Plant growth regulator extracts from seaweeds promote plant growth and confer drought tolerance in canola (Brassica napus)

Brassica napus, commonly known as canola, is an important oilseed crop in Canada contributing over $29.9 billion CAD to the Canadian economy annually. A major challenge facing Canadian canola is drought, which has become increasingly prevalent in recent years due to the changing climate. Research investigating novel agronomic techniques in mitigating drought is key to securing yields and sustainability in canola and other crops. One such technique is the use of bio-stimulant sprays to help offset biotic and abiotic stresses in plants through promoting stand establishment. Previous studies have shown that the application of seaweed extracts as bio-stimulant sprays to Brassicaceae has been successful in improving plant growth and development along with stress tolerance. However, this method has yet to be tested on canola. The organic nutrients that are waste products from processed seaweed help stimulate plant growth, yielding higher quality plants as a result. In association with Le Groupe Roullier, this study demonstrates that the Roullier extracts (RE) help increase plant growth characteristics and drought tolerance in canola when sprayed 3 times over a 3-week period. A high yielding but drought sensitive mutant of canola, d14 (developed through gene editing) was used for drought assays after 8 weeks of growth and where water was withheld for 6 days. Application of the REs prior to drought resulted in plants having enhanced survival rate and improved biomass retention indicating high drought tolerance. Subsequent RNA sequencing and gene ontological term analysis performed using RE treated plants in triplicates, revealed substantial levels of differential expression of growth-related genes along with stress-related genes. These REs elicited responses in plants that had previously only been achieved through gene editing and transgenic methodologies. Using bio-stimulant sprays provides a novel platform to promote beneficial agronomic traits, independent of genetic manipulation.

Plant reproduction: Stigma receptors regulate reactive oxygen species to establish pollination barriers

Exciting new research highlights how stigmatic receptors purposed for recognizing self-incompatible pollen interact with the FERONIA pathway to regulate stigmatic reactive oxygen species production to enforce a barrier against self-, intra- and interspecific pollen.

Cell-cell signaling during the Brassicaceae self-incompatibility response

Self-incompatibility (SI) is a mechanism that many plant families employ to prevent self-fertilization. In the Brassicaceae, the S-haplotype-specific interaction of the pollen-borne ligand, and a stigma-specific receptor protein kinase triggers a signaling cascade that culminates in the rejection of self-pollen. While the upstream molecular components at the receptor level of the signaling pathway have been extensively studied, the intracellular responses beyond receptor activation were not as well understood. Recent research has uncovered several key molecules and signaling events that operate in concert for the manifestation of the self-incompatible responses in Brassicaceae stigmas. Here, we review the recent discoveries in both the compatible and self-incompatible pathways and provide new perspectives on the early stages of Brassicaceae pollen-pistil interactions.

Identification of Arabidopsis Protein Kinases That Harbor Functional Type 1 Peroxisomal Targeting Signals

Peroxisomes are eukaryotic specific organelles that perform diverse metabolic functions including fatty acid β-oxidation, reactive species metabolism, photorespiration, and responses to stress. However, the potential regulation of these functions by post-translational modifications, including protein phosphorylation, has had limited study. Recently, we identified and catalogued a large number of peroxisomal phosphorylated proteins, implicating the presence of protein kinases in this organelle. Here, we employed available prediction models coupled with sequence conservation analysis to identify 31 protein kinases from the Arabidopsis kinome (all protein kinases) that contain a putative, non-canonical peroxisomal targeting signal type 1 (PTS1). From this, twelve C-terminal domain-PTS1s were demonstrated to be functional in vivo, targeting enhanced yellow fluorescent protein to peroxisomes, increasing the list of presumptive peroxisomal protein kinases to nineteen. Of the twelve protein kinases with functional PTS1s, we obtained full length clones for eight and demonstrated that seven target to peroxisomes in vivo. Screening homozygous mutants of the presumptive nineteen protein kinases revealed one candidate (GPK1) that harbors a sugar-dependence phenotype, suggesting it is involved in regulating peroxisomal fatty acid β-oxidation. These results present new opportunities for investigating the regulation of peroxisome functions.

Stigma Receptivity Is Controlled by Functionally Redundant MAPK Pathway Components in Arabidopsis

In angiosperms, the process of pollination relies on species-specific interaction and signaling between the male (pollen) and female (pistil) counterparts where the interplay between several pollen and stigma proteins decides the fate of the pollen. In Brassicaceae, the dry stigmatic papillary cells control pollen germination by releasing resources only to compatible pollen thereby allowing pollen to hydrate and germinate. Despite the identification of a number of stigmatic proteins that facilitate pollination responses, the signaling mechanisms that regulate functions of these proteins have remained unknown. Here, we show that, in Arabidopsis, an extremely functionally redundant mitogen-activated protein kinase (MAPK) cascade is required for maintaining stigma receptivity to accept compatible pollen. Our genetic analyses demonstrate that in stigmas, five MAPK kinases (MKKs), MKK1/2/3/7/9 are required to transmit upstream signals to two MPKs, MPK3/4, to mediate compatible pollination. Compromised functions of these five MKKs in the quintuple mutant (mkk1/2/3RNAi/mkk7/9) phenocopied pollination defects observed in the mpk4RNAi/mpk3 double mutant. We further show that this MAPK nexus converges on Exo70A1, a previously identified stigma receptivity factor essential for pollination. Given that pollination is the crucial initial step during plant reproduction, understanding the mechanisms that govern successful pollination could lead to development of strategies to improve crop yield.

Stigma Receptivity Is Controlled by Functionally Redundant MAPK Pathway Components in Arabidopsis

In angiosperms, the process of pollination relies on species-specific interaction and signaling between the male (pollen) and female (pistil) counterparts where the interplay between several pollen and stigma proteins decides the fate of the pollen. In Brassicaceae, the dry stigmatic papillary cells control pollen germination by releasing resources only to compatible pollen thereby allowing pollen to hydrate and germinate. Despite the identification of a number of stigmatic proteins that facilitate pollination responses, the signaling mechanisms that regulate functions of these proteins have remained unknown. Here, we show that, in Arabidopsis, an extremely functionally redundant mitogen-activated protein kinase (MAPK) cascade is required for maintaining stigma receptivity to accept compatible pollen. Our genetic analyses demonstrate that in stigmas, five MAPK kinases (MKKs), MKK1/2/3/7/9 are required to transmit upstream signals to two MPKs, MPK3/4, to mediate compatible pollination. Compromised functions of these five MKKs in the quintuple mutant (mkk1/2/3RNAi/mkk7/9) phenocopied pollination defects observed in the mpk4RNAi/mpk3 double mutant. We further show that this MAPK nexus converges on Exo70A1, a previously identified stigma receptivity factor essential for pollination. Given that pollination is the crucial initial step during plant reproduction, understanding the mechanisms that govern successful pollination could lead to development of strategies to improve crop yield.

