Proteomic analysis of secreted proteins from aseptically grown rice

Plants are known to secrete a variety of compounds into the rhizosphere. These compounds are thought to play important roles in the regulation of soil chemical properties and soil microorganisms. To determine the composition of proteins secreted from rice roots, aseptic hydro culture was performed, and the collected proteins were analyzed. Over 100 proteins were identified; most were identified using the rice database (RAP-DB), and about 60% of the identified proteins were suspected to have a signal peptide. Functional categorization suggested that most were secondary metabolism- and defense-related proteins. Pathogenesis- and stress-related proteins were the major proteins found in the bathing solution under aseptic conditions. Thus, we propose that rice plants constitutively secrete a large variety of proteins to protect their roots against abiotic and/or biotic stresses in the environment.

Responses to iron limitation in Hordeum vulgare L. as affected by the atmospheric CO2 concentration

Elevated atmospheric CO2 treatments stimulated biomass production in Fe-sufficient and Fe-deficient barley plants, both in hydroponics and in soil culture. Root/shoot biomass ratio was increased in severely Fe-deficient plants grown in hydroponics but not under moderate Fe limitation in soil culture. Significantly increased biomass production in high CO2 treatments, even under severe Fe deficiency in hydroponic culture, indicates an improved internal Fe utilization. Iron deficiency-induced secretion of PS in 0.5 to 2.5 cm sub-apical root zones was increased by 74% in response to elevated CO2 treatments of barley plants in hydroponics but no PS were detectable in root exudates collected from soil-grown plants. This may be attributed to suppression of PS release by internal Fe concentrations above the critical level for Fe deficiency, determined at final harvest for soil-grown barley plants, even without additional Fe supply. However, extremely low concentrations of easily plant-available Fe in the investigated soil and low Fe seed reserves suggest a contribution of PS-mediated Fe mobilization from sparingly soluble Fe sources to Fe acquisition of the soil-grown barley plants during the preceding culture period. Higher Fe contents in shoots (+52%) of plants grown in soil culture without Fe supply under elevated atmospheric CO2 concentrations may indicate an increased efficiency for Fe acquisition. No significant influence on diversity and function of rhizosphere-bacterial communities was detectable in the outer rhizosphere soil (0-3 mm distance from the root surface) by DGGE of 16S rRNA gene fragments and analysis of marker enzyme activities for C-, N-, and P-cycles.