[show abstract][hide abstract] ABSTRACT: BACKGROUND: Plant cell walls are complex multicomponent structures that have evolved to fulfil an essential function in providing strength and protection to cells. Hemicelluloses constitute a key component of the cell wall and recently a number of the genes thought to encode the enzymes required for its synthesis have been identified in Arabidopsis. The acquisition of hemicellulose synthesis capability is hypothesised to have been an important step in the evolution of higher plants. RESULTS: Analysis of the Physcomitrella patens genome has revealed the presence of homologs for all of the Arabidopsis glycosyltransferases including IRX9, IRX10 and IRX14 required for the synthesis of the glucuronoxylan backbone. The Physcomitrella IRX10 homolog is expressed in a variety of moss tissues which were newly formed or undergoing expansion. There is a high degree of sequence conservation between the Physcomitrella IRX10 and Arabidopsis IRX10 and IRX10-L. Despite this sequence similarity, the Physcomitrella IRX10 gene is only able to partially rescue the Arabidopsis irx10 irx10-L double mutant indicating that there has been a neo- or sub-functionalisation during the evolution of higher plants. Analysis of the monosaccharide composition of stems from the partially rescued Arabidopsis plants does not show any significant change in xylose content compared to the irx10 irx10-L double mutant. Likewise, knockout mutants of the Physcomitrella IRX10 gene do not result in any visible phenotype and there is no significant change in monosaccharide composition of the cell walls. CONCLUSIONS: The fact that the Physcomitrella IRX10 (PpGT47A) protein can partially complement an Arabidopsis irx10 irx10-L double mutant suggests that it shares some function with the Arabidopsis proteins, but the lack of a phenotype in knockout lines shows that the function is not required for growth or development under normal conditions in Physcomitrella. In contrast, the Arabidopsis irx10 and irx10 irx10-L mutants have strong phenotypes indicating an important function in growth and development. We conclude that the evolution of vascular plants has been associated with a significant change or adaptation in the function of the IRX10 gene family.
[show abstract][hide abstract] ABSTRACT: Development in plants is controlled by abiotic environmental cues such as day length, light quality, temperature, drought, and salinity. These signals are sensed by a variety of systems and transmitted by different signal transduction pathways. Ultimately, these pathways are integrated to control expression of specific target genes, which encode proteins that regulate development and differentiation. The molecular mechanisms for such integration have remained elusive. We here show that a linear 130-amino-acids-long sequence in the Med25 subunit of the Arabidopsis thaliana Mediator is a common target for the drought response element binding protein 2A, zinc finger homeodomain 1, and Myb-like transcription factors which are involved in different stress response pathways. In addition, our results show that Med25 together with drought response element binding protein 2A also function in repression of PhyB-mediated light signaling and thus integrate signals from different regulatory pathways.
Proceedings of the National Academy of Sciences 05/2011; 108(20):8245-50. · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: Hexokinase catalyzes the phosphorylation of glucose and fructose, but it is also involved in sugar sensing in both fungi and plants. We have previously described two types of hexokinases in the moss Physcomitrella. Type A, exemplified by PpHxk1, the major hexokinase in Physcomitrella, is a soluble protein that localizes to the chloroplast stroma. Type B, exemplified by PpHxk2, has an N-terminal membrane anchor. Both types are found also in vascular plants, and localize to the chloroplast stroma and mitochondrial membranes, respectively.
We have now characterized all 11 hexokinase encoding genes in Physcomitrella. Based on their N-terminal sequences and intracellular localizations, three of the encoded proteins are type A hexokinases and four are type B hexokinases. One of the type B hexokinases has a splice variant without a membrane anchor, that localizes to the cytosol and the nucleus. However, we also found two new types of hexokinases with no obvious orthologs in vascular plants. Type C, encoded by a single gene, has neither transit peptide nor membrane anchor, and is found in the cytosol and in the nucleus. Type D hexokinases, encoded by three genes, have membrane anchors and localize to mitochondrial membranes, but their sequences differ from those of the type B hexokinases. Interestingly, all moss hexokinases are more similar to each other in overall sequence than to hexokinases from other plants, even though characteristic sequence motifs such as the membrane anchor of the type B hexokinases are highly conserved between moss and vascular plants, indicating a common origin for hexokinases of the same type.
We conclude that the hexokinase gene family is more diverse in Physcomitrella, encoding two additional types of hexokinases that are absent in vascular plants. In particular, the presence of a cytosolic and nuclear hexokinase (type C) sets Physcomitrella apart from vascular plants, and instead resembles yeast, where all hexokinases localize to the cytosol. The fact that all moss hexokinases are more similar to each other than to hexokinases from vascular plants, even though both type A and type B hexokinases are present in all plants, further suggests that the hexokinase gene family in Physcomitrella has undergone concerted evolution.
[show abstract][hide abstract] ABSTRACT: The moss Physcomitrella is unique among plants in that it permits efficient gene targeting by homologous recombination. Furthermore, transformed DNA can replicate episomally in Physcomitrella. Here we show that episomally replicating DNA can be rescued back into Escherichia coli, and we use such rescue to study the fate of the transformed DNA. Significantly, plasmids rescued from moss transformed with circular DNA are identical to the original plasmid, whereas plasmids rescued from moss transformed with linearized DNA frequently have deletions created by direct repeat recombination. These events are highly predictable in that they target the longest direct repeat on the plasmid if this repeat is at least 12 bp. Episomal transformants obtained with linearized DNA show a more than 1,000-fold amplification of the DNA whereas transformants obtained with circular DNA have much lower copy numbers. Most episomal transformants quickly lose the plasmid in the absence of selection, but a semistable type of transformant that loses the plasmid at a much lower frequency was also observed. The consistent rescue of the original plasmid, or of predictable derivatives thereof, suggests that molecular genetics methods which rely on shuttle plasmids are feasible in Physcomitrella.
Proceedings of the National Academy of Sciences 11/2009; 106(46):19444-9. · 9.74 Impact Factor