Identification and characterization of a female gametophyte defect in sdk1-7 +/- abi3-6 +/- heterozygotes of Arabidopsis thaliana

Successful reproduction in angiosperms is dependent on the highly synchronous development of their male and female gametophytes and the ensuing fusion of the gametes from these reproductive tissue types. When crossing a T-DNA insertion line sdk1-7-/-(Salk_024564), one of the S-domain receptor kinases involved in ABA responses with a fast neutron deletion line abi3-6-/-, the F1 heterozygotes (sdk1-7+/-abi3-6 +/-) displayed 50% ovule abortion suggesting a likely gametophytic defects. We identified and characterized an early stage female gametophyte developmental defect in the heterozygous mutant ovules. Recombination frequency analysis of the F2 progenies from selfed heterozygotes revealed a possible pseudo-linkage of sdk1-7 and abi3-6 suggesting a reciprocal translocation event in the heterozygote. Our study emphasizes the importance of robust analysis to distinguish gametophytic defect phenotypes caused by genetic interactions and that resulting from possible chromosomal translocation events.

Spatial regulation of alpha-galactosidase activity and its influence on raffinose family oligosaccharides during seed maturation and germination in Cicer arietinum

Alpha-galactosides or Raffinose Family Oligosaccharides (RFOs) are enriched in legumes and are considered as anti-nutritional factors responsible for inducing flatulence. Due to a lack of alpha-galactosidases in the stomachs of humans and other monogastric animals, these RFOs are not metabolized and are passed to the intestines to be processed by gut bacteria leading to distressing flatulence. In plants, alpha(α)-galactosides are involved in desiccation tolerance during seed maturation and act as a source of stored energy utilized by germinating seeds. The hydrolytic enzyme alpha-galactosidase (α-GAL) can break down RFOs into sucrose and galactose releasing the monosaccharide α-galactose back into the system. Through characterization of RFOs, sucrose, reducing sugars, and α-GAL activity in maturing and germinating chickpeas, we show that stored RFOs are likely required to maintain a steady-state level of reducing sugars. These reducing sugars can then be readily converted to generate energy required for the high energy-demanding germination process. Our observations indicate that RFO levels are lowest in imbibed seeds and rapidly increase post-imbibition. Both RFOs and the α-GAL activity are possibly required to maintain a steady-state level of the reducing monosaccharide sugars, starting from dry seeds all the way through post-germination, to provide the energy for increased germination vigor.

Nucleus- and plastid-targeted annexin 5 promotes reproductive development in Arabidopsis and is essential for pollen and embryo formation

BACKGROUND

Pollen development is a strictly controlled post-meiotic process during which microspores differentiate into microgametophytes and profound structural and functional changes occur in organelles. Annexin 5 is a calcium- and lipid-binding protein that is highly expressed in pollen grains and regulates pollen development and physiology. To gain further insights into the role of ANN5 in Arabidopsis development, we performed detailed phenotypic characterization of Arabidopsis plants with modified ANN5 levels. In addition, interaction partners and subcellular localization of ANN5 were analyzed to investigate potential functions of ANN5 at cellular level.

RESULTS

Here, we report that RNAi-mediated suppression of ANN5 results in formation of smaller pollen grains, enhanced pollen lethality, and delayed pollen tube growth. ANN5 RNAi knockdown plants also displayed aberrant development during the transition from the vegetative to generative phase and during embryogenesis, reflected by delayed bolting time and reduced embryo size, respectively. At the subcellular level, ANN5 was delivered to the nucleus, nucleolus, and cytoplasm, and was frequently localized in plastid nucleoids, suggesting a likely role in interorganellar communication. Furthermore, ANN5-YFP co-immunoprecipitated with RABE1b, a putative GTPase, and interaction in planta was confirmed in plastidial nucleoids using FLIM-FRET analysis.

CONCLUSIONS

Our findings let us to propose that ANN5 influences basal cell homeostasis via modulation of plastid activity during pollen maturation. We hypothesize that the role of ANN5 is to orchestrate the plastidial and nuclear genome activities via protein-protein interactions however not only in maturing pollen but also during the transition from the vegetative to the generative growth and seed development.

Abiotic Stress Signaling in Wheat - An Inclusive Overview of Hormonal Interactions During Abiotic Stress Responses in Wheat

Rapid global warming directly impacts agricultural productivity and poses a major challenge to the present-day agriculture. Recent climate change models predict severe losses in crop production worldwide due to the changing environment, and in wheat, this can be as large as 42 Mt/°C rise in temperature. Although wheat occupies the largest total harvested area (38.8%) among the cereals including rice and maize, its total productivity remains the lowest. The major production losses in wheat are caused more by abiotic stresses such as drought, salinity, and high temperature than by biotic insults. Thus, understanding the effects of these stresses becomes indispensable for wheat improvement programs which have depended mainly on the genetic variations present in the wheat genome through conventional breeding. Notably, recent biotechnological breakthroughs in the understanding of gene functions and access to whole genome sequences have opened new avenues for crop improvement. Despite the availability of such resources in wheat, progress is still limited to the understanding of the stress signaling mechanisms using model plants such as Arabidopsis, rice and Brachypodium and not directly using wheat as the model organism. This review presents an inclusive overview of the phenotypic and physiological changes in wheat due to various abiotic stresses followed by the current state of knowledge on the identified mechanisms of perception and signal transduction in wheat. Specifically, this review provides an in-depth analysis of different hormonal interactions and signaling observed during abiotic stress signaling in wheat.

Integration of amplification efficiency in qPCR analysis allows precise and relative quantification of transcript abundance of genes from large gene families using RNA isolated from difficult tissues

The performance of the quantitative polymerase chain reaction (qPCR) assay in the analysis of gene expression belonging to multigene families in tissues rich in secondary metabolites is technically complicated. Here, we present the qPCR analysis of PMT2 gene, a predominant member of a multigene family from tobacco, expressed in the root tissues and is involved in the biosynthesis of nicotine. Consequently, we provide insight into the effect of polymerase chain reaction (PCR) amplification efficiency (AE) of reference and target genes of calibrator and test samples on the qPCR assay performance. Obviously, we found PCR AE as a critical indicator of qPCR assay performance involving multigene families and secondary metabolite-rich root tissues of tobacco. The integration of consistent and uniform PCR amplification efficiencies of reference and target genes of the samples into the relative quantification analysis is emphasized.

