Extracellular enzymes in the rhizosphere: Who is producing them and why

Background/Question/Methods

Plant and microbial competition for soil nutrients is most intense in the rhizosphere. Evidence has been mounting that roots are not passive competitors, but can actively compete with microorganisms for nutrients. Root exudates increase rhizosphere carbon (C) availability and microbial nutrient immobilization, which can stimulate microorganisms to produce extracellular enzymes (e.g. proteases and phosphatases), to acquire limiting nutrients from organic matter. As a result of increased enzyme activities, more nutrients can become available in the rhizosphere, where they may also become available to plants. However, the relationships between labile C, enzyme activities, and nutrient availability in the rhizosphere are still poorly understood. Furthermore, roots can also produce extracellular enzymes to acquire nutrients from organic sources. However, little is known about the extent of root enzyme activity and the degree to which these enzymes help roots exploit pools of organic nutrients in the soil. My objective is to summarize recent findings on enzyme production by roots and rhizosphere microorganisms, and the relationships between rhizosphere enzyme activities and C and nutrient availability.

Results/Conclusions

Previous studies have concluded that microbial activity increases nitrogen (N) availability in response to rhizodeposition and N immobilization, and elevated rhizosphere protease activity with increased C availability and N demand has been observed. This suggests that if rhizosphere nutrient availability cannot meet demand stimulated by rhizodeposits, microbes must produce enzymes to acquire nutrients. I hypothesize that the ratio of available C to N and phosphorus (P) determines whether rhizodeposition will increase or decrease nutrient acquisition and availability in the rhizosphere. I predict that soil dissolved N concentrations will be highest at intermediate levels of C availability.

While recent results suggest that rhizosphere C availability controls microbial enzyme production, it less clear what controls root enzyme activities. Our research has found that high activities of N and P acquiring root extracellular enzymes are common across a diverse range of plants. However, relationships between root enzyme activities and rhizosphere nutrient availability were inconsistent. At times our results contradicted the prediction that enzyme activities would be negatively correlated with the nutrients they acquire. I hypothesize that root enzymes may acquire nutrients in excess of demand or may have functions other than nutrient acquisition, such as producing chitinases to protect against fungal pathogens.