Rhizosphere bacterial sulfatases and their control by interactions with plants

Plants require sulfur for growth, but much of the sulfur in agricultural soils is present in a bound form ('sulfate esters') that can be used by some bacteria, but not directly by plants. Many of these bacteria live in close association with plant roots, forming dense communities on the root itself and in the soil immediately surrounding it (the 'rhizosphere'). They are actively nourished by the plant, which uses photosynthesis to produce sugars and amino acids that are released from the root to promote bacterial growth, and in return many of these root-associated bacteria promote plant growth by mobilizing mineral nutrients (including sulfur) from the soil for plant utilization. This interaction relies on a network of communication between plants and microbes, with specialist groups of bacteria responsible for certain functions in the soil, and the plant using specific signals to stimulate these bacteria to produce the nutrients it needs. This project will examine two main aspects of sulfur metabolism in the rhizosphere. First, we will investigate which bacterial families are important in promoting the release of bound sulfur from the soil for plants to use. Because many soil bacteria cannot be cultivated in the laboratory, we will start by establishing a database of sulfate ester utilization genes from cultivable soil bacteria that can metabolize sulfate esters. We will then use this database to investigate the overall microbial community in the rhizosphere of the three most important agricultural crops that are grown in the United Kingdom, wheat, barley and oilseed rape. This will provide vital new information to identify the most important bacterial species for sulfate ester metabolism in the rhizosphere of our major crops. To increase the predictive power of this data we will then validate these findings in the rhizospheres of the most commercially important cultivars of these crops, on field trial sites across the UK from Penzance to Aberdeen. Having identified the bacteria that are most active in soil sulfate ester metabolism, the next step is to characterize how plants control their activity. Our preliminary findings show that plants can promote the production of the most important bacterial enzyme for sulfate ester metabolism, arylsulfatase, and can also stimulate its activity once produced. The mechanisms by which these two processes occur will be investigated in a range of plants, including both crops and other species, by isolating the Arabidopsis protein that is responsible for stimulating arylsulfatase activity, and taking the first steps in elucidating how plant root exudates can override the normal bacterial control of arylsulfatase production. The results will provide new insights into how plants control nutrient cycling in the rhizosphere environment, with potential applications in promoting sustainable agriculture in the United Kingdom.

Sulfur is an essential element for plant growth, and is assimilated primarily as inorganic sulfate. However, in most agricultural soils sulfur is present mainly as sulfate esters or sulfonates, which are subject to continual cycling within agricultural soils in microbially mediated processes. This project will use a combination of cultivation-dependent and cultivation-independent methods to identify the main bacterial groupings that are responsible for cycling of sulfate ester-sulfur in the rhizospheres of the major UK crops (wheat, barley, oilseed rape). Cultivable arylsulfatase-producing bacteria will be isolated from rhizosphere soils, and a database of rhizosphere arylsulfatase genes (atsA) constructed. The most active arylsulfatase producers will be enriched by FACS after labeling the microbial fraction from rhizosphere soil with methylumbelliferylsulfate. The dominant atsA genes in the most fluorescent fraction found will be used to identify the main species responsible for sulfate ester cycling in the overall community. Changes in the bacterial community structure with altered sulfur supply under controlled conditions will be examined by T-RFLP. In order to strengthen the predictive value of this information, community analysis of sulfatase-producing bacteria in the rhizospheres of commercially important wheat, barley and oilseed rape cultivars will be carried out at field sites located across the United Kingdom. Initial data show that plants control the activity of arylsulfatases in the rhizosphere at both transcriptional level (overriding sulfate repression of the atsA gene) and post-translational level (stimulating in vitro activity of bacterial arylsulfatase). These two aspects will be explored further, identifying the sulfatase stimulating factor, and exploring the role of plant root exudates in promoting atsA expression. The results will provide exciting new data on how crop plants control nutrient cycling in the environment.