Leaf Membrane Stability under High Temperatures as an Indicator of Heat Tolerance in Potatoes and Genome-Wide Association Studies to Understand the Underlying Genetics

Abstract

High temperatures during the crop growing season are becoming more frequent and unpredictable, resulting in reduced crop productivity and quality. Heat stress disrupts plant metabolic processes that affect cell membrane composition and integrity. Cell membrane permeability, ion leakage, and heat shock proteins have been evaluated to screen for heat tolerance in plants. In potatoes, it is unclear whether leaf membrane stability under heat stress is correlated with underground tuber productivity and quality. The main goal of this study was to evaluate if leaf membrane relative electrolyte conductivity (REC) under high temperatures could be used to identify heat-tolerant potato genotypes. Electrolyte leakage assays, correlation estimations, and genome-wide association studies were carried out in 215 genotypes. Expression levels of small heat shock protein 18 (sHSP18) were evaluated in the heat-sensitive potato variety Russet Burbank and compared with those of the heat-tolerant variety Vanguard Russet using Western blotting. Significant differences were observed among genotypes for leaf membrane REC under extreme heat (50°C); REC values ranged from 47.0-99.5%. Leaf membrane REC was positively correlated with tuber external and internal defects and negatively correlated with yield. REC was negatively correlated with the content of several tuber minerals, such as nitrogen, magnesium, and manganese. Eleven quantitative trait loci (QTLs) were identified for leaf membrane REC, explaining up to 13.8% of the phenotypic variance. Gene annotation in QTL areas indicated associations with genes controlling membrane solute transport and plant responses to abiotic stresses. Vanguard Russet had lower leaf REC and higher expression of sHSP18 under high-temperature stress. Our findings indicate that leaf membrane REC under high temperatures can be used as an indicator of potato heat tolerance.

Keywords: Solanum tuberosum; abiotic stress; heat shock proteins; heat stress; relative electrolyte conductivity.

Figure 1

Leaf membrane relative electrolyte conductivity (REC) of Russet Burbank, Atlantic, Sierra Gold^TM, Vanguard Russet, and Reveille Russet in the initial experiment: (A) changes in REC with increasing temperatures (30–70°C); (B) temperatures at which 50% of the leaf electrolytes leaked (LT50). Values with the same letter were not significantly different at p ≤ 0.05.

Figure 2

Leaf membrane relative electrolyte conductivity (REC) of Russet Burbank, Atlantic, Sierra Gold^TM, Vanguard Russet, and Reveille Russet at 50°C in the main experiment. Values with the same letter were not significantly different at p ≤ 0.05.

Figure 3

The overall distribution of 215 potato clones (advanced clones from the Texas A&M Potato Program and commercial varieties), based on leaf membrane relative leaf electrolyte conductivity (REC) measured from leaf disks exposed to four hours of heat stress in the water bath at 50°C.

Figure 4

Distribution of 215 potato clones (advanced clones from the Texas A&M Potato Program and commercial varieties) according to individual market groups, namely (A) russets, (B) chips, (C) yellows, (D) reds, and (E) purples, based on relative leaf electrolyte conductivity (REC) measured from leaf tissues exposed to four hours of heat stress in the water bath at 50°C.

Figure 4

Distribution of 215 potato clones (advanced clones from the Texas A&M Potato Program and commercial varieties) according to individual market groups, namely (A) russets, (B) chips, (C) yellows, (D) reds, and (E) purples, based on relative leaf electrolyte conductivity (REC) measured from leaf tissues exposed to four hours of heat stress in the water bath at 50°C.

Figure 5

QQ plots of leaf relative electrolyte conductivity (REC) values (A) and Manhattan plot showing −log10 (p) values corresponding to SNPs across the 12 potato chromosomes for relative electrolyte conductivity using additive and dominant models (B). Genome-wide association studies (GWASs) were based on 215 potato clones evaluated for relative electrolyte conductivity (REC) after four hours of incubating leaf tissues in a water bath at 50°C. The Bonferroni threshold was 5.30 for the additive, 4.9 for 1-dom-alt, and 5.1 for the 1-dom-ref models.

Figure 6

Location of significant QTLs on potato chromosomes for leaf membrane relative electrolyte conductivity (REC) based on evaluations of 215 potato genotypes. ChromoMap (Version 4.1.1) was used to develop the graph.

Figure 7

Heat stress-induced accumulation of cytosolic sHSP18 in (1) Russet Burbank—control, 25°C, RB-NT; (2) Vanguard Russet—control, 25°C; VR-NT, (3) Russet Burbank—heat stress, 35°C, RB-HS; and (4) Vanguard Russet—heat stress, 35°C, VR-HS grown under controlled growth chamber conditions: (A) total protein before Western blotting; (B–D) expression of sHSP 18 in Exp 1–3; (E) estimation of sHSP 18 based on band volume of Exp. 1. Expression of HSP18 was examined by Western blotting analysis in leaves of plants exposed to heat stress (35°C for 18 h) and control plants (25°C for 18 h). The blots were probed with the anti-Arabidopsis thaliana HSP17.6-CI polyclonal antibody. An equal amount of protein (20 µg) was loaded in each lane. The relative levels of HSP18 were estimated by determining band density after probing using ImageJ software (ver. 5.2, Molecular Dynamics, Sunnyvale, CA, USA). Values followed by the same letter in a column are not statistically different at p ≤ 0.05. Figure 8 summarizes the results obtained from electrolyte leakage assay, correlation analysis, and Western blotting in a schematic diagram.

Figure 8

Under high temperatures, heat-tolerant potato varieties exhibited lower leaf electrolyte leakage, better membrane stability, and higher expression of small heat shock protein 18 (sHSP 18) compared to heat-sensitive varieties. Correlation analysis revealed that heat-sensitive varieties had higher tuber external and internal defect rates, lower yields, and reduced levels of essential minerals such as nitrogen (N), magnesium (Mg), manganese (Mn), and sulfur (S). {↑ = higher; ↓ = lower}.