Advances and Future Directions in Biotic and Abiotic Stress Responses of Horticultural Plants

1. Introduction

Horticultural plants are fundamental to global nutrition, economic stability, and ecosystem services. However, their productivity and quality are perpetually threatened by a complex array of biotic and abiotic stresses. Biotic stresses, including pests and pathogens, cause substantial pre- and post-harvest losses [1,2], while abiotic stresses such as drought, salinity, and extreme temperatures, intensified by climate change, pose ever-growing challenges to sustainable cultivation [3,4]. Understanding the intricate physiological, molecular, and biochemical mechanisms that underpin plant stress responses is paramount for developing resilient crop varieties and innovative management strategies. This Special Issue, “Biotic and Abiotic Stress Responses of Horticultural Plants”, brings together cutting-edge research and comprehensive reviews that illuminate these mechanisms and present pathways toward enhanced horticultural sustainability.

2. Biotic Stress Responses: From Pest Management to Molecular Defense

The battle against biotic stressors requires a multi-pronged approach, integrating traditional management with modern biotechnology. Invasive species like the oriental fruit fly (Bactrocera dorsalis) continue to devastate fruit production, necessitating advanced control strategies ranging from biological controls to RNA interference (RNAi) and CRISPR-Cas9 gene editing, as detailed by Jaffar et al. [5]. Similarly, pervasive pests such as aphids, whiteflies, and spider mites demand constant vigilance and integrated pest management (IPM) protocols [6]. Beyond pests, pathogens including bacteria, fungi, and viruses trigger sophisticated plant immune responses. Shamshiri et al. (contribution 1) demonstrated that infection with Cucumber Mosaic Virus (CMV) and Turnip Mosaic Virus (TuMV) significantly reduced the yield and quality of saffron by negatively impacting its morphological and physiological traits and altering the biosynthesis of its key apocarotenoid metabolites and associated gene expression. Zhang et al. (contribution 2) revealed that the MIR396d-p3 negatively regulated apple resistance to the fungal pathogen Colletotrichum gloeosporioides by targeting and suppressing the expression of the disease resistance-related genes MdUGT89A2 and MdRGA3. Ma et al. (contribution 3) reported that resistance to clubroot disease in radish involved rapid and coordinated activation of defense pathways, including R gene-mediated recognition, MAPK-Ca²⁺-ROS signaling, and jasmonic acid regulation, while susceptibility was characterized by delayed responses and pathogen-induced metabolic hijacking. Ginger rhizomes activated the plant–pathogen interaction pathway, including PTI and ETI immune responses, along with ROS and NO signaling, to defend against Fusarium solani infection during postharvest storage (contribution 4). Jones et al. [7] synthesized 50 years of research to show that plant immunity, mediated by cell surface and intracellular receptors, provides the foundational knowledge for designing durable resistance, such as stacking immune receptor genes to control crop diseases. Furthermore, the menace of root-knot nematodes (Meloidogyne spp.) underscores the need for innovative control measures. Vashisth et al. [8] reviewed the potential of genetic engineering and RNAi technology to develop nematode-resistant crops, highlighting a promising frontier in durable resistance breeding.

3. Abiotic Stress Responses: Unraveling Mechanisms for Climate Resilience

Abiotic stresses trigger a cascade of molecular events, from stress perception to the activation of protective genes and metabolites. Waadt et al. [9] comprehensively synthesized recent advances in understanding how plant hormones, particularly abscisic acid, mediate sophisticated sensing, signaling, and response mechanisms to confer tolerance against major abiotic stresses such as drought, salinity, and flooding. It further highlighted the critical role of hormonal crosstalk, innovative biosensors for monitoring hormone dynamics, and the potential for translating these molecular insights into developing climate-resilient crops. Kumar et al. [10] reviewed how histone acetylation dynamics, regulated by HATs and HDACs, control plant development and stress responses by modulating chromatin structure and gene expression. Laloum et al. [11] indicated the critical role of alternative splicing in fine-tuning plant abiotic stress tolerance, particularly through its regulation of ABA signaling components and splicing factors. Wang et al. (contribution 5) reported that specific members of the ABF gene family in kiwifruit (Actinidia chinensis), particularly AcABF2, AcABF3, AcABF8, AcABF9, and AcABF10, are upregulated under drought stress and are positively correlated with increased abscisic acid (ABA) levels, suggesting their crucial role in enhancing drought tolerance through the ABA signaling pathway. Wang et al. (contribution 6) demonstrated that cucumber seedlings’ tolerance to high-temperature and high-humidity stress increases with their leaf stage, with four-leaf seedlings being the most resilient due to less severe photosynthetic inhibition and oxidative damage compared to the more sensitive one- and two-leaf stages. Red light enhances the salt tolerance of hydroponically grown pea seedlings by improving chlorophyll content, antioxidant enzyme activity, ion homeostasis, and reducing oxidative damage under saline conditions (contribution 7). The application of biochar and hydroretentive polymers mitigated water-deficit stress in Satureja rechingeri by improving water status and reducing oxidative stress, as evidenced by decreased antioxidant enzyme activity and malondialdehyde content (contribution 8).

4. Sustainable Practices and Integrated Future Strategies

The transition to sustainable horticulture is imperative. Practices like intercropping not only improve resource efficiency but also suppress diseases. Lv et al. [12] found that intercropping faba bean with wheat suppressed Fusarium wilt by altering the rhizosphere microbiome and root exudate profile. The integration of multi-omics data—genomics, transcriptomics, proteomics, and metabolomics—is revolutionizing our understanding. Gao et al. [13] comprehensively summarized the role of various plant microRNAs in regulating salt stress tolerance through genetic engineering, highlighting their molecular mechanisms, target genes, and potential applications in developing salt-resistant crops. Moreover, optimizing irrigation and fertilization management, as evaluated by Hui et al. [14], remains crucial for improving seed yield by enhancing topsoil nutrient availability, leaf photosynthetic efficiency, and seed morphological traits.

5. Conclusions

This Special Issue encapsulates the significant strides made in deciphering how horticultural plants perceive and respond to environmental challenges. The collective works underscore that solutions lie at the intersection of fundamental discovery and applied innovation. By leveraging advanced biotechnological tools, adopting sustainable agronomic practices, and fostering interdisciplinary collaboration, we can fortify our horticultural systems against an uncertain climate future. The journey toward stress-resilient horticulture is ongoing, and the insights gathered here provide a robust foundation for the research and actions that will follow.