Involvement of lignin and hormones in the response of woody poplar taproots to mechanical stress

Effect of Polyethylene Glycol-Simulated Drought Stress on Stomatal Opening in "Modern" and "Ancient" Wheat Varieties

Climate change is leading to an increase in the intensity, duration, and frequency of severe droughts, especially in southern and southeastern Europe, thus aggravating water scarcity problems. Water deficit stress harms the growth, physiology, and yield of crops like durum wheat. Hence, studying ancient wheat varieties' stress responses could help identify genetic traits to enhance crop tolerance to environmental stresses. In this background, this study aimed to investigate the effects of PEG 6000-stimulated drought stress in the ancient wheat variety Saragolla and the modern one Svevo by analyzing various biochemical and molecular parameters that can especially condition the stomatal movement. Our data revealed that drought stress caused a significant increase in the levels of total soluble sugars, ABA, and IAA in both selected cultivars to a greater extent in the Saragolla than in the Svevo. We demonstrated that, under water deficit stress, calcium dynamics as well as the expression of ERF109, MAPK3/6, MYB60, and TaTPC1, involved in the activation of drought-related calcium-sensitive pathways, display significant differences between the two varieties. Therefore, our study provided further evidence regarding the ability of the ancient wheat variety Saragolla to better cope with drought stress compared to the modern variety Svevo.

Phenotypical and biochemical characterization of tomato plants treated with triacontanol

Biostimulants are heterogeneous products designed to support plant development and to improve the yield and quality of crops. Here, we focused on the effects of triacontanol, a promising biostimulant found in cuticle waxes, on tomato growth and productivity. We examined various phenological traits related to vegetative growth, flowering and fruit yield, the metabolic profile of fruits, and the response of triacontanol-treated plants to salt stress. Additionally, a proteomic analysis was conducted to clarify the molecular mechanisms underlying triacontanol action. Triacontanol application induced advanced and increased blooming without affecting plant growth. Biochemical analyses of fruits showed minimal changes in nutritional properties. The treatment also increased the germination rate of seeds by altering hormone homeostasis and reduced salt stress-induced damage. Proteomics analysis of leaves revealed that triacontanol increased the abundance of proteins related to development and abiotic stress, while down-regulating proteins involved in biotic stress resistance. The proteome of the fruits was not significantly affected by triacontanol, confirming that biostimulation did not alter the nutritional properties of fruits. Overall, our findings provide evidence of the effects of triacontanol on growth, development, and stress tolerance, shedding light on its mechanism of action and providing new insights into its potential in agricultural practices.

An artificial intelligence-integrated analysis of the effect of drought stress on root traits of "modern" and "ancient" wheat varieties

Due to drought stress, durum wheat production in the Mediterranean basin will be severely affected in the coming years. Durum wheat cultivation relies on a few genetically uniform "modern" varieties, more productive but less tolerant to stresses, and "traditional" varieties, still representing a source of genetic biodiversity for drought tolerance. Root architecture plasticity is crucial for plant adaptation to drought stress and the relationship linking root structures to drought is complex and still largely under-explored. In this study, we examined the effect of drought stress on the roots' characteristics of the "traditional" Saragolla cultivar and the "modern" Svevo. By means of "SmartRoot" software, we demonstrated that drought stress affected primary and lateral roots as well as root hair at different extents in Saragolla and Svevo cultivars. Indeed, we observed that under drought stress Saragolla possibly revamped its root architecture, by significantly increasing the length of lateral roots, and the length/density of root hairs compared to the Svevo cultivar. Scanning Electron Microscopy analysis of root anatomical traits demonstrated that under drought stress a greater stele area and an increase of the xylem lumen size vessel occurred in Saragolla, indicating that the Saragolla variety had a more efficient adaptive response to osmotic stress than the Svevo. Furthermore, for the analysis of root structural data, Artificial Intelligence (AI) algorithms have been used: Their application allowed to predict from root structural traits modified by the osmotic stress the type of cultivar observed and to infer the relationship stress-cultivar type, thus demonstrating that root structural traits are clear and incontrovertible indicators of the higher tolerance to osmotic stress of the Saragolla cultivar. Finally, to obtain an integrated view of root morphogenesis, phytohormone levels were investigated. According to the phenotypic effects, under drought stress,a larger increase in IAA and ABA levels, as well as a more pronounced reduction in GA levels occurred in Saragolla as compared to Svevo. In conclusion, these results show that the root growth and hormonal profile of Saragolla are less affected by osmotic stress than those of Svevo, demonstrating the great potential of ancient varieties as reservoirs of genetic variability for improving crop responses to environmental stresses.

