Plant growth-promoting inoculants in Australian agriculture

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Plant Growth-Promoting Inoculants in Australian Agriculture

Steven A. Wakelin, Post-Doctoral Fellow, CSIRO Land and Water, PMB 2 Glen Osmond, South Australia 5064; and Maarten H. Ryder, Research Group Leader, CSIRO Land and Water, PMB 2 Glen Osmond, South Australia 5064

Abstract

Microbial inoculants have an important role in Australian agriculture. Presently, the inoculant industry is almost exclusively based around the manufacture of rhizobia for legume inoculation. The N inputs and disease break afforded by legume rotation is fundamental to the environmental and economic sustainability of many farming systems. However, a number of other plant growth-promoting microbial inoculants have been developed or are under investigation. One example is a P-solubilizing strain of Penicillium radicum (Pr70Release). On acidic soils with high P-retaining characteristics, inoculation with P. radicum increases the growth and yield of wheat and other crop species. Inoculants are also being investigated to control a range of plant diseases. Examples include Trichoderma koningii Tk7a and species of endophytic actinomycetes for control of cereal root pathogens in broad-scale agriculture. Various other biocontrol agents are commercially available for specific applications.

Microbial Inoculants in Australian Farming Systems

Plant growth-promoting microbial inoculants play an important role in maintaining sustainable agricultural production in Australia. Most soils in Australia have a genesis that combines nutrient-poor parent materials with strong weathering and leaching. Accordingly, many agricultural soils are infertile and inherently support low-productivity ecosystems. To sustain economically viable rates of agricultural production, high inputs of fertilizers (especially P and N), in conjunction with pesticides to control weeds and diseases, are necessary. With ongoing intensification to satisfy demands for increased agricultural production, the capacity of these relatively poor soils to sustainably support production is questionable. Furthermore, inputs such as fertilizers are disruptive to biological processes if they enter natural ecosystems (e.g., by erosion or leaching).

Microbial inoculants can help Australian agriculture remain productive while minimizing impacts to natural ecosystems. This is demonstrated by the widespread exploitation of the Rhizobium-legume symbiosis to manage N inputs in farming systems. However, inoculants that promote more efficient utilization of P fertilizer inputs (thereby reducing their loading in soils), or control plant root pathogens have also been developed. These are examples of new inoculants that may have an important role in future farming systems.

Symbiotic Nitrogen Fixation

The use of microbial inoculants in Australia is dominated by the use of rhizobia bacteria (i.e., species of Rhizobium and closely related genera) to fix atmospheric nitrogen (N₂) in association with leguminous plants. The symbiosis provides the host plant with most or all of its N requirements, reducing the need for N fertilizer. N held in the legume is mineralized upon decomposition in the soil, supplying N to the following non-leguminous crop, typically wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), or canola (Brassica napus L.). As virtually all the agriculturally important legume species in Australia have been introduced, there has been an associated need for the introduction of strains of rhizobia specific to legume species so that maximum N₂-fixing benefits can be gained.

The greatest demand for rhizobium inoculants is for crop legumes (grain, pulse, and oil legumes). Approximately 4.7 million acres of crop legumes are planted annually (1), with lupin (Lupinus angustifolius L.) production accounting for half the area sown (1) (Fig. 1). Accordingly, about one third of all rhizobia inoculant manufactured is Bradyrhizobium Group G (currently strain WU425), specific for lupin (Fig. 2). Many other crop legumes which also require Rhizobium inoculation are widely grown (Fig. 1). These include field pea (Pisum sativum L.), chickpea (Cicer arietinum L.), faba bean (Vicia faba L.), lentil (Lens culinaris Medik), soybean (Glycine max L.), peanut (Arachis spp.), and many others.


Fig. 1. Area planted to grain legumes in Australia, 2001.		Fig. 2. Rhizobia-based inoculant production by Bio-Care Technology Pty Ltd, the largest inoculant producer in Australia. "Others" mainly consists of various pasture legumes such as medic and serradella.

Pastures are a common component of Australian agricultural systems that include livestock enterprises. In the semi-arid zones, legume-based annual pasture systems dominate, providing a N benefit and disease break to the following crop. In higher rainfall regions, perennial pastures serve as a resource for animal production, and generally combine a high proportion of perennial pasture legumes with regenerating annuals. Approximately 25% of the rhizobia-inoculant manufactured is specific for pasture legumes. These include lucerne (Medicago sativa L.), subterranean clover (Trifolium subterraneum L.), white clover (T. repens L.), serradella (Ornithopus spp., annual pasture legume species particularly suited to sandy acidic soils), medic (Medicago spp.), and many others.

