Physiological and transcriptomic analysis of responses to different levels of iron excess stress in various rice tissues

Iron (Fe) toxicity is a major nutritional disorder of plants and affects rice yield and production in rainfed and irrigated lowland rice grown in acid soils. Rice plants are reported to have exclusion and inclusion adaptation strategies for preventing damage from excess Fe. However, the molecular mechanisms behind the Fe toxicity response and the identities of the genes involved remain largely unknown. To reveal these mechanisms, we exposed rice plants to different levels of ferrous (Fe²⁺) excess treatment for 14 days and analyzed their growth, bronzing score, and mineral concentrations. Then, gene expression patterns in various tissues (roots, discrimination center [DC], stems, old leaves [OLs], and newest leaves [NLs]) in response to different levels of Fe excess (×1, ×10, ×20, ×50, and ×70 Fe) were examined using microarray analysis. Our results showed that the higher levels of Fe excess led to more Fe being preferentially translocated to OLs, thus avoiding Fe excess damage in the NL. We proposed three zones of Fe excess levels: the non-affected, affected, and dead zones. As an exclusion strategy, Fe uptake- and transport-related genes were suppressed in roots since in the non-affected zone. Roots are important for preventing Fe uptake to the plant body under Fe excess stress. As inclusion strategies, first, some genes highly induced in various tissues under Fe excess, such as OsNAS3, OsVIT2, and rice ferritin genes (OsFers), may be important for detoxification or isolation of excess Fe within the plant body. OsZIPs may contribute to the maintenance of zinc homeostasis. Second, the plant induces the expression of oxygen and electron transfer genes, cytochrome P450 family proteins, or some NAC-type transcription factors to avoid reactive oxygen species and abiotic stress caused by Fe excess in the affected zone. The plant may use similar Fe homeostasis mechanisms in the non-affected and affected zones in the NL and roots but employ different mechanisms in the OL, DC, and stem tissues. Our results will contribute to current screening and breeding efforts, which aim to develop Fe excess tolerance in diverse rice cultivars, thus increasing rice production in lowland fields.