A previous study showed that MKK8 and MKK10 have not expressed in shoot apices (including flower buds), mesophyll cells, mature leaves [26] and flowers  (including developing young siliques) [42]. Based on the expression **data** of Arabidopsis from PLEXdb (AT40), we found that all Arabidopsis MKK genes are expressed (Fig. 6a). Except AtMKK8, most MKK genes have transcribed in the flowering stage. With the exception of MKK10, the Arabidopsis MKKs have a consensus S/ T-X 5 -S/T in their activation domain. Even MKK10 have an atypical S/T-X 5 -S/T motif show detectable expression, indicating that they possibly have a new function other than becoming silenced. It is interesting that the expression of AtMKK10 is much higher in pollen compared with other tissues. Arabidopsis MKKs genetically exhibit the crosstalk in a variety of signaling pathways, therefore, a single mkk mutant under normal growth conditions is very difficult to obtain a specific phenotype [43]. A previous study showed that the ap1-15, ap2-6, ap3-6, ag-12, clv3-7, lfy-12 and ufo-1 mutants involve with flower development, whereas AtMKK10 was down regulated in these mutants (Fig. 6b), consistent with their expression in the flower. CLV has a potential role in the regulation of meristem size and has little expressed in the center of the meristem [44,45]. The bam (CLV homologs) mutant exhibited rather weak phenotypes and regulated the male gametophyte development, leading to almost completely sterile plants [45]. Therefore, AtMKK10 which is regulated by CLV proteins may participate in the process of pollen development (Fig. 6c).