Root Hydraulic and Aquaporin Responses to N Availability. In: Chaumont F, Tyerman SD, editors. Plant Aquaporins. Cham: Springer International Publishing; 2017. pp. 207-36. [DOI: 10.1007/978-3-319-49395-4_10]

Root hydraulic properties: An exploration of their variability across scales

Root hydraulic properties are key physiological traits that determine the capacity of root systems to take up water, at a specific evaporative demand. They can strongly vary among species, cultivars or even within the same genotype, but a systematic analysis of their variation across plant functional types (PFTs) is still missing. Here, we reviewed published empirical studies on root hydraulic properties at the segment-, individual root-, or root system scale and determined its variability and the main factors contributing to it. This corresponded to a total of 241 published studies, comprising 213 species, including woody and herbaceous vegetation. We observed an extremely large range of variation (of orders of magnitude) in root hydraulic properties, but this was not caused by systematic differences among PFTs. Rather, the (combined) effect of factors such as root system age, driving force used for measurement, or stress treatments shaped the results. We found a significant decrease in root hydraulic properties under stress conditions (drought and aquaporin inhibition, p < .001) and a significant effect of the driving force used for measurement (hydrostatic or osmotic gradients, p < .001). Furthermore, whole root system conductance increased significantly with root system age across several crop species (p < .01), causing very large variation in the data (>2 orders of magnitude). Interestingly, this relationship showed an asymptotic shape, with a steep increase during the first days of growth and a flattening out at later stages of development. We confirmed this dynamic through simulations using a state-of-the-art computational model of water flow in the root system for a variety of crop species, suggesting common patterns across studies and species. These findings provide better understanding of the main causes of root hydraulic properties variations observed across empirical studies. They also open the door to better representation of hydraulic processes across multiple plant functional types and at large scales. All data collected in our analysis has been aggregated into an open access database (https://roothydraulic-properties.shinyapps.io/database/), fostering scientific exchange.

Effects of low nitrogen supply on biochemical and physiological parameters related to nitrate and water, involving nitrate transporters and aquaporins in Citrus macrophylla

A reduction in chemical N-based fertillizer was investigated in Citrus plants. As N and water uptake are connected, the relationship between the physiological response to reductions in N was studied in relation to N metabolism and water. We examined the response of new and mature leaves and roots of Citrus macrophylla, grown under controlled conditions, and given different concentrations of N: 16, 8 or 4 mM. Differences in growth and development were determined for biochemical (mineral content, photosynthetic pigments, proteins and nitrate and nitrite reductase activity), physiological (photosynthesis and transpiration), and molecular (relative expression of nitrate transporters and aquaporins) parameters. Only plants given 4 mM N showed a reduction in growth. Although there were changes in NR activity, protein synthesis, and chlorophyll content in both 8 and 4 mM N plants that were highly related to aquaporin and nitrate transporter expression. The results revealed new findings on the relationship between aquaporins and nitrate transporters in new leaves of Citrus, suggesting a mechanism for ensuring growth under low N when new tissues are being formed.

Nitrogen-mediated metabolic patterns of susceptibility to Botrytis cinerea infection in tomato (Solanum lycopersicum) stems

Severe N stress allows an accumulation of C-based compounds but impedes that of N-based compounds required to lower the susceptibility of tomato stem to Botrytis cinerea. Botrytis cinerea, a necrotrophic filamentous fungus, forms potentially lethal lesions on the stems of infected plants. Contrasted levels of susceptibility to B. cinerea were obtained in a tomato cultivar grown on a range of nitrate concentration: low N supply resulted in high susceptibility while high N supply conferred a strong resistance. Metabolic deviations and physiological traits resulting from both infection and nitrogen limitation were investigated in the symptomless stem tissue surrounding the necrotic lesion. Prior to infection, nitrogen-deficient plants showed reduced levels of nitrogen-based compounds such as amino acids, proteins, and glutathione and elevated levels of carbon-based and defence compounds such as α-tomatine and chlorogenic acid. After B. cinerea inoculation, all plants displayed a few common responses, mainly alanine accumulation and galactinol depletion. The metabolome of resistant plants grown under high N supply showed no significant change after inoculation. On the contrary, the metabolome of susceptible plants grown under low N supply showed massive metabolic adjustments, including changes in central metabolism around glutamate and respiratory pathways, suggesting active resource mobilization and production of energy and reducing power. Redox and defence metabolisms were also stimulated by the infection in plants grown under low N supply; glutathione and chlorogenic acid accumulated, as well as metabolites with more controversial defensive roles, such as polyamines, GABA, branched-chain amino acids and phytosterols. Taken together, the results showed that nitrogen deficiency, although leading to an increase in secondary metabolites even before the pathogen attack, must have compromised the constitutive levels of defence proteins and delayed or attenuated the induced responses. The involvement of galactinol, alanine, cycloartenol and citramalate in the tomato stem response to B. cinerea is reported here for the first time.

