Mitogen-activated protein kinase activity and reporter gene assays in plants

Experimental toolbox for quantitative evaluation of clathrin-mediated endocytosis in the plant model Arabidopsis

Clathrin-mediated endocytosis (CME) is a crucial cellular process implicated in many aspects of plant growth, development, intra- and intercellular signaling, nutrient uptake and pathogen defense. Despite these significant roles, little is known about the precise molecular details of how CME functions in planta To facilitate the direct quantitative study of plant CME, we review current routinely used methods and present refined, standardized quantitative imaging protocols that allow the detailed characterization of CME at multiple scales in plant tissues. These protocols include: (1) an efficient electron microscopy protocol for the imaging of Arabidopsis CME vesicles in situ, thus providing a method for the detailed characterization of the ultrastructure of clathrin-coated vesicles; (2) a detailed protocol and analysis for quantitative live-cell fluorescence microscopy to precisely examine the temporal interplay of endocytosis components during single CME events; (3) a semi-automated analysis to allow the quantitative characterization of global internalization of cargos in whole plant tissues; and (4) an overview and validation of useful genetic and pharmacological tools to interrogate the molecular mechanisms and function of CME in intact plant samples.This article has an associated First Person interview with the first author of the paper.

Evolutionarily unique mechanistic framework of clathrin-mediated endocytosis in plants

In plants, clathrin mediated endocytosis (CME) represents the major route for cargo internalisation from the cell surface. It has been assumed to operate in an evolutionary conserved manner as in yeast and animals. Here we report characterisation of ultrastructure, dynamics and mechanisms of plant CME as allowed by our advancement in electron microscopy and quantitative live imaging techniques. Arabidopsis CME appears to follow the constant curvature model and the bona fide CME population generates vesicles of a predominantly hexagonal-basket type; larger and with faster kinetics than in other models. Contrary to the existing paradigm, actin is dispensable for CME events at the plasma membrane but plays a unique role in collecting endocytic vesicles, sorting of internalised cargos and directional endosome movement that itself actively promote CME events. Internalized vesicles display a strongly delayed and sequential uncoating. These unique features highlight the independent evolution of the plant CME mechanism during the autonomous rise of multicellularity in eukaryotes.

Coevolving MAPK and PID phosphosites indicate an ancient environmental control of PIN auxin transporters in land plants

Plant growth flexibly adapts to environmental conditions, implying cross-talk between environmental signalling and developmental regulation. Here, we show that the PIN auxin efflux carrier family possesses three highly conserved putative mitogen-activated protein kinase (MAPK) sites adjacent to the phosphorylation sites of the well-characterised AGC kinase PINOID, which regulates the polar localisation of PINs and directional auxin transport, thereby underpinning organ growth. The conserved sites of PIN1 are phosphorylated in vitro by two environmentally activated MAPKs, MPK4 and MPK6. In contrast to AGC kinases, MAPK-mediated phosphorylation of PIN1 at adjacent sites leads to a partial loss of the plasma membrane localisation of PIN1. MAPK-mediated modulation of PIN trafficking may participate in environmental adjustment of plant growth.

Kinase-Associated Phosphoisoform Assay: a novel candidate-based method to detect specific kinase-substrate phosphorylation interactions in vivo

BACKGROUND

Protein kinases are important components of signalling pathways, and kinomes have remarkably expanded in plants. Yet, our knowledge of kinase substrates in plants is scarce, partly because tools to analyse protein phosphorylation dynamically are limited. Here we describe Kinase-Associated Phosphoisoform Assay, a flexible experimental method for directed experiments to study specific kinase-substrate interactions in vivo. The concept is based on the differential phosphoisoform distribution of candidate substrates transiently expressed with or without co-expression of activated kinases. Phosphorylation status of epitope-tagged proteins is subsequently detected by high-resolution capillary isoelectric focusing coupled with nanofluidic immunoassay, which is capable of detecting subtle changes in isoform distribution.

RESULTS

The concept is validated by showing phosphorylation of the known mitogen-activated protein kinase (MAPK) substrate, ACS6, by MPK6. Next, we demonstrate that two transcription factors, WUS and AP2, both of which are shown to be master regulators of plant development by extensive genetic studies, exist in multiple isoforms in plant cells and are phosphorylated by activated MAPKs.

CONCLUSION

As plant development flexibly responds to environmental conditions, phosphorylation of developmental regulators by environmentally-activated kinases may participate in linking external cues to developmental regulation. As a counterpart of advances in unbiased screening methods to identify potential protein kinase substrates, such as phosphoproteomics and computational predictions, our results expand the candidate-based experimental toolkit for kinase research and provide an alternative in vivo approach to existing in vitro methodologies.

Dual role of tree florigen activation complex component FD in photoperiodic growth control and adaptive response pathways

A complex consisting of evolutionarily conserved FD, flowering locus T (FT) proteins is a regulator of floral transition. Intriguingly, FT orthologs are also implicated in developmental transitions distinct from flowering, such as photoperiodic control of bulbing in onions, potato tuberization, and growth cessation in trees. However, whether an FT-FD complex participates in these transitions and, if so, its mode of action, are unknown. We identified two closely related FD homologs, FD-like 1 (FDL1) and FD-like 2 (FDL2), in the model tree hybrid aspen. Using gain of function and RNAi-suppressed FDL1 and FDL2 transgenic plants, we show that FDL1 and FDL2 have distinct functions and a complex consisting of FT and FDL1 mediates in photoperiodic control of seasonal growth. The downstream target of the FT-FD complex in photoperiodic control of growth is Like AP1 (LAP1), a tree ortholog of the floral meristem identity gene APETALA1. Intriguingly, FDL1 also participates in the transcriptional control of adaptive response and bud maturation pathways, independent of its interaction with FT, presumably via interaction with abscisic acid insensitive 3 (ABI3) transcription factor, a component of abscisic acid (ABA) signaling. Our data reveal that in contrast to its primary role in flowering, FD has dual roles in the photoperiodic control of seasonal growth and stress tolerance in trees. Thus, the functions of FT and FD have diversified during evolution, and FD homologs have acquired roles that are independent of their interaction with FT.

Suppression of xylan endotransglycosylase PtxtXyn10A affects cellulose microfibril angle in secondary wall in aspen wood

Certain xylanases from family GH10 are highly expressed during secondary wall deposition, but their function is unknown. We carried out functional analyses of the secondary-wall specific PtxtXyn10A in hybrid aspen (Populus tremula × tremuloides). PtxtXyn10A function was analysed by expression studies, overexpression in Arabidopsis protoplasts and by downregulation in aspen. PtxtXyn10A overexpression in Arabidopsis protoplasts resulted in increased xylan endotransglycosylation rather than hydrolysis. In aspen, the enzyme was found to be proteolytically processed to a 68 kDa peptide and residing in cell walls. Its downregulation resulted in a corresponding decrease in xylan endotransglycosylase activity and no change in xylanase activity. This did not alter xylan molecular weight or its branching pattern but affected the cellulose-microfibril angle in wood fibres, increased primary growth (stem elongation, leaf formation and enlargement) and reduced the tendency to form tension wood. Transcriptomes of transgenic plants showed downregulation of tension wood related genes and changes in stress-responsive genes. The data indicate that PtxtXyn10A acts as a xylan endotransglycosylase and its main function is to release tensional stresses arising during secondary wall deposition. Furthermore, they suggest that regulation of stresses in secondary walls plays a vital role in plant development.