Farnesylation-mediated subcellular localization is required for CYP85A2 function

Protein farnesylation refers to the addition of a 15-carbon farnesyl isoprenoid to the cysteine residue of the CaaX motif at the carboxy terminus of target proteins. In spite of its known roles in plant development and abiotic stress tolerance, how these processes are precisely regulated by farnesylation had remained elusive. We recently showed that CYP85A2, the cytochrome P450, which converts castasterone to brassinolide in the last step of brassinosteroid synthesis must be farnesylated in order to function in this pathway. Lack of either CYP85A2 or the farnesylation motif of CYP85A2 resulted in reduced brassinolide accumulation, hypersensitivity to ABA, and increased plant drought tolerance. In this study, we have assessed the influence of the N-terminal secretory signal and the C-terminal CaaX motif of CYP85A2 in mediating CYP85A2 function and targeting to endomembrane compartments. We show that CaaX motif could still target CYPA85A2 in the absence of an intact N-terminal secretory signal to the respective membrane compartments and partially rescue cyp85a2-2 phenotypes. However, in the absence of both the CaaX motif and the secretory signal, CYP85A2 is not targeted to the membranes and becomes unstable.

Glyoxalase Goes Green: The Expanding Roles of Glyoxalase in Plants

The ubiquitous glyoxalase enzymatic pathway is involved in the detoxification of methylglyoxal (MG), a cytotoxic byproduct of glycolysis. The glyoxalase system has been more extensively studied in animals versus plants. Plant glyoxalases have been primarily associated with stress responses and their overexpression is known to impart tolerance to various abiotic stresses. In plants, glyoxalases exist as multigene families, and new roles for glyoxalases in various developmental and signaling pathways have started to emerge. Glyoxalase-based MG detoxification has now been shown to be important for pollination responses. During self-incompatibility response in Brassicaceae, MG is required to target compatibility factors for proteasomal degradation, while accumulation of glyoxalase leads to MG detoxification and efficient pollination. In this review, we discuss the importance of glyoxalase systems and their emerging biological roles in plants.

Activation of Mitochondrial Protein Phosphatase SLP2 by MIA40 Regulates Seed Germination

Reversible protein phosphorylation catalyzed by protein kinases and phosphatases represents the most prolific and well-characterized posttranslational modification known. Here, we demonstrate that Arabidopsis (Arabidopsis thaliana) Shewanella-like protein phosphatase 2 (AtSLP2) is a bona fide Ser/Thr protein phosphatase that is targeted to the mitochondrial intermembrane space (IMS) where it interacts with the mitochondrial oxidoreductase import and assembly protein 40 (AtMIA40), forming a protein complex. Interaction with AtMIA40 is necessary for the phosphatase activity of AtSLP2 and is dependent on the formation of disulfide bridges on AtSLP2. Furthermore, by utilizing atslp2 null mutant, AtSLP2 complemented and AtSLP2 overexpressing plants, we identify a function for the AtSLP2-AtMIA40 complex in negatively regulating gibberellic acid-related processes during seed germination. Results presented here characterize a mitochondrial IMS-localized protein phosphatase identified in photosynthetic eukaryotes as well as a protein phosphatase target of the highly conserved eukaryotic MIA40 IMS oxidoreductase.

Farnesylation mediates brassinosteroid biosynthesis to regulate abscisic acid responses

Protein farnesylation is a post-translational modification involving the addition of a 15-carbon farnesyl isoprenoid to the carboxy terminus of select proteins(1-3). Although the roles of this lipid modification are clear in both fungal and animal signalling, many of the mechanistic functions of farnesylation in plant signalling are still unknown. Here, we show that CYP85A2, the cytochrome P450 enzyme that performs the last step in brassinosteroid biosynthesis (conversion of castasterone to brassinolide)(4), must be farnesylated to function in Arabidopsis. Loss of either CYP85A2 or CYP85A2 farnesylation results in reduced brassinolide accumulation and increased plant responsiveness to the hormone abscisic acid (ABA) and overall drought tolerance, explaining previous observations(5). This result not only directly links farnesylation to brassinosteroid biosynthesis but also suggests new strategies to maintain crop yield under challenging climatic conditions.

Degradation of glyoxalase I in Brassica napus stigma leads to self-incompatibility response

Self-incompatibility (rejection of 'self'-pollen) is a reproductive barrier that allows hermaphroditic flowering plants to prevent inbreeding, to promote outcrossing and hybrid vigour. The self-incompatibility response in Brassica involves allele-specific interaction between the pollen small cysteine-rich, secreted protein ligand (SCR/SP11) and the stigmatic S-receptor kinase (SRK), which leads to the activation of the E3 ubiquitin ligase ARC1 (Armadillo repeat-containing 1), resulting in proteasomal degradation of compatibility factors needed for successful pollination. Despite this, targets of ARC1 and the intracellular signalling network that is regulated by these targets, have remained elusive. Here we show that glyoxalase I (GLO1), an enzyme that is required for the detoxification of methylglyoxal (MG, a cytotoxic by-product of glycolysis), is a stigmatic compatibility factor required for pollination to occur and is targeted by the self-incompatibility system. Suppression of GLO1 was sufficient to reduce compatibility, and overexpression of GLO1 in self-incompatible Brassica napus stigmas resulted in partial breakdown of the self-incompatibility response. ARC1-mediated destruction of GLO1 after self-pollination results in increased MG levels and a concomitant increase in MG-modified proteins (including GLO1), which are efficiently targeted for destruction in the papillary cells, leading to pollen rejection. Our findings demonstrate the elegant nature of plants to use a metabolic by-product to regulate the self-incompatibility response.