Cadmium promotes the absorption of ammonium in hyperaccumulator Solanum nigrum L. mediated by ammonium transporters and aquaporins

Cadmium (Cd) is a toxic heavy metal affecting the normal growth of plants. Nitrate (NO₃^-) and ammonium (NH₄⁺) are the primary forms of inorganic nitrogen (N) absorbed by plants. However, the mechanism of N absorption and regulation under Cd stress remains unclear. This study found that: (1) Cd treatment affected the biomass, root length, and Cd²⁺ flux in Solanum nigrum seedling roots. Specifically, 50 μM Cd significantly inhibited NO₃^- influx while increased NH₄⁺ influx compared with 0 and 5 μM Cd treatments measured by non-invasive micro-test technology. (2) qRT-PCR analysis showed that 50 μM Cd inhibited the expressions of nitrate transporter genes, SnNRT2;4 and SnNRT2;4-like, increased the expressions of ammonium transporter genes, SnAMT1;2 and SnAMT1;3, in the roots. (3) Under NH₄⁺ supply, 50 μM Cd significantly induced the expressions of the aquaporin genes, SnPIP1;5, SnPIP2;7, and SnTIP2;1. Our results showed that 50 μM Cd stress promoted NH₄⁺ absorption by up-regulating the gene expressions of NH₄⁺ transporter and aquaporins, suggesting that high Cd stress can affect the preference of N nutrition in S. nigrum.

Exposure to high nitrogen triggered a genotype-dependent modulation of cell and root hydraulics, which can involve aquaporin regulation

Root nitrogen acquisition has been proposed to be regulated by mass flow, a process by which water flow brings nutrients to the root surface, depending on a concerted regulation of the root hydraulic properties and stomatal conductance. As aquaporins play an important role in regulating transcellular water flow, we aimed at evaluating the short-term effect of high nitrogen (HN) availability on the dynamics of hydraulic parameters at both the root and cell level and the regulation of aquaporins. The effect of short-term HN (8 mM NO₃ ^- ) treatment was investigated on 12 diverse 15-day-old maize genotypes. Root exposure to HN triggered a rapid (<4 h) increase in the root hydraulic conductivity (L_pr ) in seven genotypes while no L_pr variation was recorded for the others, allowing the separation of the genotypes into two groups (HN-responsive and HN-nonresponsive). A remarkable correlation between L_pr and the cortex cell hydraulic conductivity (L_pc ) was observed. However, while differences in gas exchange parameters were also observed, the variations were genotype-specific and not always correlated with the root hydraulic parameters. We then investigated whether HN-induced L_pr variations were linked to the activity and regulation of plasma membrane PIP aquaporins. While some changes in PIP mRNA levels were detected, this was not correlated with the protein levels. On the other hand, the rapid variation in L_pr observed in the B73 genotype was correlated with the PIP protein abundance in the plasma membrane, highlighting PIP posttranslational mechanisms in the short-term regulation of root hydraulic parameters in response to HN treatment.

Genome-Wide Identification, Structure Characterization, and Expression Pattern Profiling of the Aquaporin Gene Family in Betula pendula

