The grapevine root-specific aquaporin VvPIP2;4N controls root hydraulic conductance and leaf gas exchange under well-watered conditions but not under water stress

Real-Time Dynamics of Water Transport in the Roots of Intact Maize Plants in Response to Water Stress: The Role of Aquaporins and the Contribution of Different Water Transport Pathways

Using an original methodological and technical approach, we studied the real-time dynamics of radial water transfer in roots and transpiration rate in intact maize plants in response to water stress. It was shown that the response of maize plants to water stress, induced by 10% PEG 6000, was accompanied by changes in the intensity and redistribution of water transfer along different pathways of radial water transport in the roots. It was shown that during the first minutes of water stress impact, the intensity of transcellular and symplastic water transport in the roots decreased with a parallel short-term increase in the transpiration rate in leaves and, presumably, in apoplastic transport in roots. Further, after a decrease in transpiration rate, the intensity of transcellular and symplastic water transport was restored to approximately the initial values and was accompanied by parallel upregulation of some PIP aquaporin genes in roots and leaves, changes in aquaporin localization in root tissues, and changes in xylem sap pH. Under water stress conditions, cell-to-cell water transport in roots becomes dominant, and aquaporins contribute to the simultaneous regulation of water transport in roots and shoots under water stress.

Biochemical Response and Gene Expression to Water Deficit of Croatian Grapevine Cultivars (Vitis vinifera L.) and a Specimen of Vitis sylvestris

The biochemical response and gene expression in different grapevine cultivars to water deficit are still not well understood. In this study, we investigated the performance of four traditional Croatian Vitis vinifera L. cultivars ('Plavac mali crni', 'Istrian Malvasia', 'Graševina', and 'Tribidrag'), and one wild (Vitis vinifera subsp. sylvestris) genotype exposed to water deficit (WD) for nine days under semi-controlled conditions in the greenhouse. Sampling for biochemical and gene expression analyses was performed at days six and nine from the beginning of WD treatment. The WD affected the accumulation of metabolites with a significant increase in abscisic acid (ABA), salicylic acid (SA), and proline in the leaves of the stressed genotypes when the WD continued for nine days. Lipid peroxidation (MDA) was not significantly different from that of the control plants after six days of WD, whereas it was significantly lower (297.40 nmol/g dw) in the stressed plants after nine days. The cultivar 'Istrian Malvasia' responded rapidly to the WD and showed the highest and earliest increase in ABA levels (1.16 ng mg-1 dw, i.e., 3.4-fold increase compared to control). 'Graševina' differed significantly from the other genotypes in SA content at both time points analyzed (six and nine days, 47.26 and 49.63 ng mg-1 dw, respectively). Proline level increased significantly under WD (up to 5-fold at day nine), and proline variation was not genotype driven. The expression of aquaporin genes (TIP2;1 and PIP2;1) was down-regulated in all genotypes, coinciding with the accumulation of ABA. The gene NCED1 (9-cis-epoxycarotenoid dioxygenase) related to ABA was up-regulated in all genotypes under stress conditions and served as a reliable marker of drought stress. This work suggests that the stress response in metabolite synthesis and accumulation is complex, treatment- and genotype-dependent.

The Rootstock Genotypes Determine Drought Tolerance by Regulating Aquaporin Expression at the Transcript Level and Phytohormone Balance

Grapevine rootstocks may supply water to the scion according to the transpiration demand, thus modulating plant responses to water deficit, but the scion variety can alter these responses, as well. The rootstock genotypes' effect on the scion physiological response, aquaporin expression, and hormone concentrations in the xylem and the leaf was assessed under well watered (WW) and water stress (WS) conditions. Under WW, vines grafted onto 1103P and R110 rootstocks (the more vigorous and drought-tolerant) showed higher photosynthesis (A_N), stomatal conductance (g_s), and hydraulic conductance (Kh_plant) compared with the less vigorous and drought-sensitive rootstock (161-49C), while under WS, there were hardly any differences between vines depending on the rootstock grafted. Besides, stomatal traits were affected by drought, which was related to g_s, but not by the rootstock. Under WS conditions, all VvPIP and VvTIP aquaporins were up-regulated in the vines grafted onto 1103P and down-regulated in the ones grafted onto 161-49C. The 1103P capability to tolerate drought was enhanced by the up-regulation of all VvPIP and VvTIP aquaporins, lower ABA synthesis, and higher ACC/ABA ratios in leaves during WS compared with 161-49C. It was concluded that, under WW conditions, transpiration and stomatal control were rootstock-dependent. However, under WS conditions, alterations in the molecular components of water transport and hormone concentration of the scion resulted in similar gas exchange values in the studied scions grafted onto different rootstocks.

Root pruning improves maize water-use efficiency by root water absorption

Root systems are an important component of plants that impact crop water-use efficiency (WUE) and yield. This study examined the effects of root pruning on maize yield, WUE, and water uptake under pot and hydroponic conditions. The pot experiment showed that root pruning significantly decreased root/shoot ratio. Both small root pruning (cut off about 1/5 of the root system, RP1) and large root pruning (cut off about 1/3 of the root system, RP2) improved WUE and root hydraulic conductivity (Lpr) in the residual root system. Compared with that in the un-cut control, at the jointing stage, RP1 and RP2 increased Lpr by 43.9% and 31.5% under well-watered conditions and 27.4% and 19.8% under drought stress, respectively. RP1 increased grain yield by 12.9% compared with that in the control under well-watered conditions, whereas both pruning treatments did not exhibit a significant effect on yield under drought stress. The hydroponic experiment demonstrated that root pruning did not reduce leaf water potential but increased residual root hydraulic conductivity by 26.2% at 48 h after root pruning under well-watered conditions. The foregoing responses may be explained by the upregulation of plasma membrane intrinsic protein gene and increases in abscisic acid and jasmonic acid in roots. Increased auxin and salicylic acid contributed to the compensated lateral root growth. In conclusion, root pruning improved WUE in maize by root water uptake.

Stomatal conductance tracks soil-to-leaf hydraulic conductance in faba bean and maize during soil drying

Although regulation of stomatal conductance is widely assumed to be the most important plant response to soil drying, the picture is incomplete when hydraulic conductance from soil to the leaf, upstream of the stomata, is not considered. Here, we investigated to what extent soil drying reduces the conductance between soil and leaf, whether this reduction differs between species, how it affects stomatal regulation, and where in the hydraulic pathway it occurs. To this end, we noninvasively and continuously measured the total root water uptake rate, soil water potential, leaf water potential, and stomatal conductance of 4-week-old, pot-grown maize (Zea mays) and faba bean (Vicia faba) plants during 4 days of water restriction. In both species, the soil-plant conductance, excluding stomatal conductance, declined exponentially with soil drying and was reduced to 50% above a soil water potential of -0.1 MPa, which is far from the permanent wilting point. This loss of conductance has immediate consequences for leaf water potential and the associated stomatal regulation. Both stomatal conductance and soil-plant conductance declined at a higher rate in faba bean than in maize. Estimations of the water potential at the root surface and an incomplete recovery 22 h after rewatering indicate that the loss of conductance, at least partly, occurred inside the plants, for example, through root suberization or altered aquaporin gene expression. Our findings suggest that differences in the stomatal sensitivity among plant species are partly explained by the sensitivity of root hydraulic conductance to soil drying.

