Functions and Applications of Plant Growth Promoting Bacteria (PGPR) in Highly Technified Crops
Ortega R1*, Miralles I1, Domene MA2, Sanchez-Maranon M3 and Soriano M1,4
1Departament of Agronomy, University of Almeria, Spain
2Department of Health and Nutrition, Experimental Station of Cajamar Foundation, Spain
3Department of Soil Science and Agricultural Chemistry, University of Granada, Spain
4University of Granada Junta de Andaluc^a Pfizer Center of Genomics and Oncological Research, Spain
Submission: October 21, 2016; Published: October 30, 2017
*Corresponding author: Ortega R, Department of Agronomy, University of Almeria, Spain, Tel: +34686624073; Fax: +34950015939; Email: rortega@ual.es
How to cite this article: Ortega R, Miralles I, Domene MA, Sánchez-Marañón M, Soriano M. Functions and Applications of Plant Growth Promoting Bacteria (PGPR) in Highly Technified Crops. Agri Res & Tech: Open Access J. 2017; 12(1): 555837. DOI: 10.19080/ARTOAJ.2017.12.555837
Abstract
The need of higher agricultural productivity in recent decades has led to a great technification in crops. However, this had significant environmental costs due mainly to the indiscriminate use of fertilizers and pesticides. In response, a new trend seeks to establish more environmentally friendly cultivation methods where plant-growth-promoting bacteria (PGPR) can play a key role. PGPR influence a large number of plant-growth and developmental factors (N and P assimilation, phyto hormones, etc)) and help protect against other harmful organisms (antagonistic and bio-control effects, ISR and SAR). Thus, cultivation of PGPR for introduction into crops can result in large commercial opportunities. However, the implantation of these microorganisms is complex, making research on microorganisms-plant interactions a topic of great current interest.
Keywords: Plant growth promoting bacteria; NP assimilation; Phytohormones; Antagonism; bio-control; ISR & SAR
Introduction
Human and animal feeding needs have led to rapid technological development in crops during the recent decades [1], even allowing the crops to become practically independent of the environment when using greenhouses where inputs and outputs and multitude of environmental factors are controlled. This has great advantages, since it allows, on one hand, to control a multitude of aspects related to the crops and, on the other, to minimize harmful external vectors such as insect pests that can devour plants and transmit diseases by viruses, bacteria and fungi. Crops are controlled under the most aseptic conditions possible and in fact there are soilless cultivation techniques (hydroponic crops) or in artificially created and periodically sterilized soils. In these cases, the inputs of nutrients to plants are generally provided by fertirrigation with inorganic fertilizers dissolved in the irrigation water.
However, intensive use of fertilizers and pesticides has significant environmental ramifications, leading to a new trend to reduce the application of synthetic fertilizers and pesticides. For this, studies are valuable on the beneficial effects of certain microorganisms and the use of fresh vegetable remains and compost-like amendments [2], which in turn introduce large amounts of microorganisms into the soil.
Although microorganisms can be found everywhere in soils, it is close to the roots of plants where concentrations can rise 10 to 100 times higher (Weller and Thomashow, 1994). This area is known as the rhizosphere, and although initially described at the beginning of the 20th century [3] like the soil zone where microbial populations are stimulated by root exudates (i.e. amino acids and sugars), this concept has been extended to the soil zone around roots where physical, chemical, and biological properties have changed due to root growth and activity [4]. The organisms present in therhizosphere include bacteria, fungi, protozoa and algae, although the bacteria are the most abundant and are selected by plants according to their needs.
On the other hand, microorganisms not only benefit from the root exudates but also have an ecological niche, where they can develop and even (in the case of endophytic bacteria) a place inside plant cells where they do not have to compete with any other microorganism [5]. Antoun & Kloepper [6] estimated that about 1-2% of the rhizosphere bacteria have a special role in strengthening and protecting plants. This group of bacteria is known as plant-growth-promoting rhizobacteria (PGPR) and species of the genera Pseudomonas, Azospirillum, Azotobacter, Klebsiella, Enterobacter, Alcaligenes, Arthrobacter, Burkholderia, Bacillus, Rhizobium and Serretia have been described within this group [7,8].
Effects of PGPR
PGPR affect plants both directly and indirectly. PGPR directly provide the plant with substances that are synthesised by the bacteria or facilitate the absorption of certain plant nutrients from the environment. The indirect promotion of plant growth occurs when PGPR prevents the deleterious effects of one or more phytopathogenic microorganisms [9-13].
