Chromosomal Distribution of Genes Conferring Tolerance to Abiotic Stresses Versus That of Genes Controlling Resistance to Biotic Stresses in Plants

Tolerance to abiotic stresses caused by environmental conditions can prevent yield loss in crops for sustaining agricultural productivity [...].


Introduction
Tolerance to abiotic stresses caused by environmental conditions can prevent yield loss in crops for sustaining agricultural productivity [1]. Resistance to biotic stresses caused by diseases and insects can prevent or reduce yield loss in crops [2]. For each crop or plant species, there are many abiotic threats, such as changes in temperature, soil salinity/alkalinity, water shortage, and soil contaminants, as well as biotic challenges from pathogens (bacteria, viruses, and fungi), insects, and nematodes. Plants need to possess genes conferring tolerance to these abiotic stresses to adapt to the changing environment, due to global climate changes, in which they are growing. Due to the coevolution of plants and stress-causing organisms [3], plants need to possess multiple resistance genes to deal with the rise of new virulence in stress-causing organisms. Plant breeders are constantly looking for new resistance genes to combat evolving organisms that pose a threat to susceptible crops. As a result, plant geneticists have identified many resistance genes in various crops, and molecular geneticists have developed molecular markers for most of those genes. Similarly, researchers are investigating plant mechanisms and underlying genetic systems involved in plant tolerance to abiotic stresses, hoping to breed crops resilient to adverse environmental conditions.
With the advent of whole-genome sequencing in many important crops, it is time to map the detailed chromosomal locations of known genes that are involved in tolerance to various abiotic stresses as well as in the resistance to biotic stresses in important plant species. In the Special Issue, "Mapping Abiotic Stress-Tolerance Genes in Plants" of International Journal of Molecular Sciences, 21 papers, including two reviews and 19 research articles, were published . Eleven research articles [3,[25][26][27][28][29][30][31][32][33][34] were published in the Special Issue "Mapping Plant Genes that Confer Resistance to Biotic Stress." In this editorial, I firstly express my appreciation to all authors for their contribution to the two Special Issues. Secondly, I will compare the chromosomal distribution patterns of genes for the two types of stresses that plants faced (Tables 1 and 2). The evidence obtained supports my long-held hypothesis that genes conferring resistance to biotic stresses are more likely to be located in the distal portion of chromosomes than the proximal portion in order to adapt to the host-pest coevolution. On the other hand, abiotic-stress tolerance genes should have a lower ratio of distal to proximal distribution than that for biotic stresses to maintain the stability of genes regulating plant growth and development. Knowing the relationship between gene functions and their chromosomal distribution patterns, plant breeders can select the most appropriate and efficient method to improve crops for withstanding stresses and ensuring productivity and food security.  Triticum aestivum 124 genomic regions associated with various diseases; several genes in those significant genomic regions had gene annotations suggesting their involvement in disease resistance.
Five genes were annotated as the leucine-rich repeat protein family and six genes were annotated as the F-box family protein, which were also reported to be involved in abiotic stress tolerance such as drought; Calcium-binding protein; ARM repeat superfamily protein; Elongation factor 1 alpha; Peroxidase; WAT1-related protein/EamA-like transporter family.

Transcription Factor
VqMYB14 bacterial flagellin peptide flg22 and harpins (glycine-rich and heat-stable proteins that are secreted through type III secretion system in gram-negative plant-pathogenic bacteria).
The promoter of VqMYB14 is induced by the elicitors flg22 to confer basal immunity (also called pathogen-associated molecular pattern (PAMP)-triggered immunity, PTI) and triggered by harpin to confer effector-triggered immunity (ETI). Overexpression of VqMYB14 enhance the main stilbene contents and expression of stilbene biosynthesis genes.

Chromosomal Distribution Patterns of Genes for Abiotic-Stress Tolerance vs. Biotic-Stress Resistance
Studying abiotic-stress tolerance, the authors of these 21 articles in this Special Issue covered Hordeum vulgare, Gossypium hirsutum, Pyrus pyrifolia, Oryza sativa, Glycine max, Fragaria vesca, Cucumis sativus, Dianthus caryophyllus, Brassica oleracea, B. napus, Sorghum bicolor, Triticum aestivum, Zea mays, Raphanus sativus, and the model plant Arabidopsis thaliana ( Table 1). The abiotic stresses studied include cold, heat, drought, salt, iron deficiency, nitrogen deficiency, UV irradiation, DNA damage, reducing agent, phytohormones (GA, SA, JA, ABA, ethylene, 2,4-D, and NAA), and heavy metals (cadmium, nickel and cobalt). Two [6,7] of the 21 articles did not present information on the chromosomal locations of genes for abiotic-stress tolerance, and one [18] did not map the BocMBF1c gene to the target species B. oleracea but did locate the orthologous gene identified in A. thaliana to the proximal section of chromosome 3.
Many transcription factor gene families (TFs) were studied in the majority of these 21 articles [6,8,9,11,[13][14][15][16][17][18][19][20]23,24]. Various putative stress-related and hormone-responsive cis-acting regulatory elements were identified in the promotor of these TFs. "The cis-regulatory sequences are linear nucleotide fragments of non-coding DNA with the main role of regulating gene expression and in turn, controls the development and physiology of an organism" [35]. Therefore, variations among members of TFs observed in those studies might account for the varying regulation of gene expression in different organs and tissues or at different developmental stages to respond to different stresses.
For genes controlling tolerance to abiotic stresses, an averaged 2.2 to 1 ratio of distal to proximal chromosomal distribution was obtained from the 21 articles (Table 1). In comparison, the 11 articles on genes conferring resistance to biotic stresses resulted in a 3.3 to 1 ratio (Table 2). Therefore, 77% of genes conferring resistance to biotic stresses were located in the distal section of chromosomes, while 69% of those for abiotic-stress tolerance were distally located. This slightly higher number of genes in the distal section of chromosomes is advantageous for plant adaptation, because genetic variability generated from the high recombination rate in distal recombination hotspots enables plants to deal with environmental changes and new virulent pests.

Conflicts of Interest:
The author declares no conflicts of interest.