Iron and Zinc in the Embryo and Endosperm of Rice (Oryza sativa L.) Seeds in Contrasting 2′-Deoxymugineic Acid/Nicotianamine Scenarios

Abstract

Iron and Zn deficiencies are worldwide nutritional disorders that can be alleviated by increasing the metal concentration of rice (Oryza sativa L.) grains via bio-fortification approaches. The overproduction of the metal chelator nicotianamine (NA) is among the most effective ones, but it is still unclear whether this is due to the enrichment in NA itself and/or the concomitant enrichment in the NA derivative 2′-deoxymugineic acid (DMA). The endosperm is the most commonly consumed portion of the rice grain and mediates the transfer of nutrients from vegetative tissues to the metal rich embryo. The impact of contrasting levels of DMA and NA on the metal distribution in the embryo and endosperm of rice seeds has been assessed using wild-type rice and six different transgenic lines overexpressing nicotianamine synthase (OsNAS1) and/or barley nicotianamine amino transferase (HvNAATb). These transgenic lines outlined three different DMA/NA scenarios: (i) in a first scenario, an enhanced NA level (via overexpression of OsNAS1) would not be fully depleted because of a limited capacity to use NA for DMA synthesis (lack of -or low- expression of HvNAATb), and results in consistent enrichments in NA, DMA, Fe and Zn in the endosperm and NA, DMA and Fe in the embryo; (ii) in a second scenario, an enhanced NA level (via overexpression of OsNAS1) would be depleted by an enhanced capacity to use NA for DMA synthesis (via expression of HvNAATb), and results in enrichments only for DMA and Fe, both in the endosperm and embryo, and (iii) in a third scenario, the lack of sufficient NA replenishment would limit DMA synthesis, in spite of the enhanced capacity to use NA for this purpose (via expression of HvNAATb), and results in decreases in NA, variable changes in DMA and moderate decreases in Fe in the embryo and endosperm. Also, quantitative LA-ICP-MS metal map images of the embryo structures show that the first and second scenarios altered local distributions of Fe, and to a lesser extent of Zn. The roles of DMA/NA levels in the transport of Fe and Zn within the embryo are thoroughly discussed.

Bioimaging of multiple elements by high‐resolution LA‐ICP‐MS reveals altered distribution of mineral elements in the nodes of rice mutants

“…Therefore, this technique can provide both quantitative data and spatially revolved imaging of multiple elements. So far, this technique has been applied in analysis of plant dried sample such as seeds (Wang et al ., ; Diaz‐Benito et al ., ), and different organs including stems and leaves (Tian et al ., ). However, there is a limit to apply this technique to map elemental distribution in fresh plant tissues due to element loss and structure destruction during sample preparation.…”

Genome-wide analysis of NAAT, DMAS, TOM, and ENA gene families in maize reveals their roles in regulating iron homeostasis

“…In summary, these observations imply that regulation of NA-PS metabolism is crucial for Fe-homeostasis in graminaceous plants. In line with these speculation, recent studies revealed that different DMA/NA ratio impacted the content of Zn and Fe in embryo and endosperm, and altered the local distribution patterns of Fe in embryo [34], shedding light on the balance between DMA and NA affects Fe-homeostasis.…”

“…It is noteworthy that excessive production of NA lead to subsequently increasing of DMA, which may increase Fe content in seed [34]. Consistently, different NA to DMA ratio regulated by NAS and NAAT had different effects on Fe content in grains [34,59]. Therefore, exploring enzymes involved in the synthesis and transport of NA and DMA may provide a theoretical basis to optimize iron bioforti cation in cereals.…”

“…To date, several approaches were taken to enhance iron content in seeds, including increasing the transcript accumulation of genes associated with Fe uptake and transport, as well as modifying expression of endosperm speci c genes [54, 57,58]. It is noteworthy that excessive production of NA lead to subsequently increasing of DMA, which may increase Fe content in seed [34]. Consistently, different NA to DMA ratio regulated by NAS and NAAT had different effects on Fe content in grains [34,59].…”

Genetic Biofortification to Enrich Rice and Wheat Grain Iron: From Genes to Product

“…Better results were achieved in the OE OsNAS2 plants with a Fe level-up to 19 μg/g in polished rice (Johnson et al, 2011) in comparison with the Nipponbare wildtype (4.5 μg/g), or in the OsNAS3 and OsNAS2 activation tag plants with 12 and 10 μg/g Fe in milled rice respectively compared to the wildtype (4 μg/g) (Lee et al, 2009b, 2012). A high Fe increase up to 55 μg/g was reported in the endosperm of japonica rice by OE of OsNAS1 and HvNAAT genes (Diaz-Benito et al, 2018). The Fe level is exceptionally high for the starchy endosperm suggesting either Fe contamination or the presence aleurone layer.…”