New Technologies and Strategies for Grapevine Breeding Through Genetic Transformation

Grapevine, as other woody perennials, has been considered a recalcitrant crop to produce transgenic plants. Since the production of transgenic and/or edited plants requires the ability to regenerate plants from transformed tissues, this step is often the biggest bottleneck in the process. The objective of this work is to review the state of the art technologies and strategies for the improvement of grapevine transformation and regeneration, focusing on three aspects: (i) problems associated with grapevine transformation; (ii) genes that promote grapevine regeneration; and (iii) vehicles for gene delivery. Concerning the first aspect, it is well documented that one of the main factors explaining the low success rate in obtaining transgenic plants is the regeneration process. After transgenic integration into receptor cells, tissue culture is required to regenerate transgenic seedlings from transformed cells. This process is time consuming and often requires the addition of environmentally damaging reagents (antibiotics and herbicides) to the culture medium to select transgenic plants. On the other hand, the expression of genes such as the so-called developmental regulators (DR), which induce specific development programs, can be used to avoid traditional tissue culture methods. The ectopic expression of specific combinations of DR in somatic cells has the potential to induce de novo meristems in diverse crops, including grapevine. Successful genome editing by de novo reprogramming of plant meristems in somatic tissues has been reported. Moreover, it has been shown that the expression of certain transcription factors can increase the regeneration efficiency in wheat, citrus, and rice. Finally, recent reports showed the use of nanoparticles, such as carbon dots (CDs), as an attractive alternative to Agrobacterium- and biolistic-mediated plant genetic transformation. In this way, the use of antibiotics in culture media is avoided, overcoming the loss of viability of plant tissues and accelerating the regeneration processes. It has been shown that CDs can act as a vehicle to transport plasmids to plant cells in transient transformation in several crops without negative impacts on photosynthesis or growth. Based on these advances, it is possible to combine these new available strategies and technologies to overcome the regeneration problems of species such as grapevine and other crops considered as recalcitrant.

Aquaporins in regulation of plant protective responses to drought

Plasmolemma permeability is an integral indicator of the functional state of plant cells under stress. Aquaporins (AQPs), specialized transmembrane proteins that form water channels and play an important role in the adaptation of plants to adverse conditions and, in particular, to lack or excess of water, are involved in the formation of the response to drought. The main function of AQPs is to facilitate the movement of water across cell membranes and maintain aqueous cell homeostasis. Under stressful conditions, there is both an increase and decrease in the expression of individual aquaporin genes. Analysis of the data revealed differences in the expression of AQPs genes in stable and sensitive plant genotypes. It turned out that aquaporins in different stress-resistant varieties of the same species also respond differently to drought. The review provides brief information on the history of the discovery of aquaporins, the structure and function of these proteins, summarizes the latest information on the role of aquaporins in the regulation of metabolism and the response of plants to stressors, with particular emphasis on aquaporins in drought protection. The discovery and study of AQPs expands the possibilities of using genetic engineering methods for the selection of new plant species, in particular, more resistant to drought and salinization of the soil, as well as to increase their productivity. The use of aquaporins in biotechnology to improve drought resistance of various species has many prospects.

Grapevine aquaporins: Diversity, cellular functions, and ecophysiological perspectives

High-scored premium wines are typically produced under moderate drought stress, suggesting that the water status of grapevine is crucial for wine quality. Aquaporins greatly influence the plant water status by facilitating water diffusion across the plasma membrane in a tightly regulated manner. They adjust the hydraulic conductance of the plasma membrane rapidly and reversibly, which is essential in specific physiological events, including adaptation to soil water scarcity. The comprehension of the sophisticated plant-water relations at the molecular level are thus important to optimize agricultural practices or to assist plant breeding programs. This review explores the recent progresses in understanding the water transport in grapevine at the cellular level through aquaporins and its regulation. Important aspects, including aquaporin structure, diversity, cellular localization, transport properties, and regulation at the cellular and whole plant level are addressed. An ecophysiological perspective about the roles of grapevine aquaporins in plant response to drought stress is also provided.

Poplar aquaporin PIP1;1 promotes Arabidopsis growth and development

BACKGROUND

Root hydraulic conductance is primarily determined by the conductance of living tissues to radial water flow. Plasma membrane intrinsic proteins (PIPs) in root cortical cells are important for plants to take up water and are believed to be directly involved in cell growth.

RESULTS

In this study, we found that constitutive overexpression of the poplar root-specific gene PtoPIP1;1 in Arabidopsis accelerated bolting and flowering. At the early stage of the developmental process, PtoPIP1;1 OE Arabidopsis exhibited faster cell growth in both leaves and roots. The turgor pressure of plants was correspondingly increased in PtoPIP1;1 OE Arabidopsis, and the water status was changed. At the same time, the expression levels of flowering-related genes (CRY1, CRY2 and FCA) and hub genes in the regulatory networks underlying floral timing (FT and SOC1) were significantly upregulated in OE plants, while the floral repressor FLC gene was significantly downregulated.

CONCLUSIONS

Taken together, the results of our study indicate that constitutive overexpression of PtoPIP1;1 in Arabidopsis accelerates bolting and flowering through faster cell growth in both the leaf and root at an early stage of the developmental process. The autonomous pathway of flowering regulation may be executed by monitoring developmental age. The increase in turgor and changes in water status with PtoPIP1;1 overexpression play a role in promoting cell growth.

Plant aquaporins alleviate drought tolerance in plants by modulating cellular biochemistry, root-architecture, and photosynthesis

Water is a vital resource for plants to grow, thrive, and complete their life cycle. In recent years, drastic changes in the climate, especially drought frequency and severity, have increased, which reduces agricultural productivity worldwide. Aquaporins are membrane channels belonging to the major intrinsic protein superfamily, which play an essential role in cellular water and osmotic homeostasis of plants under both control and water deficit conditions. A genome-wide search reveals the vast availability of aquaporin isoforms, phylogenetic relationships, different families, conserved residues, chromosomal locations, and gene structure of aquaporins. Furthermore, aquaporins gating and subcellular trafficking are commonly controlled by phosphorylation, cytosolic pH, divalent cations, reactive oxygen species, and stoichiometry. Researchers have identified their involvement in regulating hydraulic conductance, root system architecture, modulation of abiotic stress-related genes, seed viability and germination, phloem loading, xylem water exit, photosynthetic parameters, and post-drought recovery. Remarkable effects following the change in aquaporin activity and/or gene expression have been observed on root water transport properties, nutrient acquisition, physiology, transpiration, stomatal aperture, gas exchange, and water use efficiency. The present review highlights the role of different aquaporin homologs under water-deficit stress condition in model and crop plants. Moreover, the opportunity and challenges encountered to explore aquaporins for engineering drought-tolerant crop plants are also discussed here.

Molecular Tools for Adapting Viticulture to Climate Change

Adaptation of viticulture to climate change includes exploration of new geographical areas, new training systems, new management practices, or new varieties, both for rootstocks and scions. Molecular tools can be defined as molecular approaches used to study DNAs, RNAs, and proteins in all living organisms. We present here the current knowledge about molecular tools and their potential usefulness in three aspects of grapevine adaptation to the ongoing climate change. (i) Molecular tools for understanding grapevine response to environmental stresses. A fine description of the regulation of gene expression is a powerful tool to understand the physiological mechanisms set up by the grapevine to respond to abiotic stress such as high temperatures or drought. The current knowledge on gene expression is continuously evolving with increasing evidence of the role of alternative splicing, small RNAs, long non-coding RNAs, DNA methylation, or chromatin activity. (ii) Genetics and genomics of grapevine stress tolerance. The description of the grapevine genome is more and more precise. The genetic variations among genotypes are now revealed with new technologies with the sequencing of very long DNA molecules. High throughput technologies for DNA sequencing also allow now the genetic characterization at the same time of hundreds of genotypes for thousands of points in the genome, which provides unprecedented datasets for genotype-phenotype associations studies. We review the current knowledge on the genetic determinism of traits for the adaptation to climate change. We focus on quantitative trait loci and molecular markers available for developmental stages, tolerance to water stress/water use efficiency, sugar content, acidity, and secondary metabolism of the berries. (iii) Controlling the genome and its expression to allow breeding of better-adapted genotypes. High-density DNA genotyping can be used to select genotypes with specific interesting alleles but genomic selection is also a powerful method able to take into account the genetic information along the whole genome to predict a phenotype. Modern technologies are also able to generate mutations that are possibly interesting for generating new phenotypes but the most promising one is the direct editing of the genome at a precise location.

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.

Candidate genes and SNPs associated with stomatal conductance under drought stress in Vitis

BACKGROUND

Understanding the complexity of the vine plant's response to water deficit represents a major challenge for sustainable winegrowing. Regulation of water use requires a coordinated action between scions and rootstocks on which cultivars are generally grafted to cope with phylloxera infestations. In this regard, a genome-wide association study (GWAS) approach was applied on an 'ad hoc' association mapping panel including different Vitis species, in order to dissect the genetic basis of transpiration-related traits and to identify genomic regions of grape rootstocks associated with drought tolerance mechanisms. The panel was genotyped with the GrapeReSeq Illumina 20 K SNP array and SSR markers, and infrared thermography was applied to estimate stomatal conductance values during progressive water deficit.

