Overexpression of a Malus xiaojinensis Nas1 Gene Influences Flower Development and Tolerance to Iron Stress in Transgenic Tobacco

Iron (Fe) is one of the essential micronutrients required by all living organisms. Expression of MxNas1 of Malus xiaojinensis is enriched in active tissues, including leaves, roots, and phloem, and these seedlings are highly affected by Fe stress and indoleacetic acid treatment, but weakly affected by abscisic acid treatment. Subcellular localization has revealed that MxNas1 is preferentially localized in cell membranes. When this gene is introduced into tobacco, it promotes synthesis of nicotianamine (NA) synthase and increases NA content. Overexpression of MxNas1 improves tolerance to Fe stress in transgenic tobacco, but also leads to misshapen flowers. Higher levels of MxNas1 expression in transgenic tobacco plants also contributes to delayed flowering and increased levels of Fe and Zn in flowers. In addition to its role in metal transport in plants, NA may be involved in the regulation of metal transfer within cells. These results suggest that NA excess influences functions of metal-requiring proteins, including some of the transcription factors.