Stem bending generates electrical response in poplar.

Under natural conditions, plants experience external mechanical stresses such as wind and touch that impact their growth. A remarkable feature of this mechanically induced growth response is that it may occur at a distance from the stimulation site, suggesting the existence of a signal propagating through the plant. In this study, we investigated the electrical response of poplar trees to a transient controlled bending stimulation of the stem that mimics the mechanical effect of wind. Stem bending was found to cause an electrical response that we called "gradual" potential, similar in shape to an action potential. However, this signal can be distinguished from the well-known plant action potential by its propagation up to 20 cm along the stem and its strong dumping in velocity and amplitude. Two hypotheses regarding the mode of propagation of the "gradual" potential are discussed.

Between Stress and Response: Function and Localization of Mechanosensitive Ca2+ Channels in Herbaceous and Perennial Plants.

Over the past three decades, how plants sense and respond to mechanical stress has become a flourishing field of research. The pivotal role of mechanosensing in organogenesis and acclimation was demonstrated in various plants, and links are emerging between gene regulatory networks and physical forces exerted on tissues. However, how plant cells convert physical signals into chemical signals remains unclear. Numerous studies have focused on the role played by mechanosensitive (MS) calcium ion channels MCA, Piezo and OSCA. To complement these data, we combined data mining and visualization approaches to compare the tissue-specific expression of these genes, taking advantage of recent single-cell RNA-sequencing data obtained in the root apex and the stem of Arabidopsis and the Populus stem. These analyses raise questions about the relationships between the localization of MS channels and the localization of stress and responses. Such tissue-specific expression studies could help to elucidate the functions of MS channels. Finally, we stress the need for a better understanding of such mechanisms in trees, which are facing mechanical challenges of much higher magnitudes and over much longer time scales than herbaceous plants, and we mention practical applications of plant responsiveness to mechanical stress in agriculture and forestry.

PtxtPME1 and homogalacturonans influence xylem hydraulic properties in poplar.

While the xylem hydraulic properties, such as vulnerability to cavitation (VC), are of paramount importance in drought resistance, their genetic determinants remain unexplored. There is evidence that pectins and their methylation pattern are involved, but the detail of their involvement and the corresponding genes need to be clarified. We analyzed the hydraulic properties of the 35S::PME1 transgenic aspen that ectopically under- or over-express a xylem-abundant pectin methyl esterase, PtxtPME1. We also produced and analyzed 4CL1::PGII transgenic poplars expressing a fungal polygalacturonase, AnPGII, under the control of the Ptxa4CL1 promoter that is active in the developing xylem after xylem cell expansion. Both the 35S::PME1 under- and over-expressing aspen lines developed xylem with lower-specific hydraulic conductivity and lower VC, while the 4CL1::PGII plants developed xylem with a higher VC. These xylem hydraulic changes were associated with modifications in xylem structure or in intervessel pit structure that can result in changes in mechanical behavior of the pit membrane. This study shows that homogalacturonans and their methylation pattern influence xylem hydraulic properties, through its effect on xylem cell expansion and on intervessel pit properties and it show a role for PtxtPME1 in the xylem hydraulic properties.

The Hevea brasiliensis XIP aquaporin subfamily: genomic, structural and functional characterizations with relevance to intensive latex harvesting.

X-Intrinsic Proteins (XIP) were recently identified in a narrow range of plants as a full clade within the aquaporins. These channels reportedly facilitate the transport of a wide range of hydrophobic solutes. The functional roles of XIP in planta remain poorly identified. In this study, we found three XIP genes (HbXIP1;1, HbXIP2;1 and HbXIP3;1) in the Hevea brasiliensis genome. Comprehensive bioinformatics, biochemical and structural analyses were used to acquire a better understanding of this AQP subfamily. Phylogenetic analysis revealed that HbXIPs clustered into two major groups, each distributed in a specific lineage of the order Malpighiales. Tissue-specific expression profiles showed that only HbXIP2;1 was expressed in all the vegetative tissues tested (leaves, stem, bark, xylem and latex), suggesting that HbXIP2;1 could take part in a wide range of cellular processes. This is particularly relevant to the rubber-producing laticiferous system, where this isoform was found to be up-regulated during tapping and ethylene treatments. Furthermore, the XIP transcriptional pattern is significantly correlated to latex production level. Structural comparison with SoPIP2;1 from Spinacia oleracea species provides new insights into the possible role of structural checkpoints by which HbXIP2;1 ensures glycerol transfer across the membrane. From these results, we discuss the physiological involvement of glycerol and HbXIP2;1 in water homeostasis and carbon stream of challenged laticifers. The characterization of HbXIP2;1 during rubber tree tapping lends new insights into molecular and physiological response processes of laticifer metabolism in the context of latex exploitation.

