Identification of poplar and spruce GDP-mannose pyrophosphorylases involved in mannan and ascorbic acid biosynthesis

Mannans are hemicellulosic polysaccharides integral to plant growth and development, yet their functions in trees and the regulation of their synthesis remain poorly understood. Here, we investigated the biosynthesis of GDP-mannose (GDP-Man), the primary sugar donor required for mannan synthesis. Using phylogenetic analyses, protein domain characterization, and RNAseq-based expression profiling, putative GDP-mannose pyrophosphorylase (GMP) candidates were identified in poplar and spruce. Functional validation of candidate genes was performed by complementation of the Arabidopsis vtc1-1 mutant, and by generating CRISPR/Cas9 knockout and ectopic overexpression lines in poplar. Our results showed that poplar PtaGMP1 and spruce PgGMP1, successfully complemented the vtc1-1 mutant, restoring mannan content to wild-type levels. In poplar, CRISPR/Cas9 knockouts of PtaGMP1 reduced mannan by 20% and ascorbic acid by 47%, whereas ectopic overexpression of PgGMP1 increased mannan by 14% without significantly affecting ascorbic acid content. In both cases, these changes did not significantly affect tree growth. Overall, our study provides new insights into the regulation of mannan biosynthesis in trees and the contribution of GMPs to both mannan and ascorbic acid production in poplar.