Ph.D. Candidate University of Georgia Athens, Georgia
Body of Abstract: Nucleoredoxin (NRX) is evolutionarily conserved from animals to plants and thought to play a role in nuclear redox homeostasis. However, functional characterization of plant NRXs remains limited. We previously identified NRX1 as the only thioredoxin small redox protein family that is positively regulated by salicylic acid in Populus tremula x Populus alba (INRA 717-1B4, hereafter 717). NRX1 is encoded by seven tandemly duplicated genes in poplar 717. Phylogenomic analysis revealed the presence of NRX1 tandem duplicates in all genome-sequenced woody perennial trees and shrubs. NRX1 tandem duplication is also found in herbaceous perennials and biennials, which hints at a link between NRX1 tandem duplication and perenniality throughout angiosperm evolution. All seven NRX1 tandem duplicates are transcriptionally active in poplar 717 with divergent tissue preferential expression. Interestingly, the most abundantly expressed gene, NRX1.2, harbors an internal truncation and is predicted to encode a novel NRX1 with two TRX domains but not the TRX-like domain. When expressed in E. coli, both typical NRX1.3 and atypical NRX1.2 catalyze the reduction of insulin disulfides. Translational fusion with a redox-sensitive GFP revealed nuclear localization of NRX1.3 and NRX1.2 but with differential response to changing redox environment. Moreover, the atypical NRX1.2 exhibited a redox substrate profile which is different from that of the typical NRX1.3, presumably due to the truncation and the resultant conformational change. Finally, the survey of sequenced poplar genomes and population resequencing data showed considerable copy number variation of the NRX1 tandem duplicates, with the novel NRX1.2 detected only in European aspens. The recent birth of a novel NRX1 and the recurring tandem duplications in perennial trees and herbs point toward an adaptive role of NRX1 in the perennial lifestyle.