Group D MKKs
Limited functional information is available for group D MKKs, of which there are five encoded representatives in the poplar genome (PtMKK7, PtMKK9, PtMKK10, PtMKK11-1 and PtMKK11-2). However, our RTqPCR analysis suggests that only PtMKK7 and PtMKK9 are clearly expressed (Figure 10). The other three genes may therefore be pseudogenes, or be expressed only under circumstances that were not tested in our survey. The pseudogene hypothesis is also supported by the observation of structural differences in normally conserved motifs within the predicted PtMKK10, 11-1 and 11-2 protein kinase domains, and by the apparent absence of expression data for the Arabidopsis (AtMKK10) and rice (OsMKK10-1/10-2) putative orthologs [39].
While both PtMKK7 and PtMKK9 expression could be detected, the patterns differ significantly across organs and developmental stages (Figure 10). PtMKK9 expression is generally more pronounced except in secondary xylem, cell suspensions, primary phloem and xylem cambium-enriched, where PtMKK7 predominates. PtMKK9 is particularly highly expressed in mature leaves (LPI 12), where it reaches close to 10 000 transcript molecules per ng of total RNA, in contrast to the younger leaf sample (LPI1). The levels of transcript accumulation for PtMKK7 are on the other hand relatively constant across most organs. This striking difference in expression pattern has also been observed in Arabidopsis expression databases for AtMKK7 and AtMKK9 [53] where AtMKK9 transcription is most strongly associated with mature or senescing leaves.