Protein kinase SnRK2. 4 is a key regulator of aquaporins and root hydraulics in Arabidopsis

Soil water uptake by roots is a key component of plant water homeostasis contributing to plant growth and survival under ever-changing environmental conditions. The water transport capacity of roots (root hydraulic conductivity; Lpr ) is mostly contributed by finely regulated Plasma membrane Intrinsic Protein (PIP) aquaporins. In this study, we used natural variation of Arabidopsis for the identification of quantitative trait loci (QTLs) contributing to Lpr . Using recombinant lines from a biparental cross (Cvi-0 x Col-0), we show that the gene encoding class 2 Sucrose-Non-Fermenting Protein kinase 2.4 (SnRK2.4) in Col-0 contributes to >30% of Lpr by enhancing aquaporin-dependent water transport. At variance with the inactive and possibly unstable Cvi-0 SnRK2.4 form, the Col-0 form interacts with and phosphorylates the prototypal PIP2;1 aquaporin at Ser121 and stimulates its water transport activity upon coexpression in Xenopus oocytes and yeast cells. Activation of PIP2;1 by Col-0 SnRK2.4 in yeast also requires its protein kinase activity and can be counteracted by clade A Protein Phosphatases 2C. SnRK2.4 shows all hallmarks to be part of core abscisic acid (ABA) signaling modules. Yet, long-term (>3 h) inhibition of Lpr by ABA possibly involves a SnRK2.4-independent inhibition of PIP2;1. SnRK2.4 also promotes stomatal aperture and ABA-induced inhibition of primary root growth. The study identifies a key component of Lpr and sheds new light on the functional overlap and specificity of SnRK2.4 with respect to other ABA-dependent or independent SnRK2s

Hormonal and environmental signaling pathways target membrane water transport

International audiencePlant water transport and its molecular components including aquaporins are responsive, across diverse time scales, to an extremely wide array of environmental and hormonal signals. These include water deficit and abscisic acid (ABA) but also more recently identified stimuli such as peptide hormones or bacterial elicitors. The present review makes an inventory of corresponding signalling pathways. It identifies some main principles, such as the central signalling role of ROS, with a dual function of aquaporins in water and hydrogen peroxide transport, the importance of aquaporin phosphorylation that is targeted by multiple classes of protein kinases, and the emerging role of lipid signalling. More studies including systems biology approaches are now needed to comprehend how plant water transport can be adjusted in response to combined stresses

Regulation of root water transport under flooding

Regulation of root water transport under flooding. XIX International Botanical Congres

Genetics of root hydraulics in Arabidopsis thaliana . Genomic, physiological and breeding approaches for enhancing drought resistance in crops

Genetics of root hydraulics in Arabidopsis thaliana . Genomic, physiological and breeding approaches for enhancing drought resistance in crops. Environmental Workshop of Universidad Internacional de Andaluci

Identification of genetic determinants of root hydraulics in Arabidopsis thaliana

Identification of genetic determinants of root hydraulics in Arabidopsis thaliana. 26th International Conference on Arabidopsis Research

Regulation of plant aquaporins in response to water stress

International audienceAquaporins facilitate the uptake of soil water and mediate the regulation of root hydraulic conductivity (Lp(r)) in response to a large variety of environmental stresses. Here, we use Arabidopsis (Arabidopsis thaliana) plants to dissect the effects of salt on both Lp(r) and aquaporin expression and investigate possible molecular and cellular mechanisms of aquaporin regulation in plant roots under stress. Treatment of plants by 100 mm NaCl was perceived as an osmotic stimulus and induced a rapid (half-time, 45 min) and significant (70%) decrease in Lp(r), which was maintained for at least 24 h. Macroarray experiments with gene-specific tags were performed to investigate the expression of all 35 genes of the Arabidopsis aquaporin family. Transcripts from 20 individual aquaporin genes, most of which encoded members of the plasma membrane intrinsic protein (PIP) and tonoplast intrinsic protein (TIP) subfamilies, were detected in nontreated roots. All PIP and TIP aquaporin transcripts with a strong expression signal showed a 60% to 75% decrease in their abundance between 2 and 4 h following exposure to salt. The use of antipeptide antibodies that cross-reacted with isoforms of specific aquaporin subclasses revealed that the abundance of PIP1s decreased by 40% as early as 30 min after salt exposure, whereas PIP2 and TIP1 homologs showed a 20% to 40% decrease in abundance after 6 h of treatment. Expression in transgenic plants of aquaporins fused to the green fluorescent protein revealed that the subcellular localization of TIP2;1 and PIP1 and PIP2 homologs was unchanged after 45 min of exposure to salt, whereas a TIP1;1-green fluorescent protein fusion was relocalized into intracellular spherical structures tentatively identified as intravacuolar invaginations. The appearance of intracellular structures containing PIP1 and PIP2 homologs was occasionally observed after 2 h of salt treatment. In conclusion, this work shows that exposure of roots to salt induces changes in aquaporin expression at multiple levels. These changes include a coordinated transcriptional down-regulation and subcellular relocalization of both PIPs and TIPs. These mechanisms may act in concert to regulate root water transport, mostly in the long term (> or =6 h)

