CRISPR knockout of winter-biased SUT4 alters phenology, biomass accrual, and fertility of field-grown hybrid poplar

Woody perennial trees annually transition between photoautotrophic and heterotrophic states. As carbohydrate accumulation and utilization patterns shift, so does the expression of genes encoding various sucrose-proton symporters (SUTs). The poplar genome contains five SUT gene family members involved in intracellular and long-distance sucrose transport. Among these, only the single-copy SUT4 and the SUT5/SUT6 genome duplicates are expressed year-round, with SUT4 strongly expressed in winter. Using CRISPR mutagenesis, we investigated the role of winter-expressed SUTs in carbohydrate allocation, biomass accrual, seasonal phenology, and sexual reproduction in Populus tremula × P. alba under field conditions. In sut4 mutants, fall leaf senescence occurred earlier and spring bud flush was delayed compared to controls, resulting in reduced diameter and overall height growth. These mutants also exhibited altered sugar partitioning and reduced levels of raffinose family oligosaccharides, which are typically associated with cold and desiccation tolerance. Knockouts of SUT5 and SUT6 , whether single or double, did not affect seasonal phenology or overall tree growth. All genotypes flowered precociously after two years in the field, but sut4 catkins accumulated abnormally high levels of sucrose, galactinol, and raffinose. Transcriptome profiling showed down-regulation of genes associated with proanthocyanidin biosynthesis and vacuolar transport, and up-regulation of genes involved in protein oligomerization, misfolding, and refolding responses. Development of sut4 ovules was compromised, and no seeds were produced in sut4 catkins following controlled pollination. Our findings suggest that the single-copy tonoplast SUT4 may be more critical than other SUTs for mitigating stress associated with winter fitness and growth transitions during cool seasons.