The zinc finger protein PtaZFP2 negatively controls stem growth and gene expression responsiveness to external mechanical loads in poplar.

Mechanical cues are essential signals regulating plant growth and development. In response to wind, trees develop a thigmomorphogenetic response characterized by a reduction in longitudinal growth, an increase in diameter growth, and changes in mechanical properties. The molecular mechanisms behind these processes are poorly understood. In poplar, PtaZFP2, a C2H2 transcription factor, is rapidly up-regulated after stem bending. To investigate the function of PtaZFP2, we analyzed PtaZFP2-overexpressing poplars (Populus tremula × Populus alba). To unravel the genes downstream PtaZFP2, a transcriptomic analysis was performed. PtaZFP2-overexpressing poplars showed longitudinal and cambial growth reductions together with an increase in the tangent and hardening plastic moduli. The regulation level of mechanoresponsive genes was much weaker after stem bending in PtaZFP2-overexpressing poplars than in wild-type plants, showing that PtaZFP2 negatively modulates plant responsiveness to mechanical stimulation. Microarray analysis revealed a high proportion of down-regulated genes in PtaZFP2-overexpressing poplars. Among these genes, several were also shown to be regulated by mechanical stimulation. Our results confirmed the important role of PtaZFP2 during plant acclimation to mechanical load, in particular through a negative control of plant molecular responsiveness. This desensitization process could modulate the amplitude and duration of the plant response during recurrent stimuli.

Tree shoot bending generates hydraulic pressure pulses: a new long-distance signal?

When tree stems are mechanically stimulated, a rapid long-distance signal is induced that slows down primary growth. An investigation was carried out to determine whether the signal might be borne by a mechanically induced pressure pulse in the xylem. Coupling xylem flow meters and pressure sensors with a mechanical testing device, the hydraulic effects of mechanical deformation of tree stem and branches were measured. Organs of several tree species were studied, including gymnosperms and angiosperms with different wood densities and anatomies. Bending had a negligible effect on xylem conductivity, even when deformations were sustained or were larger than would be encountered in nature. It was found that bending caused transient variation in the hydraulic pressure within the xylem of branch segments. This local transient increase in pressure in the xylem was rapidly propagated along the vascular system in planta to the upper and lower regions of the stem. It was shown that this hydraulic pulse originates from the apoplast. Water that was mobilized in the hydraulic pulses came from the saturated porous material of the conduits and their walls, suggesting that the poroelastic behaviour of xylem might be a key factor. Although likely to be a generic mechanical response, quantitative differences in the hydraulic pulse were found in different species, possibly related to differences in xylem anatomy. Importantly the hydraulic pulse was proportional to the strained volume, similar to known thigmomorphogenetic responses. It is hypothesized that the hydraulic pulse may be the signal that rapidly transmits mechanobiological information to leaves, roots, and apices.

Growth and molecular responses to long-distance stimuli in poplars: bending vs flame wounding.

