Plant biostimulants are diverse substances and microorganisms used to enhance plant growth. The global market for biostimulants is projected to increase 12 % per year and reach over $2,200 million by 2018. Despite the growing use of biostimulants in agriculture, many in the scientific community consider biostimulants to be lacking peer-reviewed scientific evaluation. This article describes the emerging definitions of biostimulants and reviews the literature on five categories of biostimulants: i. microbial inoculants, ii. humic acids, iii. fulvic acids, iv. protein hydrolysates and amino acids, and v. seaweed extracts. The large number of publications cited for each category of biostimulants demonstrates that there is growing scientific evidence supporting the use of biostimulants as agricultural inputs on diverse plant species. The cited literature also reveals some commonalities in plant responses to different biostimulants, such as increased root growth, enhanced nutrient uptake, and stress tolerance.

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Agricultural uses of plant biostimulants

Plant growth-promoting rhizobacteria (PGPR) are soil bacteria that are able to colonize rhizosphere and to enhance plant growth by means of a wide variety of mechanisms like organic matter mineralization, biological control against soil-borne pathogens, biological nitrogen fixation, and root growth promotion. A very interesting feature of PGPR is their ability of enhancing nutrient bioavailability. Several bacterial species have been characterized as P-solubilizing microorganisms while other species have been shown to increase the solubility of micronutrients, like those that produce siderophores for Fe chelation. The enhanced amount of soluble macro- and micronutrients in the close proximity of the soil-root interface has indeed a positive effect on plant nutrition. Furthermore, several pieces of evidence highlight that the inoculation of plants with PGPR can have considerable effects on plant at both physiological and molecular levels (e.g., induction of rhizosphere acidification, up- and downregulation of genes involved in ion uptake, and translocation), suggesting the possibility that soil biota could stimulate plants being more efficient in retrieving nutrients from soil and coping with abiotic stresses. However, the molecular mechanisms underlying these phenomena, the signals involved as well as the potential applications in a sustainable agriculture approach, and the biotechnological aspects for possible rhizosphere engineering are still matters of discussion.

Microbial interactions in the rhizosphere: beneficial influences of plant growth-promoting rhizobacteria on nutrient acquisition process. A review

The United Nations declared 2016 as the International Year of Pulses (grain legumes) under the banner ‘nutritious seeds for a sustainable future’. A second green revolution is required to ensure food and nutritional security in the face of global climate change. Grain legumes provide an unparalleled solution to this problem because of their inherent capacity for symbiotic atmospheric nitrogen fixation, which provides economically sustainable advantages for farming. In addition, a legume-rich diet has health benefits for humans and livestock alike. However, grain legumes form only a minor part of most current human diets, and legume crops are greatly under-used. Food security and soil fertility could be significantly improved by greater grain legume usage and increased improvement of a range of grain legumes. The current lack of coordinated focus on grain legumes has compromised human health, nutritional security and sustainable food production.

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Neglecting legumes has compromised human health and sustainable food production

https://www.cell.com/trends/plant-science/pdf/S1360-1385(12)00098-2.pdf

Uptake, allocation and signaling of nitrate

A unified nomenclature of NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER family members in plants

Nitrate Transport, Sensing, and Responses in Plants

Nitrogen (N) is one of the key mineral nutrients for plants and its availability has a major impact on their growth and development. Most often N resources are limiting and plants have evolved various strategies to modulate their root uptake capacity to compensate for both spatial and temporal changes in N availability in soil. The main N sources for terrestrial plants in soils of temperate regions are in decreasing order of abundance, nitrate, ammonium and amino acids. N uptake systems combine, for these different N forms, high- and low-affinity transporters belonging to multige families. Expression and activity of most uptake systems are regulated locally by the concentration of their substrate, and by a systemic feedback control exerted by whole-plant signals of N status, giving rise to a complex combinatory network. Besides modulation of the capacity of transport systems, plants are also able to modulate their growth and development to maintain N homeostasis. In particular, root system architecture is highly plastic and its changes can greatly impact N acquisition from soil. In this review, we aim at detailing recent advances in the identification of molecular mechanisms responsible for physiological and developmental responses of root N acquisition to changes in N availability. These mechanisms are now unravelled at an increasing rate, especially in the model plant Arabidopsis thaliana L.. Within the past decade, most root membrane transport proteins that determine N acquisition have been identified. More recently, molecular regulators in nitrate or ammonium sensing and signalling have been isolated, revealing common regulatory genes for transport system and root development, as well as a strong connection between N and hormone signalling pathways. Deciphering the complexity of the regulatory networks that control N uptake, metabolism and plant development will help understanding adaptation of plants to sub-optimal N availability and fluctuating environments. It will also provide solutions for addressing the major issues of pollution and economical costs related to N fertilizer use that threaten agricultural and ecological sustainability.

