INFLUENCE OF RHIZOSPHERE PH ON WHOLE-ROOT FERRIC REDUCTASE ACTIVITY IN DIVERSE ACCESSIONS OF PHASEOLUS VULGARIS

Authors: WATERS, BRIAN - BAYLOR COLLEGE MED; LI, CHEE-MING - BAYLOR COLLEGE MED; BLAIR, MATTHEW - CIAT; BEEBE, STEVE - CIAT; Grusak, Michael

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 7/7/2006
Publication Date: 7/7/2006
Citation: Waters, B.M., Li, C.-L., Blair, M., Beebe, S., Grusak, M.A. 2006. Influence of rhizosphere pH on whole-root ferric reductase activity in diverse accessions of Phaseolus vulgaris. In: Proceedings of the Thirteenth International Symposium on Iron Nutrition and Interactions in Plants, July 3-7, 2006, Montpellier, France. p. 89.

Interpretive Summary:

Technical Abstract: In alkaline and calcareous soils, iron (Fe) availability to plants is greatly reduced. Under these conditions, plants frequently exhibit Fe-deficiency induced chlorosis, which inhibits plant productivity and ultimately seed Fe concentration. To address the question of how diverse accessions would perform in different soil environments, a number of distinct bean accessions were tested for ferric reductase activity at acid and neutral pH. Bean (Phaseolus vulgaris) accessions collected from several geographical locations were obtained from the USDA National Plant Germplasm System, and grown hydroponically at pH 5.5 using a low Fe concentration (2 µM Fe-EDDHA). Under these conditions, shoots generally remain green, but root ferric reductase activity is elevated relative to bean plants grown at higher Fe concentrations. After 14d of growth, each accession was assayed for ferric reductase activity at pH 4.5, and the same root systems were then assayed again at pH 7.0. These pH values were chosen based on detailed studies conducted with the commercial cultivar Hystyle, in which ferric reduction was measured at several pH values and was found to be highest at pH 4.5, and approximately 50% of maximum at pH 7. Preliminary experiments also demonstrated that ferric reductase activity was linear for measurement periods up to 90 min, and that at least six sequential assays of single root systems gave consistent results. For the 23 diverse bean accessions, ferric reductase activity at pH 4.5 ranged from 0.31 to 1.77 µmol Fe reduced/g FW/hr, while at pH 7.0 the values were lower, ranging from 0.21 to 1.16 µmol Fe reduced/g FW/hr. Overall, root reductase activity at pH 4.5 was positively and significantly correlated with activity at pH 7.0 across the accessions; however, when the ratio of reductase activity at pH 4.5/pH 7.0 was calculated, a range of ratios was observed. Three accessions had ratios in the range of 1.1-1.3, while two accessions had ratios in the range of 1.9-2.2. Most of the remaining accessions exhibited values in the range of 1.5 to 1.9. A ratio closer to 1.0 may indicate an accession that would be able to maintain a more consistent reductase capacity over a wider range of soil pH conditions, and thus may be useful in a bean breeding program. In addition to the reductase measurements, we have sequenced a ferric reductase gene from bean, PsFRO1, and have identified a number of polymorphisms between four genotypes that form the basis of two mapping populations. Our next step is to sequence the FRO1 genes from accessions with low and high (pH 4.5/pH 7.0) ratios to determine whether polymorphisms in this gene may lead to differences in protein sequence, which might correlate with the different pH-dependent ferric reductase ratios. We will discuss these results in the context of the basic functioning of the reductase protein, as well as their potential utility in a breeding program designed to enhance the nutritional value of bean. This work was supported in part by funds from USDA-ARS under Agreement No. 58-6250-6-001 and from the Harvest Plus Project under Agreement No. 58-6250-4-F029 to MAG.