Hexose transporters CsHT3/16 involved in post-phloem transport and affected cucumber fruit development

Hexoses are crucial for plant growth and fruit development. However, the role of hexose transporters in post-phloem sugar transport and maintenance of cellular sugar homeostasis in rapidly growing fruits, such as cucumber, is not yet fully understood. To clarify the impact of hexose transporters in cucumber fruits, we conducted systematic analyses of their tissue expression, localization, transport characteristics and physiological functions. The study revealed that CsHT3, CsHT12 and CsHT16 are the primary hexose transporter genes expressed in cucumber fruit. During the ovary and young fruit stages, CsHT3 and CsHT16 were located in the SE/CC system, but as the cucumber fruit developed and expanded, both transporters shifted to phloem parenchyma cells. The knock-out mutants of CsHT16 display shorter fruits with a larger circumference, likely due to impaired homeostasis of sugars and hormones. Simultaneously reducing the expression of CsHT3, CsHT12 and CsHT16 leads to decreased fruit size. Conversely, overexpression of CsHT3 results in increased fruit size and higher fruit sugar levels. Our data suggest that CsHT16 plays an important role in maintaining sugar homeostasis to shape the fruit, while CsHT3, CsHT12 and CsHT16 together determine the carbohydrates requirement of the enlarged cucumber fruit.