Abstract
Carotenoids are ubiquitous precursors of important metabolites including hormones, such as strigolactones (SLs) and abscisic acid (ABA), and signaling and regulatory molecules, such as the recently discovered zaxinone. Strigolactones and ABA are key regulators of plant growth and development, adaptation to environmental changes and response to biotic and abiotic stress. Previously, we have shown that zaxinone, an apocarotenoid produced in rice by the enzyme zaxinone synthase (ZAS) that is common in mycorrhizal plants, is required for normal rice growth and development, and a negative regulator of SL biosynthesis. Zaxinone is also formed in Arabidopsis, which lacks ZAS, via an unknown route. In the present study, we investigated the biological activity of zaxinone in Arabidopsis, focusing on its effect on SL and ABA biosynthesis. For this purpose, we quantified the content of both hormones and determined the levels of related transcripts in Arabidopsis (Arabidopsis thaliana), roots upon zaxinone treatment. For SL quantification, we also employed Striga seed germination bioassay. Results obtained show that zaxinone application to hydroponically grown Arabidopsis seedlings enhanced transcript levels of key biosynthetic genes of both hormones, led to higher root ABA and SL (methyl carlactonoate, MeCLA) content, and increased SL release, even under sufficient phosphate supply. Using the SL insensitive (max2-1) and the ABA deficient (aba1-6, aba2-1, and nced3) mutants, we also show that zaxinone application reduced hypocotyl growth and that this effect is caused by increasing ABA content. Our results suggest that zaxinone is a regulatory metabolite also in Arabidopsis, which triggers the biosynthesis of both carotenoid-derived hormones, SLs and ABA, in roots. In the non-mycotrophic plant Arabidopsis, zaxinone does not increase growth and may be perceived as a stress signal, while it acts as a growth-promoting metabolite and suppressor of SL biosynthesis in rice.
Highlights
Plants produce hormones and chemical signals, which regulate growth, development, and different physiological processes, mediate interactions with surrounding organisms and coordinate plants response to biotic and abiotic stress stimuli (Chaiwanon et al, 2016)
Our results showed that zaxinone positively regulates the transcript level of two key SL biosynthetic genes (MAX3 and MORE AXILLARY GROWTH 4 (MAX4)) in Arabidopsis under normal and Pi deficient conditions
Strigolactone quantification and studies performed with SL biosynthesis and perception rice mutants suggested that zaxinone is a negative regulator of SL biosynthesis, which represses levels of SL biosynthetic transcripts under Pi deficient conditions, and that the growth promoting effect of this apocarotenoid likely requires functional SL biosynthesis (Wang et al, 2019)
Summary
Plants produce hormones and chemical signals, which regulate growth, development, and different physiological processes, mediate interactions with surrounding organisms and coordinate plants response to biotic and abiotic stress stimuli (Chaiwanon et al, 2016). SLs regulate a series of further developmental processes (AlBabili and Bouwmeester, 2015; Waters et al, 2017), secondary growth, and the establishment of root system architecture, and senescence (Al-Babili and Bouwmeester, 2015; Waters et al, 2017; Jia et al, 2019), and are involved in pathogen defense and abiotic stress response (Van Ha et al, 2014; Decker et al, 2017) Besides these hormonal functions, SLs are released through plant roots as intra-specific communication signal that attracts beneficial mycorrhizal fungi for building the arbuscular mycorrhizal (AM) symbiosis (Akiyama et al, 2005). We set out to investigate whether zaxinone is a regulatory metabolite in the ZAS-free, non-mycotrophic plant A. thaliana
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
