Use of plant growth-promoting rhizobacteria to control stress responses of plant roots

Ethylene is a key gaseous hormone that controls various physiological processes in plants including growth, senescence, fruit ripening, and responses to abiotic and biotic stresses. In spite of some of these positive effects, the gas usually inhibits plant growth. While chemical fertilizers help plants grow better by providing soil-limited nutrients such as nitrogen and phosphate, over-usage often results in growth inhibition by soil contamination and subsequent stress responses in plants. Therefore, controlling ethylene production in plants becomes one of the attractive challenges to increase crop yields. Some soil bacteria among plant growth-promoting rhizobacteria (PGPRs) can stimulate plant growth even under stressful conditions by reducing ethylene levels in plants, hence the term “stress controllers” for these bacteria. Thus, manipulation of relevant genes or gene products might not only help clear polluted soil of contaminants but contribute to elevating the crop productivity. In this article, the beneficial soil bacteria and the mechanisms of reduced ethylene production in plants by stress controllers are discussed.     

A review of isothiocyanates biofumigation activity on plant parasitic nematodes

Natural isothiocyanates (ITCs) are toxic to a range of soil-borne pest and pathogens, including nematodes and fungi, and can thus be used as natural fumigants called biofumigants. Glucosinolates, β-thioglucoside N-hydroxysulfates, are secondary metabolites of Brassicales plants, stored in the S-cells vacuoles. Upon plant tissue damage myrosinase (thioglucoside glycohydrolase, EC 3.2.3.1), stored in contiguous cells, hydrolyses glucosinalates to an unstable aglycone that eventually eliminates sulfate group producing a wide range of different volatile isothiocyanates that are extremely toxic to root-knot nematodes. In fact, among synthetic commercial nematicidal formulates we can find isothiocyanates as active ingredients. Conventional nematode control practices have included soil sterilants of great environmental impact, most of which are now banned making mandatory the development of eco-sustainable alternative tools. We reviewed the nematicidal activity of isothiocyanates as components of botanical matrixes in the frame of a holistic nematode control approach encompassing secondary beneficial effects on soil structure and microbiology, beneficial preservation, enhanced residual life of biological activity and plant growth.     

Causes and consequences of plant-associated biofilms

The rhizosphere is the critical interface between plant roots and soil where beneficial and harmful interactions between plants and microorganisms occur. Although microorganisms have historically been studied as planktonic (or free-swimming) cells, most are found attached to surfaces, in multicellular assemblies known as biofilms. When found in association with plants, certain bacteria such as plant growth promoting rhizobacteria not only induce plant growth but also protect plants from soil-borne pathogens in a process known as biocontrol. Contrastingly, other rhizobacteria in a biofilm matrix may cause pathogenesis in plants. Although research suggests that biofilm formation on plants is associated with biological control and pathogenic response, little is known about how plants regulate this association. Here, we assess the biological importance of biofilm association on plants.     

Bacteria in the plant tissue culture environment

Bacteria and plants are joined in various symbiotic relationships that have developed over millennia and have influenced the evolution of both groups. Bacteria inhabit the surfaces of most plants and are also present inside many plant organs. These bacteria may have positive, neutral or negative impacts on their plant hosts. Probiotic effects may improve plant nutrition or increase resistance to biotic and abiotic stresses. Conversely pathogenic bacteria may kill or reduce the vigor of plant hosts. In addition some bacteria inhabit plants and profit from excess metabolites or shelter while not injuring the plant. Micropropagation of plants is based on the stimulation of organogenesis or embryogenesis from explants that are superficially decontaminated and placed into a sterile environment. If successful, this process removes bacteria from surfaces, but those inhabiting inner tissues and organs are usually not affected by these steriliants. In vitro conditions are designed for optimal plant growth and development, however these conditions are also often ideal for bacterial multiplication. The presence of bacteria in the in vitro environment was almost universally considered negative for plant culture, but more recently this view has been questioned. Certain bacteria appear to have a beneficial effect on the explants in culture; increasing multiplication and rooting, increasing explant quality, and organo- and embryogenesis of recalcitrant genotypes. The most important role of beneficial bacteria for micropropagated plants is likely to be during acclimatization, when growth is resumed under natural conditions. This review includes the role of bacterial interactions in plants, especially those grown in vitro.     

