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Leaf hydraulic conductance (Kleaf) and capacitance (Cleaf) are among the key parameters in plant-water regulation. Understanding the responses of these hydraulic traits to drought conditions remains a challenge for describing comprehensive plant-water relationships. The ability of an organism to resist and/or tolerate embolism events, which may occur at high negative pressure caused by hydric stress, relies on how well it can sustain a hydraulic system in a dynamic equilibrium. Populus deltoides is a water-saving tree species with a stomatal conductance that declines rapidly with reduced water availability. Under unfavorable conditions, the stomatal control of transpiration is known to be closely coordinated with a loss of plant hydraulic functioning that can ultimately result in hydraulic failure through xylem embolism, notably in leaves. The effects of drought on leaf hydraulics are also related to regulation in water permeases such as the aquaporins. To describe the responses linked to leaf hydraulics under severe drought and rewatering conditions, water-stressed poplars were monitored daily on an ecophysiological and a molecular scale. A structural and expression analysis on a set of aquaporins was carried out in parallel by in situ hybridization analysis and quantitative PCR. In complement, water distribution in water-challenged leaves was investigated using X-ray microtomography. A general depression of leaf hydraulic conductance and relative water content occurred under drought, but was reversed when plants were rewatered. More interestingly, (i) extreme leaf water deficiency led to marked xylem and lamina embolism, but a degree of hydric integrity in the midrib extraxylem territories and the bundle sheath of the minor veins was maintained, and (ii) the sub-tissue water allocation correlated well with an over-accumulation of several PIP and TIP aquaporins. Our multi-facet molecular ecophysiological approach revealed that leaves were able to secure a certain level of hydric status, in particular in cell territories near the “living ribs”, which provided rapid hydric adjustment responses once favorable conditions were restored. These findings contribute to an integrated approach to leaf hydraulics, thus favoring a better understanding of the cell mechanisms involved in tree vulnerability to climate changes.

Major intrinsic proteins (MIP) are characterized by a transmembrane pore-type architecture that facilitates transport across biomembranes of water and a variety of low molecular weight solutes. They are found in all parts of life, with remarkable protein diversity. Very little is known about MIP from fungi. And yet, it can legitimately be stated that MIP are pivotal molecular components in the privileged relationships fungi enjoy with plants or soil fauna in various environments. To date, MIP have never been studied in a mycoparasitism situation. In this study, the diversity, expression and functional prediction of MIP from the genus Trichoderma were investigated. Trichoderma spp. genomes have at least seven aquaporin genes. Based on a phylogenetic analysis of the translated sequences, members were assigned to the AQP, AQGP and XIP subfamilies. In in vitro and in planta assays with T. harzianum strain Ths97, expression analyses showed that four genes were constitutively expressed. In a mycoparasitic context with Fusarium solani, the causative agent of fusarium dieback on olive tree roots, these genes were up-regulated. This response is of particular interest in analyzing the MIP promoter cis-regulatory motifs, most of which are involved in various carbon and nitrogen metabolisms. Structural analyses provide new insights into the possible role of structural checkpoints by which these members transport water, H2O2, glycerol and, more generally, linear polyols across the membranes. Taken together, these results provide the first evidence that MIP may play a key role in Trichoderma mycoparasitism lifestyle.