Abscisic acid (ABA) is an important carotenoid-derived phytohormone that plays essential roles in plant response to biotic and abiotic stresses as well as in various physiological and developmental processes. In Arabidopsis, ABA biosynthesis starts with the epoxidation of zeaxanthin by the ABA DEFICIENT 1 (ABA1) enzyme, leading to epoxycarotenoids; e.g., violaxanthin. The oxidative cleavage of 9-cis-epoxycarotenoids, a key regulatory step catalyzed by 9-CIS-EPOXYCAROTENOID DIOXYGENASE, forms xanthoxin, which is converted in further reactions mediated by ABA DEFICIENT 2 (ABA2), ABA DEFICIENT 3 (ABA3), and ABSCISIC ALDEHYDE OXIDASE 3 (AAO3) into ABA. By combining genetic and biochemical approaches, we unravel here an ABA1-independent ABA biosynthetic pathway starting upstream of zeaxanthin. We identified the carotenoid cleavage products (i.e., apocarotenoids, β-apo-11-carotenal, 9-cis-β-apo-11-carotenal, 3-OH-β-apo-11-carotenal, and 9-cis-3-OH-β-apo-11-carotenal) as intermediates of this ABA1-independent ABA biosynthetic pathway. Using labeled compounds, we showed that β-apo-11-carotenal, 9-cis-β-apo-11-carotenal, and 3-OH-β-apo-11-carotenal are successively converted into 9-cis-3-OH-β-apo-11-carotenal, xanthoxin, and finally into ABA in both Arabidopsis and rice. When applied to Arabidopsis, these β-apo-11-carotenoids exert ABA biological functions, such as maintaining seed dormancy and inducing the expression of ABA-responsive genes. Moreover, the transcriptomic analysis revealed a high overlap of differentially expressed genes regulated by β-apo-11-carotenoids and ABA, suggesting that β-apo-11-carotenoids exert ABA-independent regulatory activities. Taken together, our study identifies a biological function for the common plant metabolites, β-apo-11-carotenoids, extends our knowledge about ABA biosynthesis, and provides new insights into plant apocarotenoid metabolic networks.

脱落酸 (ABA) 是一种重要的类胡萝卜素衍生植物激素，在植物对生物和非生物胁迫的反应以及各种生理和发育过程中起重要作用。在拟南芥中，ABA 的生物合成始于 ABA DEFICIENT 1 (ABA1) 酶对玉米黄质的环氧化，从而产生环氧类胡萝卜素；例如，紫黄质。9-顺式环氧类胡萝卜素的氧化裂解，9-顺式催化的关键调控步骤-环氧类胡萝卜素二加氧酶形成黄素，其在 ABA 缺乏 2 (ABA2)、ABA 缺乏 3 (ABA3) 和脱落醛氧化酶 3 (AAO3) ​​介导的进一步反应中转化为 ABA。通过结合遗传和生化方法，我们在此解开了从玉米黄质上游开始的不依赖 ABA1 的 ABA 生物合成途径。我们鉴定了类胡萝卜素裂解产物（即，类胡萝卜素、β-apo-11-胡萝卜素、9 - cis -β-apo-11-carotenal、3-OH-β-apo-11-carotenal 和 9- cis -3- OH-β-apo-11-carotenal) 作为这种不依赖 ABA1 的 ABA 生物合成途径的中间体。使用标记的化合物，我们发现 β-apo-11-carotenal、9 - cis -β-apo-11-carotenal 和 3-OH-β-apo-11-carotenal 依次转化为 9- cis-3-OH-β-apo-11-胡萝卜素、黄素，最后在拟南芥和水稻中转化为 ABA。当应用于拟南芥时，这些 β-apo-11-类胡萝卜素发挥 ABA 生物学功能，例如维持种子休眠和诱导 ABA 反应基因的表达。此外，转录组学分析显示受 β-apo-11-类胡萝卜素和 ABA 调控的差异表达基因高度重叠，表明 β-apo-11-类胡萝卜素发挥非 ABA 的调节活性。总之，我们的研究确定了常见植物代谢物 β-apo-11-类胡萝卜素的生物学功能，扩展了我们对 ABA 生物合成的了解，并为植物类胡萝卜素代谢网络提供了新的见解。