RESULTS

In the association panel the level of genetic diversity was substantially lower for SNPs loci (0.32) than for SSR (0.87). GWAS detected 24 significant marker-trait associations along the various stages of drought-stress experiment and 13 candidate genes with a feasible role in drought response were identified. Gene expression analysis proved that three of these genes (VIT_13s0019g03040, VIT_17s0000g08960, VIT_18s0001g15390) were actually induced by drought stress. Genetic variation of VIT_17s0000g08960 coding for a raffinose synthase was further investigated by resequencing the gene of 85 individuals since a SNP located in the region (chr17_10,497,222_C_T) was significantly associated with stomatal conductance.

CONCLUSIONS

Our results represent a step forward towards the dissection of genetic basis that modulate the response to water deprivation in grape rootstocks. The knowledge derived from this study may be useful to exploit genotypic and phenotypic diversity in practical applications and to assist further investigations.

Phosphorylation of plasma membrane aquaporin PIP2;1 in C-terminal affects light-induced stomatal opening in Arabidopsis

Guard cells undergo quick volume changes during stomatal movements. However, the contribution of aquaporins to stomatal movements has not been well understood. The plasma membrane aquaporin PIP2;1in Arabidopsis has been found to mediate abscisic acid-induced or flag22-induced stomatal closure. In this research, we investigated the role of PIP2;1 in light-induced stomatal opening by measuring the stomatal apertures of the pip2;1 mutant and PIP2;1 overexpression lines after light treatment. pip2;1 mutant exhibited a larger stomatal aperture, and the overexpression lines displayed a smaller stomatal aperture. It has been reported that the phosphorylation at Ser-280 and Ser-283 of PIP2;1 in rosette tissue increased in response to darkness, whereas osmotic water permeability (P_f) in mesophyll protoplasts in darkness was lower than that under light, suggesting that phosphorylation at Ser-280 and Ser-283 of PIP2;1 affected P_f in mesophyll protoplasts. Therefore, we obtained the pip2;1 mutant expressing phosphorylation-deficient (PIP2;1 AA) or phosphomimetic (PIP2;1 DD) forms of PIP2;1. The PIP2;1 AA lines exhibited a larger stomatal aperture as pip2;1 mutant, whereas PIP2;1 DD lines exhibited a smaller stomatal aperture as PIP2;1 overexpression lines under light. These results suggest that PIP2;1 plays a negative role in light-induced stomatal opening, and phosphorylation of PIP2;1 at Ser-280 and Ser-283 causes reduced water absorption in guard cells and decreased stomatal opening.

Physiological and PIP Transcriptional Responses to Progressive Soil Water Deficit in Three Mulberry Cultivars

Although mulberry cultivars Wubu, Yu711, and 7307 display distinct anatomical, morphological, and agronomic characteristics under natural conditions, it remains unclear if they differ in drought tolerance. To address this question and elucidate the underlying regulatory mechanisms at the whole-plant level, 2-month old saplings of the three mulberry cultivars were exposed to progressive soil water deficit for 5 days. The physiological responses and transcriptional changes of PIPs in different plant tissues were analyzed. Drought stress led to reduced leaf relative water content (RWC) and tissue water contents, differentially expressed PIPs, decreased chlorophyll and starch, increased soluble sugars and free proline, and enhanced activities of antioxidant enzymes in all plant parts of the three cultivars. Concentrations of hydrogen peroxide (H2O2), superoxide anion (O2 •-), and malonaldehyde (MDA) were significantly declined in roots, stimulated in leaves but unaltered in wood and bark. In contrast, except the roots of 7307, soluble proteins were repressed in roots and leaves but induced in wood and bark of the three cultivars in response to progressive water deficit. These results revealed tissue-specific drought stress responses in mulberry. Comparing to cultivar Yu711 and 7307, Wubu showed generally slighter changes in leaf RWC and tissue water contents at day 2, corresponding well to the steady PIP transcript levels, foliar concentrations of chlorophyll, O2 •-, MDA, and free proline. At day 5, Wubu sustained higher tissue water contents in green tissues, displayed stronger responsiveness of PIP transcription, lower concentrations of soluble sugars and starch, lower foliar MDA, higher proline and soluble proteins, higher ROS accumulation and enhanced activities of several antioxidant enzymes. Our results indicate that whole-plant level responses of PIP transcription, osmoregulation through proline and soluble proteins and antioxidative protection are important mechanisms for mulberry to cope with drought stress. These traits play significant roles in conferring the relatively higher drought tolerance of cultivar Wubu and could be potentially useful for future mulberry improvement programmes.

CO2 enrichment enhanced drought resistance by regulating growth, hydraulic conductivity and phytohormone contents in the root of cucumber seedlings

The coordinated effects of CO2 enrichment and drought stress on cucumber leaves have attracted increasing research attention, but few studies have investigated the effects of CO2 enrichment on the root system under drought stress. So we analyzed the morphological parameters, hydraulic conductivity, aquaporin-related gene expression, and endogenous phytohormone contents in roots of cucumber seedlings cultured under different CO2 concentrations (approximately 400 and 800 ± 40 μmol mol-1) and drought stresses simulated by polyethylene glycol 6000 (0%, 5%, and 10%). The results showed that under drought stress, regardless of the CO2 concentration, the root biomass and hydraulic conductivity decreased, the contents of auxin (IAA), zeatin nucleoside (ZR), and gibberellin (GA) decreased, the abscisic acid (ABA) content and the transcript levels of the aquaporin-related genes CsPIP2-4 increased, and the transcript levels of the aquaporin-related genes CsPIP2-5 and CsPIP2-7 decreased compared with no drought stress. Under moderate drought stress, CO2 enrichment decreased ABA content and the transcript level of CsPIP2-4, increased root biomass and GA content and the transcript level of CsPIP2-7, improved contribution rate of cell-to-cell water transport (mediated by aquaporins) and roots hydraulic conductivity. In summary, drought stress changed the water transport capacity of the roots and inhibited the growth of cucumber seedlings. CO2 enrichment regulated phytohormone contents and aquaporin-related gene expression, maintained the normal contribution rate of cell-to-cell water transport, and improved the root biomass and hydraulic conductivity, thereby alleviated the negative effects of drought stress on cucumber seedlings.

Does Molecular and Structural Evolution Shape the Speedy Grass Stomata?

It has been increasingly important for breeding programs to be aimed at crops that are capable of coping with a changing climate, especially with regards to higher frequency and intensity of drought events. Grass stomatal complex has been proposed as an important factor that may enable grasses to adapt to water stress and variable climate conditions. There are many studies focusing on the stomatal morphology and development in the eudicot model plant Arabidopsis and monocot model plant Brachypodium. However, the comprehensive understanding of the distinction of stomatal structure and development between monocots and eudicots, especially between grasses and eudicots, are still less known at evolutionary and comparative genetic levels. Therefore, we employed the newly released version of the One Thousand Plant Transcriptome (OneKP) database and existing databases of green plant genome assemblies to explore the evolution of gene families that contributed to the formation of the unique structure and development of grass stomata. This review emphasizes the differential stomatal morphology, developmental mechanisms, and guard cell signaling in monocots and eudicots. We provide a summary of useful molecular evidences for the high water use efficiency of grass stomata that may offer new horizons for future success in breeding climate resilient crops.

Versatile roles of aquaporin in physiological processes and stress tolerance in plants

Aquaporins are pore-forming transmembrane proteins that facilitate the movement of water and many other small neutral solutes across the cells and intracellular compartments. Plants exhibits high diversity in aquaporin isoforms and broadly classified into five different subfamilies on the basis of phylogenetic distribution and subcellular occurrence: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin 26-like proteins (NIPs), small basic intrinsic proteins (SIPs) and uncharacterized intrinsic proteins (XIPs). The gating mechanism of aquaporin channels is tightly regulated by post-translational modifications such as phosphorylation, methylation, acetylation, glycosylation, and deamination. Aquaporin expression and transport functions are also modulated by the various phytohormones-mediated signalling in plants. Combined physiology and transcriptome analysis revealed the role of aquaporins in regulating hydraulic conductance in roots and leaves. The present review mainly focused on aquaporin functional activity during solute transport, plant development, abiotic stress response, and plant-microbe symbiosis. Genetically modified plants overexpressing aquaporin-encoding genes display improved agronomic and abiotic stress tolerance.

