Genome-wide identification of MAPKKK genes and their responses to phytoplasma infection in Chinese jujube (Ziziphus jujuba Mill.).

BACKGROUND: Mitogen-activated protein kinase (MAPK) cascades play vital roles in signal transduction in response to a wide range of biotic and abiotic stresses. In a previous study, we identified ten ZjMAPKs and five ZjMAPKKs in the Chinese jujube genome. We found that some members of ZjMAPKs and ZjMAPKKs may play key roles in the plant's response to phytoplasma infection. However, how these ZjMAPKKs are modulated by ZjMAPKKKs during the response process has not been elucidated. Little information is available regarding MAPKKKs in Chinese jujube. RESULTS: A total of 56 ZjMAPKKKs were identified in the jujube genome. All of these kinases contain the key S-TKc (serine/threonine protein kinase) domain, which is distributed among all 12 chromosomes. Phylogenetic analyses show that these ZjMAPKKKs can be classified into two subfamilies. Specifically, 41 ZjMAPKKKs belong to the Raf subfamily, and 15 belong to the MEKK subfamily. In addition, the ZjMAPKKKs in each subfamily share the same conserved motifs and gene structures. Only one pair of ZjMAPKKKs (15/16, on chromosome 5) was found to be tandemly duplicated. Using qPCR, the expression profiles of these MAPKKKs were investigated in response to infection with phytoplasma. In the three main infected tissues (witches' broom leaves, phyllody leaves, and apparently normal leaves), ZjMAPKKK26 and - 45 were significantly upregulated, and ZjMAPKKK3, - 43 and - 50 were significantly downregulated. ZjMAPKKK4, - 10, - 25 and - 44 were significantly and highly induced in sterile cultivated tissues infected by phytoplasma, while ZjMAPKKK6, - 7, - 17, - 18, - 30, - 34, - 35, - 37, - 40, - 41, - 43, - 46, - 52 and - 53 were significantly downregulated. CONCLUSIONS: For the first time, we present an identification and classification analysis of ZjMAPKKKs. Some ZjMAPKKK genes may play key roles in the response to phytoplasma infection. This study provides an initial understanding of the mechanisms through which ZjMAPKKKs are involved in the response of Chinese jujube to phytoplasma infection.