Inter-organ communication is essential for plants to coordinate development and acclimate to mechanical environmental fluctuations. The aim of this study was to investigate long-distance signaling in trees. We compared on young poplars the short-term effects of local flame wounding and of local stem bending for two distal responses: (1) stem primary growth and (2) the expression of mechanoresponsive genes in stem apices. We developed a non-contact measurement method based on the analysis of apex images in order to measure the primary growth of poplars. The results showed a phased stem elongation with alternating nocturnal circumnutation phases and diurnal growth arrest phases in Populus tremula × alba clone INRA 717-1B4. We applied real-time polymerase chain reaction (RT-PCR) amplifications in order to evaluate the PtaZFP2, PtaTCH2, PtaTCH4, PtaACS6 and PtaJAZ5 expressions. The flame wounding inhibited primary growth and triggered remote molecular responses. Flame wounding induced significant changes in stem elongation phases, coupled with inhibition of circumnutation. However, the circadian rhythm of phases remained unaltered and the treated plants were always phased with control plants during the days following the stress. For bent plants, the stimulated region of the stem showed an increased PtaJAZ5 expression, suggesting the jasmonates may be involved in local responses to bending. No significant remote responses to bending were observed.

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.

Fungal X-Intrinsic Protein Aquaporin from Trichoderma atroviride: Structural and Functional Considerations.

The major intrinsic protein (MIP) superfamily is a key part of the fungal transmembrane transport network. It facilitates the transport of water and low molecular weight solutes across biomembranes. The fungal uncharacterized X-Intrinsic Protein (XIP) subfamily includes the full protein diversity of MIP. Their biological functions still remain fully hypothetical. The aim of this study is still to deepen the diversity and the structure of the XIP subfamily in light of the MIP counterparts-the aquaporins (AQPs) and aquaglyceroporins (AQGPs)-and to describe for the first time their function in the development, biomass accumulation, and mycoparasitic aptitudes of the fungal bioagent Trichoderma atroviride. The fungus-XIP clade, with one member (TriatXIP), is one of the three clades of MIPs that make up the diversity of T. atroviride MIPs, along with the AQPs (three members) and the AQGPs (three members). TriatXIP resembles those of strict aquaporins, predicting water diffusion and possibly other small polar solutes due to particularly wider ar/R constriction with a Lysine substitution at the LE2 position. The XIP loss of function in ∆TriatXIP mutants slightly delays biomass accumulation but does not impact mycoparasitic activities. ∆TriatMIP forms colonies similar to wild type; however, the hyphae are slightly thinner and colonies produce rare chlamydospores in PDA and specific media, most of which are relatively small and exhibit abnormal morphologies. To better understand the molecular causes of these deviant phenotypes, a wide-metabolic survey of the ∆TriatXIPs demonstrates that the delayed growth kinetic, correlated to a decrease in respiration rate, is caused by perturbations in the pentose phosphate pathway. Furthermore, the null expression of the XIP gene strongly impacts the expression of four expressed MIP-encoding genes of T. atroviride, a plausible compensating effect which safeguards the physiological integrity and life cycle of the fungus. This paper offers an overview of the fungal XIP family in the biocontrol agent T. atroviride which will be useful for further functional analysis of this particular MIP subfamily in vegetative growth and the environmental stress response in fungi. Ultimately, these findings have implications for the ecophysiology of Trichoderma spp. in natural, agronomic, and industrial systems.

Acclimation kinetics of physiological and molecular responses of plants to multiple mechanical loadings.

During their development, plants are subjected to repeated and fluctuating wind loads, an environmental factor predicted to increase in importance by scenarios of global climatic change. Notwithstanding the importance of wind stress on plant growth and development, little is known about plant acclimation to the bending stresses imposed by repeated winds. The time-course of acclimation of young poplars (Populus tremula L.xP. alba L.) to multiple stem bendings is studied here by following diameter growth and the expression of four genes PtaZFP2, PtaTCH2, PtaTCH4, and PtaACS6, previously described to be involved in the mechanical signalling transduction pathway. Young trees were submitted either to one transient bending per day for several days or to two bendings, 1-14 days apart. A diminution of molecular responses to subsequent bending was observed as soon as a second bending was applied. The minimum rest periods between two successive loadings necessary to recover a response similar to that observed after a single bending, were 7 days and 5 days for growth and molecular responses, respectively. Taken together, our results show a desensitization period of a few days after a single transitory bending, indicating a day-scale acclimation of sensitivity to the type of wind conditions plants experience in their specific environment. This work establishes the basic kinetics of acclimation to low bending frequency and these kinetic analyses will serve as the basis of ongoing work to investigate the molecular mechanisms involved. Future research will also concern plant acclimation to higher wind frequencies.

