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.

Biomechanical study of the effect of a controlled bending on tomato stem elongation: global mechanical analysis.

An experiment was designed to apply a controlled bending to a tomato stem and simultaneously to measure its effect on stem elongation. Stem elongation was measured over 2 d until steady and equal rates were obtained for the control and the treated plants. Thereafter, the basal part of the stem was submitted to a transient controlled bending at constant displacement rate using a motorized dynamometer. After load removal, stem elongation was again measured for 2 d. The tested plants were mature (height visible internodes) and only the basal part of the stem, which had already finished elongation, was loaded (hypocotyl and the first three internodes). A few minutes after the application of bending, elongation stopped completely for 60 min. Thereafter it took 120-1000 min to recover a rate of elongation similar to the control. The growth response was exclusively due to the bending of the basal part of the stem. It was shown that the side mechanical perturbations on the roots and on the stem tissues interacting directly with the clamp were not significantly involved on the elongation response. These results give evidence for mechanical perception and plant signalling from the basal stem to the upper elongating zone. However, none of the variables characterizing the global mechanical state of the bent part of the stem (i.e. the maximal force, bending moment, inclination, mean curvature of the stem, stored mechanical energy) could quantitatively explain the variability of the growth response. A more local mechanical analysis is therefore needed to elucidate how the mechanical stimulus is perceived.

Morphological responses and molecular modifications in tomato plants after mechanical stimulation.

Study of the growth responses of Lycopersicon esculentum (Mill. cv. VFN8) to mechanical stimulation applied to a single young internode showed a rapid and sharp decrease in stem elongation and an inhibition of elongation of several internodes, indicative of information transmission in the plant. A new tomato cDNA partial clone encoding calmodulin was isolated and used to study the time course of the gene induction in response to the rubbing treatment. Northern blot analysis showed a maximum accumulation of calmodulin mRNA 2 h after mechanical stimulation, not only in the rubbed internode, but also in upper and lower internodes and in young leaves. Treatment of the plant with calcium and EGTA showed the involvement of calcium and, in particular, intracellular calcium in calmodulin gene expression and cellular response.

Is membrane potential involved in calmodulin gene expression after external stimulation in plants?

In Bidens pilosa (cv. radiata), a non-injurious stimulus induces a local and transient change in membrane potential, and an injurious stimulus induces a transmitted electrical signal described as the combination of an action potential and a slow wave. We have studied calmodulin gene expression after these stimuli. When the stimulus is non-injurious, calmodulin mRNA accumulation is only increased in the stimulated region. In contrast, when the stimulus is injurious, mRNA accumulation takes place in both wounded and distant, unwounded tissue. We propose that the slow wave plays a role in the long-distance transmission of a wound-induced information in plants.