AcoMYB4, anAnanas comosusL. MYB Transcription Factor, Functions in Osmotic Stress through Negative Regulation of ABA Signaling

Drought and salt stress are the main environmental cues affecting the survival, development, distribution, and yield of crops worldwide. MYB transcription factors play a crucial role in plants' biological processes, but the function of pineapple MYB genes is still obscure. In this study, one of the pineapple MYB transcription factors,AcoMYB4, was isolated and characterized. The results showed thatAcoMYB4is localized in the cell nucleus, and its expression is induced by low temperature, drought, salt stress, and hormonal stimulation, especially by abscisic acid (ABA). Overexpression ofAcoMYB4in rice and Arabidopsis enhanced plant sensitivity to osmotic stress; it led to an increase in the number stomata on leaf surfaces and lower germination rate under salt and drought stress. Furthermore, inAcoMYB4OE lines, the membrane oxidation index, free proline, and soluble sugar contents were decreased. In contrast, electrolyte leakage and malondialdehyde (MDA) content increased significantly due to membrane injury, indicating higher sensitivity to drought and salinity stresses. Besides the above, both the expression level and activities of several antioxidant enzymes were decreased, indicating lower antioxidant activity in AcoMYB4 transgenic plants. Moreover, under osmotic stress, overexpression ofAcoMYB4inhibited ABA biosynthesis through a decrease in the transcription of genes responsible for ABA synthesis (ABA1andABA2) and ABA signal transduction factorABI5. These results suggest that AcoMYB4 negatively regulates osmotic stress by attenuating cellular ABA biosynthesis and signal transduction pathways.