Mapping of sugar and amino acid availability in soil around roots with bacterial sensors of sucrose and Tryptophan

We developed a technique to map the availability of sugars and amino acids along live roots in an intact soil-root matrix with native microbial soil f lora and fauna present. It will allow us to study interactions between root exudates and soil microorganisms at the fine spatial scale necessary to ev aluate mechanisms of nitrogen cycling in the rhizosphere. Erwinia herbicola 299R harboring a promoterless ice nucleation reporter gene, driven by eith er of two nutrient-responsive promoters, was used as a biosensor. Strain 29 9RTice exhibits tryptophan-dependent ice nucleation activity, while strain 299R(p61RYice) expresses ice nucleation activity proportional to sucrose co ncentration in its environment. Both biosensors exhibited up to 100-fold di fferences in ice nucleation activity in response to varying substrate abund ance in culture. The biosensors were introduced into the rhizosphere of the annual grass Avena barbata and, as a control, into bulk soil. Neither stra in exhibited significant ice nucleation activity in the bulk soil. Both try ptophan and sucrose were detected in the rhizosphere, but they showed diffe rent spatial patterns. Tryptophan was apparently most abundant in soil arou nd roots 12 to 16 cm from the tip, while sucrose was most abundant in soil near the root tip. The largest numbers of bacteria (determined by acridine orange staining and direct microscopy) occurred near root sections with the highest apparent sucrose or tryptophan exudation. High sucrose availabilit y at the root tip is consistent with leakage of photosynthate from immature , rapidly growing root tissues, while tryptophan loss from older root secti ons may result from lateral root perforation of the root epidermis.