In plants, aquaporins play a crucial role in regulating root water transport in response to environmental and physiological cues. Controls achieved at the post-translational level are thought to be of critical importance for regulating aquaporin function. To investigate the general molecular mechanisms involved, we performed, using the model species Arabidopsis, a comprehensive proteomic analysis of root aquaporins in a large set of physiological contexts. We identified nine physiological treatments that modulate root hydraulics in time frames of minutes (NO and H 2 O 2 treatments), hours (mannitol and NaCl treatments, exposure to darkness and reversal with sucrose, phosphate supply to phosphate-starved roots), or days (phosphate or nitrogen starvation). All treatments induced inhibition of root water transport except for sucrose supply to darkgrown plants and phosphate resupply to phosphatestarved plants, which had opposing effects. Using a robust label-free quantitative proteomic methodology, we identified 12 of 13 plasma membrane intrinsic protein (PIP) aquaporin isoforms, 4 of the 10 tonoplast intrinsic protein isoforms, and a diversity of post-translational modifications including phosphorylation, methylation, deamidation, and acetylation. A total of 55 aquaporin peptides displayed significant changes after treatments and enabled the identification of specific and as yet unknown patterns of response to stimuli. The data show that the regulation of PIP and tonoplast intrinsic protein abundance was involved in response to a few treatments (i.e. NaCl, NO, and nitrate starvation), whereas changes in the phosphorylation status of PIP aquaporins were positively correlated to changes in root hydraulic conductivity in the whole set of treatments The absorption of soil water by roots is crucial in order for plants to maintain their water status. Studies in various plant species have shown that the root water permeability (root hydraulic conductivity Lp r ) is constantly adjusted depending on the developmental stage of the plant, its nutritional or hormonal status, or multiple environmental stimuli (1). Despite their importance in plant growth and adaptation, these multiple responses have not been investigated in a single plant species yet. Aquaporins form a large class of channel proteins that facilitate the diffusion of water and small neutral solutes across cell membranes and, among many other functions, contribute to root water uptake (2, 3). Aquaporins are 25-to 30-kDa proteins with six membrane-spanning domains and five connecting loops (A-E), with N-and C-terminal tails exposed to the cytosol (4, 5). Plant aquaporins show a high multiplicity of isoforms. Thirty-five homologs belonging to four homology subclasses have been identified in Arabidopsis. The plasma membrane intrinsic proteins (PIPs) 1 (with 13 isoforms further subdivided into the PIP1 and PIP2 subgroups) and the tonoplast intrinsic proteins (TIPs) with 10 homologs are the most abundant aquaporins in the plasma membrane and the tonoplast, respectivel...

To test the hypothesis that, depending on the fungal symbiont, ectomycorrhizas are differentially affected by severe drought stress, we developed a simple method to quantify the loss of vitality of excised ectomycorrhizal tips subjected to drying under controlled conditions. The method uses 96-well microtitration plates with one single ectomycorrhizal tip per well, and is based on measuring the loss of volume and the loss of electrolytes before and after the imposed stress. This approach very significantly discriminated the two ectomycorrhizal morphotypes formed with beech (Fagus silvatica) by Lactarius subdulcis and Cenococcum geophilum, which confirmed the ability of the latter fungal species to protect roots against desiccation already suggested by previous works. The new method should contribute to the present effort in deciphering the functional diversity of complex ectomycorrhizal communities.

An excess of NaCl in the soil is detrimental for plant growth. It interferes with mineral nutrition and water uptake and leads to accumulation of toxic ions in the plant. Understanding the response of roots to NaCl stress may facilitate the development of crops with increased tolerance to this and other stresses. Since controls achieved at the posttranslational level are of critical importance for regulating protein function, the present work used a robust label-free quantitative proteomic methodology to quantify phosphorylation events that affect root membrane proteins in Arabidopsis, in response to short-term (up to 2 h) NaCl treatments. This work identified 302 proteotypic phosphopeptides including 77 novel phosphorylated sites. NaCl treatment significantly altered the abundance of 74 phosphopeptides, giving novel insights into the regulation of major classes of membrane proteins, including ATPases, sodium transporters, and aquaporins. The data provide a unique access to phosphorylation reprogramming of ionic equilibrium in plant cells under NaCl stress. The use of predictive bioinformatic tools for kinase motifs suggested that root membrane proteins are substrates of cAMP-dependent protein kinase, cGMP-dependent protein kinase, and protein kinase C families, also called AGC kinases, arguing for an important role of lipid signaling in abiotic stress responses. It also pointed to cross-talks between protein kinase families during NaCl stress.