David Lopez

Insights into Populus XIP aquaporins: evolutionary expansion, protein functionality, and environmental regulation

A novel category of major intrinsic proteins which share weak similarities with previously identified aquaporin subfamilies was recently identified in land plants, and named X (for unrecognized) intrinsic proteins (XIPs). Because XIPs are still ranked as uncharacterized proteins, their further molecular characterization is required. Herein, a systematic fine-scale analysis of XIP sequences found in flowering plant databases revealed that XIPs are found in at least five groups. The phylogenetic relationship of these five groups with the phylogenetic organization of angiosperms revealed an original pattern of evolution for the XIP subfamily through distinct angiosperm taxon-specific clades. Of all flowering plant having XIPs, the genus Populus encompasses the broadest panel and the highest polymorphism of XIP isoforms, with nine PtXIP sequences distributed within three XIP groups. Comprehensive PtXIP gene expression patterns showed that only two isoforms (PtXIP2;1 and PtXIP3;2) were transcribed in vegetative tissues. However, their patterns are contrasted, PtXIP2;1 was ubiquitously accumulated whereas PtXIP3;2 was predominantly detected in wood and to a lesser extent in roots. Furthermore, only PtXIP2;1 exhibited a differential expression in leaves and stems of drought-, salicylic acid-, or wounding-challenged plants. Unexpectedly, the PtXIPs displayed different abilities to alter water transport upon expression in Xenopus laevis oocytes. PtXIP2;1 and PtXIP3;3 transported water while other PtXIPs did not.

Light-mediated K leaf induction and contribution of both the PIP1s and PIP2s aquaporins in five tree species: walnut ( Juglans regia) case study

Understanding the response of leaf hydraulic conductance (K(leaf)) to light is a challenge in elucidating plant-water relationships. Recent data have shown that the effect of light on K(leaf) is not systematically related to aquaporin regulation, leading to conflicting conclusions. Here we investigated the relationship between light, K(leaf), and aquaporin transcript levels in five tree species (Juglans regia L., Fagus sylvatica L., Quercus robur L., Salix alba L. and Populus tremula L.) grown in the same environmental conditions, but differing in their K(leaf) responses to light. Moreover, the K(leaf) was measured by two independent methods (high-pressure flow metre (HPFM) and evaporative flux method (EFM)) in the most (J. regia) and least (S. alba) responsive species and the transcript levels of aquaporins were analyzed in perfused and unperfused leaves. Here, we found that the light-induced K(leaf) value was closely related to stronger expression of both the PIP1 and PIP2 aquaporin genes in walnut (J. regia), but to stimulation of PIP1 aquaporins alone in F. sylvatica and Q. robur. In walnut, all newly identified aquaporins were found to be upregulated in the light and downregulated in the dark, further supporting the relationship between the light-mediated induction of K(leaf) and aquaporin expression in walnut. We also demonstrated that the K(leaf) response to light was quality-dependent, K(leaf) being 60% lower in the absence of blue light. This decrease in K(leaf) was correlated with strong downregulation of three PIP2 aquaporins and of all the PIP1 aquaporins tested. These data support a relationship between light-mediated K(leaf) regulation and the abundance of aquaporin transcripts in the walnut tree.

Aquaporins and Leaf Hydraulics: Poplar Sheds New Light

To help understand leaf hydraulic conductance (Kleaf) modulation under high irradiance, well-watered poplars (Populus trichocarpa Torr. & Gray ex Hook and Populus nigra L.) were studied diurnally at molecular and ecophysiological scales. Transcriptional and translational modulations of plasma membrane intrinsic protein (PIP) aquaporins were evaluated in leaf samples during diurnal time courses. Among the 15 poplar PIP genes, a subset of two PIP1s and seven PIP2s are precociously induced within the first hour of the photoperiod concomitantly with a Kleaf increase. Since expression patterns were cyclic and reproducible over several days, we hypothesized that endogenous signals could be involved in PIP transcriptional regulation. To address this question, plants were submitted to forced darkness during their subjective photoperiod and compared with their control counterparts, which showed that some PIP1s and PIP2s have circadian regulation while others did not. Promoter analysis revealed that a large number of hormone, light, stress response and circadian elements are present. Finally, involvement of aquaporins is supported by the reduction of Kleaf by HgCl2 treatment.

Modulation of bud survival in Populus nigra sprouts in response to water stress-induced embolism

Understanding drought tolerance mechanisms requires knowledge about the induced weakness that leads to tree death. Bud survival is vital to sustain tree growth across seasons. We hypothesized that the hydraulic connection of the bud to stem xylem structures was critical for its survival. During an artificial drastic water stress, we carried out a census of bud metabolic activity of young Populus nigra L. trees by microcalorimetry. We monitored transcript expression of aquaporins (AQPs; plasma membrane intrinsic proteins (PIPs), X intrinsic proteins (XIPs) and tonoplast membrane intrinsic proteins (TIPs)) and measured local water status within the bud and tissues in the bearer shoot node by nuclear magnetic resonance (NMR) imaging. We found that the bud respiration rate was closely correlated with its water content and decreased concomitantly in buds and their surrounding bearer tissues. At the molecular level, we observed a modulation of AQP pattern expressions (PIP, TIP and XIP subfamilies) linked to water movements in living cells. However, AQP functions remain to be investigated. Both the bud and tree died beyond a threshold water content and respiration rate. Nuclear magnetic resonance images provided relevant local information about the various water reservoirs of the stem, their dynamics and their interconnections. Comparison of pith, xylem and cambium tissues revealed that the hydraulic connection between the bud and saturated parenchyma cells around the pith allowed bud desiccation to be delayed. At the tree death date, NMR images showed that the cambium tissues remained largely hydrated. Overall, the respiration rate (Rco2) and a few AQP isoforms were found to be two suitable, complementary criteria to assess the bud metabolic activity and the ability to survive a severe drought spell. Bud moisture content could be a key factor in determining the capacity of poplar to recover from water stress.

Gravity sensing, a largely misunderstood trigger of plant orientated growth

Gravity is a crucial environmental factor regulating plant growth and development. Plants have the ability to sense a change in the direction of gravity, which leads to the re-orientation of their growth direction, so-called gravitropism. In general, plant stems grow upward (negative gravitropism), whereas roots grow downward (positive gravitropism). Models describing the gravitropic response following the tilting of plants are presented and highlight that gravitropic curvature involves both gravisensing and mechanosensing, thus allowing to revisit experimental data. We also discuss the challenge to set up experimental designs for discriminating between gravisensing and mechanosensing. We then present the cellular events and the molecular actors known to be specifically involved in gravity sensing.

The molecular mechanisms of reaction wood induction

Reaction wood originates from cambial activity which adjust cell division activity, proportion of fibres, cell wall structure and properties, so that the resulting growth event will be the appropriate response to endogenous and environmental stimuli.