Contrasting strategies to cope with chilling stress among clones of a tropical tree, Hevea brasiliensis.

Eight Hevea brasiliensis Muell. Arg. clones (GT1, YUNYAN77-4, IRCA707, IRCA317, PB217, PB260, PR107 and RRIM600) were compared for their tolerance towards chilling stress. Net photosynthesis (Pn), stomatal conductance (Gs), optimal and effective photochemical efficiencies (F(v)/F(m) and ), non-photochemical quenching, cellular lysis and leaf necrosis were measured on trees chilled at 10 °C for 96 h, as well as upon recovery at 28 °C. In addition, ascorbate peroxidase, catalase, dehydroascorbate reductase, glutathione reductase, monodehydroascorbate reductase and superoxide dismutase activities were monitored. Clone RRIM600 appeared to be the most tolerant, because it showed no cellular lysis or leaf necrosis and the best recovery as revealed by Pn, Gs, F(v)/F(m) and . Its ability to sustain chilling stress seemed related in part to the fast closure of stomata, suggesting an 'avoidance strategy' for this clone. IRCA707, GT1 and YUNYAN77-4 were also tolerant to the cold treatment as only a few leaf injuries were observed. However, YUNYAN77-4 showed a particular behaviour with a large stomata opening during the first hour of chilling, some photosynthetic activity after 96 h at 10 °C, but the slowest recovery in Pn. The greatest cell or leaf damage was observed on PB260, IRCA317, PR107 and PB217 clones, thus classified as sensitive to chilling. These clones showed the strongest decrease in Pn, F(v)/F(m) and and the slowest recovery for F(v)/F(m) and , indicating a high sensitivity of photosystem II to cold temperatures. Punctual increases of various enzymatic activities were observed for all clones during chilling kinetics. During recovery, the strongest increases in enzymatic activity were observed for the most tolerant clones, suggesting that efficient reactive oxygen species elimination is a crucial step for determining chilling tolerance in Hevea although the enzymes implicated varied from one tolerant clone to another. This study points out contrasted strategies of the Hevea clones in copping with chilling stress and recovery.

Sucrose importation into laticifers of Hevea brasiliensis, in relation to ethylene stimulation of latex production.

BACKGROUND AND AIMS: The major economic product of Hevea brasiliensis is a rubber-containing cytoplasm (latex), which flows out of laticifers (latex cells) when the bark is tapped. The latex yield is stimulated by ethylene. Sucrose, the unique precursor of rubber synthesis, must cross the plasma membrane through specific sucrose transporters before being metabolized in the laticifers. The relative importance of sucrose transporters in determining latex yield is unknown. Here, the effects of ethylene (by application of Ethrel on sucrose transporter gene expression in the inner bark tissues and latex cells of H. brasiliensis are described. METHODS: Experiments, including cloning sucrose transporters, real time RT-PCR and in situ hybridization, were carried out on virgin (untapped) trees, treated or untreated with the latex yield stimulant Ethrel. KEY RESULTS: Seven putative full-length cDNAs of sucrose transporters were cloned from a latex-specific cDNA library. These transporters belong to all SUT (sucrose transporter) groups and differ by their basal gene expression in latex and inner soft bark, with a predominance of HbSUT1A and HbSUT1B. Of these sucrose transporters, only HbSUT1A and HbSUT2A were distinctly increased by ethylene. Moreover, this increase was shown to be specific to laticifers and to ethylene application. CONCLUSION: The data and all previous information on sucrose transport show that HbSUT1A and HbSUT2A are related to the increase in sucrose import into laticifers, required for the stimulation of latex yield by ethylene in virgin trees.

Proteome analysis of apical and basal regions of poplar stems under gravitropic stimulation.

