Andrew J. Wood

Plant Receptor-Like Serine Threonine Kinases: Roles in Signaling and Plant Defense

Plants are hosts to a wide array of pathogens from all kingdoms of life. In the absence of an active immune system or combinatorial diversifications that lead to recombination-driven somatic gene flexibility, plants have evolved different strategies to combat both individual pathogen strains and changing pathogen populations. The receptor-like kinase (RLK) gene-family expansion in plants was hypothesized to have allowed accelerated evolution among domains implicated in signal reception, typically a leucine-rich repeat (LRR). Under that model, the gene-family expansion represents a plant-specific adaptation that leads to the production of numerous and variable cell surface and cytoplasmic receptors. More recently, it has emerged that the LRR domains of RLK interact with a diverse group of proteins leading to combinatorial variations in signal response specificity. Therefore, the RLK appear to play a central role in signaling during pathogen recognition, the subsequent activation of plant defense mechanisms, and developmental control. The future challenges will include determinations of RLK modes of action, the basis of recognition and specificity, which cellular responses each receptor mediates, and how both receptor and kinase domain interactions fit into the defense signaling cascades. These challenges will be complicated by the limited information that may be derived from the primary sequence of the LRR domain. The review focuses upon implications derived from recent studies of the secondary and tertiary structures of several plant RLK that change understanding of plant receptor function and signaling. In addition, the biological functions of plant and animal RLK-containing receptors were reviewed and commonalities among their signaling mechanisms identified. Further elucidated were the genomic and structural organizations of RLK gene families, with special emphasis on RLK implicated in resistance to disease and development.

Desiccation-tolerance in bryophytes: a review

Abstract Desiccation-tolerance (DT), the ability to lose virtually all free intracellular water and then recover normal function upon rehydration, is one of the most remarkable features of bryophytes. The physiology of bryophytes differs in major respects from that of vascular plants by virtue of their smaller size; unlike vascular plants, the leafy shoots of bryophytes equilibrate rapidly with the water potential in their surroundings and tend to be either fully hydrated or desiccated and metabolically inactive. The time required to recover from desiccation increases and degree of recovery decreases with length of desiccation; both also depend upon temperature and intensity of desiccation. Tolerance in at least some species shows phenotypic plasticity. Recovery of respiration, photosynthesis and protein synthesis takes place within minutes or an hour or two; recovery of the cell cycle, food transport and the cytoskeleton may take 24 hours or more. Positive carbon balance is essential to survival of repeated cycles of drying and wetting; significant growth requires continuously wet periods of a few days or more. Male and female gametophytes, and gametophyte and sporophyte, may differ in tolerance. Desiccation-tolerance is essential to dispersal and establishment of spores and vegetative propagules. The mechanisms of DT in bryophytes, including expression of LEA proteins, high content of non-reducing sugars and effective antioxidant and photo-protection, are at least partly constitutive, allowing survival of rapid drying, but changes in gene expression resulting from mRNA sequestration and alterations in translational controls elicited upon rehydration are also important to repair processes following re-wetting. Phylogenetic and ecological considerations suggest that DT is a primitive character of land plants, lost in the course of evolution of the homoiohydric vascular-plant shoot system, but retained in spores, pollen and seeds, and re-evolved in the vegetative tissues of vascular “resurrection plants.” Bryophytes have retained the poikilohydry and DT that are probably the optimal pattern of adaptation at their scale, but modern bryophytes are specialized and diverse, and are removed by the same span of evolutionary time as the flowering plants from their primitive origins.

