Studies on Mapping Plant Genes That Confer Tolerance to Abiotic Stresses

Climate change is affecting the Earth's environment through temperature fluctuation, rainfall patterns, wind, and radiation [...].

The expansin gene family of moso bamboo is composed of four subfamilies, alphaexpansin, beta-expansin, epansin-like A, and expansin-like B, which have 45, 29, 7, and 1 genes, respectively [3]. These 82 genes were located to 32 scaffolds, instead of chromosomes, because the whole-genome sequencing work has not linked the scaffolds to chromosomes yet. Overall, there were 37 proximally located and 45 distally located genes. This suggests that many of the 82 expansin or expansin-like genes are not directly responsible for tolerance to abiotic stresses. Further research on the gene function of the individual genes is needed.
Benzoxazinoids (BXs) of rye (Secale cereale L.) could protect plants against nematodes and weeds through allelopathy [4]. However, there was no reported information on the number of genes and their chromosomal locations. DUF569 (AT1G69890) of approximal 3600 domain of unknown function (DUF) genes enhanced drought tolerance in Arabidopsis thaliana by positively regulating the production of ABA, polyphenols, flavonoids, carotenoids, and chlorophylls. This gene was distally located on chromosome 1 [5]. In A. thaliana, 10 genes (TAR2, At4g24670; YUC5, At5g43890; IAR3, At1g51760; GH3.1, At2g14960; GH3.3, At2g23170; GH3.12, At5g13320; UGT74E2, At1g05680; DAO2, At1g14120, PIN2, At5g57090, and PIN4, At2g01420) were involved in responses to salt and mannitol treatment [6]. These 10 genes disrupt auxin biosynthesis and were found on chromosomes 1, 2, 4, and 5 with the 5:5 proximal to distal ratio. The SIMILAR TO RCD-ONEs (SROs) gene family in sesame has two subfamilies, SiSRO1 and SiSRO2, which have two genes each; but SiSRO2a performed its function by regulating defense response and hydrogen peroxide metabolic process, while SiSRO2b associated genes mainly participated in biological processes related to hormonal and stress responses [7]. Three of these four genes are located in linkage groups (LGs) of sesame; SiSRO1a on LG2, SiSRO1b and SiSRO2a on LG3, while SiSRO2b on a scaffold. Both genes on LG3 were distal but at the same distance from the opposite ends. In watermelon, the Trehalose-6-Phosphate Synthetase (TPS) gene family has seven genes (ClTPS1 to 7) that confer tolerance to salt and other stresses by maintaining osmotic pressure, protecting membrane structure, and participating in the signal transduction [8]. Trehalose upregulated the activities of antioxidant enzymes, such as superoxide dismutase (SOD), ascorbic acid peroxidase (APX), peroxidase (POD), and catalase (CAT); thus, increasing reactive oxygen species (ROS) scavenging capacity. All ClTPS genes, except ClTPS2, are distally located (Table 1).
Of the 109 genes mapped to chromosomes (or scaffolds), 47 were regarded as proximally located and 62 distally located ( Table 1). The 1.32:1 distal/proximal ratio for genes reported in this 2021 Special Issue is lower than that determined in the 2020 issue, 2.20:1 [10]. It is possible that more of the genes studied in this 2021 Special Issue are involved in essential biological pathways that control plant growth and development rather than in those functions that directly respond to abiotic stresses.
In conclusion, plant genes that confer tolerance to abiotic stresses tend to be distally located on physical chromosomes. Their upstream promoter regions contain many cisacting regulatory elements that respond to phytohormones, abiotic stresses, and biotic stresses. These plant genes are characterized by spatial-temporal expression. With knowledge of the gene locations and expression profiles, plant breeders can develop efficient strategies to utilize the available genes that confer tolerance to abiotic stresses caused by climate changes.
Funding: This research received no external funding. APC was waived by IJMS.

Data Availability Statement:
The data in Table 1 were obtained from the cited publications in the special issue.

Conflicts of Interest:
The authors declare no conflict of interest.