“Reductants” Released by Roots of Fe‐Deficient Soybeans¹

Abstract

Iron‐stressed (deficient) ‘Hawkeye’ (HA) releases more “reductant” into nutrient solutions than PI‐54619‐5‐1 (PI) soybean [Glycine max (L.) Merrill] roots. These two plants differ in their response to Fe stress and their susceptibility to Fe chlorosis. The objective of this study was to characterize the “reductant” and to determine its role in the uptake of Fe by HA and PI soybeans. Reducing capacity of the ‘reductant’ was determined by its ability to reduce ferric to ferrous iron. Fe2+ was measured using 2,4,6,‐tripyrldyl‐s‐triazine (TPTZ) to form the color complex Fe2+(TPTZ)2. The reductant compounds were separated using paper chromatography (BAW‐4:1:5) and located on the paper as prussian blue spots after the papers were placed in ferricyanide‐ferrichloride solutions. Reductant from 16 HA reduced 90 μmoles (Fe3+ to Fe2+) compared to 5 μmoles for PI soybeans. The chelating agents HEDTA and DTPA (3 μmoles) decreased the reducing capacity from 1.8 μumoles Fe2+/ml concentrated reductant to 0.2 μmoles/ml. Adding more reductant partially overcome the interfering effects of the chelating agents. Adding the concentrated reductant to nutrient solutions increased the amount of Fe~+ iron in solution, but it did not increase the uptake of Fe by the plant. Something other than reductant in nutrient solution appears to be the controlling factor in the uptake of Fe by PI and HA soybeans. We believe the reductant can aid in releasing Fe from chelating agents to the root and that it can maintain Fe2+ in the reduced state in the root. The latter may be its more important role in Fe nutrition.

Mechanism of Iron Uptake by Peanut Plants

“…In sunflower under conditions of iron deficiency, there was an accumulation of o-diphenolics (14), as well as a release of reducing substances into the nutrient solution (10). A similar release was reported for soybeans (2,3) and tomatoes (12). In tomato, caffeic acid and three other p-coumaric acid derivates were released into the nutrient solution (12).…”

“…These changes include the formation of rhizodermal transfer cells (7), the accumulation of reducing substances in the rhizodermis, and an enhanced proton efflux: they facilitate iron uptake (2,14).…”

Mechanism of Short Term FeIII Reduction by Roots

“…In support of this, it was found that when plants were grown in nutrient solutions which had pH values below 4.0, compounds accumulated in the nutrient solutions which were capable of reducing inorganic Fe"' to Fe" over a 20-to 24-h period. The release of reductants in the nutrient solutions was greater with Fe-deficient than Fe-adequate plants (2,17,20,23). An investigation with HPLC suggested that the major secreted reductants were caffeic acid and p-coumaric acid (20).…”

Reduction-based iron uptake revisited

“…4 Excretion of organic compounds, mainly phenolics or flavins, have been associated with the Fe deficiency response since the early seventies, 5,6 but were later considered to be of minor importance and, with some notable exceptions (e.g., the massive production of riboflavin and riboflavin derivatives by some species), 7,8 were largely neglected in studies related to iron acquisition processes in model species such as Arabidopsis. Nevertheless, the frequent observation of this phenomenon led to speculations on the function of these compounds in Fe acquisition, comprising effects on the microbiome in the rhizosphere, a function (for flavins) in electron transport during the reduction of Fe(III), chelation of Fe, and chemical reduction of Fe(III).…”