Title: Genome-Wide Survey and Expression Analysis of NIN-Like Protein (NLP) Genes Reveals Its Potential Roles in the Response to Nutrition Deficiency in Tomato

Background: Tomato (Solanum lycopersicum) is one of the most important horticultural crops, with a marked preference of nitrate as inorganic nitrogen source. The molecular mechanisms of nitrate uptake and assimilation are poorly understood in tomato. NIN-Like Proteins (NLPs) are conserved, plant-specific transcription factors that play crucial roles in nitrate signaling. Results: In this study, genome-wide analysis revealed six NLP members in tomato genome. They were clustered into three clades in a phylogenic tree. Comparative genomic analysis showed that SlNLP genes had collinear relationships to NLPs in Arabidopsis, canola, maize and rice, and that the expansion of the SlNLP family mainly resulted from segmental duplications in tomato genome. Tissue-specific expression analysis showed that the close homologues of AtNLP6/7, SlNLP3, was strongly expressed in roots during both seedling and flowering stages; SlNLP4 and SlNLP6 exhibited preferential expression in stems and leaves; and SlNLP6 were expressed in high levels in fruits. Further, the nitrate uptake in tomato roots and expression patterns of SlNLP genes were measured under nitrogen/phosphate/potassium deficiency and nitrate resupply conditions. The transcript abundance of SlNLP3 decreased to 70% under phosphate/potassium deficiency. Most of SlNLPs were up-regulated after nitrogen starvation. SlNLP1 and SlNLP5 were induced rapidly and temporally by nitrate. Conclusions: These results provided significant insights into the potential diverse functions of SlNLPs to regulate nitrate uptake.