Gravity is a constant force guiding the direction of plant growth. In young poplar stem, reorientation of the apical region is mainly obtained by differential growth of elongating primary tissues. At the base, where elongation is achieved but where the cambium is active, reorientation is due to asymmetrical formation of reaction wood. After 45 min of gravistimulation, the stem showed no reorientation, but 1 week later, reaction wood was observed at the base of the stem. To determine the molecular mechanisms taking place at the top and base of the stem, after 45 min or 1 week of inclination, the changes induced in protein accumulation were studied by two-dimensional polyacrylamide gel electrophoresis and quantitatively analyzed using image analysis software. Around 300 protein spots were reproducibly detected and analyzed. Forty percent of these proteins showed significant changes after inclination. Mass spectrometry analysis of 135 spots led to the identification of 60 proteins involved in a wide range of activities and metabolisms. Very different patterns of protein expression were obtained according to conditions tested, highlighting the complexity of gravitropic responses. Our results suggest that primary and secondary tissues present specific mechanisms to sense reorientation and to respond to inclination. Some selected proteins are discussed.

Characterization and expression analysis under bending and other abiotic factors of PtaZFP2, a poplar gene encoding a Cys2/His2 zinc finger protein.

In plants, mechanoperception and transduction of mechanical signals have been studied essentially in Arabidopsis thaliana L. and Lycopersicon esculentum L. plants, i.e., in nonwoody plants. Here, we have described the isolation of both the full-length cDNA and the regulatory region of PtaZFP2, encoding a member of Cys2/His2 zinc finger protein (ZFP) family in Populus tremula L. x Populus alba L. Time course analysis of expression demonstrated that PtaZFP2 mRNA accumulated as early as 5 min in response to a controlled stem bending and is restricted to the organ where the mechanical stimulus is applied. The real-time quantitative Reverse Transcriptase Polymerase Chain Reaction experiments showed that PtaZFP2 was also rapidly up-regulated in poplar stems in response to gravitropism suggesting that PtaZFP2 is induced by different mechanical signals. Abundance of PtaZFP2 transcripts also increased highly in response to wounding and to a weaker extent to salt treatment and cold, which is consistent with the numerous putative cis-elements found in its regulatory region. As in other species, these data suggest that Cys2/His2 ZFPs could function in poplar as key transcriptional regulators in the acclimation response to different environmental factors.

Jr-ZFP2, encoding a Cys2/His2-type transcription factor, is involved in the early stages of the mechano-perception pathway and specifically expressed in mechanically stimulated tissues in woody plants.

Plants respond to environmental mechanical stimulation, such as wind, by modifying their growth and development. To study the molecular effects of stem bending on 3-week-old walnut trees, a cDNA-AFLP approach was developed. This study allowed the identification of a cDNA, known as Jr-ZFP2, encoding a Cys2/His2-type two-zinc-fingered transcription factor. Reverse transcriptase-polymerase chain reaction analysis confirmed that Jr-ZFP2 mRNA accumulation is rapidly and transiently induced after mechanical stimulation. After bending, Jr-ZFP2 transcript increase was restricted to the stem, the organ where the mechanical solicitation was applied. Furthermore, other abiotic factors, such as cold or salt, did not modify Jr-ZFP2 mRNA accumulation in walnut stems under our experimental conditions, whereas growth studies demonstrated that salt stress was actually perceived by the plants. These results suggest that the regulation of Jr-ZFP2 expression is more sensitive to mechanical stimulus. This gene will be a good marker for studying the early stages of mechanical perception in woody plants.

MIP diversity from Trichoderma: Structural considerations and transcriptional modulation during mycoparasitic association with Fusarium solani olive trees.

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.

Spatial activity and expression of plasma membrane H+-ATPase in stem xylem of walnut during dormancy and growth resumption.

Plasma membrane H+-ATPase (PM H+-ATPase) plays a key role in nutrient transport, stress responses and growth. To evaluate proton motive force differences between apical and basal parts of acrotonic 1-year-old shoots of walnut (Juglans regia L. cv 'Franquette') trees, spatial and seasonal changes in PM H+-ATPase were studied in mature xylem tissues. During both the dormancy and growth resumption periods, and in both the apical and basal parts of the stem, PM H+-ATPase activity showed positive correlations with the amount of immunodetectable protein. In spring, at the time of growth resumption, higher activities and immunoreactivities of PM H+-ATPase were found in the apical part of the stem than in the basal part of the stem. In spring, the decrease in xylem sugar concentration reflected the high sugar uptake rate. Our data suggest that PM H+-ATPase plays a major role in the uptake of carbohydrates from xylem vessels during growth resumption. These results are discussed in the context of the acrotonic tendency of walnut shoots.

