Abstract
Mitogen-activated protein kinase kinase kinases (MAPKKKs) are pivotal components of Mitogen-activated protein kinase (MAPK) cascades, which play a significant role in many biological processes. Although genome-wide analysis of MAPKKKs has been conducted in many species, extant results in apple are scarce. In this study, a total of 72 putative MdMAPKKKs in Raf-like group, 11 in ZIK-like group and 37 in MEEK were identified in apple firstly. Predicted MdMAPKKKs were located in 17 chromosomes with diverse densities, and there was a high-level of conservation in and among the evolutionary groups. Encouragingly, transcripts of 12 selected MdMAPKKKs were expressed in at least one of the tested tissues, indicating that MdMAPKKKs might participate in various physiological and developmental processes in apple. Moreover, they were found to respond to drought stress in roots and leaves, which suggested a possible conserved response to drought stress in different species. Overexpression of MdRaf5 resulted in a hyposensitivity to drought stress, which was at least partially due to the regulation of stomatal closure and transpiration rates. To the best of our knowledge, this is the first genome-wide functional analysis of the MdMAPKKK genes in apple, and it provides valuable information for understanding MdMAPKKKs signals and their putative functions.
Highlights
(Gossypium spp) genome[12,13]
A Mitogenactivated protein kinase (MAPK) cascade is composed of three kinases, MAPK, MAPKK, and MAPKKK14,45
The three kinases form the basic module of a MAPK cascade, Mitogen-activated protein kinase kinase kinases (MAPKKKs)-MAPKK-MAPK, which is activated via phosphorylation
Summary
(Gossypium spp) genome[12,13]. MAPKKKs activated MAPKKs, themselves, by phosphorylating the conserved serine and/or threonine residues in their T-loop, MAPKKs in turn activate a specific MAPK4,12,14. Compared to MAPKs and MAPKKs, the MAPKKKs have more members and greater variety in primary structures and domain composition[15]. They consist of long N- or C- terminal regions that can be subdivided into MEKK, Raf-like and ZIK-like groups according to their sequence alignment[7,16]. Phylogenetic analysis of the MAPKKK genes in various species shows that the diversity exists in plants. There is little genome-wide information currently available on apple MAPKKK genes. We predicted the chromosome location and gene structure of putative MAPKKKs through genome-wide surveys of apple genomic sequences. To the best of our knowledge, the current study is the first genome-wide analysis of the MAPKKK genes in apple. Information gained through this study may contribute to our understanding of the classification and putative functions of MdMAPKKKs in apple
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