Responses of water accumulation and solute metabolism in tomato fruit to water scarcity and implications for main fruit quality variables

Fruit is important for human health, and applying deficit irrigation in fruit production is a strategy to regulate fruit quality and support environmental sustainability. Responses of different fruit quality variables to deficit irrigation have been widely documented, and much progress has been made in understanding the mechanisms of these responses. We review the effects of water shortage on fruit water accumulation considering water transport from the parent plant into the fruit determined by hydraulic properties of the pathway (including xylem water transport and transmembrane water transport regulated by aquaporins) and the driving force for water movement. We discuss water relations and solute metabolism that affect the main fruit quality variables (e.g. size, flavour, nutrition, and firmness) at the cellular level under water shortage. We also summarize the most recent advances in the understanding of responses of the main fruit quality variables to water shortage, considering the effects of variety, the severity of water deficit imposed, and the developmental stage of the fruit. We finally identify knowledge gaps and suggest avenues for future research. This review provides new insights into the stress physiology of fleshy fruit, which will be beneficial for the sustainable production of high-quality fruit under deficit irrigation.

Local root ABA/cytokinin status and aquaporins regulate poplar responses to mild drought stress independently of the ectomycorrhizal fungus Laccaria bicolor

The relatively better performance of mycorrhizal plants subjected to drought stress has commonly been linked to improved root water uptake through the fungal regulation of plant aquaporins and hormones. In this study, we examined the role of ectomycorrhizal fungi in plant water relations and plant hormonal balance under mild drought using split-root seedlings of Populus trichocarpa × deltoides either with or without inoculation with Laccaria bicolor. The root compartments where the drought treatment was applied had higher ABA and lower cytokinin tZR contents, and greater expression of the plant aquaporins PtPIP1;1, PtPIP1;2, PtPIP2;5, and PtPIP2;7. On the other hand, the presence of L. bicolor within the roots down-regulated PtPIP1;4, PtPIP2;3, and PtPIP2;10, and reduced the abundance of PIP2 proteins. In addition, expression of the fungal aquaporins JQ585595 and JQ585596 were positively correlated with root ABA content, while tZR content was positively correlated with PtPIP1;4 and negatively correlated with PtPIP2;7. The results demonstrate a coordinated plant-fungal system that regulates the different mechanisms involved in water uptake in ectomycorrhizal poplar plants.

Exploiting natural variation and genetic manipulation of stomatal conductance for crop improvement

Rising global temperatures and more frequent episodes of drought are expected to drive reductions in crop yield, therefore new avenues for improving crop productivity must be exploited. Stomatal conductance (gs) balances plant CO2 uptake and water loss, therefore, greatly impacting the cumulative rate of photosynthesis and water use over the growing season, which are key determinants of crop yield and productivity. Considerable natural variation exists in stomatal anatomy, biochemistry and behavioural characteristics that impact on the kinetics and magnitude of gs and thus gaseous exchange between the plant and atmosphere. Exploiting these differences in stomatal traits could provide novel breeding targets for new crop varieties that are potentially more water use efficient and have the ability to maintain and/or maximize yield in a range of diverse environments. Here we provide an overview of variation in stomatal traits and the impact these have on gs behaviour, as well as the potential to exploit such variation and genetic manipulation for crop improvement.

Rhizobial symbiosis modifies root hydraulic properties in bean plants under non-stressed and salinity-stressed conditions

Rhizobial symbiosis improved the water status of bean plants under salinity-stress conditions, in part by increasing their osmotic root water flow. One of the main problems for agriculture worldwide is the increasing salinization of farming lands. The use of soil beneficial microorganisms stands up as a way to tackle this problem. One approach is the use of rhizobial N₂-fixing, nodule-forming bacteria. Salinity-stress causes leaf dehydration due to an imbalance between water lost through stomata and water absorbed by roots. The aim of the present study was to elucidate how rhizobial symbiosis modulates the water status of bean (Phaseolus vulgaris) plants under salinity-stress conditions, by assessing the effects on root hydraulic properties. Bean plants were inoculated or not with a Rhizobium leguminosarum strain and subjected to moderate salinity-stress. The rhizobial symbiosis was found to improve leaf water status and root osmotic water flow under such conditions. Higher content of nitrogen and lower values of sodium concentration in root tissues were detected when compared to not inoculated plants. In addition, a drop in the osmotic potential of xylem sap and increased amount of PIP aquaporins could favour higher root osmotic water flow in the inoculated plants. Therefore, it was found that rhizobial symbiosis may also improve root osmotic water flow of the host plants under salinity stress.

Aquaporin regulation in roots controls plant hydraulic conductance, stomatal conductance, and leaf water potential in Pinus radiata under water stress

Stomatal regulation is crucial for forest species performance and survival on drought-prone sites. We investigated the regulation of root and shoot hydraulics in three Pinus radiata clones exposed to drought stress and its coordination with stomatal conductance (g_s ) and leaf water potential (Ψ_leaf ). All clones experienced a substantial decrease in root-specific root hydraulic conductance (K_root-r ) in response to the water stress, but leaf-specific shoot hydraulic conductance (K_shoot-l ) did not change in any of the clones. The reduction in K_root-r caused a decrease in leaf-specific whole-plant hydraulic conductance (K_plant-l ). Among clones, the larger the decrease in K_plant-l , the more stomata closed in response to drought. Rewatering resulted in a quick recovery of K_root-r and g_s . Our results demonstrated that the reduction in K_plant-l , attributed to a down regulation of aquaporin activity in roots, was linked to the isohydric stomatal behaviour, resulting in a nearly constant Ψ_leaf as water stress started. We concluded that higher K_plant-l is associated with water stress resistance by sustaining a less negative Ψ_leaf and delaying stomatal closure.

Aquaporins as a link between water relations and photosynthetic pathway in abiotic stress tolerance in plants

Plant aquaporins constitute a large family of proteins involved in facilitating the transport of water and small neutral molecules across biological membranes. In higher plants they are divided into several sub-families, depending on membrane-type localization and permeability to specific solutes. They are abundantly expressed in the majority of plant organs and tissues, and play a function in primary biological processes. Many studies revealed the significant role of aquaporins in acquiring abiotic stresses' tolerance. This review focuses on aquaporins belonging to PIPs sub-family that are permeable to water and/or carbon dioxide. Isoforms transporting water are involved in hydraulic conductance regulation in the leaves and roots, whereas those transporting carbon dioxide control stomatal and mesophyll conductance in the leaves. Changes in PIP aquaporins abundance/activity in stress conditions allow to maintain the water balance and photosynthesis adjustment. Broad analyses showed that tight control between water and carbon dioxide supplementation mediated by aquaporins influences plant productivity, especially in stress conditions. Involvement of aquaporins in adaptation strategies to dehydrative stresses in different plant species are discussed in this review.

Dissecting the rootstock control of scion transpiration using model-assisted analyses in grapevine

How rootstocks contribute to the control of scion transpiration under drought is poorly understood. We investigated the role of root characteristics, hydraulic conductance and chemical signals (abscisic acid, ABA) in the response of stomatal conductance (gs) and transpiration (E) to drought in Cabernet Sauvignon (Vitis vinifera) grafted onto drought-sensitive (Vitis riparia) and drought-tolerant (Vitis berlandieri × Vitis rupestris 110R) rootstocks. All combinations showed a concomitant reduction in gs and E, and an increase in xylem sap ABA concentration during the drought cycle. Cabernet Sauvignon grafted onto 110R exhibited higher gs and E under well-watered and moderate water deficit, but all combinations converged as water deficit increased. These results were integrated into three permutations of a whole-plant transpiration model that couples both chemical (i.e., ABA) and hydraulic signals in the modelling of stomatal control. Model comparisons revealed that both hydraulic and chemical signals were important for rootstock-specific stomatal regulation. Moreover, model parameter comparison and sensitivity analysis highlighted two major parameters differentiating the rootstocks: (i) ABA biosynthetic activity and (ii) the hydraulic conductance between the rhizosphere and soil-root interface determined by root system architecture. These differences in root architecture, specifically a higher root length area in 110R, likely explain its higher E and gs observed at low and moderate water deficit.

