Disruption of the rice <i>4-DEOXYOROBANCHOL HYDROXYLASE</i> unravels specific functions of canonical strigolactones

Abstract

Strigolactones (SLs) regulate many developmental processes, including shoot-branching/tillering, and mediate rhizospheric interactions. SLs originate from carlactone (CL) and are structurally diverse, divided into a canonical and a noncanonical subfamily. Rice contains two canonical SLs, 4-deoxyorobanchol (4DO) and orobanchol (Oro), which are common in different plant species. The cytochrome P450 OsMAX1-900 forms 4DO from CL through repeated oxygenation and ring closure, while the homologous enzyme OsMAX1-1400 hydroxylates 4DO into Oro. To better understand the biological function of 4DO and Oro, we generated CRISPR/Cas9 mutants disrupted in <i>OsMAX1-1400</i> or in both <i>OsMAX1-900</i> and <i>OsMAX1-1400</i>. The loss of OsMAX1-1400 activity led to a complete lack of Oro and an accumulation of its precursor 4DO. Moreover, <i>Os1400</i> mutants showed shorter plant height, panicle and panicle base length, but no tillering phenotype. Hormone quantification and transcriptome analysis of <i>Os1400</i> mutants revealed elevated auxin levels and changes in the expression of auxin-related, as well as of SL biosynthetic genes. Interestingly, the <i>Os900/1400</i> double mutant lacking both Oro and 4DO did not show the observed <i>Os1400</i> architectural phenotypes, indicating their being a result of 4DO accumulation. Treatment of wild-type plants with 4DO confirmed this assumption. A comparison of the <i>Striga</i> seed germinating activity and the mycorrhization of <i>Os900, Os900/1400</i>, and <i>Os1400</i> loss-of-function mutants demonstrated that the germination activity positively correlates with 4DO content while disrupting <i>OsMAX1-1400</i> has a negative impact on mycorrhizal symbiosis. Taken together, our paper deciphers the biological function of canonical SLs in rice and reveals their particular contributions to establishing architecture and rhizospheric communications.