Analysis of the Arabidopsis NAC gene superfamily in plant development

There are a vast number of transcription factors that regulate plant growth and development. The NAC gene superfamily is one of the largest families of transcription factors in the plant kingdom. NAC gene expression profiles using Affymetrix ATH1 gene chips were obtained for different plant organs: heart embryo, mature embryo, leaf, root and flower. NAC gene expression profiles proved to be very complex, except for one NAC gene detected only in floral tissue, At1g61110. At1g61110 was shown to be specifically expressed in the anther tapetum of Arabidospis; therefore, its name was changed to TAPNAC. TAPNAC became the focus of our studies. We identified a tapnac T-DNA knockout (KO) line, SALK_069450. A molecular phenotype was observed. Several oligopeptide, sugar and metal transporters were differentially expressed. Coincidentally, a wheat NAC gene, named TaNAM-B1 for its high sequence similarity to ATNAM, TAPNAC and At3g15510 was found to be involved in nutrient remobilization. PHOSPHOLIPASE D?1 (PLD?1) was also found to be down-regulated in the tapnac KO. PLD?1 is an enzyme which hydrolyzes phospholipids that are part of tapetal cell membranes and tapetal lipid bodies. Once these tapetal cell structures are disrupted, the secretion of the compounds that form part of the pollen coat (i.e. proteins, flavonoids and lipids) into the anther locule is facilitated. Promoter deletion analysis using a GUS reporter and later GUS immuno-localization confirmed the findings of Wellmer and others. TAPNAC is a tapetal specific gene. The cis-regulatory sequence that enhances tapetal expression in the TAPNAC promoter was identified. The consensus motif TCGTGT increased tapetal expression of a GUS reporter gene, only when flanked by the TAPNAC minimal promoter region (-217 bp to +51 bp). In summary, TAPNAC transcription factor has been characterized and data indicates that it could play a role in nutrient remobilization from the tapetum to the pollen grains, particularly during late floral stages. Also, important information on tapetal specifcation cis-regulatory sequences was discovered. The consensus motif TCGTGT, present in TAPNAC promoter, was shown to enhance tapetal expression of a GUS reporter gene.