5.3 Seed and fruit-setting
Compared to other reproductive phases, the early seed and fruit-setting stages are more sensitive to temperature changes (Table 1 ). One of the consequences of warm temperature or heat stress is the accelerated senescence of terminal leaves, in which the majority of assimilates that are translocated to the seeds or fruit are fixed, which leads to loss of photosynthetic ability (Barlow, Christy, O’Leary, Riffkin, & Nuttall, 2015; Gourdji et al., 2013; Ishibashi, Yuasa, & Iwaya-Inoue, 2018; Marcelis & Baan Hofman‐Eijer, 1993; Pimentel et al., 2015; Stratonovitch & Semenov, 2015; Suwa et al., 2010; Y. Wang et al., 2019; Xu et al., 2020). On the other hand, the expansion of tomato fruits is positively regulated by increasing temperature (10-30°C) and less related to assimilate supply. However, fruit maturity is hastened by elevated temperature, resulting in a reduction of final mean weight in tomato fruits (Adams, Cockshull, & Cave, 2001; Pearce, Grange, & Hardwick, 1993). Additionally, a shortened seed or fruit-setting duration by high temperature, also results in a significant reduction in final weight (Boden et al., 2013; Sato et al., 2002; Shi et al., 2017).
Short periods or pulses of high temperature result in uneven ripening and softness of tomato fruit, reducing the fruit quality (Mulholland, Edmondson, Fussell, Basham, & Ho, 2003). In addition, high temperature-induced inactive PHYA and PHYB1/B2 leads to the reduction of tomato fruit carotenoid content (Bianchetti et al., 2020). It is noteworthy that the effect of increased temperature (> 35°C) on ethylene production, color development, and softening is reversible (Lurie, Handros, Fallik, & Shapira, 1996).
Chromatin remodeling plays an essential role in gene expression (Eckardt, 2007; Ojolo et al., 2018), and also during high temperature-controlled seed setting (Boden et al., 2013). InBrachypodium , the H2A.Z transcript level is stable with temperature, but at high temperature H2A.Z occupation is reduced and chromatin accessibility for RNA polymerase II is increased, which upregulates the transcription of starch catabolism-related genes (e.g.beta-amylase (AMY1 ) and UDP-glucose pyrophosphorylase (UDP-GPP )) during the seed-setting stage and strongly reduces yield (Boden et al., 2013) (Figure 4D) . Additionally, Brachypodium seed weight and overall yield is reduced in ACTIN-RELATED PROTEIN6 (ARP6 ) knock-down lines under high temperature, where ARP6 is required for proper H2A.Z deposition. Indeed, also in Arabidopsis H2A.Z is involved in temperature-dependent flowering (Kumar & Wigge, 2010), making this a more general regulatory mechanism at high temperature. In addition, alternative splicing has been shown to play an essential role in temperature sensing and adaption during seed setting (Xu et al., 2020; Zhang et al., 2014). The lower activity of OsbZIP58 is induced by alternative splicing under high temperature and inhibits the accumulation of storage materials (such as starch and lipids) during the seed-setting stage (Xu et al., 2020). High temperature also promotes the splicing efficiency of the rice Wx gene to maintain proper amylose content during the seed-setting stage (Zhang et al., 2014).