1 Introduction
Root
anatomical phenes play a pivotal role in capturing soil resources, and
the root cortex is a vital anatomical phene found between the epidermis
and stele. The root cortex consists of multiple layers of thin-walled
cells differentiating from the primary meristem, occupying a significant
proportion of the root cross-section volume. The cortex is a lateral
pathway for water and solute transport from root hairs to the central
cylinder, serving as the primary site for nutrient storage, aeration,
and secretion of nutrients and growth regulators (Lynch, 2015).
Root
cortical senescence (RCS) is a form of programmed cell death occurring
in the cortical cells of the root system (Schneider and Lynch, 2018).
During RCS, the root external appearance remains healthy and white, but
the cortical cells predominantly lose their nuclei (Bingham, 2007), a
phenomenon commonly referred to as ”non-pathogenic root cortex death”
(Deacon and Henry, 1978).
Accurately quantifying RCS is essential for comprehensively
understanding plant growth and physiological status. Histological
techniques utilizing cell viability staining have become the predominant
approach for studying RCS phenotypes. Commonly used dyes, including
neutral red (Beckel, 1956), Feulgen reagent (Holden, 1975), toluidine
blue (Brown and Hornby, 1987), acridine orange (Henry and Deacon, 1981),
and other cell viability stains, have demonstrated efficacy in RCS
assessment. While concerns were raised by Wenzel and McCully (1991)
about the effectiveness of acridine orange and other cell viability
staining dyes for RCS evaluation, research by Henry and Deacon (1981)
indicated a higher potency in RCS detection when assessed using acridine
orange staining, Feulgen staining, and single-cell pressure probe
techniques (Bingham, 2007). Similar to acridine orange staining,
technologies such as single-cell turgor pressure measurements,
TUNEL-assay and Nomarski optics revealed comparable RCS patterns and
occurrence rates across various plant species (Bingham, 2007; Henry and
Deacon, 1981; Liljeroth and Bryngelsson, 2001; Wenzel and McCully,
1991). These research findings confirm the feasibility of using acridine
orange and other cell viability staining methods to evaluate RCS.
RCS causes the damage or loss of nuclei in the cortical cells, an early
indicator of cell apoptosis and RCS manifestation. Therefore, anucleated
cortical cells are the hallmark of RCS, and the number of viable cells
gradually diminishes with increasing cortical aging (Liljeroth, 1995).
RCS significantly impacts the morphology and physiological
characteristics of the root system; for example, during RCS, the
cross-sectional area of the root cortex decreases, restricting the
radial transport pathways from the cortex to the stele (Liljeroth,
1995). The occurrence of RCS also increases the proportion of aerenchyma
in the root cortex of Gramineae crops, thus inhibiting the cortex
function and increasing nutrient and water transport resistance (Hu et
al., 2014a; Schneider et al., 2017b; Galindo-Castañeda et al., 2018).
RCS has been shown to decrease the metabolic consumption of
photosynthetic products in the cortex, especially during drought stress,
with a substantial portion being utilized for new root growth (Lynch,
2015). This, in turn, promotes root elongation (Chimungu et al., 2015;
Schneider et al., 2017b) and enhances water uptake (Lynch et al., 2005).
Thus, it is evident that RCS is closely associated with the
functionality of the root system under adverse conditions.
The
RCS occurrence is influenced by multiple factors., including crop
variety, growth stage, culture medium type, position within the root
system, stress conditions, and phytohormones (Liljeroth, 1995; Schneider
et al., 2017b). For instance, wheat seedling roots exhibited the highest
rate of RCS occurrence compared to rye, barley, and oats (Henry and
Deacon, 1981). Stress conditions such as low nitrogen, low phosphorus,
and drought also induce the occurrence of RCS (Liljeroth, 1995).
Compared with the root segments without RCS, the ones with RCS decreased
by 19% and 12% under high N and low N conditions, respectively
(Schneider et al., 2017b). The occurrence rate of RCS is also regulated
by ethylene (Schneider et al., 2018), and exogenous ethylene
significantly increases the RCS occurrence in maize seedlings and
lateral roots (Schneider et al., 2018). However, the characteristics of
RCS occurrence and its response to drought stress in Malvidae cotton are
rarely reported.
Drought stress significantly constrains global agricultural productivity
(Lynch, 2007), and efficient acquisition of soil moisture is crucial for
enhancing plant drought tolerance. Cotton (Gossypium hirsutum L.)
is a valuable economic crop. As a straight-rooted crop, cotton exhibits
a well-developed root system with a deep tap root and widely distributed
lateral roots.
The
function level and physiological metabolism of the root cortex largely
represent the functionality and metabolism of the entire root system.
Previous
studies on gramineous crops found that unfavorable conditions
accelerated RCS, reduced root respiration and decreased the transfer and
allocation of photosynthetic products to fast-growing tissues (Schneider
et al.,
2017b).
The anatomical structures of the root systems of dicotyledonous and
monocots differ; however, the response mechanisms of RCS to stress
conditions have not been reported in dicotyledonous crops. Therefore,
there is a need to investigate the RCS responses of dicotyledonous crops
to stress
conditions.
There are several unresolved
questions regarding the effects of RCS in cotton: Is the susceptibility
of the aboveground parts of cotton to drought due to premature RCS? What
are the spatio-temporal variation characteristics and patterns of RCS in
cotton? Is there a correlation between the consumption of a substantial
quantity of photosynthates by RCS and the resilience of the aboveground
parts of
cotton?
We hypothesize that endogenous hormones regulate RCS in drought-tolerant
varieties under drought stress, leading to reduced root metabolic costs.
Consequently, more energy is redirected towards root growth, thereby
enhancing drought tolerance. Therefore, this study aimed to explore the
causative factors of cotton RCS and its physiological metabolism and
endogenous hormonal regulation under drought stress, clarify the
physiological role of RCS occurrence in relation to the regulation of
the whole root development, and reveal the physiological mechanism of
RCS in relation to the drought
tolerance of cotton aboveground parts. The study mainly focused on (1)
evaluating the existence and occurrence patterns of RCS in cotton and
assessing the characteristics of cotton RSC under drought stress;
(2)
clarifying the characteristics and patterns of metabolic activities and
endogenous hormones and their interrelationships during the RCS in
cotton; (3) exploring the mechanism of action and effects of RCS on the
root system and aboveground parts of cotton. This study is the first to
confirm the existence of RCS in a dicotyledonous crop, cotton, and
provides an in-depth understanding of the characteristics and patterns
of RCS under drought stress. The results form the basis for using RCS to
improve drought tolerance in cotton.