Plant growth-promoting rhizobacteria--alleviators of abiotic stresses in soil: A review

With the continuous increase in human population,there is widespread usage of chemical fertilizers that are responsible for introducing abiotic stresses in agricultural crop lands.Abiotic stresses are major constraints for crop yield and global food security and therefore require an immediate response.The implementation of plant growth-promoting rhizobacteria(PGPR)into the agricultural production system can be a profitable alternative because of its efficiency in plant growth regulation and abiotic stress management.These bacteria have the potential to promote plant growth and to aid in the management of plant diseases and abiotic stresses in the soil through production of bacterial phytohormones and associated metabolites as well as through significant root morphological changes.These changes result in improved plant-water relations and nutritional status in plants and stimulate plants’defensive mechanisms to overcome unfavorable environmental conditions.Here,we describe the significance of plant-microbe interactions,highlighting the role of PGPR,bacterial phytohormones,and bacterial metabolites in relieving abiotic environmental stress in soil.Further research is necessary to gather in-depth knowledge on PGPR-associated mechanisms and plant-microbe interactions in order to pave a way for field-scale application of beneficial rhizobacteria,with the aim of building a healthy and sustainable agricultural system.Therefore,this review aims to emphasize the role of PGPR in growth promotion and management of abiotic soil stress with the goal of developing an eco-friendly and cost-effective strategy for future agricultural sustainability.     

Beneficial elements for agricultural crops and their functional relevance in defence against stresses

The beneficial elements are not deemed essential for all crops but may be vital for particular plant taxa. The distinction between beneficial and essential is often difficult in the case of some trace elements. Elements such as aluminium (Al), cobalt (Co), sodium (Na), selenium (Se) and silicon (Si) are considered beneficial for plants. These elements are not critical for all plants but may improve plant growth and yield. Pertinently, beneficial elements reportedly enhance resistance to abiotic stresses (drought, salinity, high temperature, cold, UV stress, and nutrient toxicity or deficiency) and biotic stresses (pathogens and herbivores) at their low levels. However, the essential-to-lethal range for these elements is somewhat narrow. The effect of beneficial elements at low levels deserves more attention with regard to using them to fertilize crops to boost crop production under stress and to enhance plant nutritional value as a feed or food. A more holistic approach to plant nutrition would not only be restricted to nutrients essential to survival but would also include mineral elements at levels beneficial for best growth. Here, we describe the uptake mechanisms of various beneficial elements, their favourable aspects, and the role of these elements in conferring tolerance against abiotic and biotic stresses.     

一氧化氮在植物发育及植物–微生物互作中的作用机制研究进展

一氧化氮 (NO) 作为高活性信号分子,是调控植物生长发育的关键因子。NO可提高植物对非生物胁迫及生物胁迫的抗性,增强植物的免疫能力。最新的研究表明,NO在植物根系与微生物的互作过程中发挥着重要作用,NO能够促进植物根系与根瘤菌及丛枝菌根真菌形成共生体,从而提高植物对土壤氮磷养分的获取。NO作为信号物质调控植物对生物胁迫和非生物胁迫抗性的主要机制有:1) NO与活性氧系统互作,调节活性氧的水平,缓解氧化应激反应对植物的伤害;2) NO通过蛋白质的翻译后修饰,对植物免疫及抗逆过程进行调节;3) NO与多种植物激素互作,参与激素对植物生长发育的调节过程。而且NO可促进共生体的形成及发育相关基因表达,抑制免疫基因表达,通过NO与植物球蛋白 (phytoglobin) 的循环维持共生体的氧化还原水平及能量状态,从而促进植物–微生物共生关系。以往关于NO的研究主要集中在前3个方面,有关NO在植物–微生物互作中的作用机制的研究较少,NO参与植物–微生物互作机制的研究亟待加强。揭示NO增强植物抗逆性及其调节根系发育的机制,深入探究NO调控植物–微生物互作的机理,对于提高集约化作物生产体系中养分利用效率和作物生产力具有重要的理论与实践意义。     

The Role of Nutrient Efficient Plants in Improving Crop Yields in the Twenty First Century

