Exploring the evolutionary path of plant MAPK networks.

The evolutionarily conserved mitogen-activated protein kinase (MAPK) signaling network comprises connected protein kinases arranged in MAPK modules. In this Opinion article, we analyze MAPK signaling components in evolutionarily representative species of the plant lineage and in Naegleria gruberi, a member of an early diverging eukaryotic clade. In Naegleria, there are two closely related MAPK kinases (MKKs) and a single conventional MAPK, whereas in several species of algae, there are two distinct MKKs and multiple MAPKs belonging to different groups. This suggests that the formation of multiple MAPK modules began early during plant evolution. The expansion of MAPK signaling components through gene duplications and the evolution of interaction motifs could have contributed to the highly connected complex MAPK signaling network that we know in Arabidopsis.

Comprehensive gene expression atlas for the Arabidopsis MAP kinase signalling pathways.

* Mitogen activated protein kinase (MAPK) pathways are signal transduction modules with layers of protein kinases having c. 120 genes in Arabidopsis, but only a few have been linked experimentally to functions. * We analysed microarray expression data for 114 MAPK signalling genes represented on the ATH1 Affymetrix arrays; determined their expression patterns during development, and in a wide range of time-course microarray experiments for their signal-dependent transcriptional regulation and their coregulation with other signalling components and transcription factors. * Global expression correlation of the MAPK genes with each of the represented 21 692 Arabidopsis genes was determined by calculating Pearson correlation coefficients. To group MAPK signalling genes based on similarities in global regulation, we performed hierarchical clustering on the pairwise correlation values. This should allow inferring functional information from well-studied MAPK components to functionally uncharacterized ones. Statistical overrepresentation of specific gene ontology (GO) categories in the gene lists showing high expression correlation values with each of the MAPK components predicted biological themes for the gene functions. * The combination of these methods provides functional information for many uncharacterized MAPK genes, and a framework for complementary future experimental dissection of the function of this complex family.

Learning the lipid language of plant signalling.

Plant cells respond to different biotic and abiotic stresses by producing various uncommon phospholipids that are believed to play key roles in cell signalling. We can predict how they work because animal and yeast proteins have been shown to have specific lipid-binding domains, which act as docking sites. When such proteins are recruited to the membrane locations where these phospholipids are synthesized, the phospholipids activate them directly, by inducing a conformational change, or indirectly, by juxtaposing them with an activator protein. The same lipid-binding domains are present in Arabidopsis proteins. We believe that they represent an untapped well of information about plant lipid signalling.

Growth signalling pathways in Arabidopsis and the AGC protein kinases.

Lipid-derived signals are central to regulating a multitude of cellular processes but, in plants, little is known of the downstream signalling pathways. The Arabidopsis 3-phosphoinositide-dependent protein kinase (PDK1) could couple lipid signals to the activation of several protein kinases of the so-called AGC kinase family. The Arabidopsis AGC kinases contain sequence motives required for the docking of PDK1 and phosphorylation of their activation loop in the kinase catalytic domain. It is becoming evident that specific members of the AGC kinases are implicated in key growth signalling pathways. For example, Arabidopsis p70(S6K) might be a nodal point able to integrate hormonal and developmental signals with nutritional inputs, together with the Arabidopsis Target of Rapamycin (TOR) protein.

First glance at the plant APC/C, a highly conserved ubiquitin-protein ligase.

The anaphase-promoting complex or cyclosome (APC/C) is a cell-cycle-regulated ubiquitin-protein ligase that has been extensively studied in both fungal and animal cells. Many APC/C protein targets have been identified, and their sequential degradation during the cell cycle is essential for chromatid separation and mitotic exit. APC/C-dependent ubiquitylation of proteins not involved in cell-cycle progression has also been documented in animal cells. By contrast, the plant APC/C's structure and functions remained unexplored until recently. Here, we discuss recent developments in this field and explore the Arabidopsis genome sequence to identify plant APC/C components. Details of the APC/C ubiquitylation pathway in Arabidopsis are also available on a website that will be regularly updated.

Complexity, cross talk and integration of plant MAP kinase signalling.

Mitogen-activated protein kinases (MAPKs) link information transfer from external stimuli-activated sensors to cellular responses. The completed Arabidopsis genome sequence revealed an extraordinary complexity in MAPK-signalling components in plants. Information obtained from Arabidopsis provides a framework for a unified nomenclature and the assembly and function of MAPK-signalling pathways. Strategies and tools are evolving to connect MAPK pathways and to determine their function. As a result, MAPK signalling modules emerged, one of which appears to antagonistically regulate stress- and growth-responses and another that regulates cytokinesis.