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).