Responses to higher temperature
Under 16 °C conditions, miR172 expression is reduced and, consequently, the expression of its target genes, TOE1, TOE2, and SMZ, is increased [78]. Recent reports have revealed the mechanisms by which lower temperature reduces miR172 levels 51 and 52. Post-transcriptional processing of primary miR172 (pri-miR172: miR172 transcript that has a 5′ cap and polyadenosine tail) transcripts to mature miR172 plays a major role [52]. Even though the accumulation of miR172 is higher at 23 °C, the levels of pri-miR172 and precursor-miR172 (pre-miR172: approximately 70 bp miR172 precursor that is cut out from the pri-miR172) transcripts are not drastically altered by changing temperatures (16 °C and 23 °C). In the pri-miR172b overexpressors, mature miR172 levels still show temperature-dependent differences, but in the pre-miR172 overexpressor, miR172 levels are similar between 16 °C and 23 °C. These findings indicate that the pri-miR172 to pre-miR172 processing step is modulated by ambient temperature. In this step, FCA, an RNA-binding protein that has a central role in ambient temperature and autonomous pathways [73], directly binds to pri-miR172 transcripts in a nonsequence-specific manner and positively regulates miR172 processing [52]. At 16 °C FCA transcripts and FCA proteins are less abundant than at 23 °C [52]. Ambient temperature may also regulate the transcription of the MIR172a gene. miR172 levels are negatively regulated by SVP 51 and 52, and SVP directly binds to CArG motifs in the MIR172a promoter [51]. Another miRNA may also be involved in flowering time regulation: miR399 is reduced at 16 °C and is also regulated by FCA [52]. The miR399-target gene, PHOSPHATE2 (PHO2), which functions in the maintenance of phosphate homeostasis, modulates flowering by controlling TSF expression [79].