Group A MKKs
In other plant species, some group A MKKs appear to be functionally associated with group B MPKs [48,49] These phosphotransfer relationships have been involved in responses to abiotic stresses in Arabidopsis, and in cell development in tobacco. As in Arabidopsis, there are three group A MKK genes found in poplar. PtMKK2-2 shows low but constant levels of TA in all tested organs (Figure 10; see additional file 3), while the paralogous PtMKK2-1 is much more highly expressed. Higher expression of PtMKK6, on the other hand, seems to be associated with proliferating organs such as apex, floral and terminal buds, cell suspensions, and young leaves (LPI 1), with a 25-fold decrease in PtMKK6 expression levels observed along the foliar developmental gradient from young to mature leaves. Interestingly, the presumed Arabidopsis and tobacco orthologs of PtMKK6 have been involved in regulation of cytokinesis and cell division [13], suggesting that this protein may play an analogous role in poplar tissues.