Phylogenetic and Structural Analysis of NIN-Like Proteins With a Type I/II PB1 Domain That Regulates Oligomerization for Nitrate Response

Diversified molecular adaptations of inorganic nitrogen assimilation and signaling machineries in plants

Plants evolved sophisticated machineries to monitor levels of external nitrogen supply, respond to nitrogen demand from different tissues and integrate this information for coordinating its assimilation. Although roles of inorganic nitrogen in orchestrating developments have been studied in model plants and crops, systematic understanding of the origin and evolution of its assimilation and signaling machineries remains largely unknown. We expanded taxon samplings of algae and early-diverging land plants, covering all main lineages of Archaeplastida, and reconstructed the evolutionary history of core components involved in inorganic nitrogen assimilation and signaling. Most components associated with inorganic nitrogen assimilation were derived from the ancestral Archaeplastida. Improvements of assimilation machineries by gene duplications and horizontal gene transfers were evident during plant terrestrialization. Clusterization of genes encoding nitrate assimilation proteins might be an adaptive strategy for algae to cope with changeable nitrate availability in different habitats. Green plants evolved complex nitrate signaling machinery that was stepwise improved by domains shuffling and regulation co-option. Our study highlights innovations in inorganic nitrogen assimilation and signaling machineries, ranging from molecular modifications of proteins to genomic rearrangements, which shaped developmental and metabolic adaptations of plants to changeable nutrient availability in environments.

Conserved and unique functions of NIN-like proteins in nitrate sensing and signaling

Nitrogen is the most abundant element in the atmosphere and serves as the foundation block of life, including plants on earth. Unlike carbon fixation through photosynthesis, plants rely heavily on external supports to acquire nitrogen. To this end, plants have adapted various strategies such as forming mutualistic relationships with nitrogen-fixing bacteria and evolving a large regulatory network that includes multiple transporters, sensors, and transcription factors for fine-tuning nitrate sensing and signaling. Nodule Inception (NIN) and NIN-like protein (NLP) are central in this network by executing multiple functions such as initiating and regulating the nodule symbiosis for nitrogen fixation, acting as the intracellular sensor to monitor the nitrate fluctuations in the environment, and activating the transcription of nitrate-responsive genes for optimal nitrogen uptake, assimilation, and usage. The involvement of NLPs in intracellular nitrate binding and early nitrate responses highlight their pivotal role in the primary nitrate response (PNR). Genome-wide reprogramming in response to nitrate by NLP is highly transient and rapid, requiring regulation in a precise and dynamic manner. This review aims to summarize recent progress in the study of NIN/NLP for a better understanding of the molecular basis of their roles and regulations in nitrate sensing and signaling, with the hope of shedding light on increasing biological nitrogen fixation and improving nitrogen use efficiency (NUE) to minimize fertilizer input in agriculture.

Identification and expression characteristics of NLP (NIN-like protein) gene family in pepper (Capsicum annuum L.)

BACKGROUND

Pepper (Capsicum annum L.) is the main crop in the vegetable industry. The growth and development of peppers are regulated by nitrate, but there is limited research on the molecular mechanisms of nitrate absorption and assimilation in peppers. A plant specific transcription factor NLP plays an important role in nitrate signal transduction.

METHODS AND RESULTS

In this study, a total of 7 NLP members were identified based on pepper genome data. Two nitrogen transport elements (GCN4) were found in the CaNLP5 promoter. In the phylogenetic tree, CaNLP members are divided into three branches, with pepper NLP and tomato NLP having the closest genetic relationship. The expression levels of CaNLP1, CaNLP3, and CaNLP4 are relatively high in the roots, stems, and leaves. The expression level of CaNLP7 gene is relatively high during the 5-7 days of pepper fruit color transformation. After various non-Biotic stress and hormone treatments, the expression of CaNLP1 was at a high level. The expression of CaNLP3 and CaNLP4 was down regulated in leaves, but up regulated in roots. Under conditions of nitrogen deficiency and sufficient nitrate, the expression patterns of NLP genes in pepper leaves and roots were determined.

CONCLUSION

These results provide important insights into the multiple functions of CaNLPs in regulating nitrate absorption and transport.

Auxin response factors are keys to the many auxin doors

In plants, most developmental programs depend on the action of auxin. The best described model of the auxin signaling pathway, which explains most, but not all, of the auxin transcriptional responses, relies on a de-repression mechanism. The auxin/indole-3-acetic acid repressors (Aux/IAAs) interact with the auxin response factors (ARFs), the transcription factors of the auxin signaling pathway, leading to repression of the ARF-controlled genes. Auxin induces Aux/IAA degradation, releases ARFs and activates transcription. However, this elegant model is not suitable for all ARFs. Indeed, in Arabidopsis, which has 22 ARFs, only five of them fit into the model since they are the ones able to interact with Aux/IAAs. The remaining 17 have a limited capacity to interact with the repressors, and their mechanisms of action are still unclear. The differential interactions between ARF and Aux/IAA proteins constitute one of many examples of the biochemical and structural diversification of ARFs that affect their action and therefore affect auxin transcriptional responses. A deeper understanding of the structural properties of ARFs is fundamental to obtaining a better explanation of the action of auxin in plants.

NIN-Like Proteins: Interesting Players in Rhizobia-Induced Nitrate Signaling Response During Interaction with Non-Legume Host Arabidopsis thaliana

Nitrogen is an essential macronutrient and a key cellular messenger. Plants have evolved refined molecular systems to sense the cellular nitrogen status. This is exemplified by the root nodule symbiosis between legumes and symbiotic rhizobia, where nitrate availability inhibits this mutualistic interaction. Additionally, nitrate also functions as a metabolic messenger, resulting in nitrate signaling cascades which intensively crosstalk with other physiological pathways. Nodule inception-like proteins (NLPs) are key players in nitrate signaling and regulate nitrate-dependent transcription during legume-rhizobia interactions. Nevertheless, the coordinated interplay between nitrate signaling pathways and rhizobacteria-induced responses remains to be elucidated. In our study, we investigated rhizobia-induced changes in the root system architecture of the non-legume host arabidopsis under different nitrate conditions. We demonstrate that rhizobium-induced lateral root growth and increased root hair length and density are regulated by a nitrate-related signaling pathway. Key players in this process are AtNLP4 and AtNLP5, because the corresponding mutants failed to respond to rhizobia. At the cellular level, AtNLP4 and AtNLP5 control a rhizobia-induced decrease in cell elongation rates, while additional cell divisions occurred independently of AtNLP4. In summary, our data suggest that root morphological responses to rhizobia are coordinated by a newly considered nitrate-related NLP pathway that is evolutionarily linked to regulatory circuits described in legumes.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.