Different strategies in higher plants in mobilization and uptake of iron

Abstract

Higher plants differ considerably in their capability for mobilization of iron in the rhizospere of soils with low iron availability. In the plant kingdom at least two different Strategies exist in the Fe deficiency‐induced root responses which lead to enhancement of both iron mobilization in the rhizosphere and uptake rate of iron. Strategy I is found in all dicots and in monocots, with the exception of the grasses (graminaceous species, e.g. barley, corn, sorghum). Strategy I is characterized in all instances, by an increase in the activity of a plasma membrane‐bound reductase leading to enhanced rates of Fe‐III reduction and corresponding splitting of Fe‐III‐chelates at the plasma membrane. Often, the net rate of H+ extrusion, i.e. acidification of the rhizosphere, is also increased. This acidification facilitates iron uptake by both enhancement of the reductase activity and solubilization of iron in the rhizosphere. An additional mobilization of sparingly soluble iron in the rhizosphere may occur by release of reducing and/or chelating substances (e.g. phenolics) from the Fe deficient root in response to the acidification. The efficiency of Strategy I relies mainly on the supply of Fe chelates to the reductase and its activity respectively. Thus, high HCO, concentrations in the rhizosphere strongly depress the efficiency of Strategy I. Strategy II has been found only in grasses. This Strategy is characterized by an Fe deficiency‐induced enhancement of release of phytosiderophores (non‐pro‐teinogenic amino acids) which mobilize sparingly soluble inorganic Fe‐III compounds by complexation of Fe‐III and formation of Fe phytosiderophores. The release of phytosiderophores is only slightly depressed by high substrate pH and is positively correlated to genotypical differences between species in their resistance to “lime chlorosis”;. The uptake of Fe phytosiderophores by grasses is mediated by a highly specific system which is absent in species with Strategy I. The affinity of this specific system for synthetic or microbial Fe chelates (Fe‐siderophores) is very low, it may even be absent. The utilization of iron from these chelates depends upon their rate of decay (chelate stability) and thus the supply of inorganic Fe‐III as substrate for the phytosiderophores. High substrate pH only slightly depresses the uptake rate of Fe phytosiderophores by grasses. The principal differences between Strategy I and II have important ecological implications and also require systematic consideration in the develolpment of screening methods for higher resistance to “lime chlorosis”;.