Nodule Inception (NIN), Early nodulin 11 (Enod11), and ERF Required for Nodulation 1 (ERN1) are genes involved in the nodulation process of roots by bacterial microsymbionts such as Rhizobium. NIN is one of the major regulators required for the formation of infection threads and nodule primordia and NIN was first discovered in Lotus japonicum (Schauser et al., 1999). NIN has been reported to act as a transcriptional activator directly targeting downstream genes including LjNPL (Nodulation Pectate Lyase), LjNF-Ys, cytokinin receptor MtCRE1 and MtENOD11 (Xiao et al., 2020).

Whereas (NIN) is strictly required for nodulation in legume species, it belongs to a family of NIN-like proteins (NLPs), which have been implicated in the regulation of nitrogen homeostasis in Arabidopsis (Konishi et al., 2021). NLPs regulate nitrate-response genes and seem to be involved in the adaptation to growing on land in terms of plant root development (Hsin et al., 2021). In Medicago truncatula, mutation or downregulation of NLP genes prevents nitrate inhibition of infection, nodule formation and nitrogen fixation. NIN and NLPs physically interact through their carboxy-terminal PB1 (Phox and Bem1) domains. MtNLP1 is required for the expression of nitrate-responsive genes and that nitrate triggers MtNLP1 re-localization from the cytosol to the nucleus (Xiao et al., 2020). Of the nine NLP genes in the Arabidopsis genome there is some redundancy of function.  However, nlp2 and nlp7 single mutants showed a reduction in shoot fresh weight when grown in the presence of nitrate as the sole nitrogen source, indicating that AtNLP2, like AtNLP7, plays a major role in vegetative growth (Konishi et al., 2021).

At least nine NLPs have been identified in the maize genome (Wang et al., 2018). Comparative genomic analysis showed that most ZmNLP genes had collinear relationships to the corresponding NLPs in rice, and that the expansion of the ZmNLP family resulted from segmental duplications in the maize genome. Quantitative PCR analysis revealed the expression of ZmNLP2.1, ZmNLP2.2, ZmNLP3.1, ZmNLP3.2, ZmNLP3.3, and ZmNLP3.4 was induced by nitrate in maize roots. Ectopic expression of ZmNLP3.1 in Arabidopsis restored the N-deficient phenotypes of nlp7-1 under nitrate-replete conditions in terms of shoot biomass, root morphology and nitrate assimilation (Wang et al., 2018). A more recent study in maize revealed that in zmnlp5 mutant seedlings less nitrate and nitrite accumulated in the root tissues and less ammonium in the shoot tissues. The zmnlp5 mutant plants also accumulated less nitrogen than the WT plants in the ear leaves and seed kernels (Ge et al., 2020).

Last updated June 2023 by Ankita Abnave

References:

Schauser L, Roussis A, Stiller J, Stougaard J. A plant regulator controlling development of symbiotic root nodules. Nature. 1999 Nov 11;402(6758):191-5. doi: 10.1038/46058. PMID: 10647012.

Xiao A, Yu H, Fan Y, Kang H, Ren Y, Huang X, Gao X, Wang C, Zhang Z, Zhu H, Cao Y. Transcriptional regulation of NIN expression by IPN2 is required for root nodule symbiosis in Lotus japonicus. New Phytol. 2020 Jul;227(2):513-528. doi: 10.1111/nph.16553. Epub 2020 May 20. PMID: 32187696.

Konishi M, Okitsu T, Yanagisawa S. Nitrate-responsive NIN-like protein transcription factors perform unique and redundant roles in Arabidopsis. J Exp Bot. 2021 Jul 28;72(15):5735-5750. doi: 10.1093/jxb/erab246. PMID: 34050740.

Hsin KT, Yang TJ, Lee YH, Cheng YS. Phylogenetic and Structural Analysis of NIN-Like Proteins With a Type I/II PB1 Domain That Regulates Oligomerization for Nitrate Response. Front Plant Sci. 2021 May 31;12:672035. doi: 10.3389/fpls.2021.672035. PMID: 34135927; PMCID: PMC8200828.

Wang Z, Zhang L, Sun C, Gu R, Mi G, Yuan L. Phylogenetic, expression and functional characterizations of the maize NLP transcription factor family reveal a role in nitrate assimilation and signaling. Physiol Plant. 2018 Jan 24. doi: 10.1111/ppl.12696. Epub ahead of print. PMID: 29364528.

Ge M, Wang Y, Liu Y, Jiang L, He B, Ning L, Du H, Lv Y, Zhou L, Lin F, Zhang T, Liang S, Lu H, Zhao H. The NIN-like protein 5 (ZmNLP5) transcription factor is involved in modulating the nitrogen response in maize. Plant J. 2020 Apr;102(2):353-368. doi: 10.1111/tpj.14628. Epub 2020 Jan 4. PMID: 31793100; PMCID: PMC7217196.