不同熏蒸剂对草莓连作土壤养分和微生物群落的影响

【背景】草莓连年栽培导致土传病害问题突出,施用熏蒸剂进行土壤消毒因效果显著得以广泛应用。但不同熏蒸剂对土壤病原微生物的影响存在较大差异,同时对非靶标微生物和土壤理化性质也会有不同程度的影响。【目的】明确不同熏蒸剂对草莓连作土壤养分和土壤细菌、真菌群落结构的影响,为合理选择熏蒸剂提供科学依据。【方法】以连作土壤为材料设置5个处理：未熏蒸、石灰氮熏蒸、石灰熏蒸、棉隆熏蒸、威百亩熏蒸,测定熏蒸处理后土壤养分含量;采用PacBio测序平台分析土壤微生物多样性的变化。【结果】石灰氮和威百亩处理均增加了碱解氮含量,降低了有机质、有效磷和速效钾含量;棉隆处理土壤中各养分含量均增加;石灰处理除有机质含量增加外,碱解氮、有效磷和速效钾含量均降低;棉隆、石灰和威百亩处理显著降低pH值。5个处理草莓连作土壤样本中获得了1 164个细菌OTU和444个真菌OTU。细菌多样性和丰富度分析发现,4种熏蒸剂处理均增加了土壤细菌群落的丰富度,石灰氮、石灰和威百亩处理增加了土壤细菌菌落的多样性。4种熏蒸剂处理真菌菌落的丰富度低于对照;石灰、棉隆处理真菌菌落的多样性高于对照和石灰氮、威百亩处理低于对照,但差异不显著。在物种组成分析中,从门水平看,变形菌门(Proteobacteria)和芽单胞菌门(Gemmatimonadetes)为优势细菌门;与对照相比,石灰氮、石灰、棉隆处理变形菌门相对丰度增高,威百亩处理相对丰度降低。4种处理均降低了芽单胞菌门的相对丰度。其他门类中,4种处理均增加了浮霉菌门(Planctomycetota)、疣微菌门(Verrucomicrobia)的相对丰度。优势细菌群落分析表明土壤熏蒸减少了芽单胞菌属(Gemmatimonas)、藤黄单胞菌属(Luteimonas)、中慢生根瘤菌属(Mesorhizobium)等细菌的相对丰度,增加了噬几丁质菌属(Chitinophaga)、苍白杆菌属(Ochrobactrum)的相对丰度。子囊菌门(Ascomycota)为优势真菌,石灰氮、石灰、棉隆、威百亩4种处理均增加了子囊菌门的相对丰度。另外还检测到引起草莓根部土传病害的枝孢属(Cladosporium)和镰刀菌属(Fusarium)病菌,熏蒸处理后均降低了枝孢属和镰刀菌属的相对丰度,其中枝孢属在石灰氮、石灰、棉隆、威百亩处理中分别降低了1.35%、1.11%、0.90%和1.31%,镰刀菌属分别降低了0.71%、0.85%、0.19%和0.65%,但差异不显著。4种土壤熏蒸剂均增加了有益真菌毛壳菌属(Chaetomium)的相对丰度。【结论】采用熏蒸剂处理连作土壤改变了微生物群落构成,减少或灭杀土壤中的大部分致病菌属,起到有效防治草莓土传病害的作用,但不能灭杀所有病菌,而且对有益菌和土壤理化性质有不同程度的影响,因此处理后补充有益微生物非常关键。根据对病原菌的灭杀效果,石灰氮、威百亩处理的效果优于其他处理,可作为轮换施用的熏蒸剂,本试验条件下,棉隆是一种弱的处理剂。     

