Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/4/2009
Publication Date: 10/10/2009
Citation: Sperotto, R.O., Ricachenevsky, F.K., Duarte, G.L., Boff, T., Lopes, K.L., Sperb, E.R., Grusak, M.A., Fett, J.P. 2009. Identification of up-regulated genes in flag leaves during rice grain filling and characterization of OsNAC5, a new ABA-dependent transcription factor. Planta. 230(5):985-1002. Interpretive Summary: Rice is the staple food of half of the world’s population. To keep up with population growth and to improve the nutrition and health of rice consumers, development of high-quality rice varieties, rich in minerals, protein, and carbohydrates, becomes increasingly important. Our knowledge of nutrient delivery to rice seeds is limited, although we know that the uppermost leaf, known as the flag leaf, is an important organ for transporting minerals, amino acids, and sugars to developing seeds. Because we were interested in understanding how nutrients are mobilized from flag leaves to seeds, we identified several genes related to nutrient transport in rice, and measured their expression in different rice varieties from early to late stages of seed development. We showed that the protein product of one gene in particular, OsNAC5, plays an important role in helping to move iron, zinc, and amino acids out of the rice flag leaf. This information will be useful to plant breeders as they try to improve the nutritional quality of this important food crop.
Technical Abstract: Rice is a poor source of essential micronutrients such as iron and zinc. To help clarify the molecular mechanisms that regulate the mobilization of metals from leaves to developing seeds, we conducted suppression subtractive hybridization analysis in flag leaves of two rice cultivars. Flag leaves are the major source of phloem-delivered photoassimilates and remobilized metals for developing seeds. We isolated 78 sequences that are up-regulated in flag leaves at the grain filling stage relative to the panicle exertion stage. These sequences were grouped into functional categories, and differential expression of selected genes (seven potential transport proteins), one enzyme (OsNAS3), and one transcription factor (OsNAC5) was confirmed by quantitative RT-PCR. Several senescence-associated genes were found. We show that OsNAC5 expression is activated by natural (aging) and induced senescence processes (dark, ABA application, high salinity, cold and Fe-deficiency), and its expression is not activated in the presence of BAP (6-benzylaminopurine, a senescence inhibitor). Salt induction of OsNAC5 expression is abolished by nicotinamide, an inhibitor of ABA effects. This result and the presence of cis-acting elements in the promoter region of the OsNAC5 gene suggest an ABA-dependent activation. Using four different rice cultivars, we show that OsNAC5 activation is higher and earlier in flag leaves and panicles of IR75862 plants, which have higher concentrations of Fe, Zn, and protein in the seeds.