ZmPMI3 phosphomannose isomerase kinetics is associated with natural variation of watersoluble carbohydrate content in mature maize stem

Mature maize stems accumulate substantial amounts of water-soluble carbohydrates (WSC) providing an accessible carbon pool for industrial bioconversion. However, the genetic factors controlling WSC reserves under field conditions remain poorly resolved. In this study, we quantified total WSC including glucose, fructose, sucrose contents in the internode below the first ear of a genetically diverse maize panel at physiological maturity and associated trait variation with genome-wide SNP data. The panel displayed extensive variation in stem WSCs content, with values ranging up to 50.4 % of dry weight. Genome-wide association mapping identified 20 quantitative trait loci associated with individual sugars or total WSC content. One fructose associated region contained two adjacent gene annotations predicted to encode fragments of a phosphomannose isomerase. Sequence comparison, cDNA amplification and protein structure prediction supported a unified gene model, here designated as ZmPMI3. Biochemical characterization showed ZmPMI3 catalyzes the reversible conversion between mannose-6-phosphate and fructose-6-phosphate, a central reaction in carbohydrate metabolism. Natural protein haplotypes with distinct kinetic properties were identified correlating with significant differences in stem WSC content. Together, these findings identify ZmPMI3 as a candidate contributor to natural variation in maize stem carbohydrate reserves, potentially through altered hexose-phosphate partitioning.