植物内生菌增强植物对生物胁迫抗性的研究进展

植物在生长发育过程中因遭遇多种逆境的威胁而出现营养流失、产量大幅下降等问题，而使用传统的化学农药调控植物抗逆作用会对环境造成严重污染甚至危及人类健康，因此需要从天然成分中寻找合适的农药代替品。生活在每种植物体内的内生菌几乎都是植物微生态系统中的天然成分，因其特殊的生态位而可能对植物具有更加积极和直接的影响。然而目前，关于内生菌在提高宿主生物胁迫抗性等方面的作用机制还知之甚少。该文就植物内生菌的来源、多样性和对生物胁迫的抗性展开叙述。首先，总结了植物内生菌传播的主要方式，即水平传播和垂直传播；其次对内生菌种类的多样性以及在植物中的分布多样性进行了归纳与分析；最后，详细阐述了植物内生菌增强植物对生物胁迫应激耐受性(抗致病菌病害、抗虫害)的基本特点与作用机制，即植物内生菌可利用生态位竞争或营养位竞争产生的诱导抗性遏制病原菌感染，或合成抗生素类、生物碱类、几丁质类等次生代谢产物抑制病原菌或线虫的生长，从而防治病虫害。此外，基于内生菌增强植物生物胁迫抗性的研究现状进行了展望，为更加环保的生物防治制剂的开发与利用提供了参考。     

Modulation of host immunity by beneficial microbes

In nature, plants abundantly form beneficial associations with soilborne microbes that are important for plant survival and, as such, affect plant biodiversity and ecosystem functioning. Classical examples of symbiotic microbes are mycorrhizal fungi that aid in the uptake of water and minerals, and Rhizobium bacteria that fix atmospheric nitrogen for the plant. Several other types of beneficial soilborne microbes, such as plant-growth-promoting rhizobacteria and fungi with biological control activity, can stimulate plant growth by directly suppressing deleterious soilborne pathogens or by priming aboveground plant parts for enhanced defense against foliar pathogens or insect herbivores. The establishment of beneficial associations requires mutual recognition and substantial coordination of plant and microbial responses. A growing body of evidence suggests that beneficial microbes are initially recognized as potential invaders, after which an immune response is triggered, whereas, at later stages of the interaction, mutualists are able to short-circuit plant defense responses to enable successful colonization of host roots. Here, we review our current understanding of how symbiotic and nonsymbiotic beneficial soil microbes modulate the plant immune system and discuss the role of local and systemic defense responses in establishing the delicate balance between the two partners.     

Induction of resistance against pathogens by β-aminobutyric acid

Among the many types of plant stressors, pathogen attack, mainly fungi and bacteria can cause particularly severe damage both to individual plants and, on a wider scale, to agricultural productivity. The magnitude of these pathogen-induced problems has stimulated rapid progress in green biotechnology research into plant defense mechanisms. Plants can develop local and systemic wide-spectrum resistance induced by their exposure to virulent (systemic acquired resistance—SAR) or non-pathogenic microbes and various chemical elicitors (induced systemic resistance—ISR). β-Aminobutyric acid (BABA), non-protein amino acid, is though to be important component of the signaling pathway regulating ISR response in plants. After treatment with BABA or various chemicals, after infection by a necrotizing pathogen, colonization of the roots by beneficial microbes many plants establish a unique physiological state that is called the “primed” state of the plant. This review will focus on the recent knowledge about the role of BABA in the induction of ISR against pathogens mainly against fungi.     

Influence of Organic Pesticides on Nematode and Corn Earworm Damage and on Yield of Sweet Corn

Soil fumigants and nonvolatile pesticides increased growth and yield of sweet corn ''Seneca Chief'' over that of control plants in a 3-year study. Nematicide treatments increased average yields by 31% over controls, but did not significantly affect the mean weight per ear. Increase in yield was related to control of Belonolaimus longicaudatus, Trichodorus christiei and Pratylenchus zeae. Nonvolatile chemicals more effectively reduced populations of B. longicaudatus and T. christiei than did soil fumigants. Aldicarb did not control Criconemoides ornatus. All pesticides controlled P. zeae. Pesticides did not control Heliothis zea effectively.     

