Genome-Wide Characterization of Nitrogenase Reductase (nifH) Genes in the Sweet Potato [Ipomoea batatas (L.) Lam] and Its Wild Ancestors

The sweet potato (Ipomoea batatas (L.) Lam.) is an important and widely grown crop, and the nitrogenase reductase (nifH) gene is the most widely sequenced marker gene used to identify nitrogen-fixing bacteria and archaea. There have been many examples of the isolation of the diazotrophic endophytes in sweet potatoes, and there has been no report on whether sweet potatoes and their wild ancestors harbored nifH genes. In this study, a comprehensive analysis of nifH genes has been conducted on these species by using bioinformatics and molecular biology methods. A total of 20, 19 and 17 nifH genes were identified for the first time in sweet potatoes, I. trifida and I. triloba, respectively. Based on a phylogenetic analysis, all of the nifH genes, except for g10233.t1, itf14g14040.t1 and itb14g15470.t1, were clustered into five independent clades: I, II, III, IV and V. The nifH genes clustered in the same phylogenetic branch showed a more similar distribution of conserved motifs and exons–introns than those of the other ones. All of the identified genes were further mapped on the 15 chromosomes of the sweet potato, I. trifida and I. triloba. No segmental duplication was detected in each genome of three Ipomoea species, and 0, 8 and 7 tandemly duplicated gene pairs were detected in the genome of the sweet potato, I. trifida and I. triloba, respectively. Synteny analysis between the three Ipomoea species revealed that there were 7, 7 and 8 syntenic gene pairs of nifH genes detected between the sweet potato and I. trifida, between the sweet potato and I. triloba and between I. trifida and I. triloba, respectively. All of the duplicated and syntenic nifH genes were subjected to purifying selection inside duplicated genomic elements during speciation, except for the tandemly duplicated gene pair itf11g07340.t2_itf11g07340.t3, which was subjected to positive selection. Different expression profiles were detected in the sweet potato, I. trifida and I. triloba. According to the above results, four nifH genes of the sweet potato (g950, g16683, g27094 and g33987) were selected for quantitative real-time polymerase chain reaction (qRT-PCR) analysis in two sweet potato cultivars (Eshu 15 and Long 9) under nitrogen deficiency (N0) and normal (N1) conditions. All of them were upregulated in the N1 treatment and were consistent with the analysis of the RNA-seq data. We hope that these results will provide new insights into the nifH genes in the sweet potato and its wild ancestors and will contribute to the molecular breeding of sweet potatoes in the future.