Identification of detoxification pathways in plants that are regulated in response to treatment with organic compounds isolated from oil sands process-affected water

Bitumen mining in the Athabasca oil sands region of northern Alberta results in the accumulation of large volumes of oil sands process-affected water (OSPW). The acid-extractable organic (AEO) fraction of OSPW contains a variety of compounds, including naphthenic acids, aromatics, and sulfur- and nitrogen-containing compounds that are toxic to aquatic and terrestrial organisms. We have studied the effect of AEO treatment on the transcriptome of root and shoot tissues in seedlings of the model plant, Arabidopsis thaliana. Several genes encoding enzymes involved in the xenobiotic detoxification pathway were upregulated, including cytochrome P450s (CYPs), UDP-dependent glycosyltransferases (UGTs), glutathione-S-transferases (GSTs), and membrane transporters. In addition, gene products involved in oxidative stress, β-oxidation, and glucosinolate degradation were also upregulated, indicating other potential mechanisms of the adaptive response to AEO exposure. These results provide insight into the pathways that plants use to detoxify the organic acid component of OSPW. Moreover, this study advances our understanding of genes that could be exploited to potentially develop phytoremediation and biosensing strategies for AEO contaminants resulting from oil sands mining.

A proposed role for selective autophagy in regulating auxin-dependent lateral root development under phosphate starvation in Arabidopsis

Plants respond to limited soil nutrient availability by inducing more lateral roots (LR) to increase the root surface area. At the cellular level, nutrient starvation triggers the process of autophagy through which bulk degradation of cellular materials is achieved to facilitate nutrient mobilization. Whether there is any link between the cellular autophagy and induction of LR had remained unknown. We recently showed that the S-Domain receptor Kinase (ARK2) and U Box/Armadillo Repeat-Containing E3 ligase (PUB9) module is required for lateral root formation under phosphate starvation in Arabidopsis thaliana.(1) We also showed that PUB9 localized to autophagic bodies following either activation by ARK2 or under phosphate starvation and ark2-1/pub9-1 plants displayed lateral root defects with inability to accumulate auxin in the root tips under phosphate starvation.(1) Supplementing exogenous auxin was sufficient to rescue the LR defects in ark2-1/pub9-1 mutant. Blocking of autophagic responses in wild-type Arabidopsis also resulted in inhibition of both lateral roots and auxin accumulation in the root tips indicating the importance of autophagy in mediating auxin accumulation under phosphate starved conditions.(1) Here, we propose a model for ARK2/AtPUB9 module in regulation of lateral root development via selective autophagy.

The ARC1 E3 ligase promotes a strong and stable self-incompatibility response in Arabidopsis species: response to the Nasrallah and Nasrallah commentary

Following the identification of the male (S-locus Cysteine Rich/S-locus Protein 11) and female (S Receptor kinase [SRK]) factors controlling self-incompatibility in the Brassicaceae, research in this field has focused on understanding the nature of the cellular responses activated by these regulators. We previously identified the ARM Repeat Containing1 (ARC1) E3 ligase as a component of the SRK signaling pathway and demonstrated ARC1's requirement in the stigma for self-incompatible pollen rejection in Brassica napus, Arabidopsis lyrata, and Arabidopsis thaliana. Here, we discuss our findings on the role of ARC1 in reconstructing a strong and stable A. thaliana self-incompatibility phenotype, in the context of the putative issues outlined in a commentary by Nasrallah and Nasrallah. Additionally, with their proposed standardized strategy for studying self-incompatibility in A. thaliana, we offer our perspective on what constitutes a strong and stable self-incompatibility phenotype in A. thaliana and how this should be investigated and reported to the greater community.

High humidity partially rescues the Arabidopsis thaliana exo70A1 stigmatic defect for accepting compatible pollen

We have previously proposed that Exo70A1 is required in the Brassicaceae stigma to control the early stages of pollen hydration and pollen tube penetration through the stigmatic surface, following compatible pollination. However, recent work has raised questions regarding Arabidopsis thaliana Exo70A1's expression in the stigma and its role in stigma receptivity to compatible pollen. Here, we verified the expression of Exo70A1 in stigmas from three Brassicaceae species and carefully re-examined Exo70A1's function in the stigmatic papillae. With previous studies showing that high relative humidity can rescue some pollination defects, essentially bypassing the control of pollen hydration by the Brassicaceae dry stigma, the effect of high humidity was investigated on pollinations with the Arabidopsis exo70A1-1 mutant. Pollinations under low relative humidity resulted in a complete failure of wild-type compatible pollen acceptance by the exo70A1-1 mutant stigma as we had previously seen. However, high relative humidity resulted in a partial rescue of the exo70A1-1 stigmatic papillar defect resulting is some wild-type compatible pollen acceptance and seed set. Thus, these results reaffirmed Exo70A1's proposed role in the stigma regulating compatible pollen hydration and pollen tube entry and demonstrate that high relative humidity can partially bypass these functions.

The S-Domain Receptor Kinase Arabidopsis Receptor Kinase2 and the U Box/Armadillo Repeat-Containing E3 Ubiquitin Ligase9 Module Mediates Lateral Root Development under Phosphate Starvation in Arabidopsis

When plants encounter nutrient-limiting conditions in the soil, the root architecture is redesigned to generate numerous lateral roots (LRs) that increase the surface area of roots, promoting efficient uptake of these deficient nutrients. Of the many essential nutrients, reduced availability of inorganic phosphate has a major impact on plant growth because of the requirement of inorganic phosphate for synthesis of organic molecules, such as nucleic acids, ATP, and phospholipids, that function in various crucial metabolic activities. In our screens to identify a potential role for the S-domain receptor kinase1-6 and its interacting downstream signaling partner, the Arabidopsis (Arabidopsis thaliana) plant U box/armadillo repeat-containing E3 ligase9 (AtPUB9), we identified a role for this module in regulating LR development under phosphate-starved conditions. Our results show that Arabidopsis double mutant plants lacking AtPUB9 and Arabidopsis Receptor Kinase2 (AtARK2; ark2-1/pub9-1) display severely reduced LRs when grown under phosphate-starved conditions. Under these starvation conditions, these plants accumulated very low to no auxin in their primary root and LR tips as observed through expression of the auxin reporter DR5::uidA transgene. Exogenous auxin was sufficient to rescue the LR developmental defects in the ark2-1/pub9-1 lines, indicating a requirement of auxin accumulation for this process. Our subcellular localization studies with tobacco (Nicotiana tabacum) suspension-cultured cells indicate that interaction between ARK2 and AtPUB9 results in accumulation of AtPUB9 in the autophagosomes. Inhibition of autophagy in wild-type plants resulted in reduction of LR development and auxin accumulation under phosphate-starved conditions, suggesting a role for autophagy in regulating LR development. Thus, our study has uncovered a previously unknown signaling module (ARK2-PUB9) that is required for auxin-mediated LR development under phosphate-starved conditions.

