Responses to higher temperature
Unlike lower temperature effects, higher temperatures (27 °C) promote flowering with increased FT expression 60 and 80. Recent studies have indicated that PIF4 is the main regulator for higher temperature-induced morphological changes, including floral transition [81] (Figure 4). Under SD conditions the flowering time of the pif4 mutant at 27 °C occurs at almost the same time as it does at 23 °C, whereas the flowering of wild type plants at 27 °C is accelerated with elevated FT expression. In addition, PIF4 protein directly activates FT expression by binding to the FT promoter at 27 °C under SD conditions, and PIF4 expression increases as temperature increases [60]. However, variations in PIF4 expression under different temperature conditions are not sufficient to explain the flowering phenomenon at high temperatures. The histone H2A variant H2A.Z mediates temperature signals in Arabidopsis and plays a crucial role in temperature-dependent FT expression by PIF4 through modulating the accessibility of the PIF4-binding site at the FT promoter 60 and 82. Indeed, the occupancy of H2A.Z nucleosomes on the FT promoter is decreased at high temperatures, whereas the binding of PIF4 to the FT promoter is increased, indicating that the presence of H2A.Z nucleosomes are limiting for binding of PIF4 to FT [60] (Figure 4). At 27 °C, the level of miR172 is also higher than at 23 °C [51], and this change may decrease the amount of transcripts of AP2-related FT repressors under these conditions. As described in this section, the same regulatory modules are used for processing different external (photoperiods and temperatures) and internal (development and hormone) information to optimize the timing of flowering.