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).