Plants protect their roots by alerting the enemies of grubs

Plant roots in the soil are under attack from many soil organisms. Although many ecologists are aware of the presence and importance of natural enemies in the soil that protect the plants from herbivores, the existence and nature of tritrophic interactions are poorly understood. So far, attention has focused on how plants protect their above-ground parts against herbivorous arthropods, either directly or indirectly (i.e. by getting help from the herbivore's enemies). This article is the first in showing that indirect plant defences also operate underground. We show that the roots of a coniferous plant ( Thuja occidentalis ) release chemicals upon attack by weevil larvae ( Otiorhynchus sulcatus ) and that these chemicals thereby attract parasitic nematodes ( Heterorhabditis megidis ).     

Burkholderia sp. SCMS54 reduces cadmium toxicity and promotes growth in tomato

Cadmium (Cd) can enter soil through the use of fertilisers, calcareous, pesticides and industrial and/or domestic effluents. Cd can leach into groundwater and be taken up by plants, potentially leading to reductions in plant growth and yield. In soil, plant roots interact with heavy metal (HM)‐tolerant microorganisms that may promote plant growth. Soil microorganisms may also be able to solubilise or mobilise soil metals, thereby acting as bioremediators. A better understanding of the interaction among plants, metals, microorganisms and soil will lead to improved plant tolerance. Two multi‐tolerant bacteria from the Burkholderia genus were isolated from Cd‐contaminated and Cd‐uncontaminated soil of a coffee plantation. In addition to its high tolerance to Cd, the strain SCMS54 produces indole‐acetic acid (IAA), solubilises inorganic phosphate and produces siderophores, demonstrating its potential to contribute to beneficial plant–microorganism interactions. When interacting with tomato plants exposed to Cd, the bacterium led to decreases in plant peroxide and chlorosis levels, promoted relative plant growth and decreased the root absorption of Cd, resulting in increased plant tolerance to this highly toxic HM. The results indicated that the inoculation of tomato plants with Burkholderia sp. SCMS54 promotes better growth in plants cultivated in the presence of Cd. This phenomenon appears to be attributed to a mechanism that decreases Cd concentrations in the roots via a beneficial interaction between the bacteria and the plant roots.     

Microbial amelioration of crop salinity stress

The use of soil and irrigation water with a high content of soluble salts is a major limiting factor for crop productivity in the semi-arid areas of the world. While important physiological insights about the mechanisms of salt tolerance in plants have been gained, the transfer of such knowledge into crop improvement has been limited. The identification and exploitation of soil microorganisms (especially rhizosphere bacteria and mycorrhizal fungi) that interact with plants by alleviating stress opens new alternatives for a pyramiding strategy against salinity, as well as new approaches to discover new mechanisms involved in stress tolerance. Although these mechanisms are not always well understood, beneficial physiological effects include improved nutrient and water uptake, growth promotion, and alteration of plant hormonal status and metabolism. This review aims to evaluate the beneficial effects of soil biota on the plant response to saline stress, with special reference to phytohormonal signalling mechanisms that interact with key physiological processes to improve plant tolerance to the osmotic and toxic components of salinity. Improved plant nutrition is a quite general beneficial effect and may contribute to the maintenance of homeostasis of toxic ions under saline stress. Furthermore, alteration of crop hormonal status to decrease evolution of the growth-retarding and senescence-inducing hormone ethylene (or its precursor 1-aminocyclopropane-1-carboxylic acid), or to maintain source-sink relations, photosynthesis, and biomass production and allocation (by altering indole-3-acetic acid and cytokinin biosynthesis) seem to be promising target processes for soil biota-improved crop salt tolerance.     

Impact of soil salinity on the plant-growth – promoting and biological control abilities of root associated bacteria

The effectiveness of plant growth – promoting bacteria is variable under different biotic and abiotic conditions. Abiotic factors may negatively affect the beneficial properties and efficiency of the introduced PGPR inoculants. The aim of this study was to evaluate the effect of plant growth – promoting rhizobacteria on plant growth and on the control of foot and root rot of tomatoes caused by Fusarium solani under different soil salinity conditions. Among the five tested strains, only Pseudomonas chlororaphis TSAU13, and Pseudomonas extremorientalis TSAU20 were able to stimulate plant growth and act as biological controls of foot and root rot disease of tomato. The soil salinity did not negatively affect the beneficial impacts of these strains, as they were able to colonize and survive on the roots of tomato plants under both saline and non-saline soil conditions. The improved plant height and fruit yield of tomato was also observed for plants inoculated with P. extremorientalis TSAU20. Our results indicated that, saline condition is not crucial factor in obtaining good performance with respect to the plant growth stimulating and biocontrol abilities of PGPR strains. The bacterial inoculant also enhanced antioxidant enzymes activities thereby preventing ROS induced oxidative damage in plants, and the proline concentrations in plant tissue that play an important role in plant stress tolerance.     