Induction of Terpene Biosynthesis in Berries of Microvine Transformed with VvDXS1 Alleles

Terpenoids, especially monoterpenes, are major aroma-impact compounds in grape and wine. Previous studies highlighted a key regulatory role for grapevine 1-deoxy-D-xylulose 5-phosphate synthase 1 (VvDXS1), the first enzyme of the methylerythritol phosphate pathway for isoprenoid precursor biosynthesis. Here, the parallel analysis of VvDXS1 genotype and terpene concentration in a germplasm collection demonstrated that VvDXS1 sequence has a very high predictive value for the accumulation of monoterpenes and also has an influence on sesquiterpene levels. A metabolic engineering approach was applied by expressing distinct VvDXS1 alleles in the grapevine model system "microvine" and assessing the effects on downstream pathways at transcriptional and metabolic level in different organs and fruit developmental stages. The underlying goal was to investigate two potential perturbation mechanisms, the former based on a significant over-expression of the wild-type (neutral) VvDXS1 allele and the latter on the ex-novo expression of an enzyme with increased catalytic efficiency from the mutated (muscat) VvDXS1 allele. The integration of the two VvDXS1 alleles in distinct microvine lines was found to alter the expression of several terpenoid biosynthetic genes, as assayed through an ad hoc developed TaqMan array based on cDNA libraries of four aromatic cultivars. In particular, enhanced transcription of monoterpene, sesquiterpene and carotenoid pathway genes was observed. The accumulation of monoterpenes in ripe berries was higher in the transformed microvines compared to control plants. This effect is predominantly attributed to the improved activity of the VvDXS1 enzyme coded by the muscat allele, whereas the up-regulation of VvDXS1 plays a secondary role in the increase of monoterpenes.

Regulation of aquaporins in plants under stress

Aquaporins (AQP) are channel proteins belonging to the Major Intrinsic Protein (MIP) superfamily that play an important role in plant water relations. The main role of aquaporins in plants is transport of water and other small neutral molecules across cellular biological membranes. AQPs have remarkable features to provide an efficient and often, specific water flow and enable them to transport water into and out of the cells along the water potential gradient. Plant AQPs are classified into five main subfamilies including the plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin 26 like intrinsic proteins (NIPs), small basic intrinsic proteins (SIPs) and X intrinsic proteins (XIPs). AQPs are localized in the cell membranes and are found in all living cells. However, most of the AQPs that have been described in plants are localized to the tonoplast and plasma membranes. Regulation of AQP activity and gene expression, are also considered as a part of the adaptation mechanisms to stress conditions and rely on complex processes and signaling pathways as well as complex transcriptional, translational and posttranscriptional factors. Gating of AQPs through different mechanisms, such as phosphorylation, tetramerization, pH, cations, reactive oxygen species, phytohormones and other chemical agents, may play a key role in plant responses to environmental stresses by maintaining the uptake and movement of water in the plant body.

Ectopically expressing MdPIP1;3, an aquaporin gene, increased fruit size and enhanced drought tolerance of transgenic tomatoes

BACKGROUND

Water deficit severely reduces apple growth and production, is detrimental to fruit quality and size. This problem is exacerbated as global warming is implicated in producing more severe drought stress. Thus water-efficiency has becomes the major target for apple breeding. A desired apple tree can absorb and transport water efficiently, which not only confers improved drought tolerance, but also guarantees fruit size for higher income returns. Aquaporins, as water channels, control water transportation across membranes and can regulate water flow by changing their amount and activity. The exploration of molecular mechanism of water efficiency and the gene wealth will pave a way for molecular breeding of drought tolerant apple tree.

RESULTS

In the current study, we screened out a drought inducible aquaporin gene MdPIP1;3, which specifically enhanced its expression during fruit expansion in 'Fuji' apple (Malus domestica Borkh. cv. Red Fuji). It localized on plasma membranes and belonged to PIP1 subfamily. The tolerance to drought stress enhanced in transgenic tomato plants ectopically expressing MdPIP1;3, showing that the rate of losing water in isolated transgenic leaves was slower than wild type, and stomata of transgenic plants closed sensitively to respond to drought compared with wild type. Besides, length and diameter of transgenic tomato fruits increased faster than wild type, and in final, fruit sizes and fresh weights of transgenic tomatoes were bigger than wild type. Specially, in cell levels, fruit cell size from transgenic tomatoes was larger than wild type, showing that cell number per mm2 in transgenic fruits was less than wild type.

CONCLUSIONS

Altogether, ectopically expressing MdPIP1;3 enhanced drought tolerance of transgenic tomatoes partially via reduced water loss controlled by stomata closure in leaves. In addition, the transgenic tomato fruits are larger and heavier with larger cells via more efficient water transportation across membranes. Our research will contribute to apple production, by engineering apples with big fruits via efficient water transportation when well watered and enhanced drought tolerance in transgenic apples under water deficit.

Breeding next generation tree fruits: technical and legal challenges

The new plant breeding technologies (NPBTs) have recently emerged as powerful tools in the context of 'green' biotechnologies. They have wide potential compared to classical genetic engineering and they are attracting the interest of politicians, stakeholders and citizens due to the revolutionary impact they may have on agriculture. Cisgenesis and genome editing potentially allow to obtain pathogen-resistant plants or plants with enhanced qualitative traits by introducing or disrupting specific genes in shorter times compared to traditional breeding programs and by means of minimal modifications in the plant genome. Grapevine, the most important fruit crop in the world from an economical point of view, is a peculiar case for NPBTs because of the load of cultural aspects, varietal traditions and consumer demands, which hinder the use of classical breeding techniques and, furthermore, the application of genetic engineering to wine grape cultivars. Here we explore the technical challenges which may hamper the application of cisgenesis and genome editing to this perennial plant, in particular focusing on the bottlenecks of the Agrobacterium-mediated gene transfer. In addition, strategies to eliminate undesired sequences from the genome and to choose proper target sites are discussed in light of peculiar features of this species. Furthermore is reported an update of the international legislative frameworks regulating NPBT products which shows conflicting positions and, in the case of the European Union, a prolonged lack of regulation.

Reductions in root hydraulic conductivity in response to clay soil and treated waste water are related to PIPs down-regulation in Citrus

Citrus hydraulic physiology and PIP transcript levels were characterized in heavy (clay) and light (sandy loam) soils with and without treated waste water (TWW) irrigation after a summer irrigation season and at the end of a winter rainy season recovery period. Consistent reductions in clay soils compared to sandy loam were found for fresh water (FW) and TWW irrigation, respectively, in root water uptake, as well as in hydraulic conductivity of whole plant (Ks plant), stem (Ks stem) and root (Ks root). Transcript levels of most PIPs down-regulated following TWW irrigation in both soils, but relative gene expression of three PIPs was significantly higher in summer for sandy soil and FW than for clay soil and TWW; their mRNA levels was significantly correlated to Ks root. A pot experiment, which compared short term influences of saline and TWW found that both treatments, compared to FW, reduced root water uptake and PIPs mRNA levels by 2-fold after 20 days, and the decreases continued with time until the end of the experiment. These latter data indicated that salinity had an important influence. Our results suggest that plant hydraulic adjustment to soil texture and water quality occurs rapidly, i.e. within days, and is modulated by PIPs expression.

