Abstract
Phytoalexins are inducible secondary metabolites possessing antimicrobial activity against phytopathogens. Rice produces a wide array of phytoalexins in response to pathogen attacks and environmental stresses. With few exceptions, most phytoalexins identified in rice are diterpenoid compounds. Until very recently, flavonoid sakuranetin was the only known phenolic phytoalexin in rice. However, recent studies have shown that phenylamides are involved in defense against pathogen attacks in rice. Phenylamides are amine-conjugated phenolic acids that are induced by pathogen infections and abiotic stresses including ultra violet (UV) radiation in rice. Stress-induced phenylamides, such as N-trans-cinnamoyltryptamine, N-p-coumaroylserotonin and N-cinnamoyltyramine, have been reported to possess antimicrobial activities against rice bacterial and fungal pathogens, an indication of their direct inhibitory roles against invading pathogens. This finding suggests that phenylamides act as phytoalexins in rice and belong to phenolic phytoalexins along with sakuranetin. Phenylamides also have been implicated in cell wall reinforcement for disease resistance and allelopathy of rice. Synthesis of phenolic phytoalexins is stimulated by phytopathogen attacks and abiotic challenges including UV radiation. Accumulating evidence has demonstrated that biosynthetic pathways including the shikimate, phenylpropanoid and arylmonoamine pathways are coordinately activated for phenolic phytoalexin synthesis, and related genes are induced by biotic and abiotic stresses in rice.
Highlights
Environmental stresses and plant diseases are undoubtedly critical factors in crop production and associated losses [1,2]
Transcriptomic analysis of ultra violet (UV)-treated rice leaves showed that four putative OMTs (Os11g19840, Os09g17560, Os08g06100 and Os04g01470) similar to flavonoid OMTs and caffeate O-methyltransferase (COMT) are up-regulated in response to UV irradiation prior to accumulation of sakuranetin and phenylamide phytoalexins, which suggests their possible involvement in phenolic phytoalexin synthesis in rice [22]
It has long been known that rice phytoalexins are mostly diterpenoid compounds
Summary
Environmental stresses and plant diseases are undoubtedly critical factors in crop production and associated losses [1,2]. Recent studies have shown that several phenylamides (amine-conjugated phenolic compounds) play a role as defense related agents exhibiting antimicrobial activity against rice pathogens [20,21,22,23]. This observation suggests that, along with sakuranetin, phenylamides are members of phenolic phytoalexins in rice. A group of defense-related phenolic compounds possessing antimicrobial activity was recently identified from pathogen-infected and UV-treated rice leaves [20,21,22,23]. Amine moieties found in rice phenylamide phytoalexins include arylmonoamines tyramine, tryptamine and serotonin (Figure 1)
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