Molecular Plant Breeding 2024, Vol.15, No.2, 42-51 http://genbreedpublisher.com/index.php/mpb 46 2.4 Inorganic element stress During low phosphorus stress, WRKY28 directly regulates phosphorus uptake and translocation from roots to crowns (Zhang, 2015). Similarly, WRKY42 regulates the expression of PHO1 and PHT1;1, which are involved in the plant's response to low phosphorus stress (Su et al., 2015). Additionally, AtWRKY6 (Xu et al., 2012) and AtWRKY33 (Ding et al., 2013) are associated with low boron stress and aluminum ion stress in Arabidopsis, respectively. It has been found that WRKY transcription factors play a negative role in regulating the expression of transcription factors related to elemental stress. This means that when plants are exposed to cadmium toxicity, their respiratory system is adversely affected. AtWRKY18, AtWRKY40, and AtWRKY60 are known to negatively regulate the plant response to cadmium toxicity (Liu, 2015). Similarly, WRKY6 and WRKY42 genes are capable of directly binding to and negatively regulating the expression of the PH01 promoter, which makes the plant more sensitive to low phosphorus levels (Zhang, 2015). 2.5 Oxidative stress Plants are exposed to various environmental stressors that cause the production and accumulation of reactive oxygen species (ROS) in their mitochondria. ROS is a highly oxidative molecule that can function as a signaling molecule and either positively or negatively regulate the plant's response to oxidative stress. In a study conducted on transgenic tobacco, it was found that the overexpression of FcWRKY40 increased the plant's resistance to oxidative stress. 2.6 Other abiotic stresses Apart from the five abiotic stresses mentioned earlier, there are several other abiotic stresses that plants have to endure such as light exposure, UV-B radiation, and sugar deprivation. For instance, WRKY proteins are responsible for regulating the reaction of most bamboo species to intense light (Zhao et al., 2016). Moreover, GmWRKY30 can respond to various inducers in Gentiana macrophylla, including arachidonic acid, salicylic acid, and silver ions, in order to cope with abiotic stress (He et al., 2018). 3 WRKY Transcription Factors and Biotic Stresses Plants face two main biological stresses - pathogenic bacteria infestation and feeding by phytophagous pests. When plants are exposed to these stresses, their metabolism and signal transduction processes change. Various signaling molecules, such as SA, JA, ET (ethylene), ABA, etc., get altered. This also affects the transcription levels of a large number of genes and proteins in the plant. All these lead to the plant's defense response to biotic stress. 3.1 WRKY transcription factors and pathogenic bacteria Several studies conducted on Arabidopsis and other crops have identified multiple WRKY genes that play a vital role in regulating plant resistance to pathogenic bacteria. These genes include WRKY3, WRKY33, WRKY40, WRKY46, WRKY18, WRKY53, WRKY70, and WRKY75. Therefore, these genes can be seen as a valuable source for genetically enhancing crop resistance against bacterial infections. Several studies have shown that overexpression of certain genes in Arabidopsis can enhance its resistance to various pathogens. For instance, Arabidopsis lines overexpressing AtWRKY28 and AtWRKY75 have been found to show enhanced resistance to Dictyostelium nucleatum (Chen et al., 2013). Similarly, overexpression of AtWRKY33 in Arabidopsis thaliana has been found to confer resistance against the necrotrophic fungus Staphylococcus griseus (Sham et al., 2017). Late blight is caused by Phytophthora infestans, and transcription factors such as StWRKY5 and StWRKY59 have important regulatory roles in potato resistance to late blight. StWRKY1 may mediate defense against infection of potato by Botrytis cinerea (Tian et al., 2023). The PsnWRKY70 gene in poplar may interact with specific members of the MAPK cascade to enhance resistance to leaf blight in poplar (Populussimonii × Populusnigra) (Zhao et al., 2017). It was shown that VlWRKY3 could enhance resistance to leaf blight pathogen live nutrients by increasing the expression of PR1 and NPR1 genes
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