1. Introduction
The accelerating pace of rising atmospheric carbon dioxide
(CO2) concentration has been a serious global climatic
issue for a couple of decades. Despite the Kyoto Protocol being adopted
in 1997 and entered into force in 2005, followed by Paris Agreement in
November 2016, the atmospheric CO2 concentration
([CO2]) shows no sign of slowing down in its
increasing rate. The current historical record of atmospheric
[CO2] reached a climax of 417.07 ppm in May 2020
(ESRL, Mauna Loa, Hawaii, USA). In terms of crop production, the
elevated atmospheric CO2 concentration
(eCO2) and the consequential temperature hike will bring
in more extreme weathers (such as drought and flood, extreme heatwave
and freezing temperature, hurricane and hails), frequent crop diseases
and pests occurrences, and recurrent adverse crop growth conditions. All
these will reduce crop productivity, quality, and stability, especially
for wheat and maize (Easterling et al, 2005; Rosenzweig et al, 2014; Li
et al, 2015; Wang et al, 2019a). It seems that rice and soybean are the
mere major crops that may marginally benefit from the
eCO2, especially when considering the collateral
temperature and ozone hikes (Long et al, 2006; Ainsworth, 2008; Kimball,
2016; Usui et al, 2016; Zhao et al, 2017). To mitigate the threat and
secure the crop production, it is critically important to breed crop
varieties adaptable for the future [CO2] and
temperature conditions, and to integrate a novel cultivation management
system to take the advantage of the fertilization effect of this
irreversible eCO2 change (Long et al, 2004). However,
genes responsive for eCO2 adaptation are largely unknown
(Morita et al, 2015; Nakano et al, 2017; Hasegawa et al, 2019), thus, we
need to clarify targets for breeding purpose. Top priority should be
given to those yield-limiting agronomic characters that showing
beneficially responsive to eCO2.
Rice (Oryza sativa L.) is a staple cereal crop for more than half
of the world population, especially in the densely populated Asian
regions. Despite the divergent responses of the productivity to
eCO2 by different crops, it is generally concluded that
rice may marginally benefit from the CO2 fertilization
in temperate and tropical regimes (Ruiz-Vera et al, 2013; Rosenzweig et
al, 2014; Kimball et al, 2016). Most studies conclude that the
eCO2 increases grain yield of rice (Hasegawa et al,
2017). For example, multiple-year free-air CO2enrichment (FACE) field experiments conducted at different locations
with various indica , japonica and hybrid rice varieties
concluded that, eCO2 at 550 ppm can enhance rice grain
yield up to 5~35% compared to the ambient
[CO2] (Kim et al, 2003; Kimball et al, 2016). Among
the four major rice yield components, namely, panicles per area,
spikelets per panicle, seed setting rate (%), and grain weight,
eCO2 consistently increases panicles per area, while the
other three yield components show both negative and positive responses
(Kim et al, 2003a; Huang et al, 2004; Lai et al, 2014; Hasegawa et al,
2019). The panicle per area is a fundamental factor that being
determined at the earlier stage of rice growth. It impacts the other
three yield components at later stage of growth and arbitrates final
grain yield. Therefore, endeavor to achieve a stable panicle number is
always a top priority in rice production management.
In addition to the main stem, rice plant produces tillers (branches)
that may eventually develop into panicle florescence to generate grain
yield (Wang and Li, 2011). The tiller number of a rice plant is not only
a basis for panicle number, but also an indicator of plant growth
status. Rice plants with more tillers at the early stages usually
indicate they are in a healthy developmental path toward higher yield.
Tiller number also shows a significant accrual response to
eCO2 (Kim et al, 2003b; Huang et al. 2004). Since the
discovery of MOC1 in tillering regulation (Li et al, 2003), recent
advances in molecular genetics have clarified that more than 60 genes
involving in tiller regulation in rice plants (Wang et al, 2011; Wang et
al, 2018a). However, most of the knowledge is derived from mutant or
gene manipulation experiments, limited info is available on how they
coordinate in a regular variety (Zhang et al, 2019).
Moreover, nitrogen (N) is a major macronutrient that constrains tiller
growth. Reports have clarified that eCO2 alters the
element stoichiometry in plants, especially a reduction of N content was
consistently observed from grasses to crops and trees (Luo et al, 2006;
Norby et al, 2010; Deng et al, 2015). Insufficient N availability is a
constraint on the growth of perennial grass species in response to
eCO2 (Reich et al, 2006; Mueller et al, 2013). Multiple
reviews summarize that quite a range of crops and model plants
displaying certain pattern of interaction between N requirement and
eCO2, though interpretation differs but N constraint is
a consistency (Stitt and Krapp, 1999; Wang et al, 2010; Bloom et al,
2015; Rubio-Asensio and Bloom 2017; Andrew et al, 2019).
eCO2 reduces the N content in rice plant as well (Makino
et al, 1997; Kim et al, 2001; Lieffering et al, 2004; Zhang et al, 2013;
Wang et al, 2019b). However, low N content in rice plants is supposed to
inhibit tiller occurrence (Jiang et al, 1997). Despite the expected
inhibition effect on tillering by reduced N content in rice plants under
the eCO2, multiple reports have confirmed that tillers
are promoted by eCO2 (Jitla et al, 1997; Huang et al,
2004; Yang et al, 2007; Jiang et al, 2020). However, the underlying
physiological and molecular mechanisms remain unclear.
We hypothesized that N distribution changes in favor of tillering under
eCO2 condition. To test this, we investigated the interactive effects of
CO2 and N rate on rice tillering in growth chambers. Our
objectives were 1) to analyze the interaction effect of
eCO2 and N application rate on rice tillering at early
stage; 2) to decipher the change in the N distribution among different
organs; and 3) to investigate the molecular change in response to the
eCO2. Clarifying the molecular adaptation mechanism of
rice to the eCO2 would directly enable breeders to
target certain genes in order to breed varieties that can better benefit
from the CO2 fertilization effect. The mechanism may
also help interpreting the adaptation of other terrestrial plants to the
eCO2 and develop new approaches to mitigate the
eCO2.