4.2 Stamen and pistil growth and development
As many crop reproductive processes usually occur in late spring or early summer (Hedhly et al., 2009), these are more likely to be affected by high ambient temperature or heat stress than vegetative stages (Draeger & Moore, 2017). Obviously, temperature extremes or heat waves are more immediate and harmful to reproductive tissues than high ambient temperature (Draeger & Moore, 2017; Jagadish et al., 2010; Prasad & Djanaguiraman, 2014; Saini et al., 1983, 1984; Wang, Tao, et al., 2019). High temperature affects both male and female reproductive organs, such as anther development (Draeger & Moore, 2017; Yu et al., 2017), pollen formation and viability (Djanaguiraman, Prasad, Boyle, & Schapaugh, 2013; Feng et al., 2018; Gonzalo et al., 2020; Wang, Tao, et al., 2019), size of anther dehiscence for pollen dispersal (Jagadish et al., 2010; Matsui & Hasegawa, 2019), filament elongation (Sakata et al., 2010), germinative ability on the stigma (Begcy et al., 2019; Djanaguiraman et al., 2013; Endo et al., 2009; Gonzalo et al., 2020; Shi et al., 2018), pollen tube elongation in the pistil (Saini et al., 1983; Shi et al., 2018; Zhang et al., 2018), stigma receptivity and ovule viability (Hedhly, 2011), and pollen–pistil developmental synchrony that is prerequisite for pollination success (Hedhly et al., 2009; Herrero, 2003) (Figure 4C) . Noteworthy, all the causes of reproductive organ sterility are differentially affected by short-term or long-term high temperature exposure (Karapanos, Akoumianakis, Olympios, & Passam, 2010; Zhang et al., 2018).
The tapetum plays an important role in providing nutrients for pollen development (Shi, Cui, Yang, Kim, & Zhang, 2015; Suzuki, Takeda, Tsukaguchi, & Egawa, 2001). In rice, the leucine-rich repeat-receptor-like kinase, Thermo-Sensitive Genic Male Sterile 10 (TMS10), is essential for tapetal degeneration and pollen formation to maintain normal male fertility under high ambient temperature (Yu et al., 2017). Failure of rice pollen-stigma adhesion and germination on the stigma caused by high temperature is the result of downregulated expression of two genes, AK106843 (encoding CYP703) andAK106946 (encoding a GDSL type lipase), which are involved in controlling lipid components of the pollen wall in the tapetal cells (Endo et al., 2009). High temperature-induced impairment of maize pollen tube growth results from the mis-regulated expression of genes involved in the production of energy and lipids that mediate pollen tube growth (Begcy et al., 2019). In addition, rice sterility is caused by cessation of pollen tube elongation due to heat-induced reduction of endogenous auxin in pollinated pistils (Zhang et al., 2018). A sharp decrease in endogenous auxin resulting from warm temperature-induced downregulation of YUCCA auxin biosynthesis genes also causes male sterility in barley and Arabidopsis (Sakata et al., 2010). Application of auxin completely reversed male sterility (Sakata et al., 2010; Zhang et al., 2018). This is in sharp contrast with, for example,Arabidopsis hypocotyls where high temperature promotes auxin biosynthesis to control growth (Bellstaedt et al., 2019; Gray, Ostin, Sandberg, Romano, & Estelle, 1998).