In the 21st century, nutrient efficient plants will play a major role in increasing crop yields compared to the 20th century, mainly due to limited land and water resources available for crop production, higher cost of inorganic fertilizer inputs, declining trends in crop yields globally, and increasing environmental concerns. Furthermore, at least 60% of the world's arable lands have mineral deficiencies or elemental toxicity problems, and on such soils fertilizers and lime amendments are essential for achieving improved crop yields. Fertilizer inputs are increasing cost of production of farmers, and there is a major concern for environmental pollution due to excess fertilizer inputs. Higher demands for food and fiber by increasing world populations further enhance the importance of nutrient efficient cultivars that are also higher producers. Nutrient efficient plants are defined as those plants, which produce higher yields per unit of nutrient, applied or absorbed than other plants (standards) under similar agroecological conditions. During the last three decades, much research has been conducted to identify and/or breed nutrient efficient plant species or genotypes/cultivars within species and to further understand the mechanisms of nutrient efficiency in crop plants. However, success in releasing nutrient efficient cultivars has been limited. The main reasons for limited success are that the genetics of plant responses to nutrients and plant interactions with environmental variables are not well understood. Complexity of genes involved in nutrient use efficiency for macro and micronutrients and limited collaborative efforts between breeders, soil scientists, physiologists, and agronomists to evaluate nutrient efficiency issues on a holistic basis have hampered progress in this area. Hence, during the 21st century agricultural scientists have tremendous challenges, as well as opportunities, to develop nutrient efficient crop plants and to develop best management practices that increase the plant efficiency for utilization of applied fertilizers. During the 20th century, breeding for nutritional traits has been proposed as a strategy to improve the efficiency of fertilizer use or to obtain higher yields in low input agricultural systems. This strategy should continue to receive top priority during the 21st century for developing nutrient efficient crop genotypes. This paper over views the importance of nutrient efficient plants in increasing crop yields in modern agriculture. Further, definitions and available methods of calculating nutrient use efficiency, mechanisms for nutrient uptake and use efficiency, role of crops in nutrient use efficiency under biotic and abiotic stresses and breeding strategies to improve nutrient use efficiency in crop plants have been discussed.     

Beneficial effects of silicon on abiotic stress tolerance in legumes

Soil salinity, drought, metal toxicity, and ultraviolet-B radiation were major abiotic stresses that limit plant growth and productivity by disrupting the plants' cellular ionic and osmotic balance; legumes, a diverse plant family, suffered from these abiotic stresses. Although silicon (Si) is generally considered non-essential for plant growth and development, Si uptake by plants could facilitate plant growth by reducing biotic and abiotic stresses. There is however, a lack of systematic study on Si uptake benefits and mechanism on legumes because legumes reject Si uptake. Here, we reviewed the beneficial role of Si in enhancing abiotic stress tolerance in legumes and highlighted the mechanisms through which Si could improve abiotic stress tolerance in legumes. Future research needs for Si mediated alleviation of abiotic stresses in legumes are also discussed.     

Effects of plant growth-promoting rhizobacteria on the molecular responses of maize under drought and heat stresses: A review

Drought and heat are major environmental stresses that continually influence plant growth and development. Under field conditions, these stresses occur more frequently in combination than alone, which magnifies corresponding detrimental effects on the growth and productivity of agriculturally important crops. Plant responses to such abiotic stresses are quite complex and manifested in a range of developmental, molecular, and physiological modifications that lead either to stress sensitivity or tolerance/resistance. Maize (Zea mays L.) is known for its sensitivity to abiotic stresses, which often results in substantial loss in crop productivity. Bioaugmentation with plant growth-promoting rhizobacteria (PGPR) has the potential to mitigate the adverse effects of drought and heat stresses on plants. Hence, this is considered a promising and eco-friendly strategy to ensure sustainable and long-term maize production under adverse climatic conditions. These microorganisms possess various plant growth-promoting (PGP) characteristics that can induce drought and heat tolerance in maize plants by directly or indirectly influencing molecular, metabolic, and physiological stress responses of plants. This review aims to assess the current knowledge regarding the ability of PGPR to induce drought and heat stress tolerance in maize plants. Furthermore, the drought and heat stress-induced expression of drought and heat stress response genes for this crop is discussed with the mechanisms through which PGPR alter maize stress response gene expression.     