Regulation of root water transport under flooding

Regulation of root water transport under flooding. XIX International Botanical Congres

Hydraulic Conductivity of Root 1 controls potassium-dependent oxygen sensing to regulate root hydraulic

Soil water uptake by roots is central for plant growth and survival. Flooding, as many other environmental constraints such as drought, nutrient deprivation or oxidative stress exerts deep effects on root functions by altering root water permeability (root hydraulic conductivity; Lpr). These effects, which are mediated mainly through the regulation of aquaporins, are fundamental for adaptation of wild plant species to diverse natural habitats and a major target for crop improvement. However, the signaling mechanisms which link soil properties to root hydraulics and aquaporin functions remain largely unknown. We have been investigating the genetic bases of root hydraulics in the model plant, Arabidopsis thaliana. Using quantitative genetics approaches, including linkage mapping and genome-wide association mapping, we have identified several genes controlling Lpr. The signaling pathway for regulation of Lpr by a RAF-like MAP3 kinase named Hydraulic Conductivity of Root 1 (HCR1) will be discussed in details (Shahzad et al (2016) Cell 167: 87-98.e14). This protein kinase delineates a combinatorial signaling pathway integrating two soil signals, K+ and O2 availability, to regulate root hydraulics and hypoxia responsive genes, through the control of RAP2.12, a key transcriptional regulator of the core anaerobic response. In addition, several other candidate genes identified during this study offer interesting perspectives for understanding as yet unknown mechanisms involved in the regulation of root hydraulics

Vegetative and sperm cell-specific aquaporins of Arabidopsis thaliana highlight the vacuolar equipment of pollen and contribute to plant reproduction.

International audience: The water and nutrient status of pollen is crucial to plant reproduction. Pollen grains of Arabidopsis thaliana contain a large vegetative cell and two smaller sperm cells. Pollen grains express AtTIP1;3 and AtTIP5;1, two members of the Tonoplast Intrinsic Protein sub-family of aquaporins. To address the spatial and temporal expression pattern of the two homologues, C-terminal fusions of AtTIP1;3 and AtTIP5;1 with GFP and mCherry, respectively, were expressed in transgenic Arabidopsis under the control of their native promoter. Confocal laser scanning microscopy revealed that AtTIP1;3 and AtTIP5;1 are specific for the vacuoles of the vegetative and sperm cells, respectively. The tonoplast localization of AtTIP5;1 was established by reference to fluorescent protein markers for the mitochondria and vacuoles of sperm and vegetative cells and is at variance with a recent work (Soto et al., 2010, Plant J 64: 1038-1047) which localized AtTIP5;1 in vegetative cell mitochondria. AtTIP1;3-GFP and AtTIP5;1-mCherry showed concomitant expression, from first pollen mitosis up to pollen tube penetration in the ovule, thereby revealing the dynamics of vacuole morphology in maturating and germinating pollen. T-DNA insertion mutants for either AtTIP1;3 or AtTIP5;1 showed no apparent growth phenotype and had no significant defect in male transmission of the mutated alleles. By contrast, a double knock-out displayed an abnormal rate of barren siliques, this phenotype being more pronounced under limiting water or nutrient supply. The overall data indicate that vacuoles of vegetative and sperm cells functionally interact and contribute to male fertility in adverse environmental conditions