Brassinosteroid signal transduction--choices of signals and receptors

Sending the right signals: regulating receptor kinase activity

Knowledge of the functions of plant receptor-like-kinases (RLKs) is increasing rapidly, but how their cytoplasmic signalling activity is regulated and how signals are transduced to cytoplasmic or nuclear proteins remain important questions. Recent studies, particularly of the BRASSINOSTEROID INSENSITIVE1 RLK, have begun to shed light on the mechanistic details of RLK activation, including the possible role of ligand binding. Studies of this and other RLKs have also highlighted the potential importance of hetero-oligomerisation and receptor internalisation in RLK signalling. Finally, a range of potential regulatory proteins and putative downstream signalling substrates have been identified for various RLKs. Despite some similarities with animal receptor kinase signalling systems, mechanisms that affect the intracellular behaviour, regulation and interactions of RLKs appear to be very diverse, potentially explaining how signalling specificity is maintained at the cytoplasmic level.

Brassinosteroids regulate plasma membrane anion channels in addition to proton pumps during expansion of Arabidopsis thaliana cells

Brassinosteroids (BRs) are involved in numerous physiological processes associated with plant development and especially with cell expansion. Here we report that two BRs, 28-homobrassinolide (HBL) and its direct precursor 28-homocastasterone (HCS), promote cell expansion of Arabidopsis thaliana suspension cells. We also show that cell expansions induced by HBL and HCS are correlated with the amplitude of the plasma membrane hyperpolarization they elicited. HBL, which promoted the larger cell expansion, also provoked the larger hyperpolarization. We observed that membrane hyperpolarization and cell expansion were partially inhibited by the proton pump inhibitor erythrosin B, suggesting that proton pumps were not the only ion transport system modulated by the two BRs. We used a voltage clamp approach in order to find the other ion transport systems involved in the PM hyperpolarization elicited by HBL and HCS. Interestingly, while anion currents were inhibited by both HBL and HCS, outward rectifying K+ currents were increased by HBL but inhibited by HCS. The different electrophysiological behavior shown by HBL and HCS indicates that small changes in the BR skeleton might be responsible for changes in bioactivity.

Effect of hydrogen peroxide on catalase gene expression, isoform activities and levels in leaves of potato sprayed with homobrassinolide and ultrastructural changes in mesophyll cells

The effect of hydrogen peroxide (H₂O₂) on catalase (CAT) isoform activities and amounts and on mRNA levels was studied in leaves from potato plants untreated and treated with homobrassinolide (HBR). Northern blot analysis revealed that 100 mm H₂O₂ supplied through the leaf petiole for 4 h did not induce CAT expression. In contrast, CAT1 and CAT2 responded differently to longer treatment, as CAT2 transcript levels increased markedly whereas CAT1 transcript levels remained unchanged. Western blot analysis showed disparity between the level of CAT1 transcript and CAT1 amount, which actually decreased after 28 h. CAT2 amount correlated well with transcript accumulation and CAT2 activity as visualised by zymogram analysis. H₂O₂ modified the relative importance of CAT isoforms. After 4 h, CAT1 was prevalent in untreated and H₂O₂-treated leaves. After 28 h, CAT2 was prevalent in H₂O₂-treated leaves; therefore, the quantified increase in total CAT activity in these leaves was due to the rise in CAT2. HBR pre-treatment increased CAT2 basal level not changing the pattern of CAT responses to H₂O₂, only lowering its amplitude. Even so, ultrastructural studies showed that HBR significantly reduced H₂O₂ negative effects on cellular sub-structures, allowing better recovery of affected structures and reducing the macroscopic injury symptoms on leaves, thus data point to a HBR protective role.

Is C‐26 hydroxylation an evolutionarily conserved steroid inactivation mechanism?

Sterols are essential components of virtually all higher eukaryotic organisms, though the exact identity of the dominating sterol varies between species, from the C-27 of cholesterol in vertebrates to the C-28 and C-29 sterols of plants and invertebrates. In addition to their role as structural components of cell membranes these sterols are also converted into a variety of biologically active hormones. This conversion generally involves modifications of the basic structure of the sterol by dealkylation, hydroxylation and/or isomerization. Recent studies have demonstrated that irreversible inactivation of both plant and insect hormones is achieved by a specific C-26 hydroxylation. The concept of sterol deactivation by 26-hydroxylation appears to be an example of an evolutionarily conserved mechanism that has persisted despite the widely varying requirements for sterols in the species where it has been detected.

Microarray and proteomic analysis of brassinosteroid- and gibberellin-regulated gene and protein expression in rice

Brassinosteroid (BR) and gibberellin (GA) are two groups of plant growth regulators essential for normal plant growth and development. To gain insight into the molecular mechanism by which BR and GA regulate the growth and development of plants, especially the monocot plant rice, it is necessary to identify and analyze more genes and proteins that are regulated by them. With the availability of draft sequences of two major types, japonica and indica rice, it has become possible to analyze expression changes of genes and proteins at genome scale. In this review, we summarize rice functional genomic research by using microarray and proteomic approaches and our recent research results focusing on the comparison of cDNA microarray and proteomic analyses of BR- and GA-regulated gene and protein expression in rice. We believe our findings have important implications for understanding the mechanism by which BR and GA regulate the growth and development of rice.

It runs in the family: regulation of brassinosteroid signaling by the BZR1-BES1 class of transcription factors

Research of the molecular events underlying brassinosteroid (BR) signaling is emerging as a new paradigm for studying plant hormone signal transduction pathways. Two recent reports have shown that the BZR1-BES1 family of proteins is a class of novel plant-specific transcription factors that directly bind to and regulate BR-responsive genes, thus filling in key details of the link between hormonal signal transmission in the cytoplasm and transcriptional status change in the nucleus.

