The Role of Potassium and KUP/KT/HAK Transporters in Regulating Strawberry Fruit Development

Abstract

Potassium is the most abundant macronutrient in plants participating in essential physiological processes such as turgor maintenance. A reduction in cell turgor is a hallmark of the ripening process associated with fruit softening. The dynamic of K+ fluxes in fleshy fruits is largely unknown; however, the reallocation of K+ into the apoplast has been proposed as a contributing factor to the decrease in fruit turgor contributing to fruit softening. High-affinity K+ transporters belonging to the KUP/HT/HAK transporter family have been implicated in this process in some berry fruits such as grape. The main objective of this study was to characterize the role of KUP/KT/HAK genes in strawberry fruit development. A genome-wide analysis of the KUP/KT/HAK family of high-affinity K+ transporters in strawberry (Fragaria × ananassa Duch.) was conducted, identifying 60 putative transporter genes. The chromosomal distribution of the FaKUP gene family, phylogenetic relationship and structure of predicted proteins were thoroughly examined. Transcriptomic profiling revealed the expression of 19 FaKUP genes within the fruit receptacle, with a predominant downregulation observed during ripening. This pattern suggests their functional relevance in early fruit development and turgor maintenance. Mineral composition analyses confirmed that K+ is the most abundant macronutrient in strawberry fruits, exhibiting a slight decrease as ripening progressed. Electrophysiological assessments of membrane potential (Em) and diffusion potentials (ED) at increasing external K+ concentrations in parenchymal cells of green and white fruits revealed progressive membrane depolarization and suggest a significant diminution in cytosolic K+ levels in white compared to green fruits. Furthermore, the slope of change of ED at increasing external K+ concentration indicated a lower K+ permeability of the plasma membrane in white fruits, aligning with transcriptomic data. To further characterize the role of these transporters in strawberry fruit ripening, the coding sequences of FaKUP17 and FaKUP50 were cloned into a binary vector under the control of a fruit specific promoter from the UDP-glucose:flavonoid 3-O-glucosyltransferase (UFGT) gene. Several transgenic strawberry plants were obtained through Agrobacterium tumefaciens infection. Fruits from these transgenic lines are currently under evaluation. This study provides critical insights into the regulatory mechanisms of K+ transport during strawberry ripening and identifies potential targets for genetic modifications aimed at enhancing fruit firmness and shelf life.