A Comparison of Petiole Hydraulics and Aquaporin Expression in an Anisohydric and Isohydric Cultivar of Grapevine in Response to Water-Stress Induced Cavitation

We report physiological, anatomical and molecular differences in two economically important grapevine (Vitis vinifera L.) cultivars cv. Grenache (near-isohydric) and Chardonnay (anisohydric) in their response to water-stress induced cavitation. The aim of the study was to compare organ vulnerability (petiole and stem) to cavitation by measuring ultrasonic acoustic emissions (UAE) and percent loss of conductance of potted grapevines subject to the onset of water-stress. Leaf (ψL) and stem water potential (ψS), stomatal conductance (gs), transpiration (E), petiole hydraulics (KPet), and xylem diameter were also measured. Chardonnay displayed hydraulic segmentation based on UAE, with cavitation occurring at a less negative ψL in the petiole than in the stem. Vulnerability segmentation was not observed in Grenache, with both petioles and stems equally vulnerable to cavitation. Leaf water potential that induced 50% of maximum UAE was significantly different between petioles and stems in Chardonnay (ψ50Petiole = -1.14 and ψ50Stem = -2.24 MPa) but not in Grenache (ψ50Petiole = -0.73 and ψ50Stem = -0.78 MPa). Grenache stems appeared more susceptible to water-stress induced cavitation than Chardonnay stems. Grenache displayed (on average) a higher KPet likely due to the presence of larger xylem vessels. A close relationship between petiole hydraulic properties and vine water status was observed in Chardonnay but not in Grenache. Transcriptional analysis of aquaporins in the petioles and leaves (VvPIP1;1, VvPIP2;1, VvPIP2;2 VvPIP2;3, VvTIP1;1, and VvTIP2;1) showed differential regulation diurnally and in response to water-stress. VvPIP2;1 showed strong diurnal regulation in the petioles and leaves of both cultivars with expression highest predawn. Expression of VvPIP2;1 and VvPIP2;2 responded to ψL and ψS in both cultivars indicating the expression of these two genes are closely linked to vine water status. Expression of several aquaporin genes correlated with gas exchange measurements, however, these genes differed between cultivars. In summary, the data shows two contrasting responses in petiole hydraulics and aquaporin expression between the near-isohydric cultivar, Grenache and anisohydric cultivar, Chardonnay.

Abscisic Acid Down-Regulates Hydraulic Conductance of Grapevine Leaves in Isohydric Genotypes Only

Plants evolved different strategies to cope with water stress. While isohydric species maintain their midday leaf water potential (ΨM) under soil water deficit by closing their stomata, anisohydric species maintain higher stomatal aperture and exhibit substantial reductions in ΨM It was hypothesized that isohydry is related to a locally higher sensitivity of stomata to the drought-hormone abscisic acid (ABA). Interestingly, recent lines of evidence in Arabidopsis (Arabidopsis thaliana) suggested that stomatal responsiveness is also controlled by an ABA action on leaf water supply upstream from stomata. Here, we tested the possibility in grapevine (Vitis vinifera) that different genotypes ranging from near isohydric to more anisohydric may have different sensitivities in these ABA responses. Measurements on whole plants in drought conditions were combined with assays on detached leaves fed with ABA. Two different methods consistently showed that leaf hydraulic conductance (Kleaf) was down-regulated by exogenous ABA, with strong variations depending on the genotype. Importantly, variation between isohydry and anisohydry correlated with Kleaf sensitivity to ABA, with Kleaf in the most anisohydric genotypes being unresponsive to the hormone. We propose that the observed response of Kleaf to ABA may be part of the overall ABA regulation of leaf water status.

Source-sink interaction: a century old concept under the light of modern molecular systems biology

Many approaches to engineer source strength have been proposed to enhance crop yield potential. However, a well-co-ordinated source-sink relationship is required finally to realize the promised increase in crop yield potential in the farmer's field. Source-sink interaction has been intensively studied for decades, and a vast amount of knowledge about the interaction in different crops and under different environments has been accumulated. In this review, we first introduce the basic concepts of source, sink and their interactions, then summarize current understanding of how source and sink can be manipulated through both environmental control and genetic manipulations. We show that the source-sink interaction underlies the diverse responses of crops to the same perturbations and argue that development of a molecular systems model of source-sink interaction is required towards a rational manipulation of the source-sink relationship for increased yield. We finally discuss both bottom-up and top-down routes to develop such a model and emphasize that a community effort is needed for development of this model.

Heterologous Expression of Panax ginseng PgTIP1 Confers Enhanced Salt Tolerance of Soybean Cotyledon Hairy Roots, Composite, and Whole Plants

The Panax ginseng TIP gene PgTIP1 was previously demonstrated to have high water channel activity by its heterologous expression in Xenopus laevis oocytes and in yeast; it also plays a significant role in growth of PgTIP1-transgenic Arabidopsis plants under favorable conditions and has enhanced tolerance toward salt and drought treatment. In this work, we first investigated the physiological effects of heterologous PgTIP1 expression in soybean cotyledon hairy roots or composite plants mediated by Agrobacterium rhizogenes toward enhanced salt tolerance. The PgTIP1-transgenic soybean plants mediated by the pollen tube pathway, represented by the lines N and J11, were analyzed at the physiological and molecular levels for enhanced salt tolerance. The results showed that in terms of root-specific heterologous expression, the PgTIP1-transformed soybean cotyledon hairy roots or composite plants displayed superior salt tolerance compared to the empty vector-transformed ones according to the mitigatory effects of hairy root growth reduction, drop in leaf RWC, and rise in REL under salt stress. Additionally, declines in K+ content, increases in Na+ content and Na+/K+ ratios in the hairy roots, stems, or leaves were effectively alleviated by PgTIP1-transformation, particularly the stems and leaves of composite soybean plants. At the whole plant level, PgTIP1-trasgenic soybean lines were found to possess stronger root vigor, reduced root and leaf cell membrane damage, increased SOD, POD, CAT, and APX activities, steadily increased leaf Tr, RWC, and Pn values, and smaller declines in chlorophyll and carotenoid content when exposed to salt stress compared to wild type. Moreover, the distribution patterns of Na+, K+, and Cl- in the roots, stems, and leaves of salt-stressed transgenic plants were readjusted, in that the absorbed Na+ and Cl- were mainly restricted to the roots to reduce their transport to the shoots, and the transport of root-absorbed K+ to the shoots was simultaneously promoted. PgTIP1 transformation into soybean plants enhanced the expression of some stress-related genes (GmPOD, GmAPX1, GmSOS1, and GmCLC1) in the roots and leaves under salt treatment. This indicates that the causes of enhanced salt tolerance of heterologous PgTIP1-transformed soybean are associated with the positive regulation on water relations, ion homeostasis, and ROS scavenging under salt stress both at root-specific and whole plant levels.

The functional role of xylem parenchyma cells and aquaporins during recovery from severe water stress

Xylem parenchyma cells [vessel associated cells (VACs)] constitute a significant fraction of the xylem in woody plants. These cells are often closely connected with xylem vessels or tracheids via simple pores (remnants of plasmodesmata fields). The close contact and biological activity of VACs during times of severe water stress and recovery from stress suggest that they are involved in the maintenance of xylem transport capacity and responsible for the restoration of vessel/tracheid functionality following embolism events. As recovery from embolism requires the transport of water across xylem parenchyma cell membranes, an understanding of stem-specific aquaporin expression patterns, localization and activity is a crucial part of any biological model dealing with embolism recovery processes in woody plants. In this review, we provide a short overview of xylem parenchyma cell biology with a special focus on aquaporins. In particular we address their distributions and activity during the development of drought stress, during the formation of embolism and the subsequent recovery from stress that may result in refilling. Complemented by the current biological model of parenchyma cell function during recovery from stress, this overview highlights recent breakthroughs on the unique ability of long-lived perennial plants to undergo cycles of embolism-recovery related to drought/rewetting or freeze/thaw events.

High quality RNA from hydroponically grown grapevine roots suitable for gene expression studies

Objectives:

Grapevine root system plays a great role in sensing and adapting to abiotic and biotic stresses. Identification of candidate genes involved in the tolerance to abiotic stress is becoming a crucial strategy to select and breed resilient genotypes. However, obtaining high quality RNA from grapevine roots under hydroponic culture is difficult. Hence, we have developed a new extraction procedure to improve RNA quality for root gene expression studies.

Methods:

Conventional RNA extraction methods using CTAB are not suitable for gene expression studies and need to be improved. Here we report the application of a CTAB- based method for RNA extraction using an additional clean-up purification step.

Results:

The RIN value of the resulting RNA indicated that our procedure allowed the purification of high RNA quality and quantity. Hence, the clean-up purification step efficiently eliminated contaminants which inhibit downstream applications. Derived RNA was successfully used for differential gene expression analysis in salt stressed grapevine by Northern Blot hybridizations.

