No evidence of regulation in root-mediated iron reduction in two Strategy I cluster-rooted Banksia species (Proteaceae)

Authors: CAWTHRAY, GREGORY - University Of Western Australia; DENTON, MATTHEW - University Of Adelaide; Grusak, Michael; SHANE, MICHAEL - University Of Western Australia; VENEKLAAS, ERIK - University Of Western Australia; LAMBERS, HANS - University Of Western Australia

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2021
Publication Date: 1/27/2021
Citation: Cawthray, G.R., Denton, M.D., Grusak, M.A., Shane, M.W., Veneklaas, E.J., Lambers, H. 2021. No evidence of regulation in root-mediated iron reduction in two Strategy I cluster-rooted Banksia species (Proteaceae). Plant and Soil. https://doi.org/10.1007/s11104-021-04849-5.
DOI: https://doi.org/10.1007/s11104-021-04849-5

Interpretive Summary: Iron is an essential element needed for plant growth and reproductive success, but not all plants use the same mechanisms to acquire iron from soil. We have a good understanding of how plants get iron from common nutrient-rich soils, but we wondered whether plants adapted to nutrient-poor soils might utilize different iron acquisition strategies. We studied root properties in two small woody slow-growing shrubs that only are found in South West Australia, where heavily weathered soils are extremely nutrient poor. We grew plants in liquid solution so that we could study their roots and then measured the root’s ability to convert soil iron to an absorbable form. This ability is usually enhanced when plants are growing on low levels of iron. In this study, we found that when the woody shrubs were grown with differing levels of iron, they were able to convert iron to a useful form, but this process was not enhanced in plants grown with low levels of iron. Instead, it appeared that these plants were regulating their iron acquisition at the absorption stage, in order to succeed in their unique environment. These results will help identify ways to modify important crop species to assist their growth in marginal, nutrient poor soils.

Technical Abstract: Aims. Non-mycorrhizal species such as Banksia (Proteaceae) that depend on root exudates to acquire phosphorus (P) are prominent in south-western Australia, a global biodiversity hotspot on severely P-impoverished soils. We investigated the consequences of exudate-releasing P-mobilising strategy related to control of iron (Fe) acquisition in two Banksia species, B. attenuata R.Br. and B. laricina C. Gardner, which differ greatly in their geographical distribution and rarity. Methods. We undertook solution culture experiments to measure root-mediated Fe reduction (FeR) in non-cluster and cluster roots at four stages of cluster-root development, and whole root systems for plants grown at 2 to 300 µM Fe (as Fe-EDTA). Results. Unlike typical Strategy I species, both Banksia species showed no significant variation in FeR, for either cluster or non-cluster roots, when grown at a wide range of Fe supply. For roots of different developmental stages, we measured a range for B. attenuata cluster roots of 0.13 +/- 0.03 to 1.29 +/- 0.14 µmol Fe3+ reduced g-1 FW h-1 and 0.56 +/- 0.11 to 1.10 +/- 0.24 µmol Fe3+ reduced g-1 FW h-1 in non-cluster roots. Similarly, for B. laricina cluster-roots, FeR ranged from 0.22 +/- 0.07 to 1.21 +/- 0.37 µmol Fe3+ reduced g-1 FW h-1 and in non-cluster roots from 0.56 +/- 0.11 to 0.71 +/- 0.08 µmol Fe3+ reduced g-1 FW h-1. We also observed only minor differences for whole-root system FeR, and even though B. attenuata showed signs of leaf Fe deficiency in the 2 µM Fe treatment, its measured FeR was the lowest of both species across all treatments at 0.079 +/- 0.9 µmol Fe3+ reduced g-1 FW h-1, compared with the fastest rate of 0.20 +/- 0.14 for B. laricina in the 28 µM Fe treatment. Taking plants through a pulse from low to high Fe, then back to low Fe supply did not elucidate any significant response in FeR. Conclusions. Although Fe acquisition is tightly controlled in the investigated Banksia species, such control is not based on regulation of FeR, thus challenging the model that is commonly accepted for Strategy I species.