“To dryness and beyond” – Preparation for the dried state and rehydration in vegetative desiccation-tolerant plants

The ability of vegetative plant tissues to survive desiccation is an uncommon trait, although plants that are able to do this represent all major classes of plants. Two classes of vegetative desiccation-tolerant plants exist; those that are modified desiccation-tolerant and can only survive desiccation if drying rates are slow, and those that are fully desiccation-tolerant and can survive even rapid drying rates. Investigations into the cellular level responses of these two types of plants has lead to an understanding of the underlying mechanisms of desiccation-tolerance. The following proposed mechanisms for desiccation-tolerance are presented. Modified desiccation-tolerant plants utilize inducible cellular protection systems supplemented in part by a minor rehydration induced repair component. Fully desiccation-tolerant plants utilize a rehydration induced repair system that is complemented by a constitutive protection component. This minireview explores the evidence for these proposed mechanisms in an attempt to lay the theoretical ground work for future work in this area.

Bryophytes as experimental models for the study of environmental stress tolerance: Tortula ruralis and desiccation-tolerance in mosses

The development of a complete understanding of how plants interact with the environment at the cellular level is a crucial step in advancing our ability to unravel the complexities of plant ecology particularly with regard to the role that many of the less complex plants (i.e., algae, lichens, and bryophytes) play in plant communities and in establishing areas for colonization by their more complex brothers. One of the main barriers to the advancement of this area of plant biology has been the paucity of simple and appropriate experimental models that would enable the researcher to biochemically and genetically dissect the response of less complex plants to environmental stress. A number of bryophytes model systems have been developed and they have been powerful experimental tools for the elucidation of complex biological processes in plants. Recently there has been a resurgent interest in bryophytes as models systems due to the discovery and development of homologous recombination technologies in the moss Physcomitrella patens (Hedw.) Brach & Schimp. In this report we introduce the desiccation-tolerant moss Tortula ruralis (Hedw.) Gaert., Meyer, and Scherb, as a model for stress tolerance mechanisms that offers a great deal of promise for advancing our efforts to understand how plants respond to and survive the severest of stressful environments. T. ruralis, a species native to Northern and Western North America, has been the most intensely studied of all bryophytes with respect to its physiological, biochemical, and cellular responses, to the severest of water stresses, desiccation. It is our hope that the research conducted using this bryophyte will lay the foundationfor not only the ecology of bryophytes and other less complex plants but also for the role of desiccation-tolerance in the evolution of land plants and the determination of mechanisms by which plant cells can withstand environmental insults. We will focus the discussion on the research we and others have conducted in an effort to understand the ability of T. ruralis to withstand the complete loss of free water from the protoplasm of its cells.

The nature and distribution of vegetative desiccation-tolerance in hornworts, liverworts and mosses

Abstract Desiccation-tolerance is the unique ability to revive from the air-dried state. In this review, I summarize the distribution of vegetative desiccation-tolerance within the land plants emphasizing bryophytes, and provide a current checklist of those moss, liverwort and hornwort species with documented and experimentally determined tolerance. Desiccation-tolerant species can survive equilibration with either modestly dry air (i.e., 70–80% RH or −30 to −48 MPa) or extremely dry air (i.e., 0–30% RH or less than −162 MPa), and have been identified within seven classes of bryophytes, the Andreaeopsida, Bryopsida, Polytrichopsida and Tetraphidopsida (mosses), Jungermanniopsida and Marchantiopsida (liverworts) and the Anthocerotopsida (hornworts); 210 out of 21,000 bryophyte species (ca. 1.0%) have been experimentally determined to possess vegetative desiccation tolerance—158 species of mosses, 51 species of liverworts and one species of hornwort. Finally, I propose a comprehensive survey of mosses, liverworts and hornworts using a standard procedure that employs modulated chlorophyll fluorescence for analyzing vegetative desiccation-tolerance (i.e., the “Austin Protocol”).