Conclusion:

In this study, we developed an efficient RNA isolation protocol from hydroponic cultivated grapevine roots which yielded RNA suitable for gene expression studies. This will open large perspectives in grapevine functional genomics with the identification of pertinent genes of agronomic interest.

Fungal and plant gene expression in the Tulasnella calospora-Serapias vomeracea symbiosis provides clues about nitrogen pathways in orchid mycorrhizas

Orchids are highly dependent on their mycorrhizal fungal partners for nutrient supply, especially during early developmental stages. In addition to organic carbon, nitrogen (N) is probably a major nutrient transferred to the plant because orchid tissues are highly N-enriched. We know almost nothing about the N form preferentially transferred to the plant or about the key molecular determinants required for N uptake and transfer. We identified, in the genome of the orchid mycorrhizal fungus Tulasnella calospora, two functional ammonium transporters and several amino acid transporters but found no evidence of a nitrate assimilation system, in agreement with the N preference of the free-living mycelium grown on different N sources. Differential expression in symbiosis of a repertoire of fungal and plant genes involved in the transport and metabolism of N compounds suggested that organic N may be the main form transferred to the orchid host and that ammonium is taken up by the intracellular fungus from the apoplatic symbiotic interface. This is the first study addressing the genetic determinants of N uptake and transport in orchid mycorrhizas, and provides a model for nutrient exchanges at the symbiotic interface, which may guide future experiments.

Virus-induced plasma membrane aquaporin PsPIP2;1 silencing inhibits plant water transport of Pisum sativum

BACKGROUND

Aquaporins (AQPs) are known to facilitate water transport across cell membranes, but the role of a single AQP in regulating plant water transport, particularly in plants other than Arabidopsis remains largely unexplored. In the present study, a virus-induced gene silencing (VIGS) technique was employed to suppress the expression of a specific plasma membrane aquaporin PsPIP2;1 of Pea plants (Pisum sativum), and subsequent effects of the gene suppression on root hydraulic conductivity (Lpr), leaf hydraulic conductivity (K leaf ), root cell hydraulic conductivity (Lprc), and leaf cell hydraulic conductivity (Lplc) were investigated, using hydroponically grown Pea plants.

RESULTS

Compared with control plants, VIGS-PsPIP2;1 plants displayed a significant suppression of PsPIP2;1 in both roots and leaves, while the expression of other four PIP isoforms (PsPIP1;1, PsPIP1;2, PsPIP2;2, and PsPIP2;3) that were simultaneously monitored were not altered. As a consequence, significant declines in water transport of VIGS-PsPIP2;1 plants were observed at both organ and cell levels, i.e., as compared to control plants, Lpr and K leaf were reduced by 29 %, and Lprc and Lplc were reduced by 20 and 29 %, respectively.

CONCLUSION

Our results demonstrate that PsPIP2;1 alone contributes substantially to root and leaf water transport in Pea plants, and highlight VIGS a useful tool for investigating the role of a single AQP in regulating plant water transport.

VvPIP2;4N aquaporin involvement in controlling leaf hydraulic capacitance and resistance in grapevine

Hydraulic capacitance (C) in a plant tissue buffers the xylem tension, storing and releasing water and has been highlighted in recent years as an important factor that affects water relations such as drought tolerance and embolism formation. Aquaporins (AQPs) are well known to control leaf hydraulic resistance (Rh) but their role in the control of C is unknown. Here, we assess Rh and C on detached grapevines wild-type (WT) (cv. Brachetto) leaves and over-expressing the aquaporin gene VvPIP2;4N (OE). For this purpose, we developed a new method inspired from the pressure-volume curve technique and the rehydration-kinetic-method, which allowed us to monitor the dynamics of dehydration and rehydration in the same leaf. The recovery after dehydration was measured in dark, light non-transpirative conditions, light-transpirative conditions and light-transpirative condition adding abscisic acid. Pressurizing to dehydrate leaves in the OE line, the recorded Rh and C were respectively lower and higher than those in the WT. The same results were obtained in the dark recovery by rehydration treatment. In the presence of light, either when leaves transpired or not (by depressing vapor pressure deficit), the described effects disappeared. The change in Rh and C did not affect the kinetics of desiccation of detached leaves in dark in air, in OE plants compared to WT ones. Our study highlighted that both Rh and C were influenced by the constitutive over-expression of VvPIP2;4N. The effect of AQPs on C is reported here for the first time and may involve a modulation of cell reflection coefficient.

An Integrated View of Whole-Tree Hydraulic Architecture. Does Stomatal or Hydraulic Conductance Determine Whole Tree Transpiration?

Hydraulic conductance exerts a strong influence on many aspects of plant physiology, namely: transpiration, CO₂ assimilation, growth, productivity or stress response. However we lack full understanding of the contribution of root or shoot water transport capacity to the total water balance, something which is difficult to study in trees. Here we tested the hypothesis that whole plant hydraulic conductance modulates plant transpiration using two different seedlings of citrus rootstocks, Poncirus trifoliata (L.) Raf. and Cleopatra mandarin (Citrus reshni Hort ex Tan.). The two genotypes presented important differences in their root or shoot hydraulic conductance contribution to whole plant hydraulic conductance but, even so, water balance proved highly dependent on whole plant conductance. Further, we propose there is a possible equilibrium between root and shoot hydraulic conductance, similar to that between shoot and root biomass production, which could be related with xylem anatomy.

Knockdown of MLO genes reduces susceptibility to powdery mildew in grapevine

Erysiphe necator is the causal agent of powdery mildew (PM), one of the most destructive diseases of grapevine. PM is controlled by sulfur-based and synthetic fungicides, which every year are dispersed into the environment. This is why PM-resistant varieties should become a priority for sustainable grapevine and wine production. PM resistance can be achieved in other crops by knocking out susceptibility S-genes, such as those residing at genetic loci known as MLO (Mildew Locus O). All MLO S-genes of dicots belong to the phylogenetic clade V, including grapevine genes VvMLO7, 11 and 13, which are upregulated during PM infection, and VvMLO6, which is not upregulated. Before adopting a gene-editing approach to knockout candidate S-genes, the evidence that loss of function of MLO genes can reduce PM susceptibility is necessary. This paper reports the knockdown through RNA interference of VvMLO6, 7, 11 and 13. The knockdown of VvMLO6, 11 and 13 did not decrease PM severity, whereas the knockdown of VvMLO7 in combination with VvMLO6 and VvMLO11 reduced PM severity up to 77%. The knockdown of VvMLO7 and VvMLO6 seemed to be important for PM resistance, whereas a role for VvMLO11 does not seem likely. Cell wall appositions (papillae) were present in both resistant and susceptible lines in response to PM attack. Thirteen genes involved in defense were less upregulated in infected mlo plants, highlighting the early mlo-dependent disruption of PM invasion.

Changes in Air CO₂ Concentration Differentially Alter Transcript Levels of NtAQP1 and NtPIP2;1 Aquaporin Genes in Tobacco Leaves

The aquaporin specific control on water versus carbon pathways in leaves is pivotal in controlling gas exchange and leaf hydraulics. We investigated whether Nicotiana tabacum aquaporin 1 (NtAQP1) and Nicotiana tabacum plasma membrane intrinsic protein 2;1 (NtPIP2;1) gene expression varies in tobacco leaves subjected to treatments with different CO₂ concentrations (ranging from 0 to 800 ppm), inducing changes in photosynthesis, stomatal regulation and water evaporation from the leaf. Changes in air CO₂ concentration ([CO₂]) affected net photosynthesis (Pn) and leaf substomatal [CO₂] (Ci). Pn was slightly negative at 0 ppm air CO₂; it was one-third that of ambient controls at 200 ppm, and not different from controls at 800 ppm. Leaves fed with 800 ppm [CO₂] showed one-third reduced stomatal conductance (gs) and transpiration (E), and their gs was in turn slightly lower than in 200 ppm- and in 0 ppm-treated leaves. The 800 ppm air [CO₂] strongly impaired both NtAQP1 and NtPIP2;1 gene expression, whereas 0 ppm air [CO₂], a concentration below any in vivo possible conditions and specifically chosen to maximize the gene expression alteration, increased only the NtAQP1 transcript level. We propose that NtAQP1 expression, an aquaporin devoted to CO₂ transport, positively responds to CO₂ scarcity in the air in the whole range 0-800 ppm. On the contrary, expression of NtPIP2;1, an aquaporin not devoted to CO₂ transport, is related to water balance in the leaf, and changes in parallel with gs. These observations fit in a model where upregulation of leaf aquaporins is activated at low Ci, while downregulation occurs when high Ci saturates photosynthesis and causes stomatal closure.

