Cyclin dependent protein kinases and stress responses in plants

Plants have to adjust, grow and establish themselves in various changing environmental conditions. Additionally, the sessile life-style of plants requires the development of response mechanisms for their adaptation in such environmental cues. Under biotic and abiotic stress, plant growth is negatively affected mainly as a result of cell cycle inhibition. The perception of stress involves the activation of signaling cascades that result in a prolonged S-phase and delayed entry into mitosis. Although the molecular interactions that link the cell cycle machinery to perception of stress are not fully understood, recent studies indicated the involvement of Cyclin Dependent Kinases (CDKs) in the plant response machinery. CDKs are core cell cycle regulators but their activity has been implicated in additional diverse cellular processes. Here we review the impact of different types of abiotic stress on plant cell cycle progression and CDK activity, and discuss the contribution of CDK function in the signaling control of stress tolerance.Key words: abiotic stress, cell cycle, CDK, cyclin     

The role of mycorrhizae and plant growth promoting rhizobacteria (PGPR) in improving crop productivity under stressful environments

Both biotic and abiotic stresses are major constrains to agricultural production. Under stress conditions, plant growth is affected by a number of factors such as hormonal and nutritional imbalance, ion toxicity, physiological disorders, susceptibility to diseases, etc. Plant growth under stress conditions may be enhanced by the application of microbial inoculation including plant growth promoting rhizobacteria (PGPR) and mycorrhizal fungi. These microbes can promote plant growth by regulating nutritional and hormonal balance, producing plant growth regulators, solubilizing nutrients and inducing resistance against plant pathogens. In addition to their interactions with plants, these microbes also show synergistic as well as antagonistic interactions with other microbes in the soil environment. These interactions may be vital for sustainable agriculture because they mainly depend on biological processes rather than on agrochemicals to maintain plant growth and development as well as proper soil health under stress conditions. A number of research articles can be deciphered from the literature, which shows the role of rhizobacteria and mycorrhizae alone and/or in combination in enhancing plant growth under stress conditions. However, in contrast, a few review papers are available which discuss the synergistic interactions between rhizobacteria and mycorrhizae for enhancing plant growth under normal (non-stress) or stressful environments. Biological interactions between PGPR and mycorrhizal fungi are believed to cause a cumulative effect on all rhizosphere components, and these interactions are also affected by environmental factors such as soil type, nutrition, moisture and temperature. The present review comprehensively discusses recent developments on the effectiveness of PGPR and mycorrhizal fungi for enhancing plant growth under stressful environments. The key mechanisms involved in plant stress tolerance and the effectiveness of microbial inoculation for enhancing plant growth under stress conditions have been discussed at length in this review. Growth promotion by single and dual inoculation of PGPR and mycorrhizal fungi under stress conditions have also been discussed and reviewed comprehensively.     

A burst of plant NADPH oxidases

Reactive oxygen species (ROS) are highly reactive molecules able to damage cellular components but they also act as cell signalling elements. ROS are produced by many different enzymatic systems. Plant NADPH oxidases, also known as respiratory burst oxidase homologues (RBOHs), are the most thoroughly studied enzymatic ROS-generating systems and our understanding of their involvement in various plant processes has increased considerably in recent years. In this review we discuss their roles as ROS producers during cell growth, plant development and plant response to abiotic environmental constraints and biotic interactions, both pathogenic and symbiotic. This broad range of functions suggests that RBOHs may serve as important molecular 'hubs' during ROS-mediated signalling in plants.     

The rhizosphere microbiome: Plant–microbial interactions for resource acquisition

While horticulture tools and methods have been extensively developed to improve the management of crops, systems to harness the rhizosphere microbiome to benefit plant crops are still in development. Plants and microbes have been coevolving for several millennia, conferring fitness advantages that expand the plant’s own genetic potential. These beneficial associations allow the plants to cope with abiotic stresses such as nutrient deficiency across a wide range of soils and growing conditions. Plants achieve these benefits by selectively recruiting microbes using root exudates, positively impacting their nutrition, health and overall productivity. Advanced knowledge of the interplay between root exudates and microbiome alteration in response to plant nutrient status, and the underlying mechanisms there of, will allow the development of technologies to increase crop yield. This review summarizes current knowledge and perspectives on plant–microbial interactions for resource acquisition and discusses promising advances for manipulating rhizosphere microbiomes and root exudation.     

