Magnesium supply alleviates iron toxicity-induced leaf bronzing in rice through exclusion and tissue-tolerance mechanisms

Toavintsoa Rajonandraina; Yoshiaki Ueda; Matthias Wissuwa; Guy J D Kirk; Tovohery Rakotoson; Hanna Manwaring; Andry Andriamananjara; Tantely Razafimbelo

doi:10.3389/fpls.2023.1213456

Magnesium supply alleviates iron toxicity-induced leaf bronzing in rice through exclusion and tissue-tolerance mechanisms

Front Plant Sci. 2023 Jul 21:14:1213456. doi: 10.3389/fpls.2023.1213456. eCollection 2023.

Authors

Toavintsoa Rajonandraina¹, Yoshiaki Ueda², Matthias Wissuwa^{2

3}, Guy J D Kirk⁴, Tovohery Rakotoson¹, Hanna Manwaring⁴, Andry Andriamananjara¹, Tantely Razafimbelo¹

Affiliations

¹ Laboratoire des RadioIsotopes (LRI), Université d'Antananarivo, Antananarivo, Madagascar.
² Crop, Livestock and Environment Division, Japan International Research Center for Agricultural Sciences (JIRCAS), Tsukuba, Japan.
³ PhenoRob Cluster & Institute of Crop Science and Resource Conservation (INRES), University of Bonn, Bonn, Germany.
⁴ School of Water, Energy and Environment, Cranfield University, Cranfield, United Kingdom.

Abstract

Introduction: Iron (Fe) toxicity is a widespread nutritional disorder in lowland rice causing growth retardation and leaf symptoms referred to as leaf bronzing. It is partly caused by an imbalance of nutrients other than Fe and supply of these is known to mitigate the toxicity. But the physiological and molecular mechanisms involved are unknown.

Methods: We investigated the effect of magnesium (Mg) on Fe toxicity tolerance in a field study in the Central Highlands of Madagascar and in hydroponic experiments with excess Fe (300 mg Fe L^-1). An RNA-seq analysis was conducted in a hydroponic experiment to elucidate possible mechanisms underlying Mg effects.

Results and discussion: Addition of Mg consistently decreased leaf bronzing under both field and hydroponic conditions, whereas potassium (K) addition caused minor effects. Plants treated with Mg tended to have smaller shoot Fe concentrations in the field, suggesting enhanced exclusion at the whole-plant level. However, analysis of multiple genotypes showed that Fe toxicity symptoms were also mitigated without a concomitant decrease of Fe concentration, suggesting that increased Mg supply confers tolerance at the tissue level. The hydroponic experiments also suggested that Mg mitigated leaf bronzing without significantly decreasing Fe concentration or oxidative stress as assessed by the content of malondialdehyde, a biomarker for oxidative stress. An RNA-seq analysis revealed that Mg induced more changes in leaves than roots. Subsequent cis-element analysis suggested that NAC transcription factor binding sites were enriched in genes induced by Fe toxicity in leaves. Addition of Mg caused non-significant enrichment of the same binding sites, suggesting that NAC family proteins may mediate the effect of Mg. This study provides clues for mitigating Fe toxicity-induced leaf bronzing in rice.

Keywords: RNA-Seq; fertilizer; iron toxicity; leaf bronzing; magnesium; rice.

Grants and funding

This study was partly supported by Biotechnology and Biological Sciences Research Council (Grant number BB/R020388/1) and JIRCAS research project “Africa Rice Farming System”. MW has been partly funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC 2070 – 390732324.