Sentinel plants to improve sulfur use efficiency: living instruments for nondestructive analysis

Developing specific bioassays based on the use of “sentinel plants” to quickly determine nutrient bioavailability and/or crop nutritional status may represent a reliable and efficient strategy to obtain valuable, timely and low-cost information about changes in nutrient availabilities and nutritional requirements in a crop system. Two Arabidopsis pilot bioindicators - carrying the GUS reporter gene under the control of two portions of theintergenic region between At1g12030 and At1g12040 - have been characterized for their potential ability to provide information about the sulfur nutritional status of the plant and/or the sulfate concentration in the growing medium. For this purpose, the two lines were grown in agar plates under a continuous sulfate gradient ranging from 0 to 150 µM in order to describe the growth of both roots and shoots as a function of sulfate external concentration and to determine the critical concentration of sulfate (i.e. the minimum concentration of sulfate necessary to achieve maximum biomass) in the growing medium. The main results indicate that both the pilot lines are able to correctly indicate the critical concentration of sulfate in the external medium also in the presence of interfering metal ions (such as cadmium) able to increase the plant metabolic demand for sulfur. Moreover, the two intergenic regions are suggested as bi-directional promoters able to control the expression of two flanking genes under sulfur limitation.