Of the ~7.5 million acres sown to legumes annually, about half receive rhizobial inoculation (5). Crop legumes receive 75% of the total Rhizobium inoculum and are estimated to fix AU$200 to 300 million of N annually. However, the total amount of N fixation is worth AU$2 billion (5). Therefore, N-fixation by pasture legumes, which receive only 25% of the inoculum manufactured, contributes disproportionately more than crop legumes. Furthermore, most current N-fixation benefits can be attributed to historical inoculation practices.

Although there is no specific legislation covering standards for inoculant manufacture in Australia, virtually all rhizobia inoculant undergoes voluntary quality control through the Australian Legume Inoculants Research Unit (ALIRU) (5). To meet ALIRU-approval, inoculants must meet minimum requirements for contamination levels, strain authenticity using serology and DNA analysis, numbers of Rhizobium (colony forming units), and most importantly, ability to effectively nodulate the target legume (5). In addition to quality control, the ALIRU maintains a collection of around 1700 strains of rhizobia and provides cultures of recommended strains to inoculant manufacturers (5).

Most rhizobia inoculant is formulated in peat-based carriers. As low product contamination is a fundamental requirement for ALIRU-approval, sterilization of the milled peat base by γ-irradiation is standard practice. Liquid and granule formulations are in the early phases of commercial release, and may be preferable to peat in some situations.

The state agricultural departments, universities, and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) are involved in Rhizobium research. Most of the research is focused in the following areas: providing elite strains for specific legume species; selecting strains for use on soils with certain chemical constraints (e.g., acidity); increasing soil persistence (survival) characteristics of rhizobia; and developing polymer-based seed coat formulations with long term bacterial survival to allow the use of pre-inoculated seed and seeding into dry soil conditions.

Phosphorus Solubilizing Inoculants

The availability of P to plants is a key constraint to agricultural production across much of southern Australia (3). Depending on geochemical attributes, soils may immobilize P by binding with Ca₂,
Mg₂, Fe₃, adsorption to Fe- and Al-hydrous oxides and Al-silicates, or via precipitation reactions (e.g., Al in acidic soils or Ca in alkaline soils). On such soils the efficiency of P-based fertilizers can be low, with only 10 to 30% of the P applied available for plant uptake in the year of application (6). To offset this, approximately AU$600 million is spent annually on P-based fertilizers. The necessity to repeatedly fertilize soils with P has resulted in a buildup of P in many Australian soils.

Soil microorganisms are responsible for transforming immobilized soil P into plant-available forms (4). Research is underway to determine the potential for exploitation of these microorganisms to increase plant growth on soils high in immobilized P. Most work centers on the utilization of Penicillium fungi, as many species solubilize high amounts of inorganic phosphates (10). To elicit plant growth promotion, P solubilization must occur on or adjacent to the plant root. It is essential, therefore, that strains are also selected for the ability to associate closely with plant roots (rhizosphere competence).

Fig. 3. Penicillium radicum has been commercially developed as a phosphate-solubilising inoculant for use on broad-acre wheat in Australia. It is manufactured by Bio-Care Technology Pty Ltd under the tradename Pr70Release.

Penicillium radicum A. D. Hocking & Whitelaw is an Australian phosphate-solubilizing inoculant for wheat (Fig. 3). The fungus was discovered in 1991 during a survey of wheat rhizosphere fungi for P-solubilizing activity (12) and has the capacity to solubilize a wide range of inorganic phosphates common in soils. The prime mechanism for P solubilisation is thought to be secretion of gluconic acid (12), which can mobilize P from insoluble sources by lowering pH or by chelation-based mechanisms. Successful demonstration of plant growth promotion in glasshouse and field trials (11), culminated in the development of P. radicum as the inoculant Pr70Release (Bio-Care Technology Pty Ltd, Somersby, NSW). In 2002, sufficient inoculum was produced to treat 3000 kg of wheat seed. This production level is high enough to enable extensive testing by farmers so that the best opportunities for its widespread use in Australia can be identified (soil type, agro-ecological conditions etc).