Crystal structure and biochemical analyses reveal that the Arabidopsis triphosphate tunnel metalloenzyme AtTTM3 is a tripolyphosphatase involved in root development

The Arabidopsis protein AtTTM3 belongs to the CYTH superfamily named after its two founding members, the CyaB adenylate cyclase from Aeromonas hydrophila and the mammalian thiamine triphosphatase. In this study we report the three-dimensional structure of a plant CYTH domain protein, AtTTM3, determined at 1.9 Å resolution. The crystal structure revealed the characteristic tunnel architecture of CYTH proteins, which specialize in the binding of nucleotides and other organic phosphates and in phosphoryl transfer reactions. The β barrel is composed of eight antiparallel β strands with a cluster of conserved inwardly facing acidic and basic amino acid residues. Mutagenesis of these residues in the catalytic core led to an almost complete loss of enzymatic activity. We established that AtTTM3 is not an adenylate cyclase. Instead, the enzyme displayed weak NTP phosphatase as well as strong tripolyphosphatase activities similar to the triphosphate tunnel metalloenzyme proteins from Clostridium thermocellum (CthTTM) and Nitrosomonas europaea (NeuTTM). AtTTM3 is most highly expressed in the proximal meristematic zone of the plant root. Furthermore, an AtTTM3 T-DNA insertion knockout line displayed a delay in root growth as well as reduced length and number of lateral roots, suggesting a role for AtTTM3 in root development.

Proteomics approaches advance our understanding of plant self-incompatibility response

Self-incompatibility (SI) in plants is a genetic mechanism that prevents self-fertilization and promotes out-crossing needed to maintain genetic diversity. SI has been classified into two broad categories: the gametophytic self-incompatibility (GSI) and the sporophytic self-incompatibility (SSI) based on the genetic mechanisms involved in 'self' pollen rejection. Recent proteomic approaches to identify potential candidates involved in SI have shed light onto a number of previously unidentified mechanisms required for SI response. SI proteome research has progressed from the use of isoelectric focusing in early days to the latest third-generation technique of comparative isobaric tag for relative and absolute quantitation (iTRAQ) used in recent times. We will focus on the proteome-based approaches used to study self-incompatibility (GSI and SSI), recent developments in the field of incompatibility research with emphasis on SSI and future prospects of using proteomic approaches to study self-incompatibility.

ABI3 controls embryo degreening through Mendel's I locus

Chlorophyll (chl) is essential for light capture and is the starting point that provides the energy for photosynthesis and thus plant growth. Obviously, for this reason, retention of the green chlorophyll pigment is considered a desirable crop trait. However, the presence of chlorophyll in mature seeds can be an undesirable trait that can affect seed maturation, seed oil quality, and meal quality. Occurrence of mature green seeds in oil crops such as canola and soybean due to unfavorable weather conditions during seed maturity is known to cause severe losses in revenue. One recently identified candidate that controls the chlorophyll degradation machinery is the stay-green gene, SGR1 that was mapped to Mendel's I locus responsible for cotyledon color (yellow versus green) in peas. A defect in SGR1 leads to leaf stay-green phenotypes in Arabidopsis and rice, but the role of SGR1 in seed degreening and the signaling machinery that converges on SGR1 have remained elusive. To decipher the gene regulatory network that controls degreening in Arabidopsis, we have used an embryo stay-green mutant to demonstrate that embryo degreening is achieved by the SGR family and that this whole process is regulated by the phytohormone abscisic acid (ABA) through ABSCISIC ACID INSENSITIVE 3 (ABI3); a B3 domain transcription factor that has a highly conserved and essential role in seed maturation, conferring desiccation tolerance. Misexpression of ABI3 was sufficient to rescue cold-induced green seed phenotype in Arabidopsis. This finding reveals a mechanistic role for ABI3 during seed degreening and thus targeting of this pathway could provide a solution to the green seed problem in various oil-seed crops.

Nucleocytoplasmic partitioning of tobacco N receptor is modulated by SGT1

SGT1 (Suppressor of G2 allele of SKP1) is required to maintain plant disease Resistance (R) proteins with Nucleotide-Binding (NB) and Leucine-Rich Repeat (LRR) domains in an inactive but signaling-competent state. SGT1 is an integral component of a multi-protein network that includes RACK1, Rac1, RAR1, Rboh, HSP90 and HSP70, and in rice the Mitogen-Activated Protein Kinase (MAPK), OsMAPK6. Tobacco (Nicotiana tabacum) N protein, which belongs to the Toll-Interleukin Receptor (TIR)-NB-LRR class of R proteins, confers resistance to Tobacco Mosaic Virus (TMV). Following transient expression in planta, we analyzed the functional relationship between SGT1, SIPK - a tobacco MAPK6 ortholog - and N, using mass spectrometry, confocal microscopy and pathogen assays. Here, we show that tobacco SGT1 undergoes specific phosphorylation in a canonical MAPK target-motif by SIPK. Mutation of this motif to mimic SIPK phosphorylation leads to an increased proportion of cells displaying SGT1 nuclear accumulation and impairs N-mediated resistance to TMV, as does phospho-null substitution at the same residue. Forced nuclear localization of SGT1 causes N to be confined to nuclei. Our data suggest that one mode of regulating nucleocytoplasmic partitioning of R proteins is by maintaining appropriate levels of SGT1 phosphorylation catalyzed by plant MAPK.

The effect of oil sands process-affected water and naphthenic acids on the germination and development of Arabidopsis

Oil sands mining in the Athabasca region of northern Alberta results in the production of large volumes of oil sands process-affected water (OSPW). We have evaluated the effects of OSPW, the acid extractable organic (AEO) fraction of OSPW, and individual naphthenic acids (NAs) on the germination and development of the model plant, Arabidopsis thaliana (Arabidopsis). The surrogate NAs that were selected for this study were petroleum NAs that have been used in previous toxicology studies and may not represent OSPW NAs. A tricyclic diamondoid NA that was recently identified as a component of OSPW served as a model NA in this study. Germination of Arabidopsis seeds was not inhibited when grown on medium containing up to 75% OSPW or by 50mgL(-1) AEO. However, simultaneous exposure to three simple, single-ringed surrogate NAs or a double-ringed surrogate NA had an inhibitory effect on germination at a concentration of 10mgL(-1), whereas inhibition of germination by the diamondoid model NA was observed only at 50mgL(-1). Seedling root growth was impaired by treatment with low concentrations of OSPW, and exposure to higher concentrations of OSPW resulted in increased growth inhibition of roots and primary leaves, and caused bleaching of cotyledons. Treatment with single- or double-ringed surrogate NAs at 10mgL(-1) severely impaired seedling growth. AEO or diamondoid NA treatment was less toxic, but resulted in severely impaired growth at 50mgL(-1). At low NA concentrations there was occasionally a stimulatory effect on root and shoot growth, possibly owing to the broad structural similarity of some NAs to known plant growth regulators such as auxins. This report provides a foundation for future studies aimed at using Arabidopsis as a biosensor for toxicity and to identify genes with possible roles in NA phytoremediation.

