Signaling function of NH₄⁺ in the activation of Fe-deficiency response in cucumber (Cucumis sativus L.)

NH₄⁺ is necessary for full functionality of reduction-based Fe deficiency response in plants. Nitrogen (N) is present in soil mainly as nitrate (NO₃^-) or ammonium (NH₄⁺). Although the significance of a balanced supply of NO₃^- and NH₄⁺ for optimal growth has been generally accepted, its importance for iron (Fe) acquisition has not been sufficiently investigated. In this work, hydroponically grown cucumber (Cucumis sativus L. cv. Maximus) plants were supplied with NO₃^- as the sole N source under -Fe conditions. Upon the appearance of chlorosis, plants were supplemented with 2 mM NH₄Cl by roots or leaves. The NH₄⁺ treatment increased leaf SPAD and the HCl-extractable Fe concentration while decreased root apoplastic Fe. A concomitant increase in the root concentration of nitric oxide and activity of FRO and its abolishment by an ethylene action inhibitor, indicated activation of the components of Strategy I in NH₄⁺-treated plants. Ammonium-pretreated plants showed higher utilization capacity of sparingly soluble Fe(OH)₃ and higher root release of H⁺, phenolics, and organic acids. The expression of the master regulator of Fe deficiency response (FIT) and its downstream genes (AHA1, FRO2, and IRT1) along with EIN3 and STOP1 was increased by NH₄⁺ application. Temporal analyses and the employment of a split-root system enabled us to suggest that a permanent presence of NH₄⁺ at concentrations lower than 2 mM is adequate to produce an unknown signal and causes a sustained upregulation of Fe deficiency-related genes, thus augmenting the Fe-acquisition machinery. The results indicate that NH₄⁺ appears to be a widespread and previously underappreciated component of plant reduction-based Fe deficiency response.