The dynamics of touch-responsive gene expression in cereals

Wind, rain, herbivores, obstacles, neighbouring plants, etc. provide important mechanical cues to steer plant growth and survival. Mechanostimulation to stimulate yield and stress resistance of crops is of significant research interest, yet a molecular understanding of transcriptional responses to touch is largely absent in cereals. To address this, we performed whole-genome transcriptomics following mechanostimulation of wheat, barley, and the recent genome-sequenced oat. The largest transcriptome changes occurred ±25 min after touching, with most of the genes being upregulated. While most genes returned to basal expression level by 1-2 h in oat, many genes retained high expression even 4 h post-treatment in barley and wheat. Functional categories such as transcription factors, kinases, phytohormones, and Ca2+ regulation were affected. In addition, cell wall-related genes involved in (hemi)cellulose, lignin, suberin, and callose biosynthesis were touch-responsive, providing molecular insight into mechanically induced changes in cell wall composition. Furthermore, several cereal-specific transcriptomic footprints were identified that were not observed in Arabidopsis. In oat and barley, we found evidence for systemic spreading of touch-induced signalling. Finally, we provide evidence that both the jasmonic acid-dependent and the jasmonic acid-independent pathways underlie touch-signalling in cereals, providing a detailed framework and marker genes for further study of (a)biotic stress responses in cereals.

Foliar functional and genetic variation in a keystone Hawaiian tree species estimated through spectroscopy

Imaging spectroscopy has the potential to map closely related plant taxa at landscape scales. Although spectral investigations at the leaf and canopy levels have revealed relationships between phylogeny and reflectance, understanding how spectra differ across, and are inherited from, genotypes of a single species has received less attention. We used a common-garden population of four varieties of the keystone canopy tree, Metrosideros polymorpha, from Hawaii Island and four F1-hybrid genotypes derived from controlled crosses to determine if reflectance spectra discriminate sympatric, conspecific varieties of this species and their hybrids. With a single exception, pairwise comparisons of leaf reflectance patterns successfully distinguished varieties of M. polymorpha on Hawaii Island as well as populations of the same variety from different islands. Further, spectral variability within a single variety from Hawaii Island and the older island of Oahu was greater than that observed among the four varieties on Hawaii Island. F1 hybrids most frequently displayed leaf spectral patterns intermediate to those of their parent taxa. Spectral reflectance patterns distinguished each of two of the hybrid genotypes from one of their parent varieties, indicating that classifying hybrids may be possible, particularly if sample sizes are increased. This work quantifies a baseline in spectral variability for an endemic Hawaiian tree species and advances the use of imaging spectroscopy in biodiversity studies at the genetic level.

The Salt Tolerance-Related Protein (STRP) Is a Positive Regulator of the Response to Salt Stress in Arabidopsis thaliana

Salt stress is a major abiotic stress limiting plant survival and crop productivity. Plant adaptation to salt stress involves complex responses, including changes in gene expression, regulation of hormone signaling, and production of stress-responsive proteins. The Salt Tolerance-Related Protein (STRP) has been recently characterized as a Late Embryogenesis Abundant (LEA)-like, intrinsically disordered protein involved in plant responses to cold stress. In addition, STRP has been proposed as a mediator of salt stress response in Arabidopsis thaliana, but its role has still to be fully clarified. Here, we investigated the role of STRP in salt stress responses in A. thaliana. The protein rapidly accumulates under salt stress due to a reduction of proteasome-mediated degradation. Physiological and biochemical responses of the strp mutant and STRP-overexpressing (STRP OE) plants demonstrate that salt stress impairs seed germination and seedling development more markedly in the strp mutant than in A. thaliana wild type (wt). At the same time, the inhibitory effect is significantly reduced in STRP OE plants. Moreover, the strp mutant has a lower ability to counteract oxidative stress, cannot accumulate the osmocompatible solute proline, and does not increase abscisic acid (ABA) levels in response to salinity stress. Accordingly, the opposite effect was observed in STRP OE plants. Overall, obtained results suggest that STRP performs its protective functions by reducing the oxidative burst induced by salt stress, and plays a role in the osmotic adjustment mechanisms required to preserve cellular homeostasis. These findings propose STRP as a critical component of the response mechanisms to saline stress in A. thaliana.

