Molecular mechanisms and evolutionary adaptations of transporters for photosynthates and specialized metabolites in...

Sugar transporters are membrane proteins that play a significant role in sugar translocation between adjacent cells in plants, facilitating their growth and development. These are mainly composed of sucrose-proton symporter SUT family members and SWEET family members, each with a different sugar transport mechanism. Their gene expression typically targets the leaf, stem, and grain, which represent the source, transport, and sink organs where carbohydrates are synthesized, distributed, and stored. Photosynthates such as sucrose and starch are essential for plant energy metabolism, whereas specialized metabolites, such as alkaloids, flavonoids, and terpenoids, are involved in plant defense and signaling. At present, the substrate specificity, regulatory mechanisms of these sugar transporters in response to environmental cues and developmental stages are largely unclear. Also, the evolutionary trajectories suggesting selection pressure driving transporter specialization and diversification remain unclear for many families. Therefore, the review examines the structure and function of sugar transporters, as well as the transporters involved in photosynthate mobilization. SWEET proteins facilitate sugar efflux involved in defense and plant-pathogen interaction. MATE and ABC transporters are responsible for metabolite secretion, which also plays a role in defense mechanisms. Additionally, other transporters, such as hexose and starch transporters, are also discussed. In parallel, their responses to exposure to specific environmental cues (biotic and abiotic stress) have also been discussed. The divergence of transporter families among plant lineages and adaptations to environmental stressors are highlighted by evolutionary perspectives. The study also explores how transporter engineering can enhance resilience, crop productivity, and the development of beneficial, specialized metabolites for medicines and nutraceuticals. Finally, it addresses current challenges in transporter research, emphasizing the role of emerging technologies such as omics and CRISPR in advancing our understanding of transporter functionality. This review has discussed the expressional profiles of various sugar transporter genes for sustainability. The review concludes with a roadmap for future research.