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Phenolamides (PAs), a diverse group of specialized metabolites, including hydroxycinnamoylputrescine (HP), hydroxycinnamoylagmatine, and hydroxycinnamoyltryptamine, are important in plant resistance to biotic stress. However, the genes involved in the biosynthesis and modulation of PAs have not been fully elucidated. This study identified an HP biosynthetic gene cluster in rice (Oryza sativa) comprising one gene (OsODC) encoding a decarboxylase and two tandem-duplicated genes (OsPHT3 and OsPHT4) encoding putrescine hydroxycinnamoyl acyltransferases coexpressed in different tissues. OsODC catalyzes the conversion of ornithine to putrescine, which is used in HP biosynthesis involving OsPHT3 and OsPHT4. OsPHT3 or OsPHT4 overexpression causes HP accumulation and cell death and putrescine hydroxycinnamoyl acyltransferases (PHT) activity-dependent resistance against the fungal pathogen Magnaporthe oryzae. OsODC overexpression plants also confer enhanced resistance to M. oryzae. Notably, the basic leucine zipper transcription factor APIP5, a negative regulator of cell death, directly binds to the OsPHT4 promoter, repressing its transcription. Moreover, APIP5 suppression induces OsPHT4 expression and HP accumulation. Comparative genomic analysis revealed that the HP biosynthetic gene cluster is conserved in monocots. These results characterized a previously unidentified monocot-specific gene cluster that is involved in HP biosynthesis and contributes to defense and cell death in rice.

Diterpenoids are the major group of antimicrobial phytoalexins in rice1,2. Here, we report the discovery of a rice diterpenoid gene cluster on chromosome 7 (DGC7) encoding the entire biosynthetic pathway to 5,10-diketo-casbene, a member of the monocyclic casbene-derived diterpenoids. We revealed that DGC7 is regulated directly by JMJ705 through methyl jasmonate-mediated epigenetic control3. Functional characterization of pathway genes revealed OsCYP71Z21 to encode a casbene C10 oxidase, sought after for the biosynthesis of an array of medicinally important diterpenoids. We further show that DGC7 arose relatively recently in the Oryza genus, and that it was partly formed in Oryza rufipogon and positively selected for in japonica during domestication. Casbene-synthesizing enzymes that are functionally equivalent to OsTPS28 are present in several species of Euphorbiaceae but gene tree analysis shows that these and other casbene-modifying enzymes have evolved independently. As such, combining casbene-modifying enzymes from these different families of plants may prove effective in producing a diverse array of bioactive diterpenoid natural products. A rice diterpenoid gene cluster is reported that controls the biosynthesis of 5,10-diketo-casbene, a phytoalexin that confers resistance to rice bacterial blight and blast fungus. It recently evolved in the japonica subspecies under artificial selection.

Decoration of phytochemicals contributes to the majority of metabolic diversity in nature, whereas how this process alters the biological functions of their precursor molecules remains to be investigated. Flavones, an important yet overlooked subclass of flavonoids, are most commonly conjugated with sugar moieties by UDP-dependent glycosyltransferases (UGTs). Here, we report that the natural variation of rice flavones is mainly determined by OsUGT706D1 (flavone 7-O-glucosyltransferase) and OsUGT707A2 (flavone 5-O-glucosyltransferase). UV-B exposure and transgenic evaluation demonstrate that their allelic variation contributes to UV-B tolerance in nature. Biochemical characterization of over 40 flavonoid UGTs reveals their differential evolution in angiosperms. These combined data provide biochemical insight and genetic regulation into flavone biosynthesis and additionally suggest that adoption of the positive alleles of these genes into breeding programs will likely represent a potential strategy aimed at producing stress-tolerant plants., In contrast to flavonols, the functions of plant flavones are largely unknown. Here, the authors report the two differentially evolved glucosyltranferases (flavone 7-O-glucosyltransferase and flavone 5-O-glucosyltransferase) determine natural variation of rice flavone accumulation and UV-tolerance.