Biosynthesis of Phytoalexins and Regulatory Mechanisms of It in Rice

Abstract

We performed extensive functional characterization of diterpenoid phytoalexin biosynthetic genes in rice, and found that the genes for the biosynthesis of the major diterpenoid phytoalexins, phytocassanes and momilactones, are clustered on chromosomes 2 and 4, and that their expression is coordinately induced in rice cells after elicitation. Isopentenyl diphosphate, an early precursor of diterpenoid phytoalexins, was found to be synthesized through the plastidic methylerythritol phosphate pathway. We also found that chitin elicitor receptor kinase OsCERK1 and a mitogen-activated protein kinase cascade, the OsMKK4-OsMPK6 cascade, play essential roles in the elicitor-induced production of diterpenoid phytoalexins. In addition, a basic leucine zipper transcription factor, OsTGAP1, was identified as a key regulator of the coordinated expression of the clustered genes and the methylerythritol phosphate pathway genes. Naringenin 7-O-methyltransferase (OsNOMT) was also identified as a key enzyme in the biosynthesis of another major rice phytoalexin, sakuranetin.

Signals that stop the rot: Regulation of secondary metabolite defences in cereals

“…Flavonoid phytoalexins, such as avenanthramides in oat [23], 3-deoxyanthocyanidins in sorghum [24] and sakuranetin in rice [25] are phenolic metabolites that originate from phenylalanine [26]. Plants rely on these phenols for growth and reproduction, pigmentation and as defensive molecules against pathogens [26].…”

“…Plants rely on these phenols for growth and reproduction, pigmentation and as defensive molecules against pathogens [26]. Terpenoid phytoalexins in rice (oryzalexins, phytocassanes and momilactones) [25] as well as maize (kauralexins and zealexins) [27,28] are synthesised via the isoprenoid pathway. Saponins are glycosylated triterpenoids, protective molecules that are absent in all cereals except the genus Avena and likewise synthesised via the isoprenoid pathway using the precursor mevalonate [23,29,30].…”

“…Many rice diterpenoids have been identified, and one flavonoid, sakuranetin, has been studied in detail [25]. The diterpenoids are clustered into oryzalexins, momilactones, and phytocassanes, based on their carbon skeleton [25].…”

“…Many rice diterpenoids have been identified, and one flavonoid, sakuranetin, has been studied in detail [25]. The diterpenoids are clustered into oryzalexins, momilactones, and phytocassanes, based on their carbon skeleton [25]. M. grisea induces JA biosynthesis in rice and both sakuranetin and the diterpenoids are induced by JA as well as M. grisea infection ( Figure 1A), suggesting that phytoalexin accumulation results from increased JA (Table 1) [42,94,95].…”

“…Notably, the rice genes for the biosynthesis of phytocassanes and momilactones are clustered on chromosomes 2 and 4, which effectively enables a coordinated induction in rice cells after elicitation [25,122]. A key regulator of this coordinated expression is OsTGAP1 (Oryza sativa TGA factor for phytoalexin production 1), which is a basic leucine zipper transcription factor [25].…”