A moderate change in ambient temperature significantly affects plant physiology including flowering time. MiR399 and its target gene PHOSPHATE 2 (PHO2) are known to play a role in the maintenance of phosphate homeostasis. However, the regulation of flowering time by the miR399-PHO2 module has not been investigated. As we have previously identified miR399 as an ambient temperature-res-ponsive miRNA, we further investigated whether a change in expression of the miR399-PHO2 module affects flowering time in response to ambient temperature changes. Here, we showed that miR399b-overexpressing plants and a loss-of-function allele of PHO2 (pho2) exhibited an early flowering phenotype only at normal temperature (23^oC). Interestingly, their flowering time at lower temperature (16ºC) was similar to that of wild-type plants, suggesting that alteration in flowering time by miR399 and its target PHO2 was seen only at normal temperature (23^oC). Flowering time ratio (16^oC/23^oC) revealed that miR399b-overex-pressing plants and pho2 mutants showed increased sensitivity to ambient temperature changes. Expression analysis indicated that expression of TWIN SISTER OF FT (TSF) was increased in miR399b-overexpressing plants and pho2 mutants at 23^oC, suggesting that their early flo-wering phenotype is associated with TSF upregulation. Taken together, our results suggest that miR399, an am-bient temperature-responsive miRNA, plays a role in am-bient temperature-responsive flowering in Arabidopsis.

Keywords: ambient temperature, flowering time, MiR399, PHO2, TSF