Abstract
Absorption of macronutrients such as nitrogen is a critical process for land plants. There is little information available on the correlation between the root evolution of land plants and the protein regulation of nitrogen absorption and responses. NIN-like protein (NLP) transcription factors contain a Phox and Bem1 (PB1) domain, which may regulate nitrate-response genes and seem to be involved in the adaptation to growing on land in terms of plant root development. In this report, we reveal the NLP phylogeny in land plants and the origin of NLP genes that may be involved in the nitrate-signaling pathway. Our NLP phylogeny showed that duplication of NLP genes occurred before divergence of chlorophyte and land plants. Duplicated NLP genes may lost in most chlorophyte lineages. The NLP genes of bryophytes were initially monophyletic, but this was followed by divergence of lycophyte NLP genes and then angiosperm NLP genes. Among those identified NLP genes, PB1, a protein–protein interaction domain was identified across our phylogeny. To understand how protein–protein interaction mediate via PB1 domain, we examined the PB1 domain of Arabidopsis thaliana NLP7 (AtNLP7) in terms of its molecular oligomerization and function as representative. Based on the structure of the PB1 domain, determined using small-angle x-ray scattering (SAXS) and site-directed mutagenesis, we found that the NLP7 PB1 protein forms oligomers and that several key residues (K867 and D909/D911/E913/D922 in the OPCA motif) play a pivotal role in the oligomerization of NLP7 proteins. The fact that these residues are all conserved across land plant lineages means that this oligomerization may have evolved after the common ancestor of extant land plants colonized the land. It would then have rapidly become established across land-plant lineages in order to mediate protein–protein interactions in the nitrate-signaling pathway.
Highlights
The evolution of land plants involved a series of adaptations to living in diverse terrestrial environments, including the evolution of roots–a key organ that allows plants to absorb macronutrients
We focused on expressing Phox and Bem1 (PB1) domain, which serving as protein–protein interaction domain on Arabidopsis thaliana NLP7 (AtNLP7)
61 NODULE INCEPTION (NIN)-like protein (NLP) homologs containing the RWP-RK and PB1 domains were obtained for the 17 selected species
Summary
The evolution of land plants involved a series of adaptations to living in diverse terrestrial environments, including the evolution of roots–a key organ that allows plants to absorb macronutrients. Nitrates are known to be involved in plant growth and development, and to serve as signaling molecules that are involved in the primary nitrate response (PNR) (Gowri et al, 1992; Liu et al, 2017). Β-glucuronidase (GUS) expression in an NLP6 transgenic line of Arabidopsis can be observed after nitrate treatment, whereas no GUS signal is observed after KCl treatment (Konishi and Yanagisawa, 2013). This suggests that NLP may serve as a nitrate-response TF
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