Hormonal Crosstalk Redefines Iron Deficiency Morphophysiological Responses: Insights From Tomato Auxin- and Ethylene-Related Double Mutants.

Abstract

Plants have evolved complex strategies for iron (Fe) acquisition, yet the coordination of underlying hormonal mechanisms remains elusive. This study investigated how auxin and ethylene signaling interact to regulate Fe-deficiency responses using single (auxin-insensitive diageotropica, dgt, ethylene-insensitive Never ripe, Nr, ethylene-overproducer epinastic, epi) and double (dgt/Nr, dgt/epi) hormonal mutants of tomato (Solanum lycopersicum cv. Micro-Tom). We analyzed growth parameters, photosynthetic pigments, oxidative stress markers, antioxidant enzyme activities, nutrient profiles, and the expression of Fe acquisition genes. Results revealed that ethylene strongly contributes to the transcriptional regulation of key Fe acquisition genes, as epi and dgt/epi mutants exhibited strong upregulation of these genes under Fe deficiency, indicating that enhanced ethylene biosynthesis up-regulates Fe acquisition genes even with impaired auxin sensitivity. Conversely, root and shoot development were primarily constrained by auxin sensitivity. The dgt allele showed clear dominance over Nr and epi, as all dgt-harboring genotypes displayed significantly reduced root systems and leaf area regardless of Fe availability. Furthermore, Fe deficiency decreased the activities of antioxidant enzymes, increasing oxidative stress and altering mineral distribution, notably through enhanced Mn accumulation in leaves and Cu in roots. Our findings demonstrate that while ethylene is a central regulator of the Fe-deficiency transcriptional response, auxin sets the structural framework for root adaptation.

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