Dual roles of pea CCD7 in strigolactone-dependent and -independent processes as revealed by transcriptomic profiling and mycorrhization assays

Strigolactones (SLs) are carotenoid-derived molecules that act as both rhizosphere signals and plant hormones. Although they play pivotal roles in plant development and signaling, our understanding of their functions, particularly in response to environmental stresses, remains limited. Moreover, the SL biosynthetic enzyme CAROTENOID CLEAVAGE DIOXYGENASE 7 (CCD7) has been proposed to exert SL-independent biological roles, but experimental evidence is lacking. To address this, we analyzed root transcriptomic profiles of the wild type (WT) pea cultivar 'Tere`se' and the two near-isogenic mutants rms5, defective in CCD7, and rms1, defective in CCD8. Gene Ontology enrichment analysis revealed that SL deficiency suppresses stress-related processes, including responses to salt, heat, and hydrogen peroxide, as well as cellular functions related to cell cycle progression, cell division, and protein and amino acid metabolism. LC-MS/MS profiling revealed significant crosstalk with stress-related hormones, with SL levels positively correlated with abscisic acid and negatively correlated with jasmonic acid. Analysis of mutant-specific transcriptomic changes indicated that CCD7 contributes to several biological processes beyond its role in SL biosynthesis, including light-related functions, apocarotenoid metabolism, and mycorrhization. Colonization assays further demonstrated a SL-independent positive role of CCD7 in mycorrhization, as rms5 mutants exhibited reduced fungal colonization compared to rms1 and WT. Overall, our findings reveal a broad involvement of SLs in plant stress adaptation and uncover additional CCD7mediated pathways beyond canonical SL biosynthesis, which are potentially linked to the accumulation of 9-cis beta-carotene-derived apocarotenoids or alternative CCD7 substrates.