Abstract
The abilities to mobilize and/or sequester excess ions within and outside the plant cell are important components of salt-tolerance mechanisms. Mobilization and sequestration of Na+ involves three transport systems facilitated by the plasma membrane H+/Na+ antiporter (SOS1), vacuolar H+/Na+ antiporter (NHX1), and Na+/K+ transporter in vascular tissues (HKT1). Many of these mechanisms are conserved across the plant kingdom. While Gossypium hirsutum (upland cotton) is significantly more salt-tolerant relative to other crops, the critical factors contributing to the phenotypic variation hidden across the germplasm have not been fully unraveled. In this study, the spatio-temporal patterns of Na+ accumulation along with other physiological and biochemical interactions were investigated at different severities of salinity across a meaningful genetic diversity panel across cultivated upland Gossypium. The aim was to define the importance of holistic or integrated effects relative to the direct effects of Na+ homeostasis mechanisms mediated by GhHKT1, GhSOS1, and GhNHX1. Multi-dimensional physio-morphometric attributes were investigated in a systems-level context using univariate and multivariate statistics, randomForest, and path analysis. Results showed that mobilized or sequestered Na+ contributes significantly to the baseline tolerance mechanisms. However, the observed variance in overall tolerance potential across a meaningful diversity panel were more significantly attributed to antioxidant capacity, maintenance of stomatal conductance, chlorophyll content, and divalent cation (Mg2+) contents other than Ca2+ through a complex interaction with Na+ homeostasis. The multi-tier macro-physiological, biochemical and molecular data generated in this study, and the networks of interactions uncovered strongly suggest that a complex physiological and biochemical synergy beyond the first-line-of defense (Na+ sequestration and mobilization) accounts for the total phenotypic variance across the primary germplasm of Gossypium hirsutum. These findings are consistent with the recently proposed Omnigenic Theory for quantitative traits and should contribute to a modern look at phenotypic selection for salt tolerance in cotton breeding.
Highlights
High salt concentration in the soil impedes the ability of the roots to extract water, causing dehydration and osmotic stresses with negative impacts to cellular processes that support vegetative and reproductive growth
Our preliminary studies on a subset of test germplasm revealed that such NaCl levels imposed only mild stress that did not elicit obvious differential reactions across cultivars at the whole-plant level
Of the representative germplasm panel, which included twenty five (25) uncharacterized core-Gossypium Diversity Reference Set (GDRS) accessions and two (2) known salt-sensitive controls (TX-307 and the genome RefSeq TM1), a total of twelve (12) accessions appeared to cover the range of stress tolerance potentials relative to the extent of genetic diversity established by simple sequence repeat (SSR)-based phylogenetic studies (Figures 2A–D; Table 1; Supplementary Figure 1; Supplementary Dataset 1; Hinze et al, 2017)
Summary
High salt concentration in the soil impedes the ability of the roots to extract water, causing dehydration and osmotic stresses with negative impacts to cellular processes that support vegetative and reproductive growth. Osmotic stress tolerance mechanisms are controlled by genes involved in long distance signaling (SOS3, SnRKs), osmotic adjustment (P5CS, HKT1, SOS1), and stomatal regulation (ERA1, PP2C, AAPK, PKS3) (Berthomieu et al, 2003; Nakayama et al, 2005; Munns and Tester, 2008; Fujii and Zhu, 2012; Amini et al, 2015). In concert, these processes slow down the rate of cell expansion in roots and young leaves (Yeo et al, 1991). The capacity to enhance production of enzymatic and non-enzymatic antioxidants can improve tolerance under salinity stress and protect the photosynthetic machinery (Chakraborty et al, 2016)
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