The Course of Mechanical Stress: Types, Perception, and Plant Response

Mechanical stimuli, together with the corresponding plant perception mechanisms and the finely tuned thigmomorphogenetic response, has been of scientific and practical interest since the mid-17th century. As an emerging field, there are many challenges in the research of mechanical stress. Indeed, studies on different plant species (annual/perennial) and plant organs (stem/root) using different approaches (field, wet lab, and in silico/computational) have delivered insufficient findings that frequently impede the practical application of the acquired knowledge. Accordingly, the current work distils existing mechanical stress knowledge by bringing in side-by-side the research conducted on both stem and roots. First, the various types of mechanical stress encountered by plants are defined. Second, plant perception mechanisms are outlined. Finally, the different strategies employed by the plant stem and roots to counteract the perceived mechanical stresses are summarized, depicting the corresponding morphological, phytohormonal, and molecular characteristics. The comprehensive literature on both perennial (woody) and annual plants was reviewed, considering the potential benefits and drawbacks of the two plant types, which allowed us to highlight current gaps in knowledge as areas of interest for future research.

Network-Based Analysis to Identify Hub Genes Involved in Spatial Root Response to Mechanical Constrains

Previous studies report that the asymmetric response, observed along the main poplar woody bent root axis, was strongly related to both the type of mechanical forces (compression or tension) and the intensity of force displacement. Despite a large number of targets that have been proposed to trigger this asymmetry, an understanding of the comprehensive and synergistic effect of the antistress spatially related pathways is still lacking. Recent progress in the bioinformatics area has the potential to fill these gaps through the use of in silico studies, able to investigate biological functions and pathway overlaps, and to identify promising targets in plant responses. Presently, for the first time, a comprehensive network-based analysis of proteomic signatures was used to identify functions and pivotal genes involved in the coordinated signalling pathways and molecular activities that asymmetrically modulate the response of different bent poplar root sectors and sides. To accomplish this aim, 66 candidate proteins, differentially represented across the poplar bent root sides and sectors, were grouped according to their abundance profile patterns and mapped, together with their first neighbours, on a high-confidence set of interactions from STRING to compose specific cluster-related subnetworks (I–VI). Successively, all subnetworks were explored by a functional gene set enrichment analysis to identify enriched gene ontology terms. Subnetworks were then analysed to identify the genes that are strongly interconnected with other genes (hub gene) and, thus, those that have a pivotal role in the bent root asymmetric response. The analysis revealed novel information regarding the response coordination, communication, and potential signalling pathways asymmetrically activated along the main root axis, delegated mainly to Ca2+ (for new lateral root formation) and ROS (for gravitropic response and lignin accumulation) signatures. Furthermore, some of the data indicate that the concave side of the bent sector, where the mechanical forces are most intense, communicates to the other (neighbour and distant) sectors, inducing spatially related strategies to ensure water uptake and accompanying cell modification. This information could be critical for understanding how plants maintain and improve their structural integrity—whenever and wherever it is necessary—in natural mechanical stress conditions.

