Metallothionein family genes in kiwifruit: characterization and determining their roles in plant’s response to different stresses

Kiwifruit growth and development are severely affected by various biotic and abiotic stresses, especially cold stress and the bacterial disease caused by <i>Pseudomonas syringae</i> pv. <i>actinidiae</i> (Psa). Metallothioneins (MTs) are a group of cysteine-rich proteins that play crucial roles in stress response, metal detoxification, and homeostasis in plants. However, the protective role of these MTs in kiwifruit remains to be elucidated. In the present study, four <i>AcMT</i> genes were identified in the Hongyang kiwifruit genome, namely, two Type 2 isoforms (<i>AcMT2</i> and <i>AcMT2a</i>) and two Type 3 isoforms (<i>AcMT3a</i> and <i>AcMT3b</i>) located separately on four different chromosomes. The hormones and stress response <i>cis</i>-elements within the promoter regions of these <i>AcMTs</i> were characterized. It was revealed that the four <i>AcMT</i> genes exhibited different expression patterns in different tissues: <i>AcMT2</i> and <i>AcMT2a</i> were expressed at much higher levels in the fruit, male flower, female flower, root, and bark, while <i>AcMT3a</i> was expressed mainly in the fruit and <i>AcMT3b</i> was expressed highly in the bark. The expression patterns of these <i>AcMT</i> genes after exposure to Psa infection and different phytohormones, including gibberellic acid A3(GA₃), ethylene (ET), and abscisic acid (ABA), were evaluated. It was revealed that in response to Psa infection, the main <i>AcMTs</i> in each tissue (those with expression levels higher compared to the other <i>MTs</i> in that tissue) were downregulated during the early stage in kiwifruits, followed by a recovery phase. In addition, most <i>AcMTs</i> were downregulated after exposure to ET and GA₃, while type 2 <i>AcMTs</i> (<i>AcMT2</i> and <i>AcMT2a</i>) were upregulated after treatment with ABA. The overexpression of <i>AcMTs</i> in <i>Escherichia coli</i> presented a higher tolerance to H₂O₂, heavy metals, low temperature, and high temperature. Collectively, these findings demonstrated the protective roles of <i>AcMTs</i> in terms of stress resistance conferred through plant hormone-related signal pathways.