To cope with environmental fluctuations and to prevent invasion by pathogens, plant metabolism must be flexible and dynamic. Active oxygen species, whose formation is accelerated under stress conditions, must be rapidly processed if oxidative damage is to be averted. The lifetime of active oxygen species within the cellular environment is determined by the antioxidative system, which provides crucial protection against oxidative damage. The antioxidative system comprises numerous enzymes and compounds of low molecular weight. While research into the former has benefited greatly from advances in molecular technology, the pathways by which the latter are synthesized have received comparatively little attention. The present review emphasizes the roles of ascorbate and glutathione in plant metabolism and stress tolerance. We provide a detailed account of current knowledge of the biosynthesis, compartmentation, and transport of these two important antioxidants, with emphasis on the unique insights and advances gained by molecular exploration.

ASCORBATE AND GLUTATHIONE: Keeping Active Oxygen Under Control

https://www.cell.com/trends/plant-science/pdf/S1360-1385(02)02251-3.pdf

GATEWAY vectors for Agrobacterium-mediated plant transformation.

Jasmonates: biosynthesis, perception, signal transduction and action in plant stress response, growth and development. An update to the 2007 review in Annals of Botany

Studies of the molecular genetics, mechanisms of activation and functional connections between general phenylpropanoid metabolism and certain branch pathways in parsley, bean, potato plants and cell cultures

Physiology and Molecular Biology of Phenylpropanoid Metabolism

Plants sense potential microbial invaders by using pattern-recognition receptors to recognize pathogen-associated molecular patterns (PAMPs). In Arabidopsis thaliana, the leucine-rich repeat receptor kinases flagellin-sensitive 2 (FLS2) (ref. 2) and elongation factor Tu receptor (EFR) (ref. 3) act as pattern-recognition receptors for the bacterial PAMPs flagellin and elongation factor Tu (EF-Tu) (ref. 5) and contribute to resistance against bacterial pathogens. Little is known about the molecular mechanisms that link receptor activation to intracellular signal transduction. Here we show that BAK1 (BRI1-associated receptor kinase 1), a leucine-rich repeat receptor-like kinase that has been reported to regulate the brassinosteroid receptor BRI1 (refs 6,7), is involved in signalling by FLS2 and EFR. Plants carrying bak1 mutations show normal flagellin binding but abnormal early and late flagellin-triggered responses, indicating that BAK1 acts as a positive regulator in signalling. The bak1-mutant plants also show a reduction in early, but not late, EF-Tu-triggered responses. The decrease in responses to PAMPs is not due to reduced sensitivity to brassinosteroids. We provide evidence that FLS2 and BAK1 form a complex in vivo, in a specific ligand-dependent manner, within the first minutes of stimulation with flagellin. Thus, BAK1 is not only associated with developmental regulation through the plant hormone receptor BRI1 (refs 6,7), but also has a functional role in PRR-dependent signalling, which initiates innate immunity.

/pdf/a-flagellin-induced-complex-of-the-receptor-fls2-and-bak1-357o3yyeyl.pdf

A flagellin-induced complex of the receptor FLS2 and BAK1 initiates plant defence

Functional Analysis and Phosphorylation Site Mapping of Leucine‐Rich Repeat Receptor‐Like Kinases

The plasma membrane proteins are critical components in cellular control and differentiation and thus are of special interest to those studying signal transduction mechanisms in all organisms. When conducting proteomic studies on membrane components of cells and tissues, the complexity is not simply confined to the large number of proteins present in the sample but also to the highly hydrophobic nature of membrane proteins containing multiple transmembrane domains. Consequently, these proteins are more difficult to analyze by mass spectrometry, particularly if protein sequence coverage is to be established. This chapter contains a method for extraction, solubilization, alkylation, proteolysis, and identification of hydrophobic integral plasma membrane proteins for large-scale proteomic analysis using strong cation exchange chromatography (SCXC) and liquid chromatography-tandem mass spectrometry (LC/MS/MS). In our approach, microsomes are isolated from plant tissue and then subjected to a two-phase extraction procedure to enrich for plasma membranes. Proteins are extracted and solubilized from the membrane using a methanol-aqueous buffer system that allows for effective reduction, cysteinyl alkylation, and tryptic digestion for subsequent SCXC-LC/MS/MS analysis. Our protocol is also amenable to isotope labeling methods to quantify integral membrane protein expression and post-translational modifications. In addition to plants, the method can be applied to other systems quite readily; thus, we anticipate that it will be of general interest to those characterizing plasma membrane proteins of any organism.