Aquaporin water channels (AQPs) constitute a large family of transmembrane proteins present throughout all kingdoms of life. They play key roles in the flux of water and many solutes across the membranes. The AQP diversity, protein features, and biological functions of silver birch are still unknown. A genome analysis of Betula pendula identified 33 putative genes encoding full-length AQP sequences (BpeAQPs). They are grouped into five subfamilies, representing ten plasma membrane intrinsic proteins (PIPs), eight tonoplast intrinsic proteins (TIPs), eight NOD26-like intrinsic proteins (NIPs), four X intrinsic proteins (XIPs), and three small basic intrinsic proteins (SIPs). The BpeAQP gene structure is conserved within each subfamily, with exon numbers ranging from one to five. The predictions of the aromatic/arginine selectivity filter (ar/R), Froger's positions, specificity-determining positions, and 2D and 3D biochemical properties indicate noticeable transport specificities to various non-aqueous substrates between members and/or subfamilies. Nevertheless, overall, the BpePIPs display mostly hydrophilic ar/R selective filter and lining-pore residues, whereas the BpeTIP, BpeNIP, BpeSIP, and BpeXIP subfamilies mostly contain hydrophobic permeation signatures. Transcriptional expression analyses indicate that 23 BpeAQP genes are transcribed, including five organ-related expressions. Surprisingly, no significant transcriptional expression is monitored in leaves in response to cold stress (6 °C), although interesting trends can be distinguished and will be discussed, notably in relation to the plasticity of this pioneer species, B. pendula. The current study presents the first detailed genome-wide analysis of the AQP gene family in a Betulaceae species, and our results lay a foundation for a better understanding of the specific functions of the BpeAQP genes in the responses of the silver birch trees to cold stress.

Adaptable and Multifunctional Ion-Conducting Aquaporins

Aquaporins function as water and neutral solute channels, signaling hubs, disease virulence factors, and metabolon components. We consider plant aquaporins that transport ions compared to some animal counterparts. These are candidates for important, as yet unidentified, cation and anion channels in plasma, tonoplast, and symbiotic membranes. For those individual isoforms that transport ions, water, and gases, the permeability spans 12 orders of magnitude. This requires tight regulation of selectivity via protein interactions and posttranslational modifications. A phosphorylation-dependent switch between ion and water permeation in AtPIP2;1 might be explained by coupling between the gates of the four monomer water channels and the central pore of the tetramer. We consider the potential for coupling between ion and water fluxes that could form the basis of an electroosmotic transducer. A grand challenge in understanding the roles of ion transporting aquaporins is their multifunctional modes that are dependent on location, stress, time, and development.

Interrelations of nutrient and water transporters in plants under abiotic stress

Environmental changes cause abiotic stress in plants, primarily through alterations in the uptake of the nutrients and water they require for their metabolism and growth and to maintain their cellular homeostasis. The plasma membranes of cells contain transporter proteins, encoded by their specific genes, responsible for the uptake of nutrients and water (aquaporins). However, their interregulation has rarely been taken into account. Therefore, in this review we identify how the plant genome responds to abiotic stresses such as nutrient deficiency, drought, salinity and low temperature, in relation to both nutrient transporters and aquaporins. Some general responses or regulation mechanisms can be observed under each abiotic stress such as the induction of plasma membrane transporter expression during macronutrient deficiency, the induction of tonoplast transporters and reduction of aquaporins during micronutrients deficiency. However, drought, salinity and low temperatures generally cause an increase in expression of nutrient transporters and aquaporins in tolerant plants. We propose that both types of transporters (nutrients and water) should be considered jointly in order to better understand plant tolerance of stresses.

Nitrate Uptake and Transport Properties of Two Grapevine Rootstocks With Varying Vigor

In viticulture, rootstocks are essential to cope with edaphic constraints. They can also be used to modulate scion growth and development to help improve berry yield and quality. The rootstock contribution to scion growth is not fully understood. Since nitrogen (N) is a significant driver of grapevine growth, rootstock properties associated with N uptake and transport may play a key role in the growth potential of grafted grapevines. We evaluated N uptake and transport in a potted system using two grapevines rootstocks [Riparia Gloire (RG) and 1103 Paulsen (1103P)] grafted to Pinot noir (Pommard clone) scion. Combining results of nitrate induction and steady-state experiments at two N availability levels, we observed different responses in the uptake and utilization of N between the two rootstocks. The low vigor rootstock (RG) exhibited greater nitrate uptake capacity and nitrate assimilation in roots after nitrate resupply than the more vigorous 1103P rootstock. This behavior may be attributed to a greater root carbohydrate status observed in RG for both experiments. However, 1103P demonstrated a higher N translocation rate to shoots regardless of N availability. These distinct rootstock behaviors resulted in significant differences in biomass allocation between roots and shoots under N-limited conditions, although the overall vine biomass was not different. Under sufficient N supply, differences between rootstocks decreased but 1103P stored more N in roots, which may benefit growth in subsequent growing seasons. Overall, greater transpiration of vines grafted to 1103P rootstock causing higher N translocation to shoots could partially explain its known growth-promoting effect to scions under low and high N availability, whereas the low vigor typically conferred to scions by RG may result from the combination of lower N translocation to shoots and a greater allocation of biomass toward roots when N is low.