Modulation of class III peroxidase pathways and phenylpropanoids in Arundo donax under salt and phosphorus stress

Arundo donax L. is an invasive species that has been recently employed for biomass production due to its well-known ability to colonize harsh environment. Based on previous observations, the present study investigated the potential role of phenylpropanoids and class III peroxidases to confer adaptation through biochemical and transcriptomic analysis in A. donax after Na⁺ and P excess supply, both in single stress and in combination, and after growth at low P level. The levels of hydrogen peroxide, flavonoids (i.e., quercetin, apigenin and kaempferol derivatives) and the activity of class III peroxidases, as well as the expression of several genes encoding for their enzymes involved in their biosynthesis, increased when Na⁺ was supplied in combination with P. These results suggest that those biomolecules are involved in the response of A. donax, to the presence of +Na and P in the soil. Moreover, even though at the sampling time no significant accumulation of lignin has been determined, the trend of accumulation of such metabolite and most of all the increase of several transcripts involved in its synthesis was found. This work for the first time indicates the need for further investigation devoted to elucidating whether the strengthening of cell walls via lignin synthesis is one of the mechanisms used by A. donax to adapt to harsh environments.

Biochar and/or Compost to Enhance Nursery-Produced Seedling Performance: A Potential Tool for Forest Restoration Programs

Today, the use of nursery-produced seedlings is the most widely adopted method in forest restoration processes. To ensure and enhance the performance of transplanting seedlings into a specific area, soil amendments are often used due to their ability to improve soil physicochemical properties and, in turn, plant growth and development. The aim of the present study was to evaluate Populus euramericana growth and development on a growing substrate added with biochar and compost, both alone and in combination. To accomplish this aim, a pot experiment was performed to test biochar and/or compost effects on growing substrate physicochemical characteristics, plant morpho-physiological traits, and plant phenology. The results showed that biochar and/or compost improved growing substrate properties by increasing electrical conductivity, cation exchange capacity, and nutrient concentrations. On the one hand, these ameliorations accelerated poplar growth and development. On the other hand, amendments did not have positive effects on some plant morphological traits, although compost alone increased plant height, and very fine and fine root length. The combined use of biochar and compost did not show any synergistic or cumulative beneficial effects and led to a reduction in plant growth and development. In conclusion, compost alone seems to be the best solution in both ameliorating substrate characteristics and increasing plant growth, highlighting the great potential for its proper and effective application in large-scale forest restoration strategies.

Meristematic Connectome: A Cellular Coordinator of Plant Responses to Environmental Signals?

Mechanical stress in tree roots induces the production of reaction wood (RW) and the formation of new branch roots, both functioning to avoid anchorage failure and limb damage. The vascular cambium (VC) is the factor responsible for the onset of these responses as shown by their occurrence when all primary tissues and the root tips are removed. The data presented confirm that the VC is able to evaluate both the direction and magnitude of the mechanical forces experienced before coordinating the most fitting responses along the root axis whenever and wherever these are necessary. The coordination of these responses requires intense crosstalk between meristematic cells of the VC which may be very distant from the place where the mechanical stress is first detected. Signaling could be facilitated through plasmodesmata between meristematic cells. The mechanism of RW production also seems to be well conserved in the stem and this fact suggests that the VC could behave as a single structure spread along the plant body axis as a means to control the relationship between the plant and its environment. The observation that there are numerous morphological and functional similarities between different meristems and that some important regulatory mechanisms of meristem activity, such as homeostasis, are common to several meristems, supports the hypothesis that not only the VC but all apical, primary and secondary meristems present in the plant body behave as a single interconnected structure. We propose to name this structure “meristematic connectome” given the possibility that the sequence of meristems from root apex to shoot apex could represent a pluricellular network that facilitates long-distance signaling in the plant body. The possibility that the “meristematic connectome” could act as a single structure active in adjusting the plant body to its surrounding environment throughout the life of a plant is now proposed.

Arabidopsis Defense against the Pathogenic Fungus Drechslera gigantea Is Dependent on the Integrity of the Unfolded Protein Response

Drechslera gigantea Heald & Wolf is a worldwide-spread necrotrophic fungus closely related to the Bipolaris genus, well-known because many member species provoke severe diseases in cereal crops and studied because they produce sesterpenoid phytoxins named ophiobolins which possess interesting biological properties. The unfolded protein response (UPR) is a conserved mechanism protecting eukaryotic cells from the accumulation of unfolded/misfolded proteins in the endoplasmic reticulum (ER). In plants, consolidated evidence supports the role of UPR in the tolerance to abiotic stress, whereas much less information is available concerning the induction of ER stress by pathogen infection and consequent UPR elicitation as part of the defense response. In this study, the infection process of D. gigantea in Arabidopsis thaliana wild type and UPR-defective bzip28 bzip60 double mutant plants was comparatively investigated, with the aim to address the role of UPR in the expression of resistance to the fungal pathogen. The results of confocal microscopy, as well as of qRT-PCR transcript level analysis of UPR genes, proteomics, microRNAs expression profile and HPLC-based hormone analyses demonstrated that ophiobolin produced by the fungus during infection compromised ER integrity and that impairment of the IRE1/bZIP60 pathway of UPR hampered the full expression of resistance, thereby enhancing plant susceptibility to the pathogen.

