The Mechanisms and Pathways of Crop Responses to Stress

1. Introduction

Rice (Oryza sativa L.) and wheat (Triticum aestivum L.) are the two most important food crops and provide sustenance for billions of people worldwide [1,2,3]. It has been shown that drought and heat stress are the most important abiotic stresses affecting crop yield formation [4,5,6,7]. The world’s population is continuously growing, and the increase in the frequency of abiotic stress highlights the necessity of breeding crops with high yields under abiotic stresses using stress-resistant cultivation techniques. Thus, it is necessary to understand the mechanisms and pathways of crops’ responses to stress factors and conduct functional analyses of stress-tolerant genes to advance the breeding of stress-resistant crops and explore the corresponding cultivation techniques [8,9].

The Special Issue is titled “The Stress of Crop Adversity: The Mechanisms and Pathways of Stress Resistance”, and presents five papers focused on the physiological, gene expression, and yield formation responses of crops to environmental stresses such as drought and heat stress. They mainly discuss the physiological response mechanisms of crop growth to drought stress, the identification and expression analysis of the response of sucrose transporter genes to heat stress, and the mapping of quantitative trait loci (QTLs) related to wheat-yield-related agronomic traits. These studies can provide strong theoretical support for the future breeding of stress-resistant crops and high-yield cultivation.

2. Overview of Publications

2.1. Rice Growth and Leaf Physiology in Response to Drought Stress

Knowledge of rice’s responses to drought stress may help in maintaining or enhancing crop production and quality in drought-prone areas [10]. The first study presented in this Special Issue used a rain shelter experiment with five treatments, namely, P1 (drought stress from the tillering stage), P2 (drought stress from the jointing–booting stage), P3 (drought stress from the heading–flowering stage), P4 (drought stress from the grain filling stage), and CK treatment (sufficient water supply throughout the whole growth cycle). The results suggested that drought stress significantly affects the growth and physiological processes of rice. The parameters analyzed were plant height, tiller number and leaf physiological parameters. Notably, the research observed that the impact of drought stress during the early growth stages is more detrimental to yield components when compared to the effect of drought stress during the late growth stages. This study suggests that timely and effective drought stress management strategies will help combat damage to rice production in the future. Ultimately, its results will not only help with the assessment of rice’s adaptive capacity to adverse conditions, but also provide a scientific foundation for improving drought tolerance and optimizing yields in rice cultivation.

2.2. Sucrose Transporter Gene Identification and Expression Analysis Under Heat Stress

Grain filling is a critical stage in wheat yield formation. At this stage, the photosynthetic products are distributed mainly into the grains, and sucrose transporters (SUTs) play important roles in sucrose transportation from source tissues to sink organs [1,11,12,13]. The second study presented a genome-wide analysis of the SUT gene family in wheat, and a total of 19 SUTs were identified [11]. The study further examined the expression patterns of these genes under heat stress conditions. Notably, the expression levels of TaSUT1 were significantly higher than those of other TaSUTs, suggesting its potential role in sucrose unloading and long-distance transport. Additionally, this study also found that a reduction in TaSUT1 expression may have been the main reason for the decrease in sucrose content and grain weight under heat stress. Based on a comprehensive analysis of other relevant studies [1,13,14], future research should focus on the molecular regulatory mechanisms of the entire process from sucrose loading in photosynthetic organs to sucrose unloading at the grain.

2.3. QTL Mapping for Agronomic Important Traits in Well-Adapted Wheat Cultivars

Grain yield is a complex trait that is influenced by various other traits. Thus, genetic analyses of yield-related traits and identification of associated quantitative trait loci (QTL) are essential for breeding high-yield cultivars to increase wheat production [15]. In the third study, nine mapped QTLs were detected in multiple experiments, and they were found to explain a large percentage of phenotypic variations, which can be further fine-mapped and cloned [15]. Furthermore, Kompetitive Allele Speciffc PCR (KASP) assays were also developed and validated, which can be widely used in marker-assisted selection (MAS). In general, this study laid an important foundation for the discovery of genes underlying important traits in wheat and provided useful molecular markers for the MAS of high-yield-related traits in breeding. However, this article lacks precise gene localization and functional verification, which should be prioritized in future research.

2.4. Granule Size Distribution and Viscosity Parameters of Starch in Response to Lodging in Winter Wheat

Starch constitutes 65 to 75% of the final dry weight of wheat grains, making it a crucial component in terms of both yield and end-use quality. However, lodging is a prevalent and critical issue in major wheat-producing regions. As reported in several studies [16,17], lodging can cause a substantial decline in grain yield and negatively affect starch processing characteristics. The fourth study investigated the impact of lodging on the granule size distribution and viscosity parameters of starch in wheat. The results show that lodging significantly reduces starch content and yield while increasing protein content. Moreover, the study reveals that lodging modifies the proportion of A-type and B-type starch granules, and then decreases the peak viscosity, hold viscosity, final viscosity, breakdown viscosity, and rebound value. Based on the results of this study, we believe that under high-yield conditions, further breeding of lodging-resistant varieties and the integration of lodging-resistant cultivation techniques remain crucial research directions for future research.

2.5. Phenotypic and Gene Expression Analysis of Fruit Development

The development and maturation of fruits is controlled by inherent genetic programs. The fifth study investigated fruit development and maturation in two persimmon cultivars (Rojo Brillante and Fuyu) under different weather conditions in Spain and Japan. By comparing these differences, this study identified different factors that influence fruit growth and quality, and emphasized how environmental conditions and hormone regulation affect the key characteristics of the fruit, such as the size, color, hardness, and ripening. These findings reveal that climatic variables interact with genetic backgrounds to modulate the differential patterns of fruit growth, particularly in relation to gibberellin (GA) and oxidative-stress-related genes. By integrating physiological measurements with molecular analyses, this research also deepens our understanding of persimmon fruit development and provides actionable insights for optimizing cultivation practices.

3. Conclusions

These five articles provide significant insights into how crops respond to environmental stresses from physiological, genetic, and genomic perspectives. They emphasize the complexity of stress responses and offer strategies for stress-tolerant breeding. Based on the research presented in this Special Issue, we believe that future research should focus on the following areas: Firstly, it is crucial to explore how multiple stresses affect the yield formation of different crops. As highlighted in this Special Issue, studying the molecular regulatory mechanisms involved in leaf sucrose synthesis and grain sucrose unloading under multiple-stress conditions may represent a key area of focus. Secondly, the critical genetic variants and regulatory networks involved in abiotic stress resistance in crops require further investigation. Finally, it is essential to utilize modern breeding techniques to breed new crop varieties that can withstand multiple stresses. More specifically, the breeding of multi-stress-resistant varieties should prioritize the use of rapid breeding techniques and modern methods, such as high-throughput phenotyping, marker-assisted selection, genomic selection, and genome editing.