Plasma membrane vesicles from cauliflower meristematic tissue and their role in water passage

BACKGROUND

Cauliflower (Brassica oleracea L. var. botrytis) inflorescences are composed mainly of meristematic tissue, which has a high cellular proliferation. This considerable cellular density makes the inflorescence an organ with a large proportion of membranes. However, little is known about the specific role of the lipid and protein composition of the plasma membrane present in this organ.

RESULTS

In this work, we analyzed the lipids and proteins present in plasma membrane from two different stages of development of cauliflower inflorescence and compared them with leaf plasma membrane. For this purpose, plasma membrane vesicles were obtained by centrifugation for each sample and the vesicular diameter and osmotic permeability (Pf) were analyzed by dynamic light scattering and the stopped-flow technique, respectively. In addition, fatty acids and sterols were analyzed by gas chromatography and HPLC. The protein composition of the inflorescences and leaves was characterized by HPLC-ESI-QTOF-MS and the data obtained were compared with Brassicaceae proteins present in the UniProt database in relation to the presence of aquaporins determined by western blot analysis. The highest Pf value was found in 90 day inflorescences-derived plasma membrane vesicles (61.4 ± 4.14 μms- 1). For sterols and fatty acids, the concentrations varied according to the organ of origin. The protein profile revealed the presence of aquaporins from the PIP1 and PIP2 subfamilies in both inflorescences and leaves.

CONCLUSION

This study shows that the composition of the sterols, the degree of unsaturation of the fatty acids, and the proteins present in the membranes analyzed give them high functionality for water passage. This represents an important addition to the limited information available in this field.

Genome-wide analysis of the aquaporin genes in melon (Cucumis melo L.)

Melon (Cucumis melo L.) is a very important crop throughout the world and has great economic importance, in part due to its nutritional properties. It prefers well-drained soil with low acidity and has a strong demand for water during fruit set. Therefore, a correct water balance—involving aquaporins—is necessary to maintain the plants in optimal condition. This manuscript describes the identification and comparative analysis of the complete set of aquaporins in melon. 31 aquaporin genes were identified, classified and analysed according to the evolutionary relationship of melon with related plant species. The individual role of each aquaporin in the transport of water, ions and small molecules was discussed. Finally, qPCR revealed that almost all melon aquaporins in roots and leaves were constitutively expressed. However, the high variations in expression among them point to different roles in water and solute transport, providing important features as that CmPIP1;1 is the predominant isoform and CmTIP1;1 is revealed as the most important osmoregulator in the tonoplast under optimal conditions. The results of this work pointing to the physiological importance of each individual aquaporin of melon opening a field of knowledge that deserves to be investigated.

Apoplastic barriers, aquaporin gene expression and root and cell hydraulic conductivity in phosphate-limited sheepgrass plants

Mineral nutrient supply can affect the hydraulic property of roots. The aim of the present work on sheepgrass (Leymus chinensis L.) plants was to test whether any changes in root hydraulic conductivity (Lp; exudation analyses) in response to a growth-limiting supply of phosphate (P) are accompanied by changes in (1) cell Lp via measuring the cell pressure, (2) the aquaporin (AQP) gene expression by performing qPCR and (3) the formation of apoplastic barriers, by analyzing suberin lamella and Casparian bands via cross-sectional analyses in roots. Plants were grown hydroponically on complete nutrient solution, containing 250 µM P, until they were 31-36 days old, and then kept for 2-3 weeks on either complete solution, or transferred on solution containing 2.5 µM (low-P) or no added P (no-P). Phosphate treatments caused significant decreases in root and cell-Lp and AQP gene expression, while the formation of apoplastic barriers increased, particularly in lateral roots. Experiments using the AQP inhibitor mercury (Hg) suggested that a significant portion of radial root water uptake in sheepgrass occurs along a path involving AQPs, and that the Lp of this path is reduced under low- and no-P. It is concluded that a growth-limiting supply of phosphate causes parallel changes in (1) cell Lp and aquaporin gene expression (decrease) and (2) apoplastic barrier formation (increase), and that the two may combine to reduce root Lp. The reduction in root Lp, in turn, facilitates an increased root-to-shoot surface area ratio, which allocates resources to the root, sourcing the limiting nutrient.