Gap Size in Hyrcanian Forest Affects the Lignin and N Concentrations of the Oriental Beech (Fagus orientalis Lipsky) Fine Roots but Does Not Change Their Morphological Traits in the Medium Term

Research Highlights: Fine roots play an important role in plant growth as well as in carbon (C) and nutrient cycling in terrestrial ecosystems. Gaining a wider knowledge of their dynamics under forest gap opening would improve our understanding of soil carbon input and below-ground carbon stock accumulation. Single-tree selection is increasingly recognized as an alternative regime of selection cutting sustaining biodiversity and carbon stock, along with timber production, among ecosystem functions. However, the fine root response in terms of morphological and chemical composition to the resulting harvest-created gaps remains unclear. Background and Objectives: This paper investigates the effect in the medium term (i.e., 6 years after logging) of differently sized harvest-created gaps on fine root dynamics and chemical composition. Materials and Methods: A total of 15 differently sized gaps (86.05–350.7 m2) and the adjacent 20 m distant closed canopies (control) were selected in a temperate Fagus orientalis forest (Hyrcanian region, Iran). Eight soil cores were collected at the cardinal points of the gap edge, including four facing the gap area—the same at the adjacent intact forest. Results: For the selected edge trees, the different size of gaps, the core position, and the tree orientation did not affect the investigated morphological traits, except for the slightly higher specific root length (SRL) for the larger fine root fraction (1–2 mm) in the side facing the gap area. Differently, the investigated chemical traits such as N concentration and cellulose:lignin ratio significantly increased with increasing gap size, the opposite for C:N ratio and lignin. Moreover, N concentration and C:N significantly decreased and increased with the fine root diameter, respectively. Conclusions: This work highlighted that, in the medium term and within the adopted size range, artificial gap opening derived from single-tree selection practice affected the chemistry rather than the biomass and morphology of gap-facing fine roots of edge trees. The medium term of six years after gap creation might have been long enough for the recovery of the fine root standing biomass to the pre-harvest condition, particularly near the stem of edge trees. A clear size threshold did not come out; nevertheless, 300 m2 may be considered a possible cut-off determining a marked change in the responses of fine roots.

Reaction Wood Anatomical Traits and Hormonal Profiles in Poplar Bent Stem and Root

Reaction wood (RW) formation is an innate physiological response of woody plants to counteract mechanical constraints in nature, reinforce structure and redirect growth toward the vertical direction. Differences and/or similarities between stem and root response to mechanical constraints remain almost unknown especially in relation to phytohormones distribution and RW characteristics. Thus, Populus nigra stem and root subjected to static non-destructive mid-term bending treatment were analyzed. The distribution of tension and compression forces was firstly modeled along the main bent stem and root axis; then, anatomical features, chemical composition, and a complete auxin and cytokinin metabolite profiles of the stretched convex and compressed concave side of three different bent stem and root sectors were analyzed. The results showed that in bent stems RW was produced on the upper stretched convex side whereas in bent roots it was produced on the lower compressed concave side. Anatomical features and chemical analysis showed that bent stem RW was characterized by a low number of vessel, poor lignification, and high carbohydrate, and thus gelatinous layer in fiber cell wall. Conversely, in bent root, RW was characterized by high vessel number and area, without any significant variation in carbohydrate and lignin content. An antagonistic interaction of auxins and different cytokinin forms/conjugates seems to regulate critical aspects of RW formation/development in stem and root to facilitate upward/downward organ bending. The observed differences between the response stem and root to bending highlight how hormonal signaling is highly organ-dependent.

