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

Abstract

KEYWORDSChinese jujube, MAPKKKs, Jujube witches' broom, Phytoplasma, Expression profiles salinity, cold and pathogen attack [1][2][3]. The conserved serine/threonine MAPKKKs can be activated by plasma membrane receptors, these in turn phosphorylate the MAPKKs, which then activate the MAPKs by sequential phosphorylation. Finally, the MAPKs regulate other kinases or related transcription factors in response to various stresses [4, 5]. Each MAPK cascade family consists of a number of members, the number of members varies significantly between families. For example, the MAPKKKs family comprises a greater number of members and shows more complex sequence diversity than the other two families. The members belonging to the MAPKKKs family can be classified into the subfamilies Raf, ZIK and MEKK, according to their characteristic sequence motifs [6]. A structural diversity is found among MAPKKKs in each subfamily. The Raf subfamily has a C-terminal kinase domain and a long N-terminal regulatory domain, while ZIK proteins have only an N-terminal kinase domains and the MEKK subfamily has fewer conserved kinase domains. In addition, a long N-terminal regulatory domain forms the backbone for the Raf and ZIK subfamilies [1, 6]. The MAPK cascades have been implicated in signal transduction in distinct innate immunity [7, 8]. In Arabidopsis, the MEKK1-MKK4/5-MPK3/6-WRKY22/WRKY29/FRK1 cascade is involved in innate immunity signalling transduction and the MEKK1-MKK1/MKK2-MPK4 kinase cascade can negatively activate MEKK2 which further leads to a SUMM2-mediated immune responses [9, 10]. In tobacco, NPK1-MEK1-Ntf6 can regulate WRKY/MYB transcription factors to participate in the tobacco mosaic virus infection pathway [11]. In addition, the MAPKKKα-MKK2/MKK4-MPK2/MPK3 cascades take part in the Pto-mediated effect or triggered immunity (ETI) pathway by regulating the transcription factor TGA in tomato [12]. Hence, the MAPKs pathway is indeed involved in response to pathogen attack and may also play essential roles in the response to phytoplasma infection in Chinese jujube. The MAPKKKs family has now been characterised in the plant kingdom. A total of 80 MAPKKKs were first identified in Arabidopsis in 2002 [6, 13]. Since then, an array of different MAPKKKs have been identified from in a range of plant species including rice (75 members), Zea mays (71 members), Vitis vinifera (45 members), Malus domestica (72 members) and Musa nana (77 members) [14-17].However, little is known about the biological information and function of the MAPKKK gene family in Chinese jujube, even though detailed information for ZjMAPKKs and ZjMAPKs has previously been ZjMAPKKKs subfamily was used to find the highest number of conserved domains [30]. Sequence alignment, phylogenetic and gene duplication analysisAll the protein sequences of ZjMAPKKKs were aligned by ClusterX software using the default parameter values. The alignments of the protein sequences of 56 ZjMAPKKKs and 80 AtMAPKKKs were then subjected to phylogenetic analysis using MEGA 6.06 alignment expl...