Introduction
Mast seeding (or masting) is synchronous highly variable seed production
among years by a population of perennial plants (Kelly, 1994; Kelly,
Turnbull, Pharis, & Sarfati, 2008; Schauber et al., 2002). This results
in irregular heavy flowering and seeding events, which occur in a range
of taxa globally, including in various woody and herbaceous endemic
species in the New Zealand flora (Webb & Kelly, 1993; Schauber et al.,
2002; Kelly et al., 2008). A key question is what external and internal
factors allow the plants to synchronously trigger heavy reproduction in
only some years. Better understanding of those factors, to allow
prediction of changes under global climate change (Kelly et al., 2013;
McKone, Kelly, & Lee, 1998; Rees, Kelly, & Bjornstad, 2002) requires
clarification of the underlying genetic mechanisms which control masting
(Samarth, Kelly, Turnbull, & Jameson, 2020).
Although masting imposes costs, such as missed opportunities for
reproduction, it is selectively favoured in plants which gain benefits
from one or more Economies of Scale (EOS) (Kelly, 1994; Kelly & Sork,
2002). The two most common EOSs are predator satiation (where seed
predators are not able to consume all the seed production, ensuring
higher survival of the offspring) or more efficient wind pollination
(Kelly & Sork, 2002). In order for masting to occur, plants need some
synchronising factor, typically a weather cue. Samarth et al. (2020)
have suggested that a likely cue for masting comes from seasonal changes
in summer temperature, so it has been speculated that increases in
global temperatures may alter masting behaviour, although the nature of
this effect is uncertain (Kelly et al., 2013; Monks, Monks, &
Tanentzap, 2016). Changes in masting would affect the wider community,
potentially impacting on food availability for indigenous seed predators
and the rest of the food chain. Molecular studies, such as Kobayashi et
al. (2013), have the potential to improve mechanistic understanding that
underpins forecasting of changes in mast flowering behaviour. That in
turn can help show how changes in natural conditions may lead to
evolution of flowering-time genes and associated regulatory mechanisms.
However, there is currently very little molecular evidence on the
mechanisms for temperature-driven mast flowering in plants.
Information from model plant species provides useful background to the
special case of mast seeding species. Molecular and genetic approaches
have revealed that various external cues interact with the developmental
processes to regulate the floral transition in perennial plants (Khan,
Ai, & Zhang, 2014; Kobayashi et al., 2013). Genetic pathways
controlling flowering time in model crops and temperate grasses,
including Arabidopsis (Arabidopsis thaliana ), tomato, apple,
rice, barley, wheat and Brachypodium distachyon (purple false
brome), show a high degree of conservation between dicot and monocot
species (Shrestha, Gomez-Ariza, Brambilla, & Fornara, 2014). Both
dicots and monocots share common floral integrator genes including
homologues of florigen, the universal flowering hormone. Florigen, or
FLOWERING LOCUS T (FT) is a 175 amino acid long protein belonging to thep hosphatidyl e thanolamine b indingp rotein (PEBP) family, an evolutionarily conserved protein
family found in all taxa of organisms from bacteria to animals and
plants (Karlgren et al., 2011). Phylogenetic analysis of different
homologues of the PEBP gene sequences across the plant kingdom has
revealed three sub-families. These are MOTHER OF FT AND TFL (MFT), FT
and TERMINAL FLOWER 1 (TFL1) (Karlgren et al., 2011). FT and TFL1
protein sequences share 60% homology with highly conserved amino acid
sequences across diverse species. These genes, however, act
antagonistically to each other: FT promotes flowering whereas TFL1
represses it (Liu, Yang, Wei, & Wang, 2016).
In the current study, molecular tools were used to investigate the
regulation of flowering of the alpine snow tussock, Chionochloa
pallens (Poaceae). This species is one of the most strongly masting
plant species globally (Kelly et al., 2000), which gives the plant
selective benefits through predator satiation (Rees et al., 2002). The
possible impacts of global warming on masting in this species have been
discussed in the literature (Kelly et al., 2013; McKone et al., 1998;
Monks et al., 2016; Rees et al., 2002), but more information on its
flowering mechanisms is required. From various plants, including some
manipulated to induce or prevent flowering, we took leaf samples from
tillers (shoots), some of which subsequently flowered and some remained
vegetative. We later identified each leaf as coming from a plant that
subsequently flowered or one from a plant that remained vegetative
(Appendix S1). We then used ecological transcriptomics (Samarth, Lee,
Song, Macknight, & Jameson, 2019; Todd, Black, & Gemmell, 2016) to
identify the potential homologues of PEBP sequences involved in the
onset of flowering. Subsequent structural, functional and expression
analysis of the PEBP sequences led to the identification of an
orthologous TFL1 gene with a novel function. In addition, global
transcriptomic analysis revealed crucial transcription factors including
thermosensory and floral epigenetic genes involved in the initiation of
flowering in C. pallens .