Background: NAD(H) kinase (NADK) is the key enzyme that catalyzes de novo synthesis of NADP(H) from NAD(H) for NADP(H)-based metabolic pathways. In plants, NADKs form functional subfamilies. Studies of these families in Arabidopsis thaliana indicate that they have undergone considerable evolutionary selection; however, the detailed evolutionary history and functions of the various NADKs in plants are not clearly understood.
Principal findings: We performed a comparative genomic analysis that identified 74 NADK gene homologs from 24 species representing the eight major plant lineages within the supergroup Plantae: glaucophytes, rhodophytes, chlorophytes, bryophytes, lycophytes, gymnosperms, monocots and eudicots. Phylogenetic and structural analysis classified these NADK genes into four well-conserved subfamilies with considerable variety in the domain organization and gene structure among subfamily members. In addition to the typical NAD_kinase domain, additional domains, such as adenylate kinase, dual-specificity phosphatase, and protein tyrosine phosphatase catalytic domains, were found in subfamily II. Interestingly, NADKs in subfamily III exhibited low sequence similarity (∼30%) in the kinase domain within the subfamily and with the other subfamilies. These observations suggest that gene fusion and exon shuffling may have occurred after gene duplication, leading to specific domain organization seen in subfamilies II and III, respectively. Further analysis of the exon/intron structures showed that single intron loss and gain had occurred, yielding the diversified gene structures, during the process of structural evolution of NADK family genes. Finally, both available global microarray data analysis and qRT-RCR experiments revealed that the NADK genes in Arabidopsis and Oryza sativa show different expression patterns in different developmental stages and under several different abiotic/biotic stresses and hormone treatments, underscoring the functional diversity and functional divergence of the NADK family in plants.
Conclusions: These findings will facilitate further studies of the NADK family and provide valuable information for functional validation of this family in plants.