Bacteria with ACC deaminase can promote plant growth and help to feed the world

To feed all of the world's people, it is necessary to sustainably increase agricultural productivity. One way to do this is through the increased use of plant growth-promoting bacteria; recently, scientists have developed a more profound understanding of the mechanisms employed by these bacteria to facilitate plant growth. Here, it is argued that the ability of plant growth-promoting bacteria that produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase to lower plant ethylene levels, often a result of various stresses, is a key component in the efficacious functioning of these bacteria. The optimal functioning of these bacteria includes the synergistic interaction between ACC deaminase and both plant and bacterial auxin, indole-3-acetic acid (IAA). These bacteria not only directly promote plant growth, they also protect plants against flooding, drought, salt, flower wilting, metals, organic contaminants, and both bacterial and fungal pathogens. While a considerable amount of both basic and applied work remains to be done before ACC deaminase-producing plant growth-promoting bacteria become a mainstay of plant agriculture, the evidence indicates that with the expected shift from chemicals to soil bacteria, the world is on the verge of a major paradigm shift in plant agriculture.     

Phytoremediation of organochlorine pesticides: Concept,method, and recent developments

Rapid increase in industrialization of world economy in the past century has resulted in significantly high emission of anthropogenic chemicals in the ecosystem. The organochlorine pesticides (OCPs) are a great risk to the global environment and endanger the human health due to their affinity for dispersion, transportation over long distances, and bioaccumulation in the food chain. Phytoremediation is a promising technology that aims to make use of plants and associated bacteria for the treatment of groundwater and soil polluted by these contaminants. Processes known to be involved in phytoremediation of OCPs include phytoaccumulation, rhizoremediation, and phytotransformation. Vegetation has been accounted to considerably amplify OCP elimination from soil, in contrast to non-planted soil, attributable to both, uptake within plant tissues and high microbial degradation of OCP within the root zone. Developing transgenic plants is a promising approach to enhance phytoremediation capabilities. Recent advances in the application of phytoremediation technique for OCPs, including uptake by plants and plant–microbe association in the rhizosphere for the enhanced degradation and mineralization of these pollutants, is presented in this review. Additionally, some attempts to improve this technique using transgenesis and role of certain enzymes are also discussed.     

Multicellular behaviour and production of a wide variety of toxic substances support usage of Bacillus subtilis as a powerful biocontrol agent

Intensive cultivation of plants in the monoculture field system in order to feed the continuously growing human population creates a need for their protection from the variety of natural competitors such as: bacteria, fungi, insects as well as other plants. The increase in the use of chemical substances in the 20th century has brought many effective solutions for the agriculture. However, it was extremely difficult to obtain a substance, which would be directed solely against a specific plant pathogen and would not be harmful for the environment. In the late 1900's scientists began trying to use natural antagonisms between resident soil organism to protect plants. This phenomenon was named biocontrol. Biological control of plants by microorganisms is a very promising alternative to an extended use of pesticides, which are often expensive and accumulate in plants or soil, having adverse effects on humans. Nonpathogenic soil bacteria living in association with roots of higher plants enhance their adaptive potential and, moreover, they can be beneficial for their growth. Here, we present the current status of the use of Bacillus subtilis in biocontrol. This prevalent inhabitant of soil is widely recognized as a powerful biocontrol agent. Naturally present in the immediate vicinity of plant roots, B. subtilis is able to maintain stable contact with higher plants and promote their growth. In addition, due to its broad host range, its ability to form endospores and produce different biologically active compounds with a broad spectrum of activity, B. subtilis as well as other Bacilli are potentially useful as biocontrol agents.     

