Silicon ameliorates iron deficiency chlorosis in strategy I plants: first evidence and possible mechanism (s)

Abstract

Silicon (Si) and iron (Fe) are respectively the second and the forth most abundant minerals in the earth’s crust. While the essentiality of Fe is discovered at the middle of the 19th century, Si is still not fully accepted as an essential element for higher plants. However, Si is the only known element that alleviates multiple stresses in plants (e.g. metal excess, drought, salt, lodging, diseases and pests). Fe deficiency chlorosis is a wide-spread nutritional disorder of many crops grown in calcareous and alkaline soils. The various adaptation mechanisms are involved in Fe acquisition from rhizosphere by roots of the so-called strategy 1 plants (all dicots and monocot species, with exception of grasses which belong to strategy 2), i.e. morphological changes (e.g. lateral roots and enhanced root hair formation in the apical zones) and physiological changes such as enhanced proton excretion, FeIII reduction by a plasma membrane reductase and Fe uptake via an inducible FeII transporter (IRT1). These root responses have been studied and characterized mainly in the nutrient solutions without Si supply. Therefore, unambiguous information on an interaction between these two mineral elements is still lacking. We demonstrated for the first time that the application of Si in nutrient solution experiments also ameliorates Fe deficiency chlorosis in cucumber, a Si accumulating dicot, which is also commonly used as a model plant of strategy 1. I will present recent work from our lab in the context of the effect of Si on both physiological (e.g. FeIII reducing capacity, release of phenolics and organic acids) and molecular (e.g. expression of FRO2, HA1 and IRT1, the genes coding FeIII chelate reductase, H+-ATPase and IRT1, respectively) aspects of root responses to Fe deficiency. In particular, I will focus on the storage and utilization of root apoplastic Fe, root-to-shoot Fe transport and utilization of Fe from the leaf apoplast. Based on these investigations we propose the possible role of Si in Fe deficiency stress as 1) increasing apoplastic Fe pool in roots and 2) improving internal Fe status and thus delaying Fe chlorosis, rather than a direct regulatory/signaling effect of Si on the key Fe deficiency inducible root responses.