Proteomic analysis of Brassica stigmatic proteins following the self-incompatibility reaction reveals a role for microtubule dynamics during pollen responses

Mate selection and maintenance of genetic diversity is crucial to successful reproduction and species survival. Plants utilize self-incompatibility system as a genetic barrier to prevent self pollen from developing on the pistil, leading to hybrid vigor and diversity. In Brassica (canola, kale, and broccoli), an allele-specific interaction between the pollen SCR/SP11 (S-locus cysteine rich protein/S locus protein 11) and the pistil S Receptor Kinase, results in the activation of SRK which recruits the Arm Repeat Containing 1 (ARC1) E3 ligase to the proteasome. The targets of Arm Repeat Containing 1 are proposed to be compatibility factors, which when targeted for degradation by Arm Repeat Containing 1 results in pollen rejection. Despite the fact that protein degradation is predicted to be important for successful self-pollen rejection, the identity of the various proteins whose abundance is altered by the SI pathway has remained unknown. To identify potential candidate proteins regulated by the SI response, we have used the two-dimensional difference gel electrophoresis analysis, coupled with matrix-assisted laser desorption ionization/time of flight/MS. We identified 56 differential protein spots with 19 unique candidate proteins whose abundance is down-regulated following self-incompatible pollinations. The identified differentials are predicted to function in various pathways including biosynthetic pathways, signaling, cytoskeletal organization, and exocytosis. From the 19 unique proteins identified, we investigated the role of tubulin and the microtubule network during both self-incompatible and compatible pollen responses. Moderate changes in the microtubule network were observed with self-incompatible pollinations; however, a more distinct localized break-down of the microtubule network was observed during compatible pollinations, that is likely mediated by EXO70A1, leading to successful pollination.

Cellular pathways regulating responses to compatible and self-incompatible pollen in Brassica and Arabidopsis stigmas intersect at Exo70A1, a putative component of the exocyst complex

In the Brassicaceae, compatible pollen-pistil interactions result in pollen adhesion to the stigma, while pollen grains from unrelated plant species are largely ignored. There can also be an additional layer of recognition to prevent self-fertilization, the self-incompatibility response, whereby self pollen grains are distinguished from nonself pollen grains and rejected. This pathway is activated in the stigma and involves the ARM repeat-containing 1 (ARC1) protein, an E3 ubiquitin ligase. In a screen for ARC1-interacting proteins, we have identified Brassica napus Exo70A1, a putative component of the exocyst complex that is known to regulate polarized secretion. We show through transgenic studies that loss of Exo70A1 in Brassica and Arabidopsis thaliana stigmas leads to the rejection of compatible pollen at the same stage as the self-incompatibility response. A red fluorescent protein:Exo70A1 fusion rescues this stigmatic defect in Arabidopsis and is found to be mobilized to the plasma membrane concomitant with flowers opening. By contrast, increased expression of Exo70A1 in self-incompatible Brassica partially overcomes the self pollen rejection response. Thus, our data show that the Exo70A1 protein functions at the intersection of two cellular pathways, where it is required in the stigma for the acceptance of compatible pollen in both Brassica and Arabidopsis and is negatively regulated by Brassica self-incompatibility.

Suppression of MKK5 reduces ozone-induced signal transmission to both MPK3 and MPK6 and confers increased ozone sensitivity in Arabidopsis thaliana

In Arabidopsis thaliana, ozone-induced signaling has been shown to involve the mitogen-activated protein kinases (MAPKs) MPK3 and MPK6. To identify a possible ozone-induced mitogen-activated protein kinase kinase (MAPKK) involved in the activation of these specific MAPKs, we employed RNA interference-(RNAi)-based suppression of MKK5, a known cognate MAPKK to both MPK3 and MPK6. When exposed to ozone, activation of both MPK3 and MPK6 was markedly reduced in the MKK5-suppressed plants compared to WT. Additionally, the MKK5-suppressed plants were found to be highly sensitive to ozone as determined by visible leaf damage concomitant with elevated levels of leaf-localised H(2)O(2). Taken together, our data suggest MKK5 functions both in ozone-induced activation of MPK3 and MPK6 and in integrating ROS homeostasis during ozone stress.

Quantitative proteomics identifies oxidant-induced, AtMPK6-dependent changes in Arabidopsis thaliana protein profiles

In Arabidopsis thaliana, oxidant-induced signalling has been shown to utilize the mitogen-activated protein kinase (MAPK), AtMPK6. To identify proteins whose accumulation is altered by ozone in an AtMPK6-dependent manner we employed isotope-coded affinity tagging (ICAT) technology to investigate the impact of AtMPK6-suppression on the protein profiles in Arabidopsis both before (air control) and during continuous ozone (O(3)) fumigation (500 nL L(-1) for 8 h). Among the 150 proteins positively identified and quantified in the O(3)-treated plants, we identified thirteen proteins whose abundance was greater in the AtMPK6-suppressed genotype than in wild-type (WT). These include the antioxidant proteins, monodehydroascorbate reductase, peroxiredoxin Q, and glutathione reductase. A further eighteen proteins were identified whose abundance was lower in the ozone-treated AtMPK6-suppressed line relative to ozone-exposed WT plants. These predominantly comprised proteins involved in carbohydrate-, energy-, and amino acid metabolism, and tetrapyrrole biosynthesis. In control plants, five proteins increased, and nine proteins decreased in abundance in the AtMPK6-suppressed genotype compared to that of the WT, reflecting changes in the protein composition of plants that have AtMPK6 constitutively suppressed. Since a number of these proteins are part of the redox response pathway, and loss of AtMPK6 renders Arabidopsis more susceptible to oxidative stress, we propose that AtMPK6 plays a key role in the plant's overall ability to manage oxidative stress.

