Response of Rice to Insect Elicitors and the Role of OsJAR1 in Wound and Herbivory-Induced JA-lle Accumulation

Plants produce jasmonic acid （JA） and its amino acid conjugate, jasmonoyI-L-isoleucine （JA-Ile） as major defense signals in response to wounding and herbivory. In rice （Oryza sativa）, JA and JA-Ile rapidly increased after mechanical damage, and this increase was further amplified when the wounds were treated with oral secretions from generalist herbivore larvae, lawn armyworms （Spodoptera mauritia）, revealing for the first time active perception mechanisms of herbivore-associated elicitor（s） in rice. In the rice genome, two OsJAR genes can conjugate JA and lie and form JA-Ile in vitro; however, their function in herbivory- induced accumulation of JA-Ile has not been investigated. By functional characterization of TOS17 retrotransposon-tagged Osjarl plants and their response to simulated herbivory, we show that OsJAR1 is essential for JA-Ile production in herbivore-attacked, field-grown plants. In addition, OsJAR1 was required for normal seed development in rice under field conditions. Our results suggest that OsJAR1 possesses at least two major functions in rice defense and development that cannot be complemented by the additional OsJAR2 gene function, although this gene previously showed overlapping enzyme activity in vitro.

Molecular evidence for biochemical diversification of phenolamide biosynthesis in rice plants

Two phenolamides （PAs）, p-coumaroylputrescine and feruloylputrescine strongly accumulate in rice （Oryza sativa cv. Nipponbare） leaves subjected to attack of chewing and sucking herbivores. Here we identified and characterized in vitro three novel rice genes that mediated coumaroyI-CoA/ feruloyI-CoA conjugation to polyamines, putrescine and agmatine. Interestingly, two genes were highly specific for their polyamine substrates, encoding putresdne N-hydrox- ycinnamoyltransferase and agmatine N-hydroxycinnamoyl- transferase, while the third enzyme could use both polyamines and it was therefore annotated as putrescine/ agmatine N-hydroxycinnamoyltransferase. All genes were preferentially expressed in rice roots and developing flowers, and in addition, the putrescine/agmatine N-hydroxycinnamoyl- transferase transcripts were strongly induced by wounding in the young rice leaves. Because the wound response of this gene was only partially suppressed in the jasmonoyI-L-isoleucine deficient plants （Osjarl）, it suggests that its upregulation （as well as inducible PAs in rice） may be largely independent of jasmonoyI-L-isoleucine signaling pathway. The finding of three closely related genes with a similar and/or overlapping activity in PA biosynthesis provides another striking example of rapid diversification of plant metabolism in response to environmental stresses in nature.

Farinose fiavonoids are associated with high freezing tolerance in fairy primrose (Primula malacoides) plants

The deposition of surface （farinose） flavonoids on aerial parts of some Primula species is a well-documented but poorly understood phenomenon. Here, we show that flavonoid deposition on the leaves and winter buds may contribute strongly to preventing freezing damage in these plants. The ice nucleation temperature of fairy primrose （Primula malacoides） leaves covered with natural flavone was approximately 6~C lower compared to those that had their flavone artificially removed. Additionally, farinose flavonoids on the leaves reduced subse- quent electrolyte leakage （EL） from the cells exposed to freezing temperatures. Interestingly, exogenous application of flavone at 4 mg/g fresh weight to P. malacoides leaves, which had the original flavone mechanically removed, restored freezing tolerance, and diminished EL from the cells to pretreatment values. Our results suggest that farinose flavonoids may function as mediators of freezing tolerance in P. malacoides, and exogenous application of flavone could be used to reduce freezing damage during sudden but predictable frost events in other plant species.

Integrated view of plant metabolic defense with particular focus on chewing herbivores

Success of plants largely depends on their ability to defend against herbivores.Since emergence of the first voracious consumers,plants maintained adapting their structures and chemistry to escape from extinction.The constant pressure was further accelerated by adaptation of herbivores to plant defenses,which all together sparked the rise of a chemical empire comprised of thousands of specialized metabolites currently found in plants.Metabolic diversity in the plant kingdom is truly amazing,and although many plant metabolites have already been identified,a large number of potentially useful chemicals remain unexplored in plant bioresources.Similarly,biosynthetic routes for plant metabolites involve many enzymes,some of which still wait for identification and biochemical characterization.Moreover,regulatory mechanisms that control gene expression and enzyme activities in specialized metabolism of plants are scarcely known.Finally,understanding of how plant defense chemicals exert their toxicity and/or repellency against herbivores remains limited to typical examples,such as proteinase inhibitors,cyanogenic compounds and nicotine.In this review,we attempt summarizing the current status quo in metabolic defense of plants that is predominantly based on the survey of ubiquitous examples of plant interactions with chewing herbivores.