Zaxinone, a Natural Apocarotenoid, Regulates Growth and Strigolactone Biosynthesis in Rice

Abstract

Carotenoids are the precursor of several metabolites with regulatory functions, which include the plant hormones abscisic acid (ABA) and strigolactones (SLs), and signaling molecules, such as β-cyclocitral. These carotenoid-derivatives originate from oxidative breakdown of the double bond resulting in carbonyl cleavage-products designated as apocarotenoids. The cleavage reaction causing apocarotenoid formation is catalyzed frequently by Carotenoid Cleavage Dioxygenases (CCDs). Several lines of evidence indicate the presence of yet unidentified apocarotenoids with regulatory or signaling function. Here, we first characterized the biological functions of the apocarotenoid zaxinone formed by ZAXINONE SYNTHASE (ZAS), a member of an overlooked, widely distributed plant CCD clade. The loss-of-function rice zas mutant contains less zaxinone, exhibiting retarded growth with elevated levels of SLs that determines plant architecture, mediates mycorrhization, and facilitates the germination of root parasitic seeds, such as Striga spp. The zaxinone treatment rescued zas phenotypes, down-regulated SL biosynthesis and release, and enhanced root growth in the wild-type rice seedlings. Next, we performed multi-omics analysis demonstrating zaxinone increased sugar metabolism and induced photosynthesis in a manner that led to phenotypical changes in rice roots. Besides, transcriptome analysis showed that zaxinone upregulated CYTOKININ GLUCOSYLTRANSFERASES expression in roots, which might explain the increase in the apex and meristem length, and in the number of cellular layers. Finally, the investigation of zaxinone biology and the utilization of its application is constrained by its laborious organic synthesis and low abundance in natural sources. Therefore, we developed easy-to-synthesize and highly efficient Mimic of Zaxinone (MiZax), based on the structure-activity-relationship study using a series of apocarotenoids. Activity-based experiments unraveled MiZax3 and MiZax5 were at least as active as zaxinone in rescuing root phenotypes of the zas mutant, promoting root growth in wild-type seedlings, and reducing SL biosynthesis and release. Taken together, zaxinone is a key regulator of rice growth and development, which regulates sugar metabolism, suppresses SL biosynthesis, fine-tunes cytokinins level, and modulates biotic interactions with arbuscular mycorrhizal (AM) fungi. Our work also provides easy-to-synthesize mimics for illuminating zaxinone biology and as a tool to improve crop growth and reduce the infestation by Striga hermonthica, a severe threat to food security worldwide.