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Nitrogen acquisition by roots: physiological and developmental mechanisms ensuring plant adaptation to a fluctuating resource

In Arabidopsis the plasma membrane nitrate transceptor (transporter/receptor) NRT1.1 governs many physiological and developmental responses to nitrate. Alongside facilitating nitrate uptake, NRT1.1 regulates the expression levels of many nitrate assimilation pathway genes, modulates root system architecture, relieves seed dormancy and protects plants from ammonium toxicity. Here, we assess the functional and phenotypic consequences of point mutations in two key residues of NRT1.1 (P492 and T101). We show that the point mutations differentially affect several of the NRT1.1-dependent responses to nitrate, namely the repression of lateral root development at low nitrate concentrations, and the short-term upregulation of the nitrate-uptake gene NRT2.1, and its longer-term downregulation, at high nitrate concentrations. We also show that these mutations have differential effects on genome-wide gene expression. Our findings indicate that NRT1.1 activates four separate signalling mechanisms, which have independent structural bases in the protein. In particular, we present evidence to suggest that the phosphorylated and non-phosphorylated forms of NRT1.1 at T101 have distinct signalling functions, and that the nitrate-dependent regulation of root development depends on the phosphorylated form. Our findings add to the evidence that NRT1.1 is able to trigger independent signalling pathways in Arabidopsis in response to different environmental conditions.

Multiple mechanisms of nitrate sensing by Arabidopsis nitrate transceptor NRT1.1.

Nitrate sensing and signaling in plants.

In Arabidopsis (Arabidopsis thaliana), the NRT21 gene codes for the main component of the root nitrate (NO3−) high-affinity transport system (HATS) Due to the strong correlation generally found between high-affinity root NO3− influx and NRT21 mRNA level, it has been postulated that transcriptional regulation of NRT21 is a key mechanism for modulation of the HATS activity However, this hypothesis has never been demonstrated, and is challenged by studies suggesting the occurrence of posttranscriptional regulation at the NRT21 protein level To unambiguously clarify the respective roles of transcriptional and posttranscriptional regulations of NRT21, we generated transgenic lines expressing a functional 35S::NRT21 transgene in an atnrt21 mutant background Despite a high and constitutive NRT21 transcript accumulation in the roots, the HATS activity was still down-regulated in the 35S::NRT21 transformants in response to repressive nitrogen or dark treatments that strongly reduce NRT21 transcription and NO3− HATS activity in the wild type In some treatments, this was associated with a decline of NRT21 protein abundance, indicating posttranscriptional regulation of NRT21 However, in other instances, NRT21 protein level remained constant Changes in abundance of NAR21, a partner protein of NRT21, closely followed those of NRT21, and thus could not explain the close-to-normal regulation of the HATS in the 35S::NRT21 transformants Even if in certain conditions the transcriptional regulation of NRT21 contributes to a limited extent to the control of the HATS, we conclude from this study that posttranscriptional regulation of NRT21 and/or NAR21 plays a predominant role in the control of the NO3− HATS in Arabidopsis

Regulation of High-Affinity Nitrate Uptake in Roots of Arabidopsis Depends Predominantly on Posttranscriptional Control of the NRT2.1/NAR2.1 Transport System

Eléonore Bouguyon

Papers

Nitrate Transport, Sensing, and Responses in Plants

Nitrogen acquisition by roots: physiological and developmental mechanisms ensuring plant adaptation to a fluctuating resource

Multiple mechanisms of nitrate sensing by Arabidopsis nitrate transceptor NRT1.1.

Nitrate sensing and signaling in plants.

Regulation of High-Affinity Nitrate Uptake in Roots of Arabidopsis Depends Predominantly on Posttranscriptional Control of the NRT2.1/NAR2.1 Transport System