PMC:1574314 / 23631-27439 JSONTXT

{"target":"https://pubannotation.org/docs/sourcedb/PMC/sourceid/1574314","sourcedb":"PMC","sourceid":"1574314","source_url":"https://www.ncbi.nlm.nih.gov/pmc/1574314","text":"MAPKKs\n\nGroup A MKKs\nIn 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.\n\nGroup B MKKs\nGroup B MKKs in poplar are represented by a single gene, PtMKK3. This is also seen in Arabidopsis (AtMKK3) and rice (OsMKK3) [39]. MKKs of this class are unique in encoding a characteristic MKK protein kinase domain fused in C-terminal to a putative nuclear transport factor 2 (NTF2) domain [4]. No biological functions have yet been assigned to plant MKK3s, but PtMKK3 has moderate to relatively high expression levels across all organs (TA between 400 to 2600), with the highest levels detected in female floral buds, lateral foliar buds and cell suspensions (Figure 10).\n\nGroup C MKKs\nAmong the plant MKKs, attention has been largely focused on those found in group C because of their demonstrated roles in stress signaling. Ectopic expression of constitutively-activated versions of the tobacco NtMEK2 protein, and of other group C MKKs, has been used to demonstrate that they are capable of phosphorylating and thus activating stress-responsive Group A MPKs [50-52]. Poplar possesses two group C MKKs, PtMKK4 and PtMKK5, both of which are expressed. Of the two, PtMKK5 shows higher TA in most organs, while the expression of PtMKK4 only predominates in suspension-cultured cells (Figure 10).\n\nGroup D MKKs\nLimited 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].\nWhile 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.","divisions":[{"label":"title","span":{"begin":0,"end":6}},{"label":"sec","span":{"begin":8,"end":1028}},{"label":"title","span":{"begin":8,"end":20}},{"label":"p","span":{"begin":21,"end":1028}},{"label":"sec","span":{"begin":1030,"end":1616}},{"label":"title","span":{"begin":1030,"end":1042}},{"label":"p","span":{"begin":1043,"end":1616}},{"label":"sec","span":{"begin":1618,"end":2239}},{"label":"title","span":{"begin":1618,"end":1630}},{"label":"p","span":{"begin":1631,"end":2239}},{"label":"title","span":{"begin":2241,"end":2253}},{"label":"p","span":{"begin":2254,"end":2980}}],"tracks":[]}