Interactions between the S-domain receptor kinases and AtPUB-ARM E3 ubiquitin ligases suggest a conserved signaling pathway in Arabidopsis

The Arabidopsis (Arabidopsis thaliana) genome encompasses multiple receptor kinase families with highly variable extracellular domains. Despite their large numbers, the various ligands and the downstream interacting partners for these kinases have been deciphered only for a few members. One such member, the S-receptor kinase, is known to mediate the self-incompatibility (SI) response in Brassica. S-receptor kinase has been shown to interact and phosphorylate a U-box/ARM-repeat-containing E3 ligase, ARC1, which, in turn, acts as a positive regulator of the SI response. In an effort to identify conserved signaling pathways in Arabidopsis, we performed yeast two-hybrid analyses of various S-domain receptor kinase family members with representative Arabidopsis plant U-box/ARM-repeat (AtPUB-ARM) E3 ligases. The kinase domains from S-domain receptor kinases were found to interact with ARM-repeat domains from AtPUB-ARM proteins. These kinase domains, along with M-locus protein kinase, a positive regulator of SI response, were also able to phosphorylate the ARM-repeat domains in in vitro phosphorylation assays. Subcellular localization patterns were investigated using transient expression assays in tobacco (Nicotiana tabacum) BY-2 cells and changes were detected in the presence of interacting kinases. Finally, potential links to the involvement of these interacting modules to the hormone abscisic acid (ABA) were investigated. Interestingly, AtPUB9 displayed redistribution to the plasma membrane of BY-2 cells when either treated with ABA or coexpressed with the active kinase domain of ARK1. As well, T-DNA insertion mutants for ARK1 and AtPUB9 lines were altered in their ABA sensitivity during germination and acted at or upstream of ABI3, indicating potential involvement of these proteins in ABA responses.

Surviving the passage: Non-canonical stromal targeting of an Arabidopsis mitogen-activated protein kinase kinase

In plants, mitogen-activated protein kinases (MAPK) have been implicated in signalling associated with many processes, including cellular differentiation, organ development, cell death and stress/hormone signalling. While MAPK cascades are known to act in the cytosol and the nucleus, sequence analysis of the Arabidopsis MAPK cascade proteins predicts the presence of import signals that would target some of them to other organelles. In vitro uptake experiments confirm the predicted import of an oxidant-responsive MAPKK, AtMKK4, into the chloroplast. Unexpectedly, the imported MKK4 protein was not processed through stromal peptidase-dependent cleavage of the N-terminal signal peptide, thus leaving the pre-protein intact. Nevertheless, the N-terminal region was shown to be essential both for the import process and for the ability of MKK4 to activate its cognate MAPK targets in vivo. MKK4 import also occurred irrespective of the activation status of the kinase. The import of this primarily cytosolic oxidant-stimulated AtMKK4 into the chloroplasts, organelles with high redox fluxes, suggests that one of the functions of MKK4 might be to help coordinate intercompartment responses to cellular redox imbalances.

SIPK conditions transcriptional responses unique to either bacterial or oomycete elicitation in tobacco

SUMMARY The mitogen-activated protein kinase, SIPK (salicylic acid-induced protein kinase), is known to be rapidly activated in tobacco (Nicotiana tabacum) by various elicitors. However, SIPK activation induced by the oomycete elicitor, beta-megaspermin, is reported to require external calcium influx, whereas that induced by the bacterial elicitor, hrpZ(Psph), does not. This suggests that SIPK activation is involved in different elicitor-initiated signalling pathways, and raises the question of whether the role(s) of SIPK in mediating stress outcomes, including transcriptional re-programming, differs in an elicitor-specific manner. To examine this, we compared the impact of silencing SIPK on the transcript profile of tobacco suspension culture cells challenged with either hrpZ(Psph) or beta-megaspermin. SIPK-silencing was found to have a substantial impact on both hrpZ(Psph)- and beta-megaspermin-induced transcriptional responses, and these impacts included both common and elicitor-differentiated features. As well as revealing a role for SIPK in modulating expression of known redox- and defence-related genes in response to both elicitors, our analysis detected a substantial impact of SIPK silencing on transcription of 80S ribosomal subunit mRNAs. This novel observation suggests that SIPK may play a role in affecting translation efficiency as one mechanism for enacting rapid genome-wide, elicitor-specific physiological reprogramming during defence responses.

Multifunctional arm repeat domains in plants

Arm repeat domains are composed of multiple 42 amino acid Arm repeats and are found in the proteomes of all eukaryotic organisms. The Arm repeat domain is a highly conserved right-handed super helix of alpha-helices involved in protein-protein interactions. The well-characterized Arm repeat proteins in animal and plants are known to function in diverse cellular processes including signal transduction, cytoskeletal regulation, nuclear import, transcriptional regulation, and ubiquitination. While Arm repeat domains are found in all eukaryotes, plants have evolved some unique domain organizations, such as the U-box and Arm repeat domain combination, with specialized functions. The plant-specific U-box/Arm repeat proteins are the largest family of Arm repeat proteins in all the genomes surveyed, and more recent data have implicated these proteins as E3 ubiquitin ligases. While functions have not been assigned for most of the plant Arm repeat proteins, recent studies have demonstrated their importance in multiple processes such as self-incompatibility, hormone signaling, and disease resistance.

RNA interference-based (RNAi) suppression of AtMPK6, an Arabidopsis mitogen-activated protein kinase, results in hypersensitivity to ozone and misregulation of AtMPK3

The recent increase in tropospheric ozone (O(3)) concentrations promotes additional oxidative stress through the production of reactive oxygen species (ROS) in plant tissues, resulting in the activation of genes whose products enable the stressed cells to retain their integrity and function. This response is made possible by an integration of highly regulated signaling networks that mediate the perception of, and response to, this oxidative assault. In Arabidopsis thaliana, ROS-induced signaling has been shown to flow through a protein phosphorylation cascade involving the mitogen-activated protein kinases (MAPKs) AtMPK3 (MPK3) and AtMPK6 (MPK6). We found that RNAi-mediated silencing of MPK6 renders the plant more sensitive to ozone, as determined by visible leaf damage. The MPK6-RNAi genotype also displayed a more intense and prolonged activation of MPK3 compared to that of WT plants. An MPK3 loss-of-function genotype is similarly very sensitive to ozone, and displays an abnormally prolonged MPK6 activation profile, suggesting reciprocity in regulation between these two MAPKs.