Perspectives of plant-associated microbes in heavy metal phytoremediation

"Phytoremediation" know-how to do-how is rapidly expanding and is being commercialized by harnessing the phyto-microbial diversity. This technology employs biodiversity to remove/contain pollutants from the air, soil and water. In recent years, there has been a considerable knowledge explosion in understanding plant-microbes-heavy metals interactions. Novel applications of plant-associated microbes have opened up promising areas of research in the field of phytoremediation technology. Various metabolites (e.g., 1-aminocyclopropane-1-carboxylic acid deaminase, indole-3-acetic acid, siderophores, organic acids, etc.) produced by plant-associated microbes (e.g., plant growth promoting bacteria, mycorrhizae) have been proposed to be involved in many biogeochemical processes operating in the rhizosphere. The salient functions include nutrient acquisition, cell elongation, metal detoxification and alleviation of biotic/abiotic stress in plants. Rhizosphere microbes accelerate metal mobility, or immobilization. Plants and associated microbes release inorganic and organic compounds possessing acidifying, chelating and/or reductive power. These functions are implicated to play an essential role in plant metal uptake. Overall the plant-associated beneficial microbes enhance the efficiency of phytoremediation process directly by altering the metal accumulation in plant tissues and indirectly by promoting the shoot and root biomass production. The present work aims to provide a comprehensive review of some of the promising processes mediated by plant-associated microbes and to illustrate how such processes influence heavy metal uptake through various biogeochemical processes including translocation, transformation, chelation, immobilization, solubilization, precipitation, volatilization and complexation of heavy metals ultimately facilitating phytoremediation.     

Plant–microbe interactions in the apoplast: Communication at the plant cell wall

The apoplast is a continuous plant compartment that connects cells between tissues and organs and is one of the first sites of interaction between plants and microbes. The plant cell wall occupies most of the apoplast and is composed of polysaccharides and associated proteins and ions. This dynamic part of the cell constitutes an essential physical barrier and a source of nutrients for the microbe. At the same time, the plant cell wall serves important functions in the interkingdom detection, recognition, and response to other organisms. Thus, both plant and microbe modify the plant cell wall and its environment in versatile ways to benefit from the interaction. We discuss here crucial processes occurring at the plant cell wall during the contact and communication between microbe and plant. Finally, we argue that these local and dynamic changes need to be considered to fully understand plant–microbe interactions.

Plants and microbes modify the host cell wall to benefit from the interaction by altering its properties, which are defined by the biochemistry of its polysaccharides, regulated by cell wall ions and proteins.     

Glutathione and plant response to the biotic environment

Glutathione (GSH) is a major antioxidant molecule in plants. It is involved in regulating plant development and responses to the abiotic and biotic environment. In recent years, numerous reports have clarified the molecular processes involving GSH in plant–microbe interactions. In this review, we summarize recent studies, highlighting the roles of GSH in interactions between plants and microbes, whether pathogenic or beneficial to plants.     

Evolutionary ecology of plant-microbe interactions: soil microbial structure alters selection on plant traits

? Below-ground microbial communities influence plant diversity, plant productivity, and plant community composition. Given these strong ecological effects, are interactions with below-ground microbes also important for understanding natural selection on plant traits? ? Here, we manipulated below-ground microbial communities and the soil moisture environment on replicated populations of Brassica rapa to examine how microbial community structure influences selection on plant traits and mediates plant responses to abiotic environmental stress. ? In soils with experimentally simplified microbial communities, plants were smaller, had reduced chlorophyll content, produced fewer flowers, and were less fecund when compared with plant populations grown in association with more complex soil microbial communities. Selection on plant growth and phenological traits also was stronger when plants were grown in simplified, less diverse soil microbial communities, and these effects typically were consistent across soil moisture treatments. ? Our results suggest that microbial community structure affects patterns of natural selection on plant traits. Thus, the below-ground microbial community can influence evolutionary processes, just as recent studies have demonstrated that microbial diversity can influence plant community and ecosystem processes.     

