Contents lists available at ScienceDirect Plant Science journal homepage: www.elsevier.com/locate/plantsci Revisiting Fe/S interplay in tomato: A split-root approach to study the systemic and local responses Eleonora Coppa a , Silvia Celletti a , Youry Pii b , Tanja Mimmo b , Stefano Cesco b , Stefania Astolfi a, ⁎ a DAFNE, University of Tuscia, Via S.C. de Lellis, 01100, Viterbo, Italy b Faculty of Science and Technology, Free University of Bozen-Bolzano, 39100, Bolzano, Italy ARTICLE INFO Keywords: Iron-deficiency Solanum lycopersicum Split-root Strategy I Sulfur ABSTRACT Based on our previous studies demonstrating an intriguing interplay between sulfur (S) and iron (Fe), a split-root experiment was performed to determine whether plant S status and/or S external concentration could modify plant capability to take up and accumulate Fe. This split-root system allowed the roots of each tomato plant to grow in two different compartments, both Fe-deficient, but one S-sufficient, and the other one S-free. Although S was freely available to half root system and thus plant S status was preserved, S-deficient part of root apparatus exhibited a decrease of total S, thiols and protein content, an enhanced activity of both ATPsulfurylase and O-acetylserine(thiol)lyase, and a higher expression of SlST1.1, as occurring under S defi- ciency. The side of the root apparatus exposed to combined S and Fe deficiency, showed an over induction of the FeIII-reducing capacity (+40%) and of the expression levels of the gene codifying for this protein (SlFRO1), with respect to the Fe-deficient part of the root system. Interestingly, the regulation pattern of the bHLH transcription factor SlFER, controlling the expression of both SlFRO1 and SlIRT1 genes, was very close to that of SlFRO1. SlIRT1 expression levels appeared unaffected by S supply, suggesting distinct regulatory processes targeting SlFRO1 and SlIRT1. 1. Introduction The lower Fe accumulation in leaves of S-deficient maize plants was the first clear indication of an intriguing relation between sulfur (S) and iron (Fe) acquisition in plants [1] and triggered a quite intense scientific production in the following years on this topic. It is interesting to note that the same response was recorded in other crops such as barley [2], durum wheat [3] and tomato [4], regardless of which mechanism is employed by plants to cope with Fe deficiency (i.e. Strategy I, or FeIII- reduction based mechanism, or Strategy II, or FeIII-chelation based mechanism) [5]. It should be highlighted that at the field scale multiple nutrient depletions are more common than single nutritional defi- ciencies and the response of plants to a combined deficiency is com- pletely different when compared to the single one, at both physiological and molecular level [6,7]. The comprehension of the mechanisms un- derlying the responses to the combined deficiency are still mostly lacking, even if some theories have been postulated. In particular, it has been suggested that in grasses (Strategy II plants) the reduced Fe ac- cumulation in plant tissues induced by S deficiency could be ascribed to a decrease in the production and release of phytosiderophores (PS) [2], whereas in tomato (a Strategy I plant) the effect was rather due to an impaired ethylene and nicotianamine (NA) production [4]. In addition, the involvement of sulfate in transcriptional regulation of cellular Fe homeostasis was described in both barley, in which HvYS1 expression changed in response to S external supply [8], and tomato, in which S deficiency virtually abolished the expression of the NA synthase (LeNAS) gene and limited the expression of both LeIRT1 and LeFRO1 gene [4]. According to this theory, limited Fe availability results in a further sulfate demand that becomes the driving force which leading to an increased sulfate uptake and assimilation rate [6,9]. Interestingly, PS, ethylene and NA share a common precursor, namely S-adeno- sylmethionine (SAM), whose synthesis depends on the availability of methionine (Met) [10]. Furthermore, it has been shown in plants that Fe is normally linked with S when it is bound in Fe-S proteins sug- gesting that Fe-S clusters are the biggest sink for Fe within the plant [11]. However, higher S need to sustain the activation of Strategy I and II machinery cannot fully account for the observed link between the flow of these two essential nutrients in plant tissues, but also likely reflect a direct interference of Fe with the signal transduction pathway involved in S metabolism (and vice versa) and with the activation of different acquisition strategies. https://doi.org/10.1016/j.plantsci.2018.08.015 Received 5 May 2018; Received in revised form 23 August 2018; Accepted 24 August 2018 ⁎ Corresponding author at: Università degli Studi della Tuscia, DAFNE, Via San Camillo de Lellis s.n.c., 01100, Viterbo, Italy. E-mail address: sastolfi@unitus.it (S. Astolfi). Plant Science 276 (2018) 134–142 Available online 28 August 2018 0168-9452/ © 2018 Elsevier B.V. All rights reserved. T