Effect of Localized S Application on Capability to Cope with Fe Deficiency of Tomato Plants Grown In Split-Root System

It has been recently demonstrated an important interplay between sulfur (S) and iron (Fe) in tomato (Zuchi et al., 2009, 2015). Following up on our previous studies, which demonstrated that Fe use efficiency increased under adequate S supply, a split-root experiment was performed to determine whether plant S status and/or S external concentration could modify plant capability to uptake and accumulate Fe. The root system of each tomato plant was grown in two different compartments, both Fe-deficient, but one S-sufficient (F), and the other one S-deficient (D).

The split-root experiment allowed to manipulate separately the S status of the root environment and of the plant, and thus to separate systemic and local effects.

D half of the root apparatus showed a decrease of total S, thiols and protein content, which were closely related to enhanced activity of both ATPsulfurylase and O-acetylserine(thiol)lyase, two enzymes involved in the assimilatory pathway, and higher expression of SlST1.1, encoding a high affinity sulfate transporter.

Moreover, S-free medium stimulated Fe(III) reducing capacity (40%) and up-regulated expression of SlFRO1. The expression of both SlFRO1 and SlIRT1 genes is controlled by the SlFER transcription factor, whose pattern of regulation was only similar to that of SlFRO1.

The evidence that root sulfate uptake and assimilation rate was greater in D half of root system compared to F one, as commonly occurs under S deficiency condition, could be likely ascribed to the S status of the root environment rather than to plant S status. Furthermore, in contrast with our previous studies, data showed that the same D half of root apparatus showed an improved capability to cope with Fe deficiency, but the two different steps (reduction and transport) of Strategy I mechanism seem to be regulated by different mechanisms.