The tobacco Ntann12 gene, encoding an annexin, is induced upon Rhodoccocus fascians infection and during leafy gall development

Annexins are calcium-binding proteins that have been associated in plants with different biological processes such as responses to abiotic stress and early nodulation stages. Until now, the implication of annexins during plant–pathogen interactions has not been reported. Here, a novel plant annexin gene induced in tobacco BY-2 cell suspension cultures infected with the phytopathogenic bacterium Rhodococcus fascians (strain D188) has been identified . Expression of this gene, called Ntann12 , is also induced, but to a lower extent, by a strain (D188-5) that is unable to induce leafy gall formation. This gene was also induced in BY-2 cells infected with Pseudomonas syringae but not in cells infected with Agrobacterium tumefaciens or Escherichia coli. Ntann12 expression was also found to be stimulated by abiotic stress, including NaCl and abscissic acid, confirming a putative role in stress signal transduction pathways. In addition, promoter- GUS analyses using homozygous transgenic tobacco seedlings showed that the developmentally controlled expression of Ntann12 is altered upon R. fascians infection. Finally, up-regulation of Ntann12 during leafy gall ontogenesis was confirmed by RT-qPCR. Discussion is focused on the potential role of Ntann12 in biotic and abiotic stress responses and in plant development, both processes that may involve Ca²⁺-dependent signalling.     

Plant protein kinases stimulated by calcium

Calcium signals play an important role in many aspects of plant growth and development, including plant response to biotic and abiotic stress. The stimulus characteristic intracellular Ca2+ signals are generated in plant cells by a variety of stimuli, including changes in environmental conditions, interaction with microbes and growth and development processes. Cytoplasmatic calcium brings about responses by interacting with target proteins, like calcium-dependent kinases. In plant there are at least five classes of protein kinases (CDPK, CRK, CCaMK, CaMK and SnRK3), which activity is regulated by calcium ions. In this article the structure, regulation and function of calcium stimulated protein kinases are briefly reviewed.     

Plant symbionts: keys to the phytosphere

The exterior and interior of plants, aboveground and belowground, comprise a complex plant micro-ecosystem, known in recent years as the phytosphere. There are three components: the phyllosphere, endosphere, and rhizosphere. Although in comparison with other ecosystems the phytosphere is small, it similarly includes a great variety of functional microbes. Among these are certain microbes that live in symbiotic relationships with plants; these microbes are known as plant symbionts. Recent research has shown that these symbionts have tremendous effects on plant growth, confer resistance to abiotic stresses and pathogens, aid in the accumulation of metabolites, and have crucial relationships with other plant-associated microbes in the phytosphere. We review the ecological effects of plant symbionts on other microbes, and interactions between plant symbionts in the phytosphere. In addition, we discuss internal mechanisms and suggest future hot spots for research.     

Soil microbes alter plant fitness under competition and drought

Plants exist across varying biotic and abiotic environments, including variation in the composition of soil microbial communities. The ecological effects of soil microbes on plant communities are well known, whereas less is known about their importance for plant evolutionary processes. In particular, the net effects of soil microbes on plant fitness may vary across environmental contexts and among plant genotypes, setting the stage for microbially mediated plant evolution. Here, we assess the effects of soil microbes on plant fitness and natural selection on flowering time in different environments. We performed two experiments in which we grew Arabidopsis thaliana genotypes replicated in either live or sterilized soil microbial treatments, and across varying levels of either competition (isolation, intraspecific competition or interspecific competition) or watering (well‐watered or drought). We found large effects of competition and watering on plant fitness as well as the expression and natural selection of flowering time. Soil microbes increased average plant fitness under interspecific competition and drought and shaped the response of individual plant genotypes to drought. Finally, plant tolerance to either competition or drought was uncorrelated between soil microbial treatments suggesting that the plant traits favoured under environmental stress may depend on the presence of soil microbes. In summary, our experiments demonstrate that soil microbes can have large effects on plant fitness, which depend on both the environment and individual plant genotype. Future work in natural systems is needed for a complete understanding of the evolutionary importance of interactions between plants and soil microorganisms.     