Definition of Soybean Genomic Regions That Control Seed Phytoestrogen Amounts

Soybean seeds contain large amounts of isoflavones or phytoestrogens such as genistein, daidzein, and glycitein that display biological effects when ingested by humans and animals. In seeds, the total amount, and amount of each type, of isoflavone varies by 5 fold between cultivars and locations. Isoflavone content and quality are one key to the biological effects of soy foods, dietary supplements, and nutraceuticals. Previously we had identified 6 loci (QTL) controlling isoflavone content using 150 DNA markers. This study aimed to identify and delimit loci underlying heritable variation in isoflavone content with additional DNA markers. We used a recombinant inbred line (RIL) population (n=100) derived from the cross of “Essex” by “Forrest,” two cultivars that contrast for isoflavone content. Seed isoflavone content of each RIL was determined by HPLC and compared against 240 polymorphic microsatellite markers by one-way analysis of variance. Two QTL that underlie seed isoflavone content were newly discovered. The additional markers confirmed and refined the positions of the six QTL already reported. The first new region anchored by the marker BARC_Satt063 was significantly associated with genistein (P=0.009, R2=29.5%) and daidzein (P=0.007 , R2=17.0%). The region is located on linkage group B2 and derived the beneficial allele from Essex. The second new region defined by the marker BARC_Satt129 was significantly associated with total glycitein (P=0.0005 , R2=32.0%). The region is located on linkage group D1a+Q and also derived the beneficial allele from Essex. Jointly the eight loci can explain the heritable variation in isoflavone content. The loci may be used to stabilize seed isoflavone content by selection and to isolate the underlying genes.

Aldehyde dehydrogenase (ALDH) superfamily in plants: gene nomenclature and comparative genomics.

In recent years, there has been a significant increase in the number of completely sequenced plant genomes. The comparison of fully sequenced genomes allows for identification of new gene family members, as well as comprehensive analysis of gene family evolution. The aldehyde dehydrogenase (ALDH) gene superfamily comprises a group of enzymes involved in the NAD+- or NADP+-dependent conversion of various aldehydes to their corresponding carboxylic acids. ALDH enzymes are involved in processing many aldehydes that serve as biogenic intermediates in a wide range of metabolic pathways. In addition, many of these enzymes function as ‘aldehyde scavengers’ by removing reactive aldehydes generated during the oxidative degradation of lipid membranes, also known as lipid peroxidation. Plants and animals share many ALDH families, and many genes are highly conserved between these two evolutionarily distinct groups. Conversely, both plants and animals also contain unique ALDH genes and families. Herein we carried out genome-wide identification of ALDH genes in a number of plant species—including Arabidopsis thaliana (thale crest), Chlamydomonas reinhardtii (unicellular algae), Oryza sativa (rice), Physcomitrella patens (moss), Vitis vinifera (grapevine) and Zea mays (maize). These data were then combined with previous analysis of Populus trichocarpa (poplar tree), Selaginella moellindorffii (gemmiferous spikemoss), Sorghum bicolor (sorghum) and Volvox carteri (colonial algae) for a comprehensive evolutionary comparison of the plant ALDH superfamily. As a result, newly identified genes can be more easily analyzed and gene names can be assigned according to current nomenclature guidelines; our goal is to clarify previously confusing and conflicting names and classifications that might confound results and prevent accurate comparisons between studies.

How some plants recover from vegetative desiccation: A repair based strategy

Desiccation-tolerant plants can be grouped into two categories: the 1) desiccation-tolerant plants whose internal water content rapidly equilibrates to the water potential of the environment and 2) the modified desiccation-tolerant plants that all employ mechanisms to retard and control the rate of water loss. Desiccation tolerance can be achieved by mechanisms that incorporate one of two alternatives, viz. cellular protection or cellular recovery (repair). The majority of plants probably utilize aspects of both. Desiccation-tolerant species, in particular the moss Tortula ruralis, appear to utilize a tolerance strategy that combines a constitutive protection system and a rehydration-inducible recovery mechanism. The rehydration-induced recovery mechanism of Tortula ruralis relies heavily upon a change in gene expression that is mediated by posttranscriptional events rather than the slower reacting transcriptional controls. Findings indicate that it takes a certain amount of prior water loss to fully activate the protein-based portion of the recovery mechanisms upon rehydration. Utilizing cDNAs representing individual hydrins (proteins whose synthesis is hydration specific) and rehydrins (proteins whose synthesis is rehydration specific), it was determined that if drying rates were slow rehydrin transcripts selectively accumulate in the dried gametophytes. Studies revealed that this storage involves the formation of mRNPs (messenger ribonucleoprotein particles). The identity and possible functions of the rehydrins of Tortula ruralis are also under investigation, in particular Tr155, a small rehydrin (24kD) appears to be involved in antioxidant production during rehydration.

