Reactive oxygen signalling: the latest news

Global transcription profiling reveals comprehensive insights into hypoxic response in Arabidopsis

Plants have evolved adaptation mechanisms to sense oxygen deficiency in their environments and make coordinated physiological and structural adjustments to enhance their hypoxic tolerance. To gain insight into how plants respond to low-oxygen stress, gene expression profiling using whole-genome DNA amplicon microarrays was carried out at seven time points over 24 h, in wild-type and transgenic P(SAG12):ipt Arabidopsis (Arabidopsis thaliana) plants under normoxic and hypoxic conditions. Transcript levels of genes involved in glycolysis and fermentation pathways, ethylene synthesis and perception, calcium signaling, nitrogen utilization, trehalose metabolism, and alkaloid synthesis were significantly altered in response to oxygen limitation. Analysis based on gene ontology assignments suggested a significant down-regulation of genes whose functions are associated with cell walls, nucleosome structures, water channels, and ion transporters and a significant up-regulation of genes involved in transcriptional regulation, protein kinase activity, and auxin responses under conditions of oxygen shortage. Promoter analysis on a cluster of up-regulated genes revealed a significant overrepresentation of the AtMYB2-binding motif (GT motif), a sugar response element-like motif, and a G-box-related sequence, and also identified several putative anaerobic response elements. Finally, quantitative real-time polymerase chain reactions using 29 selected genes independently verified the microarray results. This study represents one of the most comprehensive analyses conducted to date investigating hypoxia-responsive transcriptional networks in plants.

Surface polysaccharides enable bacteria to evade plant immunity

Plants have an immune system to perceive pathogenic or potentially beneficial bacteria. Aspects of perception, signal transduction and the responses that the plant produces resemble features of innate immunity observed in animals. Plant reactions are various and include the production of antimicrobial compounds. Bacteria that are successful in establishing pathogenic or symbiotic interactions have developed multiple ways to protect themselves. We review the general importance of bacterial surface polysaccharides in the evasion of plant immune responses and elaborate on their role in protecting symbiotic bacteria against toxic reactive oxygen species during invasion of the host plant.

Redox regulation: a broadening horizon

Initially discovered in the context of photosynthesis, regulation by change in the redox state of thiol groups (S-S <--> 2SH) is now known to occur throughout biology. Several systems, each linking a hydrogen donor to an intermediary disulfide protein, act to effect changes that alter the activity of target proteins: the ferredoxin/thioredoxin system, comprised of reduced ferredoxin, a thioredoxin, and the enzyme, ferredoxin-thioredoxin reductase; the NADP/thioredoxin system, including NADPH, a thioredoxin, and NADP-thioredoxin reductase; and the glutathione/glutaredoxin system, composed of reduced glutathione and a glutaredoxin. A related disulfide protein, protein disulfide isomerase (PDI) acts in protein assembly. Regulation linked to plastoquinone and signaling induced by reactive oxygen species (ROS) and other agents are also being actively investigated. Progress made on these systems has linked redox to the regulation of an increasing number of processes not only in plants, but in other types of organisms as well. Research in areas currently under exploration promises to provide a fuller understanding of the role redox plays in cellular processes, and to further the application of this knowledge to technology and medicine.

Hemipterans as plant pathogens

Integration of the tools of genetics, genomics, and biochemistry has provided new approaches for identifying genes responding to herbivory. As a result, a picture of the complexity of plant-defense signaling to different herbivore feeding guilds is emerging. Plant responses to hemipteran insects have substantial overlap with responses mounted against microbial pathogens, as seen in changes in RNA profiles and emission of volatiles. Responses to known defense signals and characterization of the signaling pathways controlled by the first cloned insect R gene (Mi-1) indicate that perception and signal transduction leading to resistance may be similar to plant-pathogen interactions. Additionally, novel signaling pathways are emerging as important components of plant defense to insects. The availability of new tools and approaches will further enhance our understanding of the nature of defense in plant-hemipteran interactions.

Downstream divergence of the ethylene signaling pathway for harpin-stimulated Arabidopsis growth and insect defense

Ethylene (ET) signal transduction may regulate plant growth and defense, depending on which components are recruited into the pathway in response to different stimuli. We report here that the ET pathway controls both insect resistance (IR) and plant growth enhancement (PGE) in Arabidopsis (Arabidopsis thaliana) plants responding to harpin, a protein produced by a plant pathogenic bacterium. PGE may result from spraying plant tops with harpin or by soaking seeds in harpin solution; the latter especially enhances root growth. Plants treated similarly develop resistance to the green peach aphid (Myzus persicae). The salicylic acid pathway, although activated by harpin, does not lead to PGE and IR. By contrast, PGE and IR are induced in both wild-type plants and genotypes that have defects in salicylic acid signaling. In response to harpin, levels of jasmonic acid (JA) decrease, and the COI1 gene, which is indispensable for JA signal transduction, is not expressed in wild-type plants. However, PGE and IR are stimulated in the JA-resistant mutant jar1-1. In the wild type, PGE and IR develop coincidently with increases in ET levels and the expression of several genes essential for ET signaling. The ET receptor gene ETR1 is required because both phenotypes are arrested in the etr1-1 mutant. Consistently, inhibition of ET perception nullifies the induction of both PGE and IR. The signal transducer EIN2 is required for IR, and EIN5 is required for PGE because IR and PGE are impaired correspondingly in the ein2-1 and ein5-1 mutants. Therefore, harpin activates ET signaling while conscribing EIN2 and EIN5 to confer IR and PGE, respectively.

Signalling by tips

New molecules, including protein kinases, lipids and molecules that have neurotransmitter activities in animals have emerged as important players in tip-growing cells. Transcriptomics analysis reveals that the largest single class of genes expressed in pollen tubes encode signal transducers, reflecting the necessity to decode complex and diverse pathways that are associated with tip growth. Many of these pathways may use common intracellular second messengers, with ions and reactive oxygen species emerging as two major common denominators in many of the processes involved in tip growth. These second messengers might influence the actin cytoskeleton through known interactions with actin-binding proteins. In turn, changes in the dynamic properties of the cytoskeleton would define the basic polarity events needed to shape and modify tip-growing cells.