Abstract
Iron (Fe) is an essential element to plants, but can be harmful if accumulated to toxic concentrations. Fe toxicity can be a major nutritional disorder in rice (Oryza sativa) when cultivated under waterlogged conditions, as a result of excessive Fe solubilization of in the soil. However, little is known about the basis of Fe toxicity and tolerance at both physiological and molecular level. To identify mechanisms and potential candidate genes for Fe tolerance in rice, we comparatively analyzed the effects of excess Fe on two cultivars with distinct tolerance to Fe toxicity, EPAGRI 108 (tolerant) and BR-IRGA 409 (susceptible). After excess Fe treatment, BR-IRGA 409 plants showed reduced biomass and photosynthetic parameters, compared to EPAGRI 108. EPAGRI 108 plants accumulated lower amounts of Fe in both shoots and roots compared to BR-IRGA 409. We conducted transcriptomic analyses of roots from susceptible and tolerant plants under control and excess Fe conditions. We found 423 up-regulated and 92 down-regulated genes in the susceptible cultivar, and 42 up-regulated and 305 down-regulated genes in the tolerant one. We observed striking differences in root gene expression profiles following exposure to excess Fe: the two cultivars showed no genes regulated in the same way (up or down in both), and 264 genes were oppositely regulated in both cultivars. Plants from the susceptible cultivar showed down-regulation of known Fe uptake-related genes, indicating that plants are actively decreasing Fe acquisition. On the other hand, plants from the tolerant cultivar showed up-regulation of genes involved in root cell wall biosynthesis and lignification. We confirmed that the tolerant cultivar has increased lignification in the outer layers of the cortex and in the vascular bundle compared to the susceptible cultivar, suggesting that the capacity to avoid excessive Fe uptake could rely in root cell wall remodeling. Moreover, we showed that increased lignin concentrations in roots might be linked to Fe tolerance in other rice cultivars, suggesting that a similar mechanism might operate in multiple genotypes. Our results indicate that changes in root cell wall and Fe permeability might be related to Fe toxicity tolerance in rice natural variation.
Highlights
Iron (Fe) is an essential nutrient for plants
2.04 ± 0.21a 2.58 ± 0.2a 104 ± 2.65a decreased upon excess Fe treatment (Figures 1E,F), while no significant differences were observed in shoot and root dry weight in the tolerant cultivar
We found among the top up-regulated genes in BR-IRGA 409 a vacuolar Fe transporter, OsVIT2, that is responsive to excess Fe (Zhang et al, 2012); an uncharacterized VIT-Like (VTL) transporter (69-fold), similar to the VTL proteins known to mediate Fe transport into vacuoles in Arabidopsis (Gollhofer et al, 2014); OsASR3, which was already been shown to be responsive to drought stress (Joo et al, 2013), and aluminum toxicity (Arenhart et al, 2013); and one abscisic acid responsive protein (18-fold)
Summary
Iron (Fe) is an essential nutrient for plants. It is involved in oxireductive reactions in photosynthesis, respiration and nitrogen assimilation, as well as in other important plant biological processes. Plants have evolved mechanisms to maintain Fe homeostasis when soil concentration is low (Sperotto et al, 2012; Ricachenevsky et al, 2018). Fe can become toxic if accumulated inside the cell, acting as a potent generator of reactive oxygen species (ROS), specially the hydroxyl radical, by the Fenton reaction (Becana et al, 1998). This radical is extremely toxic to cell metabolism, leading to oxidation of biological macromolecules such as lipids, proteins and nucleic acids, causing membrane leakage and even cell death (Blokhina et al, 2003). Plants must maintain Fe concentrations within a narrow range for proper growth and development
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