Revealing on hydrogen sulfide and nitric oxide signals co-ordination for plant growth under stress conditions

In the recent times, plants are facing certain types of environmental stresses, which give rise to formation of reactive oxygen species (ROS) such as hydroxyl radicals, hydrogen peroxides, superoxide anions and so on. These are required by the plants at low concentrations for signal transduction and at high concentrations, they repress plant root growth. Apart from the ROS activities, hydrogen sulfide (H₂S) and nitric oxide (NO) have major contributions in regulating growth and developmental processes in plants, as they also play key roles as signaling molecules and act as chief plant immune defense mechanisms against various biotic as well as abiotic stresses. H₂S and NO are the two pivotal gaseous messengers involved in growth, germination and improved tolerance in plants under stressed and non-stress conditions. H₂S and NO mediate cell signaling in plants as a response to several abiotic stresses like temperature, heavy metal exposure, water and salinity. They alter gene expression levels to induce the synthesis of antioxidant enzymes, osmolytes and also trigger their interactions with each other. However, research has been limited to only cross adaptations and signal transductions. Understanding the change and mechanism of H₂S and NO mediated cell signaling will broaden our knowledge on the various biochemical changes that occur in plant cells related to different stresses. A clear understanding of these molecules in various environmental stresses would help to confer biotechnological applications to protect plants against abiotic stresses and to improve crop productivity.     

Microbial rescue to plant under habitat-imposed abiotic and biotic stresses

Habitat-imposed abiotic and biotic stress is a serious condition and is also a land-degradation problem in arid and semi-arid regions, causing major problem for crop productivity. Most of the cultivable and a least half of irrigated lands around the world are severely affected by environmental stresses. However, in these conditions, there are plant populations successfully adapted and evolutionarily different in their strategy of stress tolerance. Vascular plants do not function as autonomous individuals, but house diverse communities of symbiotic microbes. The role of these microbes can no longer be ignored. Microbial interactions are critical not only for host but also for fungal survival in stressed environments. Plants benefit extensively by harboring these associated microbes; they promote plant growth and confer enhanced resistance to various pathogens by producing antibiotics. To date, improvements in plant quality, production, abiotic and biotic stress resistance, nutrient, and water use have relied largely on manipulating plant genomes by breeding and genetic modification. Increasing evidence indicates that the function of symbiotic microbes seems to parallel more than one of these characteristics.     

影响引人微生物根部定殖的因素

从外界引入的各类有益微生物如生防菌(BCA)和根际促生菌或增产菌(PGPR,YIB)到种子表面随其生根发芽而蔓延或直接到根表沿根分布定殖．外来微生物在根际定殖的过程为与根尖接触,沿根分布,最后在根际建立自己的种群．定殖的位点以PGPR为例,是表皮细胞间隙,或侧根、根毛基部．外来微生物在根际定殖动态变化的原因,由于根际生物的和非生物的因素引起的．生物因子除去外来微生物本身的生理特性,还有根际土著微生物与外来微生物的相互作用,更重要的是植物基因型对微生物定殖的影响．非生物因子包括土壤环境、土壤结构和含水量,土壤温度和土壤pH值均能影响外来微生物在根部的定殖．     

Functions and interaction of plant lipid signalling under abiotic stresses

Lipids are the primary form of energy storage and a major component of plasma membranes, which form the interface between the cell and the extracellular environment. Several lipids — including phosphoinositide, phosphatidic acid, sphingolipids, lysophospholipids, oxylipins, and free fatty acids — also serve as substrates for the generation of signalling molecules. Abiotic stresses, such as drought and temperature stress, are known to affect plant growth. In addition, abiotic stresses can activate certain lipid-dependent signalling pathways that control the expression of stress-responsive genes and contribute to plant stress adaptation. Many studies have focused either on the enzymatic production and metabolism of lipids, or on the mechanisms of abiotic stress response. However, there is little information regarding the roles of plant lipids in plant responses to abiotic stress. In this review, we describe the metabolism of plant lipids and discuss their involvement in plant responses to abiotic stress. As such, this review provides crucial background for further research on the interactions between plant lipids and abiotic stress.     