Modification of photosynthetic regulation in tomato overexpressing glutathione peroxidase.

To investigate the function of glutathione peroxidase (GPX) in plants, we produced transgenic tomato plants overexpressing an eukaryotic selenium-independent GPX (GPX5). We show here that total GPX activity was increased by 50% in transgenic plants, when compared to control plants transformed with the binary vector without the insert (PZP111). A preliminary two-dimensional electrophoretic protein analysis of the GPX overexpressing plants showed notably a decrease in the accumulation of proteins identified as rubisco small subunit 1 and fructose-1,6-bisphosphate aldolase, two proteins involved in photosynthesis. These observations, together with the fact that in standard culture conditions, GPX-overexpressing plants were not phenotypically distinct from control plants prompted us to challenge the plants with a chilling treatment that is known to affect photosynthesis activity. We found that upon chilling treatment with low light level, photosynthesis was not affected in GPX-overexpressing plants while it was in control plants, as revealed by chlorophyll fluorescence parameters and fructose-1,6-biphosphatase activity. These results suggest that overexpression of a selenium-independent GPX in tomato plants modifies specifically gene expression and leads to modifications of photosynthetic regulation processes.

Mechanosensitive control of plant growth: bearing the load, sensing, transducing, and responding.

As land plants grow and develop, they encounter complex mechanical challenges, especially from winds and turgor pressure. Mechanosensitive control over growth and morphogenesis is an adaptive trait, reducing the risks of breakage or explosion. This control has been mostly studied through experiments with artificial mechanical loads, often focusing on cellular or molecular mechanotransduction pathway. However, some important aspects of mechanosensing are often neglected. (i) What are the mechanical characteristics of different loads and how are loads distributed within different organs? (ii) What is the relevant mechanical stimulus in the cell? Is it stress, strain, or energy? (iii) How do mechanosensing cells signal to meristematic cells? Without answers to these questions we cannot make progress analyzing the mechanobiological effects of plant size, plant shape, tissue distribution and stiffness, or the magnitude of stimuli. This situation is rapidly changing however, as systems mechanobiology is being developed, using specific biomechanical and/or mechanobiological models. These models are instrumental in comparing loads and responses between experiments and make it possible to quantitatively test biological hypotheses describing the mechanotransduction networks. This review is designed for a general plant science audience and aims to help biologists master the models they need for mechanobiological studies. Analysis and modeling is broken down into four steps looking at how the structure bears the load, how the distributed load is sensed, how the mechanical signal is transduced, and then how the plant responds through growth. Throughout, two examples of adaptive responses are used to illustrate this approach: the thigmorphogenetic syndrome of plant shoots bending and the mechanosensitive control of shoot apical meristem (SAM) morphogenesis. Overall this should provide a generic understanding of systems mechanobiology at work.

Iron-superoxide dismutase and monodehydroascorbate reductase transcripts accumulate in response to internode rubbing in tomato.

A cDNA encoding an iron-superoxide dismutase (Fe-SOD) was isolated by RACE-PCR from a Lycopersicon esculentum cDNA library. The Fe-SOD cDNA consists of a 746-bp open reading frame and is predicted to encode a protein of 249 amino acids with a calculated molecular mass of 27.9 kDa. The deduced amino acid sequence was very similar to other plant Fe-SODs and a potential chloroplastic targeting was found. To study the induction of oxidative burst in response to mechanical stimulation, the accumulation of Fe-SOD and monodehydroascorbate reductase (MDHAR) mRNAs was analysed in response to young growing internode rubbing in tomato plants. Northern analyses show that Fe-SOD mRNA and MDHAR mRNA accumulated in tomato internodes 10 min after the mechanical stimulation. These results suggest that reactive oxygen species are early involved in the response of a plant to a mechanical stimulation, such as rubbing. The nucleotide sequence data reported in this paper will appear in the NCBI Nucleotide Sequence Databases under the accession number AY262025.