4.3 RCS mediates resource redistribution for root growth and
improves drought tolerance
Under
abiotic stress, resource redistribution during RCS is more important for
increasing root absorption area and improving aboveground drought
tolerance of plants than is reducing root respiration (Postma and Lynch,
2011; Jaramillo et al., 2013; Chimungu et al., 2014b) In addition, the
enlargement of cortical cells and the reduction of cortical cell layers
are important features of RCS. Research on corn has shown that under
field water stress, varieties with large cortical cells had a root depth
of 32% to 41%, a relative water content of 22% to 30% in the leaves,
and a yield increase of 99% to 145% compared to varieties with small
cortical cells (Chimungu et al., 2014b). Meanwhile, compared with
varieties with increased cell layers in the root cortex, corn varieties
with reduced cell layers in the root cortex had a root depth of 33% to
40%, a relative water content of 10% to 35% in the leaves, a biomass
of 35% to 70% in the stems and leaves, and a yield increase of 33% to
114% (Chimungu et al., 2014a). This may be due to the release of a
large amount of nutrients from aging cells in the cortex, which may
significantly impact plant growth throughout the entire growth season by
supporting higher growth rates and subsequent larger soil exploration
(Lynch et al., 2014; Saengwilai et al., 2014). Therefore, in maize,
growth was promoted under stress by increased cortical cell size and
decreased number of root cortical cell layers. However, there are no
reports on the impact of resource reallocation during RCS on the drought
tolerance of cotton crops.
Based on the aboveground traits, this study is the first to explore the
impact of resource redistribution during RCS on cotton drought
tolerance. We found that RCS helps promote cotton growth, with ”Guoxin
02” exhibiting greater root dry weight, total root length, specific root
surface area, and convex hull area, as well as higher leaf water
potential and relative leaf water content under drought stress. This is
probably due to the changes in the root structure caused by the aging of
the root cortex tissues, such as changes in internal cell walls and
morphological adjustments. The correlation analysis indicated that RCS
was significantly positively correlated with leaf water potential, leaf
relative water content, and F v/F m (p <
0.01). These results support the hypothesis that RCS leads to more
extensive branching and deeper roots, thereby expanding the surface area
of the roots. This conferred the cotton plants with a stronger survival
advantage by fully utilizing deep-water resources, enabling the plants
to better adapt to arid environments and maintain normal aboveground
growth while improving drought tolerance (Figure 9, Postma and Lynch,
2011; Schneider et al., 2017a). We consider this effect as the
self-catalytic effect.
4.4
IAA andABA
regulate RCS
The
intrinsic developmental signals of plants work in synergy with external
environmental factors, inducing changes in different endogenous hormone
levels within the plant, thereby regulating the aging process in plant
tissues. Although some plant hormones, such as IAA and ABA, have been
shown to regulate leaf senescence, their role in root senescence has not
been fully elucidated (Schippers et al., 2015).
Our
research found that the IAA content of ”Guoxin 02” was significantly
higher than that of ”Ji 228”, and its content increased with the
increasing distance from the root tips (Figure 6C). This may be because
IAA promotes cell elongation and volume increase, which increases the
root growth to adapt to drought stress. Similar trends have also been
reported in rice, indicating the key role of IAA in enhancing drought
tolerance in rice seedlings (Zhang et al., 2009). Our study also found
that increased IAA content could improve the drought tolerance of
cotton, and the RCS degree of ”Guoxin 02” is significantly increased
with increased IAA content, indicating that IAA has a positive
regulatory effect on plant RCS (Figures S1 D-F).
As a plant sesquiterpene, ABA is involved in regulating plant growth and
development processes and responding to various biotic and abiotic
stresses (Jibran et al., 2013). Our research showed that the ABA content
decreased as the distance from the root tip increased (Figure 6E). This
result is similar to that reported in barley, whereby the ABA content
was higher in the root tip compared to the root basal zone (Liu et al.,
2019). This is probably because the root tip is the growth point of the
root system, with more active and complex cellular activities, promoting
the activity of the ABA synthesis pathway. In addition, we found that
the reduction of ABA could improve the drought tolerance of cotton and
significantly increase the RCS degree of the drought-tolerance variety,
suggesting the negative regulatory effect of ABA on cotton RCS (Figures
S1 G-I).
In conclusion, this study confirms for the first time the presence of
RCS in cotton, a dicotyledonous crop, and deeply explores the
characteristics and patterns of RCS in cotton under drought stress. We
systematically studied the characteristics, patterns, and
interrelationships of root metabolic activity and endogenous hormones
during the RCS, as well as their regulatory effects on roots and
aboveground parts. We found that RCS enhances cotton drought tolerance
by reducing root metabolic costs and regulating endogenous hormone
content (Figure 10).