Plant growth-promoting rhizobacteria: strategies to improve abiotic stresses under sustainable agriculture

The abiotic stresses like drought, heavy metal and salts directly or indirectly influence the global environmental pollution and decrease the agricultural productivity. The stress tolerant plant growth promoting rhizobacteria (PGPR) play an important role against the abiotic stresses in terms of enhancing the efficacy of soil, plant growth promotion (PGP). Stress tolerance PGPRs have certain specific PGP properties such as hormones synthesis, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, Indole-3-acetic acid (IAA) production, Abscisic acid (ABA) synthesis, enzymes production, nitrogen fixation, phosphorus (P) and Potassium (K) solubilization, as results which protect various crops during such stress conditions and consequently enhance crop sustainability. Efficient PGPRs isolated from various stress conditions have certainly, more useful against that particular stress. This article highlighted the isolation of various stresses tolerant PGPRs from varieties of crops under different stress conditions and their effect on varieties of crops to enhance their plant growth promotion.     

褪黑素调控根系生长和根际互作的机制研究进展

【目的】根系生长和根际互作是影响植物对土壤养分吸收的关键因子。根系在土壤中穿插生长，不断改变其形态可塑性，进而改变根系构型，扩大与土壤的接触面积以获取所需养分。同时根系的生理可塑性协同根系形态可塑性显著影响根际互作效应，为植物经济高效获取养分资源提供可能。探究褪黑素等内源生长调节因子对根系形态和生理可塑性的调控机制，揭示通过最大化根际效应强化根际互作的有效途径，对集约化作物体系提高养分利用效率，促进绿色增产增效，具有重要的理论与实践意义。主要进展褪黑素作为新型植物生长调节信号分子，在盐害、干旱和低温等非生物胁迫中具有增强植物抗逆性、改善植物生长等重要调节作用。褪黑素显著改变根系生长，对植物主根生长主要表现为抑制作用，对侧根及不定根的发育和生长具有浓度依赖性调节，从而深刻影响植物根系构型。褪黑素调控根系生长的机制尚不清楚，总结已有进展表明:一方面褪黑素调节光周期，影响光合产物的运输和糖信号，从而调控地下部碳分配和根系生长；另一方面，褪黑素还能与生长素等植物激素互作，参与激素对植物生长调控的信号通路，从而对植物的生长发育和新陈代谢产生影响。这些进展对深入揭示褪黑素调控根系生长发育的机制提供了重要依据。问题与展望根系的生长发育以及根系构型的改变显著影响根际过程和根际互作，褪黑素作为调控因子在不同养分环境条件下显著影响根系的形态可塑性。然而，褪黑素在根际过程和根际互作中的作用机制并不清楚，有关研究亟待加强。深入探究褪黑素参与根际互作的机制，理解褪黑素调控根系生长和根际过程的作用途径，可为集约化农业体系下精准调控作物根系生长，强化根际互作，提高养分利用效率提供科学依据。     

Role of methylotrophic bacteria in managing abiotic stresses for enhancing agricultural production

Abiotic stresses are a significant factor that considerably limits plant growth and productivity. Methylotrophs are an essential group of bacteria that utilize volatile carbon compounds, are prolific colonizers of different plant parts, and play a vital role in plant growth promotion(PGP) under stress conditions.Numerous rhizospheric and phyllosphere methylotrophs have been reported to exhibit PGP activities with superior stress-tolerating capacity against drought,heavy metal, salinity, high and...     

植物非生物逆境相关锌指蛋白基因的研究进展

植物能够适应多种逆境主要是通过改变其基因表达和代谢途径来实现的,因此研究这些基因表达和功能对提高植物耐逆性具有重要意义。锌指蛋白是一类具有手指状结构域的转录因子,这种结构域由锌离子与多个半胱氨酸和(或)组氨酸组成,锌离子在稳定其结构和发挥调控功能方面具有关键作用。植物锌指蛋白在植物耐逆性方面具有重要作用。本文综述了近几年来从拟南芥(Arabidopsis thaliana)、水稻(Oryza sativa)、小麦(Triticum aestivum)、番茄(Solanum lycopersicum)等植物中克隆的与非生物逆境相关锌指蛋白基因的研究成果,重点阐述了其基因表达部位、受逆境诱导情况及转基因植株的耐逆性等。目前的研究结果表明锌指蛋白能够调控耐逆相关基因的表达,在植物逆境代谢中发挥重要作用,因此可以利用锌指蛋白基因进行作物耐逆性的遗传改良,提高作物的耐逆能力。