The stress-responsive Tortula ruralis gene ALDH21A1 describes a novel eukaryotic aldehyde dehydrogenase protein family

Summary The desiccation-tolerant plant Tortula ruralis is an important experimental system for the study of gene control in response to severe water deficit-stress. EST gene discovery efforts have identified a cDNA ALDH21A1 encoding a member of the aldehyde dehydrogenase (ALDH) gene super family. The deduced polypeptide ALDH21A1 contains the ALDH active site signature sequence and 17 residues correlated with enzymatic activity, however ALDH21A1 is ≤30 % identical to known ALDH proteins. Based upon established nomenclature for ALDH proteins, ALDH21A1 describes a novel eukaryotic ALDH protein family designated ALDH21. ALDH21A1 has a predicted molecular mass of 52.9 kDa, and a predicted pI of 5.9. Genomic blot analysis indicated that ALDH21A1 is present in 1-to-2 copies within the haploid T. ruralis genome. RNA blot hybridizations were used to analyze expression of ALDH21A1 in response to desiccation, ABA, UV, and NaCl. ALDH21A1 steady-state transcript levels increased in response to all treatments and were more abundant within the polysomal mRNA fraction of salt-treated gametophytes. The data suggest that ALDH21A1 plays an important role in the detoxification of aldehydes generated in response to desiccation- and salinity-stress, and we postulate that ALDH21A1 expression represents a unique stress tolerance mechanism.

EsDREB2B, a novel truncated DREB2-type transcription factor in the desert legume Eremosparton songoricum, enhances tolerance to multiple abiotic stresses in yeast and transgenic tobacco

BackgroundDehydration-Responsive Element-Binding Protein2 (DREB2) is a transcriptional factor which regulates the expression of several stress-inducible genes. DREB2-type proteins are particularly important in plant responses to drought, salt and heat. DREB2 genes have been identified and characterized in a variety of plants, and DREB2 genes are promising candidate genes for the improvement of stress tolerance in plants. However, little is known about these genes in plants adapted to water-limiting environments.ResultsIn this study, we describe the characterization of EsDREB2B, a novel DREB2B gene identified from the desert plant Eremosparton songoricum. Phylogenetic analysis and motif prediction indicate that EsDREB2B encodes a truncated DREB2 polypeptide that belongs to a legume-specific DREB2 group. In E. songoricum, EsDREB2B transcript accumulation was induced by a variety of abiotic stresses, including drought, salinity, cold, heat, heavy metal, mechanical wounding, oxidative stress and exogenous abscisic acid (ABA) treatment. Consistent with the predicted role as a transcription factor, EsDREB2B was targeted to the nucleus of onion epidermal cells and exhibited transactivation activity of a GAL4-containing reporter gene in yeast. In transgenic yeast, overexpression of EsDREB2B increased tolerance to multiple abiotic stresses. Our findings indicate that EsDREB2B can enhance stress tolerance in other plant species. Heterologous expression of EsDREB2B in tobacco showed improved tolerance to multiple abiotic stresses, and the transgenic plants exhibited no reduction in foliar growth. We observed that EsDREB2B is a functional DREB2-orthologue able to influence the physiological and biochemical response of transgenic tobacco to stress.ConclusionsBased upon these findings, EsDREB2B encodes an abiotic stress-inducible, transcription factor which confers abiotic stress-tolerance in yeast and transgenic tobacco.