Heterologous Expressions of foxtail millet (Seteria italica) mitogen-activated protein kinase kinase (SiMKK) group A genes regulate root development under salt stress in Arabidopsis thaliana

Abstract The MAPK cascade is evolutionarily conserved in eukaryotes and involved in the regulation of plant growth, development, resistance to biotic and abiotic stress. However, the gene function of MAPK cascade in foxtail millet (Seteria italica) is rarely studied. In this study, RNA sequencing was performed and MAPK cascade was the main enrichment pathway in foxtail millet after salt treatment. Meanwhile, fourteen genes encoded and mitogen-activated protein kinase kinases (SiMKKs) were identified which could be divided into 4 subfamilies. Under salt treatment, the expression levels of 11 SiMKKs were upregulated and the expression level of SiMKK6-2 in group A had the biggest increase. SiMKK1 and SiMKK6-1, which were the other two member of in the same subfamily, also significantly upregulated under salt stress. Overexpression of these three genes in Arabidopsis thaliana reduced the sensitivity of roots to salt stress. Transgenic plants had more lateral roots. The decrease of primary root length of transgenic plants under salt stress was significantly lower than that of wild type plants. These three genes are involved in regulating the development of primary and lateral roots of plants, which can maintain better root development to improve plant tolerance to salt stress.