The role of microbial signals in plant growth and development

Plant growth and development involves a tight coordination of the spatial and temporal organization of cell division, cell expansion and cell differentiation. Orchestration of these events requires the exchange of signaling molecules between the root and shoot, which can be affected by both biotic and abiotic factors. The interactions that occur between plants and their associated microorganisms have long been of interest, as knowledge of these processes could lead to the development of novel agricultural applications. Plants produce a wide range of organic compounds including sugars, organic acids and vitamins, which can be used as nutrients or signals by microbial populations. On the other hand, microorganisms release phytohormones, small molecules or volatile compounds, which may act directly or indirectly to activate plant immunity or regulate plant growth and morphogenesis. In this review, we focus on recent developments in the identification of signals from free-living bacteria and fungi that interact with plants in a beneficial way. Evidence has accumulated indicating that classic plant signals such as auxins and cytokinins can be produced by microorganisms to efficiently colonize the root and modulate root system architecture. Other classes of signals, including N-acyl-L-homoserine lactones, which are used by bacteria for cell-to-cell communication, can be perceived by plants to modulate gene expression, metabolism and growth. Finally, we discuss the role played by volatile organic compounds released by certain plant growth-promoting rhizobacteria in plant immunity and developmental processes. The picture that emerges is one in which plants and microbes communicate themselves through transkingdom signaling systems involving classic and novel signals.Key words: Arabidopsis, alkamides, auxins, quorum-sensing, cytokinins     

Plant metabolomics for plant chemical responses to belowground community change by climate change

General circulation models on global climate change predict increase in surface air temperature and changes in precipitation. Increases in air temperature (thus soil temperature) and altered precipitation are known to affect the species composition and function of soil microbial communities. Plant roots interact with diverse soil organisms such as bacteria, protozoa, fungi, nematodes, annelids and insects. Soil organisms show diverse interactions with plants (eg. competition, mutualism and parasitism) that may alter plant metabolism. Besides plant roots, various soil microbes such as bacteria and fungi can produce volatile organic compounds (VOCs), which can serve as infochemicals among soil organisms and plant roots. While the effects of climate change are likely to alter both soil communities and plant metabolism, it is equally probable that these changes will have cascading consequnces for grazers and subsequent food web components aboveground. Advances in plant metabolomics have made it possibile to track changes in plant metabolomes as they respond to biotic and abiotic environmental changes. Recent developments in analytical instrumentation and bioinformatics software have established metabolomics as an important research tool for studying ecological interactions between plants and other organisms. In this review, we will first summarize recent progress in plant metabolomics methodology and subsequently review recent studies of interactions between plants and soil organisms in relation to climate change issues.     

Check-in procedures for plant cell entry by biotrophic microbes

Significant advances in the cell biology of plant-microbe interactions have been achieved recently, to a large extent based on new technical approaches such as the use of fluorescent protein tags in model plants exploited in conjunction with available genetic resources. They have highlighted the pivotal role played by epidermal cells as the first site at which direct cell-to-cell contact takes place between the plant and microbes it may host. Here, we compare the cellular aspects of early biotrophic interactions with symbiotic and pathogenic microbes and evaluate the hypothesis that their hosting by plant cells share common traits related to the necessity of preserving host-cell integrity. The cellular events that accompany cell entry by the different biotrophs are divided into three categories, depending on whether the cellular changes are triggered by diffusible molecules, direct contact, or cell lumen penetration. Similarities and differences mirror the nutritional and developmental strategies of each plant-interacting organism, underlining the fact that plant cell entry represents a key aspect in the establishment of biotrophy.