Effects of cytokinin on production of diterpenoid phytoalexins in rice

Abstract

IntroductionHigher plants that are attacked by pathogenic microorganisms respond with a variety of defense reactions, including the production of secondary metabolites called phytoalexins, which serve as plant antibiotics.In rice, 15 compounds have been identified as phytoalexins in suspension-cultured cells treated with a biotic elicitor such as a chitin oligosaccharide or a cerebroside [1][2] and/or in leaves either infected with the blast fungus Magnaporthe oryzae or irradiated with UV light. [3][4][5][6][7][8][9][10] Except for the flavonoid sakuranetin, they are all diterpenoids. The rice diterpenoid phytoalexins are classified into four groups based on their basic carbon frameworks: phytocassanes A-E, 8,9,11) oryzalexins A-F, [4][5][6]12) momilactones A and B, 3,13) and oryzalexin S, 7) the major ones being phytocassanes and momilactones.As shown in Fig. 1, the common precursor geranylgeranyl diphosphate (GGDP) is cyclized to ent-copalyl diphosphate (ent-CDP) and then to ent-cassa-12,15-diene and ent-sandaracopimaradiene, leading to phytocassanes A to E and oryzalexins A to F, respectively. GGDP is also cyclized to syn-CDP and then to 9b H-pimara-7,15-diene and stemar-13-ene, leading to momilactones A and B and oryzalexin S, respectively. By utilizing the information from the rice genome database that recently opened to the public, six diterpene cyclases have been demonstrated to be involved in the conversion of GGDP to the four diterpene hydrocarbon precursors via ent-or syn-CDP. ent-CDP synthase (OsCPS2) and syn-CDP synthase (OsCPS4) catalyze the conversion of GGDP to ent-CDP and syn-CDP (14), and ent-cassa-12,15-diene synthase (OsKSL7), ent-sandaracopimaradien synthase (OsKSL10), 9b H-pimara-7,15-diene synthase (OsKSL4), and stemar-13-ene synthase (OsKSL8) catalyze the conversion of ent-CDP or syn-CDP to the four diterpene hydrocarbons ent-cassa-12,15-diene, entsandaracopimaradiene, 9b H-pimara-7,15-diene, and stemar-13-ene, respectively. [15][16][17] The methylerythritol phosphate (MEP) pathway genes have been shown to be responsible for chitin oligosaccharide elicitor-inducible production of diterpenoid phytoalexins in suspension-cultured rice cells, 18) and microsomal P450s and a dehydrogenase have also been shown to be involved in the downstream oxidation of diterpene hydrocarbons. 19) On the other hand, secondary signaling molecules, including plant hormones, have been indicated to be involved in the defense responses caused by pathogen attacks. For example, salicylic acid, jasmonic acid (JA), and ethylene have been investigated extensively and shown to play crucial roles as sec- Tsurumi-ku, Yokohama, Kanagawa 230-0045, Japan (Received November 18, 2009; Accepted May 13, 2010) The production of phytoalexins is a well-documented defense response against pathogen infection in rice. Although jasmonic acid has been suggested to be involved in the production of phytoalexins in both suspensioncultured rice cells and rice leaves, there has been almost no information on other secondary signaling mole...

Should I fight or should I grow now? The role of cytokinins in plant growth and immunity and in the growth–defence trade-off

“…Thus, while SA may not be essential for cytokinin-induced immunity in early defence responses such as scopoletin and capsidiol biosynthesis, it certainly contributes to the overall defence response. In agreement with this, a synergistic action of cytokinins and SA was found to regulate the production of another type of phytoalexin, known as diterpenoid phytoalexins, during defence responses of rice to M. oryzae (Ko et al, 2010;Akagi et al, 2014), and this response is dependent on both SA and the SA signalling regulator WRKY45 (Akagi et al, 2014).…”

WRKY45-dependent priming of diterpenoid phytoalexin biosynthesis in rice and the role of cytokinin in triggering the reaction

“…CK was reported to induce the expression of DP biosynthetic genes and the accumulation of their products in rice (Ko et al 2010). Recently, we showed that the synthetic CK kinetin acts synergistically with the SA signaling pathway to induce PR gene expression in rice (Jiang et al 2013).…”

Biosynthesis, elicitation and roles of monocot terpenoid phytoalexins

“…Jasmonates, ET and salicylic acid (SA) are the hormones whose interactive roles in defense signaling are most often considered, but almost all phytohormones, including abscisic acid, auxins, cytokinins, brassinosteroids and GAs, have been shown to influence monocot responses to biotic attack (De Vleesschauwer et al, 2013). Similar to jasmonates, exogenous application of natural and synthetic cytokinins to rice cells and leaf disks induced accumulation of diterpenoid phytoalexins and transcripts encoding the respective biosynthetic enzymes (Ko et al, 2010). During M. oryzae infection, increased levels of endogenous cytokinins are also detectable, together with transcription of cytokinin-responsive genes sharing the OsRR6 (response regulator 6) promoter (Jiang et al, 2013).…”