Zaxinone and its Mimics: New Frontier in Apocarotenoid-Mediated Growth Regulation and Crop Improvement

The discovery of the apocarotenoid growth regulator zaxinone has fundamentally expanded the current understanding of plant growth, development, and hormone signaling. Synthesized in rice by Zaxinone Synthase (ZAS), this metabolite functions as a regulator of plant physiology, metabolism and hormone homeostasis, thereby modulating plant growth, architecture and rhizosphere communication. Recent genetic, structural and biochemical analyses have revealed its impact on primary metabolism, interference with hormonal signaling and role in determining the level of key plant hormones, including strigolactones, cytokinins and abscisic acid. Exogenous application and genetic approaches, including OsZAS1 overexpression, have demonstrated that elevated endogenous zaxinone levels promote rice growth and productivity. These studies also unveiled the complex role of zaxinone in regulating the establishment and development of arbuscular mycorrhization. To overcome the limited accessibility of zaxinone, highly efficient mimics of zaxinone (MiZax) have been developed and evaluated for their bioactivity and utility for agricultural applications. MiZax have been shown to retain the key biological activities and physiological responses of zaxinone, including the promotion of crop growth and productivity, enhancement of fertilizer usage efficiency, suppression of root parasitic weed infestation, as well as transient effects on root-associated microbial communities. This Expert View provides insights into how the discovery of zaxinone expands our conceptual landscape of plant growth substances, lays the foundation for a new type of biostimulants, and identifies the critical open questions for guiding future research into apocarotenoid-mediated growth regulation.