接种促生菌对花生根际土壤微生物及营养元素的影响

植物根际促生菌是一类可促进植物生长的有益细菌,有效的根际促生菌剂可以减少化肥施用。以束村氏菌属(Tsukamurella sp.)P9、伯克霍尔德氏菌属(Burkholderia sp.)P10、以及P9和P10混合菌液作为接种菌株,研究促生菌对花生生长、植株及土壤营养、根际土壤微生物类群及功能的影响。30 d盆栽实验结果表明,接种组的花生鲜重、株高及根长均显著提高;根际土壤细菌总数、固氮菌和溶磷菌数均明显高于未接种组;氮循环功能菌群数量有不同程度提高,土壤蔗糖酶、脲酶及过氧化氢酶均高于对照;土壤碱解氮及速效钾显著提高,植株营养指标有所提升,尤以P10接种效果更优。本研究初步结论表明2株促生菌通过活化土壤微生物、提高植株的有效营养元素含量,促进了花生的生长。     

Effect of soil fumigants on Fusarium wilt and nodulation of peas (Pisum sativum L.)

Chloropicrin, dazomet, formaldehyde, and D-D soil treatments all decreased the incidence of Fusarium wilt in a wilt-susceptible variety of pea grown in wilt-infested soil, but only chloropicrin and dazomet gave satisfactory control of the disease. All four fumigants decreased root nodulation, but no adverse effects on plant growth were detected. With dazomet, formaldehyde, and D-D, decreased nodulation is largely attributed to the extra soil nitrogen mineralized, whereas with chloropicrin the almost complete suppression of nodulation probably reflects the lethal effect of this material on the Rhizobium bacteria.     

ENDOPHYTIC BACTERIA FROM SEEDS OF NICOTIANA TABACUM CAN REDUCE CADMIUM PHYTOTOXICITY

Although endophytic bacteria seem to have a close association with their host plant, little is known about the influence of seed endophytic bacteria on initial plant development and on their interactions with plants under conditions of metal toxicity. In order to further elucidate this close relationship, we isolated endophytic bacteria from surface sterilized Nicotiana tabacum seeds that were collected from plants cultivated on a cadmium-(Cd) and zinc-enriched soil. Many of the isolated strains showed Cd tolerance. Sterilely grown tobacco plants were inoculated with either the endogenous microbial consortium, composed of cultivable and noncultivable strains; single strains; or defined consortia of the most representative cultivable strains. Subsequently, the effects of inoculation of endophytic bacteria on plant development and on metal and nutrient uptake were explored under conditions with and without exposure to Cd. In general, seed endophytes were found to have a positive effect on plant growth, as was illustrated by an increase in biomass production under conditions without Cd. In several cases, inoculation with endophytes resulted in improved biomass production under conditions of Cd stress, as well as in a higher plant Cd concentration and total plant Cd content compared to noninoculated plants. These results demonstrate the beneficial effects of seed endophytes on metal toxicity and accumulation, and suggest practical applications using inoculated seeds as a vector for plant beneficial bacteria.     

Tolerance of transgenic canola expressing 1-aminocyclopropane-1-carboxylic acid deaminase to growth inhibition by nickel.

Plant growth-promoting bacteria are useful to phytoremediation strategies in that they confer advantages to plants in contaminated soil. When plant growth-promoting bacteria contain the enzyme 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, the bacterial cell acts as a sink for ACC, the immediate biosynthetic precursor of the plant growth regulator ethylene thereby lowering plant ethylene levels and decreasing the negative effects of various environmental stresses. In an effort to gain the advantages provided by bacterial ACC deaminase in the phytoremediation of metals from the environment two transgenic canola lines with the gene for this enzyme were generated and tested. In these transgenic canola plants, expression of the ACC deaminase gene is driven by either tandem constitutive cauliflower mosaic virus (CaMV) 35S promoters or the root specific rolD promoter from Agrobacterium rhizogenes. Following the growth of transgenic and non-transformed canola in nickel contaminated soil, it was observed that the rolD plants demonstrate significantly increased tolerance to nickel compared to the non-transformed control plants.