Genetic transformation of a fruit-specific, highly expressed stilbene synthase gene from Chinese wild Vitis quinquangularis

The stilbene synthase gene VqSTS6, from Chinese wild type Vitis quinquangularis accession Danfeng-2, increases the resveratrol content and pathogen resistance of transgenic plants of V. vinifera Thompson Seedless. This study successfully created transgenic plants of V. vinifera Thompson Seedless which overexpressed VqSTS6, cloned from Chinese wild type V. quinquangularis accession Danfeng-2. Western blot and qRT-PCR showed a variable range in transcript levels among transgenic lines. The resistance to powdery mildew (Uncinula necator) was particularly enhanced in lines most highly expressing VqSTS6. Compared with the non-transformed controls, trans-resveratrol and other stilbene compounds were significantly increased in the transgenic lines. The correlation between high resveratrol content and high pathogen resistance in transgenic grapes is discussed. We hypothesize that the fruit-specific, highly expressed gene VqSTS6 from Chinese wild V. quinquangularis accession Danfeng-2, is directly involved in the resveratrol synthesis pathway in grapes, and plays an important role in the plant's defense against pathogens. Genetic transformation of VqSTS6 explored the potential of the wild Chinese grape species for use in breeding, the results of which would raise both the disease resistance and the fruit quality of V. vinifera grapevines.

Novel functional microRNAs from virus-free and infected Vitis vinifera plants under water stress

MicroRNAs (miRNAs) are small non-coding RNAs that regulate the post-transcriptional control of several pathway intermediates, thus playing pivotal roles in plant growth, development and response to biotic and abiotic stresses. In recent years, the grapevine genome release, small(s)-RNAseq and degradome-RNAseq together has allowed the discovery and characterisation of many miRNA species, thus rendering the discovery of additional miRNAs difficult and uncertain. Taking advantage of the miRNA responsiveness to stresses and the availability of virus-free Vitis vinifera plants and those infected only by a latent virus, we have analysed grapevines subjected to drought in greenhouse conditions. The sRNA-seq and other sequence-specific molecular analyses have allowed us to characterise conserved miRNA expression profiles in association with specific eco-physiological parameters. In addition, we here report 12 novel grapevine-specific miRNA candidates and describe their expression profile. We show that latent viral infection can influence the miRNA profiles of V. vinifera in response to drought. Moreover, study of eco-physiological parameters showed that photosynthetic rate, stomatal conductance and hydraulic resistance to water transport were significantly influenced by drought and viral infection. Although no unequivocal cause–effect explanation could be attributed to each miRNA target, their contribution to the drought response is discussed.

Over-expression of AQUA1 in Populus alba Villafranca clone increases relative growth rate and water use efficiency, under Zn excess condition

Transgenic Populus alba over-expressing a TIP aquaporin ( aqua1) showed a higher growth rate under Zn excess, suggesting that aqua1 could be involved in water homeostasis, rather than in Zn homeostasis. Populus is the internationally accepted model for physiological and developmental studies of tree traits under stress. In plants, aquaporins facilitate and regulate the diffusion of water, however, few poplar aquaporins have been characterized to date. In this study, we reported for the first time an in vivo characterization of Populus alba clone Villafranca transgenic plants over-expressing a TIP aquaporin (aqua1) of P. x euramericana clone I-214. An AQUA1:GFP chimeric construct, over-expressed in P. alba Villafranca clones, shows a cytoplasmic localization in roots, and it localizes in guard cells in leaves. When over-expressed in transgenic plants, aqua1 confers a higher growth rate compared to wild-type (wt) plants, without affecting chlorophyll accumulation, relative water content (RWC), and fluorescence performances, but increasing the intrinsic Transpiration Efficiency. In response to Zn (1 mM), transgenic lines did not show a significant increase in Zn accumulation as compared to wt plants, even though the over-expression of this gene confers higher tolerance in root tissues. These results suggest that, in poplar plants, this gene could be principally involved in regulation of water homeostasis and biomass production, rather than in Zn homeostasis.

MzPIP2;1: An Aquaporin Involved in Radial Water Movement in Both Water Uptake and Transportation, Altered the Drought and Salt Tolerance of Transgenic Arabidopsis

BACKGROUND

Plants are unavoidably subjected to various abiotic stressors, including high salinity, drought and low temperature, which results in water deficit and even death. Water uptake and transportation play a critical role in response to these stresses. Many aquaporin proteins, localized at different tissues, function in various transmembrane water movements. We targeted at the key aquaporin in charge of both water uptake in roots and radial water transportation from vascular tissues through the whole plant.

RESULTS

The MzPIP2;1 gene encoding a plasma membrane intrinsic protein was cloned from salt-tolerant apple rootstock Malus zumi Mats. The GUS gene was driven by MzPIP2;1 promoter in transgenic Arabidopsis. It indicated that MzPIP2;1 might function in the epidermal and vascular cells of roots, parenchyma cells around vessels through the stems and vascular tissues of leaves. The ectopically expressed MzPIP2;1 conferred the transgenic Arabidopsis plants enhanced tolerance to slight salt and drought stresses, but sensitive to moderate salt stress, which was indicated by root length, lateral root number, fresh weight and K+/Na+ ratio. In addition, the possible key cis-elements in response to salt, drought and cold stresses were isolated by the promoter deletion experiment.

CONCLUSION

The MzPIP2;1 protein, as a PIP2 aquaporins subgroup member, involved in radial water movement, controls water absorption and usage efficiency and alters transgenic plants drought and salt tolerance.

PIP1 aquaporins: Intrinsic water channels or PIP2 aquaporin modulators?

The highly conserved plant aquaporins, known as Plasma membrane Intrinsic Proteins (PIPs), are the main gateways for cell membrane water exchange. Years of research have described in detail the properties of the PIP2 subfamily. However, characterizing the PIP1 subfamily has been difficult due to the failure to localize to the plasma membrane. In addition, the discovery of the PIP1-PIP2 interaction suggested that PIP1 aquaporins could be regulated by a complex posttranslational mechanism that involves trafficking, heteromerization and fine-tuning of channel activity. This review not only considers the evidence and findings but also discusses the complexity of PIP aquaporins. To establish a new benchmark in PIP regulation, we propose to consider PIP1-PIP2 pairs as functional units for the purpose of future research into their physiological roles.

Aquaporins in Plants

Aquaporins are membrane channels that facilitate the transport of water and small neutral molecules across biological membranes of most living organisms. In plants, aquaporins occur as multiple isoforms reflecting a high diversity of cellular localizations, transport selectivity, and regulation properties. Plant aquaporins are localized in the plasma membrane, endoplasmic reticulum, vacuoles, plastids and, in some species, in membrane compartments interacting with symbiotic organisms. Plant aquaporins can transport various physiological substrates in addition to water. Of particular relevance for plants is the transport of dissolved gases such as carbon dioxide and ammonia or metalloids such as boron and silicon. Structure-function studies are developed to address the molecular and cellular mechanisms of plant aquaporin gating and subcellular trafficking. Phosphorylation plays a central role in these two processes. These mechanisms allow aquaporin regulation in response to signaling intermediates such as cytosolic pH and calcium, and reactive oxygen species. Combined genetic and physiological approaches are now integrating this knowledge, showing that aquaporins play key roles in hydraulic regulation in roots and leaves, during drought but also in response to stimuli as diverse as flooding, nutrient availability, temperature, or light. A general hydraulic control of plant tissue expansion by aquaporins is emerging, and their role in key developmental processes (seed germination, emergence of lateral roots) has been established. Plants with genetically altered aquaporin functions are now tested for their ability to improve plant tolerance to stresses. In conclusion, research on aquaporins delineates ever expanding fields in plant integrative biology thereby establishing their crucial role in plants.