Field Crop 2024, Vol.7, No.6, 287-297 http://cropscipublisher.com/index.php/fc 289 2.2 Biotic stresses Bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa) is a severe threat to kiwifruit production. Several studies have identified mechanisms and genetic loci associated with Psa resistance. For instance, quantitative trait loci (QTLs) have been mapped to identify genetic markers for Psa resistance, providing insights into breeding resistant cultivars (Tahir et al., 2019). Additionally, salicylic acid (SA) has been found to enhance resistance by upregulating defense-related proteins and pathways (Qu et al., 2023). Sulfur treatments have also been shown to induce resistance by increasing phenolic components and modifying morphological structures in kiwifruit stems (Gu et al., 2021). Pest resistance in kiwifruit involves complex interactions between the plant and various pests. While specific studies on pest resistance mechanisms in kiwifruit are limited, general plant defense responses, including the activation of stress-responsive genes and pathways, play a crucial role in mitigating pest damage. The role of transcription factors and signaling molecules in enhancing overall stress resistance can indirectly contribute to improved pest resistance (Tahir et al., 2019; Zhang et al., 2019; Qu et al., 2023). By understanding these mechanisms, researchers and breeders can develop strategies to enhance kiwifruit resistance to both abiotic and biotic stresses, ensuring sustainable production and quality. 3 Physiological Mechanisms of Stress Resistance 3.1 Water use efficiency and drought resistance Kiwifruit plants exhibit various physiological adaptations to enhance water use efficiency and drought resistance. The R1R2R3-MYB transcription factor AcMYB3R has been identified as a key player in improving drought tolerance. Overexpression of AcMYB3R in Arabidopsis thaliana resulted in upregulation of stress-responsive genes such as RD29A, RD29B, COR15A, and RD22, which are crucial for drought resistance (Zhang et al., 2019). Additionally, the abscisic acid (ABA) pathway plays a significant role in drought response, with ABA levels increasing significantly under drought conditions, leading to the upregulation of ABA-responsive genes (Wurms et al., 2023). 3.2 Temperature tolerance mechanisms Temperature tolerance in kiwifruit involves complex regulatory networks. The bZIP transcription factor AchnABF1 has been shown to enhance cold tolerance by upregulating key genes associated with ABA-dependent and ABA-independent pathways. This transcription factor also improves reactive oxygen species (ROS) scavenging ability, reducing oxidative damage under cold stress (Jin et al., 2021). Another bZIP transcription factor, AcePosF21, regulates ascorbic acid (AsA) biosynthesis, which is crucial for neutralizing ROS induced by cold stress. Overexpression of AcePosF21 leads to increased AsA levels and reduced oxidative damage (Figure 2) (Liu et al., 2023). 3.3 Salt exclusion and osmotic adjustment Salt stress tolerance in kiwifruit involves mechanisms such as salt exclusion and osmotic adjustment. The NAC domain transcription factor AvNAC030 enhances salt tolerance by improving osmotic adjustment ability and antioxidant defense mechanisms (Li et al., 2021). Additionally, the HSF gene AeHSFA2b has been shown to improve salt tolerance by increasing the expression of genes involved in osmotic adjustment and ROS scavenging (Ling et al., 2023). Comparative studies of different kiwifruit genotypes under salt stress revealed that genotypes with higher proline and total soluble sugar levels exhibited better osmotic adjustment and salt tolerance (Abid et al., 2020). 3.4 Antioxidant responses to biotic and abiotic stress Antioxidant responses are critical for kiwifruit's resistance to both biotic and abiotic stresses. The bZIP transcription factor AchnABF1 enhances ROS-scavenging ability by increasing the activity of catalase (CAT) and peroxidase (POD), thereby reducing oxidative damage under various stresses (Jin et al., 2021). Similarly, the expression of genes encoding antioxidant enzymes such as APX, GST, and GR is upregulated under salt stress, contributing to enhanced stress tolerance (Abid et al., 2020). The role of antioxidants in mitigating oxidative damage is further supported by the increased activity of peroxidase (POD) and catalase (CAT) under waterlogging
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