Novel roles of HSFs and HSPs, other than relating to heat stress, in temperature-mediated flowering

Abstract

The thermotolerant ability of heat shock factors (HSFs) and heat shock proteins (HSPs) in plants has been shown. Recently, focus has been on their function in plant growth and development under non-stress conditions. Their role in flowering has been suggested given that lower levels of HSF/HSPs resulted in altered flowering in Arabidopsis. Genetic and molecular studies of Arabidopsis HSF/HSP mutants advocated an association with temperature-mediated regulation of flowering, but the fundamental genetic mechanism behind this phenomenon remains obscure. Here we outline plausible integration between HSFs/HSPs and temperature-dependent pathways in plants regulating flowering. Moreover, we discuss how similar pathways can be present in thermoperiodic geophytic plants that require ambient high temperatures for flowering induction.

Keywords: Ambient temperature; flowering; geophytes; heat shock proteins; heat shock transcription factors; non-stress.

Fig. 1.

Illustrative representation of the genetic control underlying temperature-mediated flowering in Arabidopsis under non-stress conditions. The terrestrial ecosystem has undergone constant evolutionary changes that explain their survival (Wang et al., 2018). Several plants have demonstrated a temperature-mediated vegetative to reproductive phase transition to prevent unintentional flowering under abruptly changed environmental conditions. Few flowering pathway genes are engaged in this process, but the signalling mechanisms remain unexplored. Heat shock proteins (HSPs) and heat shock transcription factors (HSFs) are stimulated for flowering initiation under certain temperature variations. This figure is an attempt to interpret the involvement of HSFs/HSPs in temperature-mediated flowering and elucidate the genetic mechanism behind it. Ambient high temperature accelerates HSP101, HSP90 and HSP70 expression and regulates flowering via a refined mechanism consisting of various modules. Enhanced HSP101 suppresses SVP and FLC as direct or indirect downstream targets to initiate flowering. Furthermore, repressed FLC and SVP suppress a floral integrator FT (FLOWERING LOCUS T) that induces a florigen signal (Qin et al., 2021). HSP90 functions as a molecular chaperone and complexes with downstream floral integrators LFY, SOC1 and AGL24 that promote flowering (Margaritopoulou et al., 2016). In addition, apart from high ambient temperature, vernalization accelerates flowering in Arabidopsis. Low temperature inhibits AtHSFB2b expression further downregulating VIN3 (VERNALIZATION INSENSITIVE 3) and positively affecting PRC2 (Polycomb Repressive Complex 2) (Jeong et al., 2022). FLC and FT act as direct downstream targets of PRC2, and their decreased expression encourages the reproductive phase transition and flower development (Amasino and Michaels, 2010).

Fig. 2.

Temperature mediates the floral transition of saffron. (A) Ambient high temperatures during floral transition induce while low temperatures suppress flowering in saffron. (B) Expression profiling (heat map representation) of HSFs and HSPs between flowering and non-flowering apical meristem in saffron during floral induction.