Overexpression of SIPK in tobacco enhances ozone-induced ethylene formation and blocks ozone-induced SA accumulation

Ozone induces rapid activation of SIPK, a mitogen-activated protein kinase (MAPK) in tobacco. Through transgenic manipulation it has previously been shown that overexpression of SIPK leads to enhanced ozone-induced lesion formation with concomitant accumulation of ROS. In spite of this hypersensitive phenotype, the effect of this altered SIPK expression on the levels of various hormones that regulate ozone-induced cell death has remained unexplored. The response of both salicylate and ethylene, the major phytohormones that modulate ozone-induced cell death, have now been analysed in SIPK-OX tobacco plants. Ozone treatment strongly induced ethylene formation in the sensitive SIPK-OX plants at ozone concentrations that failed to elicit stress ethylene release in WT plants. By contrast, SIPK-overexpressing plants displayed no ozone-induced SA accumulation, whereas WT plants accumulated SA upon ozone exposure. Epistatic analysis of SIPK-OX function suggests that the ozone-induced cell death observed in SIPK-OX plants is either independent, or upstream, of SA accumulation.

SIPK signaling controls multiple components of harpin-induced cell death in tobacco

Harpin from Pseudomonas syringae pv. phaseolicola (HrpZ) elicits a rapid cell death response in tobacco plants. Multiple signaling components, including mitogen-activated protein kinase (MAPK), reactive oxygen species (ROS) and salicylic acid (SA), have been reported to be involved in this cell death process, but the interaction between these molecules is poorly understood. Here we show through utilizing plants manipulated in SIPK expression levels that lack of SIPK results in increased sensitivity to harpin with concomitant accumulation of higher levels of ROS. Conversely, SIPK-overexpressing plants show reduced sensitivity to harpin relative to wild-type plants, and display reduced ROS accumulation. Harpin-induced cell death was found to be conditional on the ability of the plant to accumulate SA, whereas harpin induction of MAPK activation and ROS accumulation are not. However, harpin-induced ROS accumulation is required for activation of SIPK and wound-induced protein kinase. Transcriptional profiling revealed that suppression of SIPK signaling also affects early expression of a range of pathogen- and stress-responsive genes during harpin challenge.

Activation of stress-responsive mitogen-activated protein kinase pathways in hybrid poplar (Populus trichocarpa x Populus deltoides)

Plant mitogen-activated protein kinase (MAPK) cascades are important amplifying modules that can rapidly transduce stress signals into various appropriate intracellular responses. Several extracellular regulated kinase (ERK)-type MAPKs involved in plant defense signaling have been identified in herbaceous species, but no MAPK cascade has yet been characterized in a tree species. We examined the signal transduction events that lead to activation of defense mechanisms in poplar, a major forest species of economic and ecological importance which is becoming the model tree system for studying stress and adaptation responses. We show that, in poplar cell suspensions and leaf tissue, chitosan, a non-host-specific elicitor, and ozone, a strong oxidant and atmospheric pollutant, induce rapid and transient activation of at least two myelin basic protein (MBP) kinases with apparent molecular masses of 44 and 47 kD. The chitosan- and ozone-activated kinases have characteristics of MAPKs-they preferentially phosphorylate MBP, require tyrosine and threonine phosphorylation to be activated and are specifically recognized by anti-ERK and anti-pERK antibodies. Moreover, activation of these poplar MAPKs by chitosan or ozone is dependent on the production of reactive oxygen species; the influx of calcium ions via membrane channels; the activation of an upstream, membrane-localized component; and a cognate MAPK kinase (MAPKK). These data suggest that biotic and abiotic challenges activate MAPKs in poplar, as in herbaceous species, which then function as a convergence point for pathogen defense and oxidant stress signaling cascades.

Double jeopardy: both overexpression and suppression of a redox-activated plant mitogen-activated protein kinase render tobacco plants ozone sensitive

In plants, the role of mitogen-activated protein kinase (MAPK) in reactive oxygen species (ROS)-based signal transduction processes is elusive. Despite the fact that ROS can induce MAPK activation, no direct genetic evidence has linked ROS-induced MAPK activation with the hypersensitive response, a form of programmed cell death. In tobacco, the major ROS-induced MAPK is salicylate-induced protein kinase (SIPK). We found through gain-of-function and loss-of-function approaches that both overexpression and RNA interference-based suppression of SIPK render the plant sensitive to ROS stress. Transgenic lines overexpressing a nonphosphorylatable version of SIPK were not ROS sensitive. Analysis of the MAPK activation profiles in ROS-stressed transgenic and wild-type plants revealed a striking interplay between SIPK and another MAPK (wound-induced protein kinase [WIPK]) in the different kinotypes. During continuous ozone exposure, abnormally prolonged activation of SIPK was seen in the SIPK-overexpression genotype, without WIPK activation, whereas strong and stable activation of WIPK was observed in the SIPK-suppressed lines. Thus, one role of activated SIPK in tobacco cells upon ROS stimulation appears to be control of the inactivation of WIPK.

Ozone treatment rapidly activates MAP kinase signalling in plants

Brief exposure to ozone, a potent cross-inducer of plant stress responses, leads within minutes to activation of an ERK-type MAP kinase (approximately 46 kDa) in tobacco. This activation process is calcium-dependent and can be blocked both by free radical quenchers and by a specific inhibitor of MEK-1 (MAPKK). Hydrogen peroxide and superoxide anion radicals can substitute for ozone as the activation stimulus, which does not appear to require salicylate as an intermediary. The properties of the ozone-induced MAPK suggest that it may be SIPK (salicylate-induced protein kinase), a tobacco MAPK that is activated by a variety of stress treatments. The ability of ozone to activate SIPK indicates that this protein kinase acts as a very early transducer of redox stress signals in plant cells.

The curriculum of practice: a conceptual framework for speech-language therapy and audiology practice with a black African first language clientele

This paper discusses the use of a conceptual framework (i.e., the curriculum of practice) to guide the transformation of professional practice toward equitable service provision, rooted in a research finding that South African Speech-Language Therapy & Audiology has a favourable service bias toward White English/Afrikaans first language speakers (Pillay, 1997b). Three perspectives of practice are discussed, i.e., the official, espoused and actual perspectives. Also, the various elements of practice are reviewed, and include clinical practice activities, resources and their utilization, professional education curricular and professional policies. It is asserted that re-constructing the profession's knowledge base in a presently (1996-) transforming South Africa, mostly requires the use of a critical (versus, for e.g., a scientific) paradigm.