4.1 Floral transition and development
Unlike dicots, such as Arabidopsis , in which the transition from
the vegetative to the mature reproductive stage occurs in a shorter
time, monocots (e.g. wheat and barley) show a long interval from days to
weeks between first spikelet primordia formation, inside the leaf sheath
up to the moment an ear is pushed out and the plant reaches the heading
stage (Gauley & Boden, 2019; Gol et al., 2017). Therefore, those plants
are more susceptible to supra-optimal temperature due to their long
reproductive-phase establishment, leading to considerable loss of yield
and grain quality (Bheemanahalli et al., 2019; Lohani, Singh, & Bhalla,
2019).
At the same time that warm temperatures can accelerate (e.g.Arabidopsis ) or delay flowering (e.g. Brassica rapa ), it
also modifies the response of the plant to photoperiod, another major
factor affecting floral transition (Capovilla, Schmid, & Posé, 2014;
Del Olmo, Poza-Viejo, Piñeiro, Jarillo, & Crevillén, 2019). A
moderately warm temperature can trigger Arabidopsis floral
transition under non-inductive short-day (SD) conditions
(Balasubramanian, Sureshkumar, Lempe, & Weigel, 2006; Vu et al., 2019),
or partially hasten soybean floral initiation under non-inductive
long-day (LD) (Wu et al., 2015). Differentially, elevated ambient
temperature cannot compensate for photoperiod as a floral inductive
signal in some species, which indicates that the effects of ambient
temperature on the reproductive development are highly
photoperiod-dependent (Hemming et al., 2012; Kiss et al., 2017). For
wheat, barley and Brachypodium grown under LD conditions, warm
temperatures shorten the time to floral transition (Boden et al., 2013;
Dixon et al., 2018; Dixon et al., 2019; Hemming et al., 2012). In
contrast, an increase in temperature delays the floral transition and
development under SD condition (Figure 4A ). InArabidopsis , high temperature is able to induce the expression of
flowering-promoting genes, such as FLOWERING LOCUS T (FT )
under non-inductive SD (Casal & Balasubramanian, 2019; Vu et al.,
2019). In contrast, transcript levels of the barley and wheat ortholog
of FLOWERING LOCUS T1 (FT1 ), do not increase upon high
temperature treatments in short days (Dixon et al., 2018; Hemming et
al., 2012; Kiss et al., 2017), suggesting FT1 -independent high
temperature responsiveness of flowering in cereals (Jacott & Boden,
2020). In maize, time to tasseling is advanced by higher temperature,
but there is no effect on the silking time (Wang et al., 2019). Notably,
rice floral meristem initiation and development benefits from its
flooded cultivation, since the early reproductive stages are
accomplished under water, where air high temperature is buffered, but
the heading is generally accelerated by high temperature (Chen et al.,
2018; Hu et al., 2015; Jagadish, Murty, & Quick, 2015).
Besides photoperiod-dependent temperature responses, vernalization
regulates floral transition in winter cereal crops. VERNALIZATION
1 (VRN1 ), a floral activator and its expression is induced under
cold temperatures. The flowering repressor VRN2 , decreased
expression to reduce VRN2-regulated repression of the central flowering
activator FT1 (Huan, Mao, Chong, & Zhang, 2018; Kim, Doyle,
Sung, & Amasino, 2009; Kippes et al., 2015; Oliver, Finnegan, Dennis,
Peacock, & Trevaskis, 2009; Yan et al., 2006; Yan et al., 2004; Yan et
al., 2003). Temperature shifts, such as those caused by climate change,
hinder vernalization, which is referred to as de-vernalization (Gregory
& Purvis, 1948; Mergner et al., 2020). Warm temperature-induced
interruption of vernalization accelerates flowering in wheat and barley
under LD photoperiod, resulting in the formation of additional spikelets
due to a delay in the early stages of inflorescence development (Dixon
et al., 2019; Ejaz & von Korff, 2017; Greenup et al., 2011)
(Figure 4B ).
In contrast to plastic vegetative development under high temperature,
reproductive characteristics like floral identity often show low
plasticity (or phenotypic robustness / stability) against environmental
fluctuation (Fal et al., 2019; Klingenberg, 2019). For instance, the
rice EXTRA GLUME 1 (EG1 ) gene, encodes predominantly
mitochondria-localized functional lipase, which functions upstream of
some floral identity genes (OsMADS1 , OsMADS6 andOsG1 ) to promote floral developmental robustness in a high
temperature-dependent manner (Zhang et al., 2016).