Enrichment and preparation of plasma membrane proteins from Arabidopsis thaliana for global proteomic analysis using liquid chromatography-tandem mass spectrometry

Brassinosteroids (BRs) regulate multiple aspects of plant growth and development and require an active BRASSINOSTEROIDINSENSITIVE1 (BRI1) and BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1) for hormone perception and signal transduction. Many animal receptor kinases exhibit ligand-dependent oligomerization followed by autophosphorylation and activation of the intracellular kinase domain. To determine if early events in BR signaling share this mechanism, we used coimmunoprecipitation of epitope-tagged proteins to show that in vivo association of BRI1 and BAK1 was affected by endogenous and exogenous BR levels and that phosphorylation of both BRI1 and BAK1 on Thr residues was BR dependent. Immunoprecipitation of epitope-tagged BRI1 from Arabidopsis thaliana followed by liquid chromatography‐tandem mass spectrometry (LC/MS/MS) identified S-838, S-858, T-872, and T-880 in the juxtamembrane region, T-982 in the kinase domain, and S-1168 in C-terminal region as in vivo phosphorylation sites of BRI1. MS analysis also strongly suggested that an additional two residues in the juxtamembrane region and three sites in the activation loop of kinase subdomain VII/VIII were phosphorylated in vivo. We also identified four specific BAK1 autophosphorylation sites in vitro using LC/MS/MS. Sitedirected mutagenesis of identified and predicted BRI1 phosphorylation sites revealed that the highly conserved activation loop residue T-1049 and either S-1044 or T-1045 were essential for kinase function in vitro and normal BRI1 signaling in planta. Mutations in the juxtamembrane or C-terminal regions had only small observable effects on autophosphorylation and in planta signaling but dramatically affected phosphorylation of a peptide substrate in vitro. These findings are consistent with many aspects of the animal receptor kinase model in which ligand-dependent autophosphorylation of the activation loop generates a functional kinase, whereas phosphorylation of noncatalytic intracellular domains is required for recognition and/or phosphorylation of downstream substrates.

Identification and Functional Analysis of in Vivo Phosphorylation Sites of the Arabidopsis BRASSINOSTEROID-INSENSITIVE1

The BRASSINOSTEROID INSENSITIVE1 (BRI1) receptor kinase has recently been shown to possess tyrosine kinase activity, and preventing autophosphorylation of the tyrosine-831 regulatory site by site-directed mutagenesis enhances shoot growth. In this study, we characterized the increased leaf growth of Arabidopsis (Arabidopsis thaliana) plants expressing BRI1(Y831F)-Flag compared with BRI1-Flag (both driven by the native promoter and expressed in the bri1-5 weak allele background) and provide insights into the possible mechanisms involved. On average, relative leaf growth rate was increased 16% in the Y831F plants (in the bri1-5 background), and the gain of function of the Y831F-directed mutant was dominant in the wild-type background. Leaves were larger as a result of increased cell numbers and had substantially increased vascularization. Transcriptome analysis indicated that genes associated with brassinolide biosynthesis, secondary cell wall biosynthesis and vascular development, and regulation of growth were altered in expression and may contribute to the observed changes in leaf architecture and whole plant growth. Analysis of gas exchange and chlorophyll fluorescence indicated that Y831F mutant plants had higher rates of photosynthesis, and metabolite analysis documented enhanced accumulation of starch, sucrose, and several amino acids, most prominently glycine and proline. These results demonstrate that mutation of BRI1 can enhance photosynthesis and leaf growth/vascularization and may suggest new approaches to increase whole plant carbon assimilation and growth.

Enhancing Arabidopsis Leaf Growth by Engineering the BRASSINOSTEROID INSENSITIVE1

In humans and other animals, steroid hormones regulate transcription via membrane-bound receptors. These receptors have an extracellular ligand-binding domain and an intracellular domain that are responsible for transducing the signal to the next member along the signaling pathway. Brassinosteroids (BRs) are structurally similar to the animal steroid hormones found in vertebrates and insects. Plants also use steroids as signaling molecules. BRs regulate the expression of numerous genes associated with plant development, and require the activity of a serine/threonine (Ser/ Thr) receptor kinase to realize these effects. Signaling in dicotyledonous (e.g.,Arabidopsis thaliana)and monocotyle-donous (e.g.,Oryza sativa) models, is mediated by the receptor kinases BRI1 and OsBRI1, respectively. The extracellular domain of BRI1 perceives BRs, and the signal is then mediated via an intracellular kinase domain that autophosphorylates Ser and Thr residues and potentially, other substrates. BRI1 transduces steroid signals across the plasma membrane and mediates genomic effects.

Steven D. Clouse

Papers

Functional Analysis and Phosphorylation Site Mapping of Leucine‐Rich Repeat Receptor‐Like Kinases

Enrichment and preparation of plasma membrane proteins from Arabidopsis thaliana for global proteomic analysis using liquid chromatography-tandem mass spectrometry

Identification and Functional Analysis of in Vivo Phosphorylation Sites of the Arabidopsis BRASSINOSTEROID-INSENSITIVE1

Enhancing Arabidopsis Leaf Growth by Engineering the BRASSINOSTEROID INSENSITIVE1

Brassinosteroids: Modes of BR Action and Signal Transduction