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.