Herbivore-induced, indirect plant defences

Indirect responses are defensive strategies by which plants attract natural enemies of their herbivores that act as plant defending agents. Such defences can be either constitutively expressed or induced by the combined action of mechanical damage and low- or high-molecular-weight elicitors from the attacking herbivore. Here, we focus on two induced indirect defences, namely the de novo production of volatiles and the secretion of extrafloral nectar, which both mediate interactions with organisms from higher trophic levels (i.e., parasitoids or carnivores). We give an overview on elicitors, early signals, and signal transduction resulting in a complex regulation of indirect defences and discuss effects of cross-talks between the signalling pathways (synergistic and antagonistic effects). In the light of recent findings, we review molecular and genetic aspects of the biosynthesis of herbivore-induced plant volatiles comprising terpenoids, aromatic compounds, and metabolites of fatty acids which act as infochemicals for animals and some of which even induce defence genes in neighbouring plants. Finally, ecological aspects of these two indirect defences such as their variability, specificity, evolution as well as their ecological relevance in nature are discussed.

BZR1 is a transcriptional repressor with dual roles in brassinosteroid homeostasis and growth responses

Brassinosteroid (BR) homeostasis and signaling are crucial for normal growth and development of plants. BR signaling through cell-surface receptor kinases and intracellular components leads to dephosphorylation and accumulation of the nuclear protein BZR1. How BR signaling regulates gene expression, however, remains unknown. Here we show that BZR1 is a transcriptional repressor that has a previously unknown DNA binding domain and binds directly to the promoters of feedback-regulated BR biosynthetic genes. Microarray analyses identified additional potential targets of BZR1 and illustrated, together with physiological studies, that BZR1 coordinates BR homeostasis and signaling by playing dual roles in regulating BR biosynthesis and downstream growth responses.

Toward a systems approach to understanding plant cell walls

One of the defining features of plants is a body plan based on the physical properties of cell walls. Structural analyses of the polysaccharide components, combined with high-resolution imaging, have provided the basis for much of the current understanding of cell walls. The application of genetic methods has begun to provide new insights into how walls are made, how they are controlled, and how they function. However, progress in integrating biophysical, developmental, and genetic information into a useful model will require a system-based approach.

Arabidopsis membrane steroid binding protein 1 is involved in inhibition of cell elongation

A putative Membrane Steroid Binding Protein (designated MSBP1) was identified and functionally characterized as a negative regulator of cell elongation in Arabidopsis thaliana. The MSBP1 gene encodes a 220-amino acid protein that can bind to progesterone, 5-dihydrotestosterone, 24-epi-brassinolide (24-eBL), and stigmasterol with different affinities in vitro. Transgenic plants overexpressing MSBP1 showed short hypocotyl phenotype and increased steroid binding capacity in membrane fractions, whereas antisense MSBP1 transgenic plants showed long hypocotyl phenotypes and reduced steroid binding capacity, indicating that MSBP1 negatively regulates hypocotyl elongation. The reduced cell elongation of MSBP1-overexpressing plants was correlated with altered expression of genes involved in cell elongation, such as expansins and extensins, indicating that enhanced MSBP1 affected a regulatory pathway for cell elongation. Suppression or overexpression of MSBP1 resulted in enhanced or reduced sensitivities, respectively, to exogenous progesterone and 24-eBL, suggesting a negative role of MSBP1 in steroid signaling. Expression of MSBP1 in hypocotyls is suppressed by darkness and activated by light, suggesting that MSBP1, as a negative regulator of cell elongation, plays a role in plant photomorphogenesis. This study demonstrates the functional roles of a steroid binding protein in growth regulation in higher plants.

Integrating transcriptional controls for plant cell expansion

In addition to independent transcriptional responses generated by auxin and brassinosteroid, recent microarray analyses indicate that these two plant hormones also coordinate the expression of a set of shared target genes.

The plant hormones auxin and brassinosteroid promote cell expansion by regulating gene expression. In addition to independent transcriptional responses generated by the two signals, recent microarray analyses indicate that auxin and brassinosteroid also coordinate the expression of a set of shared target genes.

Hormonal regulation of plant growth and development

Besides environmental factors, plant growth depends upon endogenous signals. Bill Gray examines what these hormonal signals are and how they act to regulate many aspects of growth and development.

LeSTIG1, an extracellular binding partner for the pollen receptor kinases LePRK1 and LePRK2, promotes pollen tube growth in vitro

As pollen tubes grow through the pistil they are thought to perceive and respond to diverse signals. The tomato pollen-specific receptor kinases LePRK1 and LePRK2 might participate in signaling during pollen tube growth. We previously showed that the extracellular domain of LePRK2 interacts with a pollen protein, LAT52, before but not after pollen germination. To determine whether LePRK2 might have different binding partner(s) after pollen germination, we characterized two more proteins that, like LAT52, were identified in yeast two-hybrid screens using the extracellular domains of LePRK1 and LePRK2 as baits. We show that LeSHY, a leucine-rich repeat protein from pollen, and LeSTIG1, a small cysteine-rich protein from pistil, can bind the extracellular domains of both LePRK1 and LePRK2 in vitro. In vitro binding assays with the extracellular domain of LePRK2 suggested that LeSTIG1 could displace binding of LAT52, consistent with the idea that LePRK1 and LePRK2 might interact with different ligands at different stages of pollen tube growth. Exogenous LeSTIG1 promotes pollen tube growth in vitro. The interaction of these pollen kinases with LeSTIG1 supports the notion that LePRK1 and LePRK2 are involved in mediating pollen-pistil interactions.