Hypothesised molecular model of flowering in C. pallens
This study showed a strong conservation of the activity of PEBP-like gene family members as floral promoters in C. pallens . The transcriptomic analysis further suggested that C. pallensundergoes flowering through the interaction of external and internal signals which can be integrated at the CpATFL1 locus to initiate flowering.
We suggest that flowering in the masting plant C. pallens is a coordinated two-step process (Fig. 5). As shown in B. distachyon , the regulatory loop controlled by the vernalisation loci mediating flowering-time is also conserved in C. pallens. This is the first stage of flowering control in C. pallens , where every year cold winter temperatures may suppress the activity of CpVRN2 by increasing the expression of CpVRN1 . The suppression ofCpVRN2 remains constant post-vernalisation, thereby leading to the elevated expression of FT -like genes in the spring each year. However, an increase in the FTs does not necessarily correlate with flowering in the next season, as C. pallens flowers intermittently. In the second stage, only plants with sufficient internal cues (higher gibberellin and sucrose levels, and competency to undergo the floral transition) can respond to the warmer summer temperatures. Warmer summer temperatures induce activation of the thermosensory genes (PIF4/5), consequently leading to a greater expression of CpATFL1. Elevated expression of CpATFL1leads on to the floral transition and flowering. This two-stage process almost certainly requires additional epigenetic factors acting onCpATFL1 to control the reproductive transition. Alternatively, plants with insufficient cues may lead to re-activation ofCpVRN2 , thus allowing plants to remain vegetative for the next season. Further analysis based on CHiP-seq may allow the identification of the specific epigenetic genes and the corresponding methylation changes in the histones to activate the flowering process in response to the environmental cues. This model also supports the known ecological models hypothesized to control flowering through changes in summer temperatures (Kelly et al., 2013, Samarth et al., 2020; Schauber et al., 2002).
Because of the potential impacts of global climate change on masting phenology (Monks et al., 2016), and the potential downstream impacts on introduced and native fauna (Griffiths & Barron, 2016), it is becoming increasingly important to predict masting years accurately. Such predictions are strongest when they are based on a detailed mechanistic understanding of the underlying control mechanisms. The current study has identified significant molecular regulators of flowering in C. pallens which can be used to explore changes in flowering gene expression under altered climates. This will allow the design of new masting models with greater confidence and to understand how mast flowering may change in the face of climate change.