Plant growth promoting bioproduct

Field of flowering potatoes The promotion of plant growth and the control of plant disease have been widely accepted as a pressing need for the 21st century, in all over the world (see citation above). The need is driven by a growing human population and the demand for high quality food, that is free from unacceptable levels of chemicals, such as herbicides and pesticides.

Paradoxically, this increased demand has led to the development of agricultural practices that increase the disease pressure on plants. It has been estimated that approximately one third of the food crop is destroyed every year, mainly due to attack by pathogenic fungi, bacteria and nemathodes. Presently, the main strategy against fungal pathogens involve the use of chemical fungicides, resistant plant cultivars and/or avoiding monocultures by mixing different cultivars on the field, a strategy which has been proved useful in suppressing Magnaporthe griseum, the primary cause of rice blast, which is damaging significantly every year rice cultures especially in the Asian regions.

However, there are major limitations in all these strategies. Firstly, they are not as efficient as desirable and do not prevent all diseases. In case of agrochemicals, toxic residues from the chemicals either used as pesticides or as fertilizers can accumulate in the soil and food chain. Consequently, the use of many agrochemicals including antibiotics also used in medicine (e.g. streptomycin, tetracyclin) has been banned or restricted because of environmental and health risks. There is a tendency to shift towards healthy food and a more sustainable agriculture and new agrochemicals have little chance of regulatory acceptance. Secondly, resistant cultivars are not effective against all diseases, their resistance is often broken by the pathogen within a few years and resistance is frequently accompanied by a reduction in yield. Topically, there is a lack of acceptance amongst the public for genetically modified (GM) crops.

In order to secure the supply of safe and wholesome food, there is clearly a need to develop new agricultural practices that supplement and, ultimately, may even replace existing plant disease control strategies. A strategy that has yet to be implemented, but which has the potential to supplement and possibly to supplant the use of chemical pesticides, is the use of formulations of selected natural soil bacteria as plant growth promoting (biofertilizer) and/or biocontrol agents. Some root colonizing bacteria (rhizobacteria) are considered as efficient microbial competitors in the root area where they are exerting their beneficial effects on plant growth and development.

Representatives of many bacterial genera have been introduced into soils, onto seeds, roots, tubers or other planting materials and shown to improve crop and various bacteria have been marketed as biopesticides and biofertilizers but long term successful application of these bacterial samples failed due to unsufficient knowledge about their physiology and behaviour in different environments and the unability to obtain long-living formulations which allowed reproducible application results.

At least the last difficulty can be overcome by use of biocontrol agents derived from Bacillus subtilis and its relatives whose are able to form heat- and desiccation resistant spores with unlimited ability to germinate and grow also after long periods of storage. For this reason it is not surprising that the most successful agents for enhancing of plant growth and protection against pathogens are prepared from bacilli (Kodiak, Gustafson Inc., and Serenade QST713, AgraQuest Inc. U.S.A.). Similarly, Bacillus derived biofertilizers and biocontrol agents were successfully distributed in China (more than 50 different samples are on the market) and Europe (e.g. Bacillus "subtilis" FZB24, Abitep G.m.b.H) Although enhancement of plant growth by root colonising Bacillus strains is well documented (Krebs et al. 1998; Kloepper et al. 2004; Yao et al 2006), very little is known about the basic molecular mechanisms responsible for beneficial action of the bacilli group of plant growth promoting bacteria, PGPR, preventing development of optimal application strategies for available formulations in agriculture and horticulture.

However, according to results obtained by the Kloepper lab and others, it becomes now increasingly clear that plant growth promotion by rhizosphere bacilli results from combined action of several factors. The volatiles 3-hydroxy-2-butanone (acetoine) and 2,3-butanediol, released by Bacillus subtilis and B. amyloliquefaciens, trigger enhanced plant growth (Ryu et al. 2003) and several bacilli were shown to trigger induced systemic resistance against pathogens most likely by the same volatiles (Kloepper et al. 2004).