Dissecting mitogen-activated protein kinase cascades involving arabidopsis MKK6

• Main Author: Zeng, Qingning
• Language: English
• Published: University of British Columbia 2011
• Online Access: http://hdl.handle.net/2429/31941

Mitogen-activated protein (MAP) kinase cascades act as critical components in the signalling pathways of all eukaryotic cells. They play a pivotal role in the transduction of extra- and intra-cellular stimuli and regulate cell growth, proliferation, differentiation and cell death, through sequential activation of MAP kinase kinase kinases (MAPKKKs), MAP kinase kinases (MKKs), and MAP kinases (MPKs). These three components form modules that control the phosphorylation of various substrates including transcription factors, enzymes, and cytoskeleton-associated proteins. In the Arabidopsis genome, over 60 MAPKKKs (AtMKKK), 10 MAPKKs (AtMKK), and 20 MAPKs (AtMPK) have been identified. The smaller number of AtMKKs suggests that diverse signals may converge and be integrated at the level of AtMKK. Among the ten AtMKKs, MKK6 has been proposed to play a role in regulating cytokinesis. However, little is known about the hierarchal phosphorylation system containing MKK6. In this Ph.D. project, I aimed to dissect the MAP kinase cascades involving MKK6 in Arabidopsis. I investigated potential targets of MKK6. Four MAP kinases were identified to interact with, and be phosphorylated by, MKK6, namely, MPK4, MPK6, MPK11, and MPK13. Among them, MPK13 is developmentally co-expressed with MKK6, and both MPK13 and MKK6 display high Promoter::GUS activity at the primary root tips and at the lateral root primordia. Partial suppression of either MKK6 or MPK13 expression significantly reduces the number of lateral roots in the transgenic lines, suggesting that the MKK6-MPK13 module positively regulates lateral root formation. Loss-of-function mutants of another potential target of MKK6, MPK4, are severely affected. In mpk4 mutant plants, anthers can develop normal microspore mother cells (MMCs) and peripheral supporting tissues, but the MMCs fail to complete meiotic cytokinesis, a phenotype reminiscent of those observed in both atnack2/tes/stud and anq1/mkk6 mutants. Biochemical analysis defines a putative signalling module linking AtNACK2/TES/STUD with ANP3, MKK6 and MPK4, suggesting a model in which the AtNACK2-ANP3-MKK6-MPK4 cascade specifically facilitates male meiotic cytokinesis in Arabidopsis. In summary, my PhD work has expanded our understanding of the diversity of MAP kinase cascades. Different MPKs can act as downstream targets of the same MKK with tight spatial and temporal regulation during plant development.