Ecophysiological responses to excess iron in lowland and upland rice cultivars.

Iron (Fe) is an essential nutrient for plants but under high concentrations, such as that found naturally in clay and waterlogged soils, its toxic effect can limit production. This study aimed to investigate the stress tolerance responses exhibited by different rice cultivars. Both lowland and upland cultivars were grown under excess Fe and hypoxic conditions. Lowland cultivars showed higher Fe accumulation in roots compared with upland cultivars suggesting the use of different strategies to tolerate excess Fe. The upland Canastra cultivar displayed a mechanism to limit iron translocation from roots to the shoots, minimizing leaf oxidative stress induced by excess Fe. Conversely, the cultivar Curinga invested in the increase of R1/A, as an alternative drain of electrons. However, the higher iron accumulation in the leaves, was not necessarily related to high toxicity. Nutrient uptake and/or utilization mechanisms in rice plants are in accordance with their needs, which may be defined in relation to crop environments. Alterations in the biochemical parameters of photosynthesis suggest that photosynthesis in rice under excess Fe is primarily limited by biochemical processes rather than by diffusional limitations, particularly in the upland cultivars. The electron transport rate, carboxylation efficiency and electron excess dissipation by photorespiration demonstrate to be good indicators of iron tolerance. Altogether, these chemical and molecular patterns suggests that rice plants grown under excess Fe exhibit gene expression reprogramming in response to the Fe excess per se and in response to changes in photosynthesis and nutrient levels to maintain growth under stress.

Comparative transcriptomics of rice plants under cold, iron, and salt stresses.

Abiotic stresses such as salinity, iron toxicity, and low temperatures are the main limiting factors of rice (Oryza sativa L.) yield. The elucidation of the genes involved in responses to these stresses is extremely important to understand the mechanisms that confer tolerance, as well as for the development of cultivars adapted to these conditions. In this study, the RNA-seq technique was used to compare the transcriptional profile of rice leaves (cv. BRS Querência) in stage V3, exposed to cold, iron, and salt stresses for 24 h. A range of 41 to 51 million reads was aligned, in which a total range of 88.47 to 89.21 % was mapped in the reference genome. For cold stress, 7905 differentially expressed genes (DEGs) were observed, 2092 for salt and 681 for iron stress; 370 of these were common to the three DEG stresses. Functional annotation by software MapMan demonstrated that cold stress usually promoted the greatest changes in the overall metabolism, and an enrichment analysis of overrepresented gene ontology (GO) terms showed that most of them are contained in plastids, ribosome, and chloroplasts. Saline stress induced a more complex interaction network of upregulated overrepresented GO terms with a relatively low number of genes compared with cold stress. Our study demonstrated a high number of differentially expressed genes under cold stress and a greater relationship between salt and iron stress levels. The physiological process most affected at the molecular level by the three stresses seems to be photosynthesis.