P-mobilization per se does not account for all of the plant growth-promoting effect observed by P. radicum. Under field conditions, P. radicum increases wheat growth under low and high P-fertilization levels (11). Other possible mechanisms for plant growth promotion, particularly phytohormone production, are being investigated.

Given the problems many Australian soils have with low P fertilizer use efficiency and subsequent P accumulation, inoculants that can solubilize P have the potential to play a valuable role in Australian agriculture.

Biological Control of Root Diseases

Despite improvements in crop rotation practices, root diseases still pose a significant constraint to cereal grain production in southern Australia. Production losses due to the soil-borne pathogens Gaeumannomyces graminis (Sacc.) von Arx & Olivier var. tritici Walker (Ggt; causative agent of take-all disease) and Rhizoctonia solani Kühn are estimated to be over AU$87 million annually (2). In contrast to foliar diseases, growers have limited options for managing root diseases. Therefore, biocontrol agents such as Trichoderma koningii Rifai strain Tk7a are being investigated for commercial development.

T. koningii Tk7a was originally isolated from soil suppressive to the saprophytic growth of Ggt in Western Australia (9). The fungus reduced the incidence of take-all on wheat plants (7), possibly through mechanisms such as mycoparasitism or by production of anti-fungal compounds (8). T. koningii Tk7a has been tested for biocontrol activity in over 50 field trials across Australia, USA, China, and New Zealand (1987-2001). A formulation developed in China was available in 1998-99 for small-scale commercial testing for control of wheat root diseases (Biocon-Tk; Min Feng Co., Shandong Province). This strain is now being considered for full-scale commercial production for control of several plant diseases in both China and Australia.

Other Biocontrol Inoculants

In addition to T. koningii Tk7a, there are many other examples of inoculants in Australia for control of plant pathogens. Important examples include the use of the entomopathogenic fungus Metarhizium for control of red-headed cockchafer (Adoryphorus coulonii Burmeister) in pastures, greyback cane grub (Dermolepida albohirtum Waterhouse) in sugarcane, and the Australian plague locust (Chortoicetes terminifera Walker). The twist fungus (Dilophospora alopecuri (Fr.) Bessey) is used to control annual-ryegrass toxicity (of livestock) caused by the bacterium Clavibacter toxicus Riley & Ophel, and inoculants based on various species of Bacillus and Trichoderma are targeted for other specific purposes.

Future Opportunities

The use of plant-growth promoting rhizobacteria (PGPR) as inoculants is a recent area of interest. Inoculants that promote rapid early seedling growth have potential to play an important role in agriculture, for example to promote early growth of grass and legume pasture species. The establishment of pastures in hostile (e.g., acidic) soils can be slow, resulting in greater weed competition and reduced opportunities to capture soil resources. Opportunities for the use of PGPRs on wheat and other cereal crops are being pursued for similar purposes.

Another emerging area is the use of endophytic microorganisms as inoculants. As endophytes occupy internal plant tissues, they are ideally situated to directly influence plant growth. Furthermore, they are protected from the external stresses and microbial competition that are characteristic of the rhizosphere. Most work is centered on the use of endophytic Actinomycetes to protect plants from fungal, nematode, and insect pests. However, the potential use of nitrogen-fixing or plant growth-promoting endophytes warrants attention.

In Australia, an increasing number of scientifically researched inoculants are undergoing commercial development. Our ability to harness soil biological activity through inoculation with specific biological treatments will increase with in-depth studies of microbial ecology and mechanisms of action. This will be further aided with new developments in fermentation and processing technologies.

Acknowledgments

The authors thank the following people for their input into this paper: David Roget (CSIRO), Ross Ballard (South Australian Research and Development Institute), Dr David Herridge (Agriculture NSW, Tamworth), Gary Bullard (Bio-Care Technology Pty Ltd, Somersby, NSW), Lucy Anderson (ASI Pty Ltd, Wodonga, VIC), and Dr. Pauline Mele (Agriculture Victoria, Rutherglen).

The work on Penicillium radicum is funded by the Grains Research Development Corporation of Australia (GRDC CS0223). Trichoderma koningii Tk7a is being developed with cooperation between CSIRO Australia, China Agricultural University, and the Shandong Academy of Sciences with financial support from the Australian Center for International Agricultural Research (ACIAR 9680) and the Australian Government Department of Education, Science and Training.

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