: metabolic pathways, proteins or enzymes, and organelle components. As expected, many of these genes were known rice blast-related genes; however, we discovered new genes with high fold changes. Most of them encoded conserved hypothetical proteins, and some were hypothetically conserved genes. Our study may contribute to finding new resistance genes and understanding the mechanism of rice blast development.

Natural variation of diterpenoid phytoalexins in cultivated and wild rice species

Rice (Oryza sativa) leaves accumulate phytoalexins in response to pathogen attack. The major phytoalexins in rice are diterpenoids such as momilactones, phytocassanes, and oryzalexins. We analyzed the abundance of momilactones A and B and phytocassanes A and D in UV-light-irradiated leaves of cultivars from the World Rice Core Collection (WRC). Both types of phytoalexins were detected in most cultivars; however, their accumulated amounts varied greatly from cultivar to cultivar. The amounts of momilactones A and B tended to be higher in japonica cultivars than those in indica cultivars. However, the accumulated amounts of phytocassanes were not related to differences in subspecies. In addition, variation in phytoalexin content was observed for seven wild rice species. During the analysis of momilactone A in cultivars from the WRC, two unknown compounds were detected in'Jaguary' and 'Basilanon'. We isolated these compounds from UV-light-irradiated leaves and determined their structures. The compound isolated from 'Jaguary' was an isomer of momilactone A that had an abietane skeleton, while that from 'Basilanon' was di-dehydrogenated phytocassane A; these compounds were denoted as oryzalactone and phytocassane G. Oryzalactone accumulated in only three cultivars, whereas phytocassane G accumulated in almost all of the cultivars from the WRC. These findings indicate the existence of large natural variation in the phytoalexin composition in rice.

Integrative Metabolomic and Transcriptomic Analyses Reveal Metabolic Changes and Its Molecular Basis in Rice Mutants of the Strigolactone Pathway

Plants have evolved many metabolites to meet the demands of growth and adaptation. Although strigolactones (SLs) play vital roles in controlling plant architecture, their function in regulating plant metabolism remains elusive. Here we report the integrative metabolomic and transcriptomic analyses of two rice SL mutants, d10 (a biosynthesis mutant) and d14 (a perception mutant). Both mutants displayed a series of metabolic and transcriptional alterations, especially in the lipid, flavonoid, and terpenoid pathways. Levels of several diterpenoid phytoalexins were substantially increased in d10 and d14, together with the induction of terpenoid gene cluster and the corresponding upstream transcription factor WRKY45, an established determinant of plant immunity. The fact that WRKY45 is a target of IPA1, which acted as a downstream transcription factor of SL signaling, suggests that SLs contribute to plant defense through WRKY45 and phytoalexins. Moreover, our data indicated that SLs may modulate rice metabolism through a vast number of clustered or tandemly duplicated genes. Our work revealed a central role of SLs in rice metabolism. Meanwhile, integrative analysis of the metabolome and transcriptome also suggested that SLs may contribute to metabolite-associated growth and defense.

Advances in biosynthesis, regulation, and metabolic engineering of plant specialized terpenoids

Plant specialized terpenoids are natural products that have no obvious role in growth and development, but play many important functional roles to improve the plant's overall fitness. Besides, plant specialized terpenoids have immense value to humans due to their applications in fragrance, flavor, cosmetic, and biofuel industries. Understanding the fundamental aspects involved in the biosynthesis and regulation of these high-value molecules in plants not only paves the path to enhance plant traits, but also facilitates homologous or heterologous engineering for overproduction of target molecules of importance. Recent developments in functional genomics and high-throughput analytical techniques have led to unraveling of several novel aspects involved in the biosynthesis and regulation of plant specialized terpenoids. The knowledge thus derived has been successfully utilized to produce target specialized terpenoids of plant origin in homologous or heterologous host systems by metabolic engineering and synthetic biology approaches. Here, we provide an overview and highlights on advances related to the biosynthetic steps, regulation, and metabolic engineering of plant specialized terpenoids.

Specialized diterpenoid metabolism in monocot crops: Biosynthesis and chemical diversity

Among the myriad specialized metabolites that plants employ to mediate interactions with their environment, diterpenoids form a chemically diverse group with vital biological functions. A few broadly abundant diterpenoids serve as core pathway intermediates in plant general metabolism. The majority of plant diterpenoids, however, function in specialized metabolism as often species-specific chemical defenses against herbivores and microbial diseases, in below-ground allelopathic interactions, as well as abiotic stress responses. Dynamic networks of anti-microbial diterpenoids were first demonstrated in rice (Oryza sativa) over four decades ago, and more recently, unique diterpenoid blends with demonstrated antibiotic bioactivities were also discovered in maize (Zea mays). Enabled by advances in -omics and biochemical approaches, species-specific diterpenoid-diversifying enzymes have been identified in these and other Poaceous species, including wheat (Triticum aestivum) and switchgrass (Panicum virgatum), and are discussed in this article with an emphasis on the critical diterpene synthase and cytochrome P450 monooxygenase families and their products. The continued investigation of the biosynthesis, diversity, and function of terpenoid-mediated crop defenses provides foundational knowledge to enable the development of strategies for improving crop resistance traits in the face of impeding pest, pathogen, and climate pressures impacting global agricultural production.

Rice Secondary Metabolites: Structures, Roles, Biosynthesis, and Metabolic Regulation

Rice (Oryza sativa L.) is an important food crop providing energy and nutrients for more than half of the world population. It produces vast amounts of secondary metabolites. At least 276 secondary metabolites from rice have been identified in the past 50 years. They mainly include phenolic acids, flavonoids, terpenoids, steroids, alkaloids, and their derivatives. These metabolites exhibit many physiological functions, such as regulatory effects on rice growth and development, disease-resistance promotion, anti-insect activity, and allelopathic effects, as well as various kinds of biological activities such as antimicrobial, antioxidant, cytotoxic, and anti-inflammatory properties. This review focuses on our knowledge of the structures, biological functions and activities, biosynthesis, and metabolic regulation of rice secondary metabolites. Some considerations about cheminformatics, metabolomics, genetic transformation, production, and applications related to the secondary metabolites from rice are also discussed.