Exploring the Relation Between Plant Phosphorus Nutrition and Growth Promotion by Bacillus subtilis/amyloliquefaciens Strains

Abstract

The use of plant growth-promoting rhizobacteria (PGPR) as biofertilizers is of increasing interest due to economic and environmental issues. Phosphorus (P) is one of the most limiting plant nutrients because of its reduced availability in soil and the limitation of its sources for fertilization. Because of this, PGPR are considered as a potential tool to reduce chemical P fertilization, either by increasing the access to normally unavailable P forms in soil or by increasing the efficiency of plant P uptake. Most of the PGPR-based products commercially available contain strains of aerobic endospore-forming bacteria, also referred to as bacilli, because of their capacity to be formulated in economic and long shelf-life products. However, inconsistent results remain as a major limitation for wide spread use of PGPR. The present work aimed at facing this problem by studying the relation between bacilli PGPR and plant P nutrition within a conceptual framework that considers plant growth promotion as a three-party interaction: host, PGPR strain, and environment (soil). In this context, the first chapter presents a literature review about the state-of-the-art knowledge on the mechanisms of action of bacilli PGPR. Here, particular attention is given to the agronomical relevance of the mechanisms described so far by analyzing the techniques and experimental systems used. The analysis of the techniques used allows determination of the presence of causal effects, while the experimental systems determine the validity of results to understanding the processes in nature. In the second chapter, two factors, one involved in the PGPR strain – soil interaction (soil P content) and the other in the PGPR strain – host interaction (inoculum concentration), were evaluated for their effect on early plant growth promotion mediated by P-solubilization. This study used a soil-plant experimental model consisting of Chinese cabbage and the phytase-producing strain Bacillus amyloliquefaciens FZB45. In the third chapter, a molecular study was conducted to explore the relation of phytase-coding DNA sequences and taxonomy in the B. subtilis/amyloliquefaciens group and the usefulness of this gene for discrimination between those two species. Finally, the fourth chapter explored the relation between PGPR-induced changes of root architecture and plant P uptake. For this, 73 bacilli strains were screened for production of key molecules involved in alteration of root architecture, and 4 of them were tested for their effect on root architecture of Chinese cabbage. In addition, the combined effects of B. amyloliquefaciens FZB42 and different soil P levels on root architecture and P uptake of Chinese cabbage were tested in a soil-plant model system.