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

The MAPK cascade, evolutionarily conserved among eukaryotes, plays a crucial role in regulating plant growth, development, and resistance to both biotic and abiotic stresses. However, the gene function of MAPK cascade in foxtail millet (Setaria italica) is poorly studied. In this study, RNA sequencing revealed that the MAPK cascade is the primary enrichment pathway in foxtail millet under salt treatment. Meanwhile, fourteen genes encoding mitogen-activated protein kinase kinases (SiMKKs) were identified and could be categorized into four subfamilies. Under salt treatment, the expression of 11 SiMKKs was upregulated, with SiMKK6-2 in group A showing the most significant increase. SiMKK1 and SiMKK6-1, the other two members of the same subfamily, were also significantly upregulated under salt stress. Overexpression of these three genes in Arabidopsis thaliana reduced the sensitivity of roots to salt stress. The transgenic plants exhibited an increase in lateral roots. Under salt stress, the decrease in primary root length of transgenic plants was significantly less 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.