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).