Insights on the SWEET Gene Role in Soluble Sugar Accumulation via the CO2 Fixation Pathway in Forage Maize Under Salt Stress

Rising soil and water salinity endanger plant growth and crop productivity, putting global food security at risk. As plants are sessile, their adaptation to rapidly changing environments is slow, endangering their survival. As a result, mitigation efforts should shift to developing smart crops capable of withstanding dynamic and heterogeneously distributed salinity. Recent breakthroughs in bioinformatics and high throughput genomics can cost-effectively accelerate the introduction of superior varieties for saline regions. Sugar plays an essential role in biomass accumulation and is thus a viable target for forage crop improvement programs. Sugars Will Eventually be Exported Transporter (SWEET) gene family transcribes for source-sink carbon allocation in the form of sugar in higher plants. However, little is known about SWEET's role in maize's phenotypes of agronomic interest for forage production. Here, through a genome-wide analysis, we identified and characterized 19 SWEET genes that are expressed across various shoot phenotypes. Eleven of the genes are salt-responsive, and ZmSWEET7 is most abundant in high-sugar-yielding varieties compared to low-sugar varieties. Homologous overexpression of the ZmSWEET7 increases the maximum quantum yield of photosystem II photochemistry (FvF(M)), CO2 assimilation rate (A), soluble sugar content, and dry matter, with the quantum yield for CO2 fixation efficiency (phiCO(2)) showing the most significant increase. There is a strong positive association between phiCO(2) and soluble sugar content, dry matter, and FV/FM in ZmSWEET7 overexpressing mutants compared to the wild. These findings indicate that ZmSWEET7-mediated CO2 fixation efficiency rather than assimilation rate plays a positive pleiotropic role in C accumulation in the form of sugar or dry matter via increased FvF(M). This work lays a strong foundation for salt-tolerant forage maize genetic improvement.