Biological fixation of N
N-fixing microorganisms can live free or in symbiosis with some plants, especially with legumes where they form nodules in their roots [14]. These microorganisms take the atmospheric N and supply it in the form of compounds assimilable by the plants and in turn receive carbohydrates from the roots [15,16].
Solubilization of phosphorus
Together with N and K, P is another of the macronutrients of plants. Although agricultural soils usually have sufficient amounts of P due to inputs from fertilizers, much of it is usually in insoluble form not available to plants. However, there are some microorganisms capable of converting insoluble phosphorus to soluble forms such as orthophosphates [17-19].
Production of stimulants of plant growth
There is evidence that PGPRs produce phytohormones such as auxins, gibberellins, cytokinins, and ethylene that influence a large number of processes such as stem and root growth, flowering, and fruit development [20-23].
Antagonistic activity and biocontrol agents
According to Beattie [24], bacteria that reduce the incidence of plant diseases are considered biocontrol agents, whereas those that exhibit antagonistic activity against plant pathogens are defined as antagonists [25,26]. The following actions can be highlighted within these activities.
Synthesis of hydrolytic enzymes: such as chitinases, glucanases, proteases and lipases that can lyse pathogenic fungal cells [27,28].
Production of siderophores: Most of Fe in the soil is usually in insoluble forms and in Fe-limiting environments PGPRs can produce siderophores, which are iron chelating compounds that trap available iron and provide it to plants, thereby promoting their growth [29-31]. It also has an antagonistic effect by preventing other harmful bacteria and fungifrom taking Fe from the soil. Bacterial siderophores can be classified into four types (carboxylate, hydroxamates, phenol catecholates and pyoverdines) [32].
Production of antibiotics: According to Haas & Defago [33] there are 6 classes of antibiotic compounds produced by PGPRs and related to the control of root diseases: phenazines, phloroglucinols, pyoluteorin, pyrrolnitrin, cyclic lipopeptides, and hydrogen cyanide. Besides other antibiotics such as polymyxin, circulin and colistin, are active against pathogenic bacteria and fungi [27].
Induced systemic and systemic acquired resistance (ISR & SAR)
These are two independent phenomena but in plants they provoke an immune response to attacks by pathogens. ISR consists of a self-plant resistance induced by non-pathogenic rhizobacteria or PGPR [34-36]. SAR is a resistance activated by exposure to a pathogen. ISR and SAR act by different metabolic pathways. While induction of SAR is through salicylic acid, ISR requires jasmonic acid [34]. ISR-mediated protection is significantly lower than that produced by SAR [37]. Nevertheless, both types of protection can occur simultaneously with a higher effect than each separately [38].
Commercial Application of PGPR
As can be concluded from the above, PGPR applications can be multiple and generally result in an environmentally more sustainable alternative than chemical fertilizers and pesticides [39-42]. According to Nakkeran et al. [43], optimal PGPR for commercialization must have the capacity to compete with other microorganisms, increase plant growth, have a broad spectrum of action, and be resistant to heat, UV radiation, and oxidizing agents. Despite that species of Bacillus, Enterobacter, Klebsiella, Azobacter, Variovorax, Azosprillum, and Serratia [6] have been applied commercially for several decades and new studies in laboratory are promising, the effects in crops are not totally satisfactory [44,45].
For example, PGPR use as fertilizers involves losses during aerial application, due to environmental factors, runoff, etc. However, there are several options to favour the establishment of PGPR. They are often applied to plant seeds [46] and, once sown, PGPR should be able to settle in the rhizosphere by taking advantage of plant exudates. On the other hand, nano encapsulation technology can be used as a tool to protect PGPRs and allow a more controlled release of PGPR [47]. Genetic modification experiments may also improve the functionalities and establishment of PGPR [48].
Despite of the large number of studies related to the mechanisms and mode of operation of PGPR, the complexity of PGPR-plant interactions makes it necessary to expand the knowledge on this topic. Molecular and genetic studies [46] should allow further comprehension of these interactions in the rhizosphere and help in the development of new commercial products. Finally, these studies can be strengthened by advances in metagenomics due to recent progress in bioinformatics, refinement of DNA amplifications and computational development [49]. This will facilitate the identification of bacteria species in experimental crops and the monitoring of the time course of populations throughout the culture cycles [50,51].
Acknowledgement
The authors are grateful for funding received from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO) under the NACAL (CGL2015-71709-R) Project. The second author is also grateful for funding received from the Ramon y Cajal Research Contract (RYC-2016-21191) from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO).
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