Activation of the Jasmonic Acid Pathway by Depletion of the Hydroperoxide Lyase OsHPL3 Reveals Crosstalk between the HPL and AOS Branches of the Oxylipin Pathway in Rice

The allene oxide synthase (AOS) and hydroperoxide lyase (HPL) branches of the oxylipin pathway, which underlie the production of jasmonates and aldehydes, respectively, function in plant responses to a range of stresses. Regulatory crosstalk has been proposed to exist between these two signaling branches; however, there is no direct evidence of this. Here, we identified and characterized a jasmonic acid (JA) overproduction mutant, cea62, by screening a rice T-DNA insertion mutant library for lineages that constitutively express the AOS gene. Map-based cloning was used to identify the underlying gene as hydroperoxide lyase OsHPL3. HPL3 expression and the enzyme activity of its product, (E)-2-hexenal, were depleted in the cea62 mutant, which resulted in the dramatic overproduction of JA, the activation of JA signaling, and the emergence of the lesion mimic phenotype. A time-course analysis of lesion formation and of the induction of defense responsive genes in the cea62 mutant revealed that the activation of JA biosynthesis and signaling in cea62 was regulated in a developmental manner, as was OsHPL3 activity in the wild-type plant. Microarray analysis showed that the JA-governed defense response was greatly activated in cea62 and this plant exhibited enhanced resistance to the T1 strain of the bacterial blight pathogen Xanthomonasoryzaepvoryzae (Xoo). The wounding response was attenuated in cea62 plants during the early stages of development, but partially recovered when JA levels were elevated during the later stages. In contrast, the wounding response was not altered during the different developmental stages of wild-type plants. These findings suggest that these two branches of the oxylipin pathway exhibit crosstalk with regards to biosynthesis and signaling and cooperate with each other to function in diverse stress responses.

脂氧素通路的丙二烯氧化物合酶（allene oxide synthase, AOS）与氢过氧化物裂解酶（hydroperoxide lyase, HPL）分支，分别介导茉莉酸类物质（jasmonates）与醛类的合成，在植物应对多种胁迫的生理过程中发挥功能。已有研究推测这两条信号分支间存在调控串扰，但尚未有直接实验证据证实该假说。本研究通过筛选水稻T-DNA插入突变体库中组成型表达AOS基因的株系，成功鉴定并表征了一株茉莉酸（jasmonic acid, JA）过量积累突变体cea62。借助图位克隆技术，我们将该突变体的核心调控基因鉴定为氢过氧化物裂解酶OsHPL3。cea62突变体中HPL3的表达水平及其编码产物(E)-2-己烯醛的酶活性均显著降低，这一变化引发了JA的显著过量积累、JA信号通路的激活以及类病斑表型的出现。对cea62突变体的病斑形成过程与防御响应基因诱导情况开展时间进程分析后发现，cea62中JA的生物合成与信号通路激活呈现发育依赖性调控模式，这与野生型植株中OsHPL3的活性变化规律一致。芯片分析（microarray analysis）结果显示，JA介导的防御响应在cea62中被显著激活，该植株对水稻白叶枯病菌（Xanthomonasoryzaepvoryzae, Xoo）的T1菌株表现出增强的抗性。cea62植株在发育早期的伤响应（wounding response）受到抑制，但在后期随着JA水平升高，伤响应得到部分恢复；而野生型植株的伤响应在不同发育阶段均未发生明显改变。上述研究结果表明，脂氧素通路的这两个分支在生物合成与信号转导层面存在串扰，并协同参与植物应对多种胁迫的生理过程。