Cortex cell hydraulic conductivity, endodermal apoplastic barriers and root hydraulics change in barley (Hordeum vulgare L.) in response to a low supply of N and P

BACKGROUND

Mineral nutrient limitation affects the water flow through plants. We wanted to test on barley whether any change in root-to-shoot ratio in response to low supply of nitrogen and phosphate is accompanied by changes in root and cell hydraulic properties and involves changes in aquaporin (AQP) gene expression and root apoplastic barriers (suberin lamellae, Casparian bands).

METHODS

Plants were grown hydroponically on complete nutrient solution or on solution containing only 3.3 % or 2.5 % of the control level of nutrient. Plants were analysed when they were 14-18 d old.

RESULTS

Nutrient-limited plants adjusted water flow to an increased root-to-shoot surface area ratio through a reduction in root hydraulic conductivity (Lp) as determined through exudation analyses. Cortex cell Lp (cell pressure probe analyses) decreased in the immature but not the mature region of the main axis of seminal roots and in primary lateral roots. The aquaporin inhibitor HgCl2 reduced root Lp most in nutrient-sufficient control plants. Exchange of low-nutrient for control media caused a rapid (20-80 min) and partial recovery in Lp, though cortex cell Lp did not increase in any of the root regions analysed. The gene expression level (qPCR analyses) of five plasma membrane-localized AQP isoforms did not change in bulk root extracts, while the formation of apoplastic barriers increased considerably along the main axis of root and lateral roots in low-nutrient treatments.

CONCLUSIONS

Decrease in root and cortex cell Lp enables the adjustment of root water uptake to increased root-to-shoot area ratio in nutrient-limited plants. Aquaporins are the prime candidate to play a key role in this response. Modelling of water flow suggests that some of the reduction in root Lp is due to increased formation of apoplastic barriers.

Root and cell hydraulic conductivity, apoplastic barriers and aquaporin gene expression in barley (Hordeum vulgare L.) grown with low supply of potassium

BACKGROUND AND AIMS

Limited supply of mineral nutrients often reduces plant growth and transpirational water flow while increasing the ratio of water-absorbing root to water-losing shoot surface. This could potentially lead to an imbalance between water uptake (too much) and water loss (too little). The aim of the present study was to test whether, as a countermeasure, the hydraulic properties (hydraulic conductivity, Lp) of roots decrease at organ and cell level and whether any decreases in Lp are accompanied by decreases in the gene expression level of aquaporins (AQPs) or increases in apoplastic barriers to radial water movement.

METHODS

Barley plants were grown hydroponically on complete nutrient solution, containing 2 mm K+ (100 %), or on low-K solution (0.05 mm K+; 2.5 %), and analysed when they were 15-18 d old. Transpiration, fresh weight, surface area, shoot water potential (ψ), K and Ca concentrations, root (exudation) and cortex cell Lp (cell pressure probe), root anatomy (cross-sections) and AQP gene expression (qPCR) were analysed.

KEY RESULTS

The surface area ratio of root to shoot increased significantly in response to low K. This was accompanied by a small decrease in the rate of water loss per unit shoot surface area, but a large (~50 %) and significant decrease in Lp at root and cortex cell levels. Aquaporin gene expression in roots did not change significantly, due to some considerable batch-to-batch variation in expression response, though HvPIP2;5 expression decreased on average by almost 50 %. Apoplastic barriers in the endodermis did not increase in response to low K.

CONCLUSIONS

Barley plants that are exposed to low K adjust to an increased ratio of root (water uptake) to shoot (water loss) surface primarily through a decrease in root and cell Lp. Reduced gene expression of HvPIP2;5 may contribute to the decrease in Lp.

Is Nitrogen a Key Determinant of Water Transport and Photosynthesis in Higher Plants Upon Drought Stress?

Drought stress is a major global issue limiting agricultural productivity. Plants respond to drought stress through a series of physiological, cellular, and molecular changes for survival. The regulation of water transport and photosynthesis play crucial roles in improving plants' drought tolerance. Nitrogen (N, ammonium and nitrate) is an essential macronutrient for plants, and it can affect many aspects of plant growth and metabolic pathways, including water relations and photosynthesis. This review focuses on how drought stress affects water transport and photosynthesis, including the regulation of hydraulic conductance, aquaporin expression, and photosynthesis. It also discusses the cross talk between N, water transport, and drought stress in higher plants.