Changes in leaf sugar content during sink transition by low temperature exposure in Phalaenopsis orchids

Phalaenopsis orchids require a prolonged period of low temperatures for flowering, which is highly correlated with soluble sugar contents in leaves. This study was conducted to investigate changes in leaf sugar content during sink transition in Phalaenopsis. Phalaenopsis Queen Beer ‘Mantefon’ clones were grown at 28 °C, followed by exposure to 20 °C for floral induction. Leaves were sampled after 0, 2, 6, and 10 weeks of the low temperature (LT) treatment for analysis of soluble sugar content and RNA-seq. Exogenous sucrose labeled with a stable carbon isotope was applied to mature leaves. Inflorescences did not yet emerge after two weeks, but they were 0.5–1 cm and 5–10 cm long after 6 and 10 weeks, respectively. The carbon isotope analysis revealed that leaf sugars were translocated from leaves to inflorescences during the floral induction, rather than vegetative organs such as newly developing leaves. After two weeks of LT, there was a considerable accumulation of sucrose in leaves, which subsequently decreased as the inflorescences developed. During the LT period, the expression of sucrose-phosphate synthase (SPS) significantly increased, whereas that of some members of the SWEET family, sugar transporters, was suppressed before inflorescence initiation. As the inflorescence initiated and elongated, the expression of SWEET family members increased again. These results indicated that exposure to low temperatures triggered sink transition and sugar accumulation in leaves, which were then translocated and utilized for inflorescence development. This finding implies a significant association between leaf sugars and floral induction, highlighting their pivotal roles in the flowering process of Phalaenopsis.