Cholla cactus frames as lightweight and torsionally tough biological materials

Biological materials tested in compression, tension, and impact inspire designs for strong and tough materials, but torsion is a relatively neglected loading mode. The wood skeletons of cholla cacti, subject to spartan desert conditions and hurricane force winds, provide a new template for torsionally resilient biological materials. Novel mesostructural characterization methods of laser-scanning and photogrammetry are used alongside traditional optical microscopy, scanning electron microscopy, and micro-computed tomography to identify mechanisms responsible for torsional resistance. These methods, in combination with finite element analysis reveal how cholla meso and macro-porosity and fibril orientation contribute to highly density-efficient mechanical behavior. Selective lignification and macroscopic tubercle pore geometry contribute to density-efficient shear stiffness, while mesoscopic wood fiber straightening, delamination, pore collapse, and fiber pullout provide extrinsic toughening mechanisms. These energy absorbing mechanisms are enabled by the hydrated material level properties. Together, these hierarchical behaviors allow the cholla to far exceed bamboo and trabecular bone in its ability to combine specific torsional stiffness, strength, and toughness. STATEMENT OF SIGNIFICANCE: The Cholla cactus experiences, due to the high velocity desert winds, high torsional loads. Our study has revealed the amazingly ingenious strategy by which the tubular structure containing arrays of voids intermeshed with wood fibers resists these high loads. Deformation is governed by compressive and tensile stresses which are greatest at 45 degrees to the cross section. It proceeds by stretching, sliding, and bending of the wood fibers which are coupled with the pore collapse, resulting in delayed failure and a high torsional toughness.

Regulatory networks controlling the development of the root system and the formation of lateral roots: a comparative analysis of the roles of pericycle and vascular cambium

Background

The production of a new lateral root from parental root primary tissues has been investigated extensively, and the most important regulatory mechanisms are now well known. A first regulatory mechanism is based on the synthesis of small peptides which interact ectopically with membrane receptors to elicit a modulation of transcription factor target genes. A second mechanism involves a complex cross-talk between plant hormones. It is known that lateral roots are formed even in parental root portions characterized by the presence of secondary tissues, but there is not yet agreement about the putative tissue source providing the cells competent to become founder cells of a new root primordium.

Scope

We suggest models of possible regulatory mechanisms for inducing specific root vascular cambium (VC) stem cells to abandon their activity in the production of xylem and phloem elements and to start instead the construction of a new lateral root primordium. Considering the ontogenic nature of the VC, the models which we suggest are the result of a comparative review of mechanisms known to control the activity of stem cells in the root apical meristem, procambium and VC. Stem cells in the root meristems can inherit various competences to play different roles, and their fate could be decided in response to cross-talk between endogenous and exogenous signals.

Conclusions

We have found a high degree of relatedness among the regulatory mechanisms controlling the various root meristems. This fact suggests that competence to form new lateral roots can be inherited by some stem cells of the VC lineage. This kind of competence could be represented by a sensitivity of specific stem cells to factors such as those presented in our models.

Unraveling physiological, biochemical and molecular mechanisms involved in olive (Olea europaea L. cv. Chétoui) tolerance to drought and salt stresses

Olive (Olea europaea L.) is an economically important crop for the Mediterranean basin, where prolonged drought and soil salinization may occur. This plant has developed a series of mechanisms to tolerate and grow under these adverse conditions. By using an integrated approach, we described in Chétoui olive cultivar the changes in plant growth, oxidative damage and osmolyte accumulation in leaves, in combination with corresponding changes in physiological parameters and proteome. Our results showed, under both stress conditions, a greater growth reduction of the aboveground plant organs than of the underground counterparts. This was associated with a reduction of all photosynthetic parameters, the integrity of photosystem II and leaf nitrogen content, and corresponding representation of photosynthetic apparatus proteins, Calvin-Benson cycle and nitrogen metabolism. The most significant changes were observed under the salinity stress condition. Oxidative stress was also observed, in particular, lipid peroxidation, which could be tentatively balanced by a concomitant photoprotective/antioxidative increase of carotenoid levels. At the same time, various compensative mechanisms to cope with nitrogen source demands and to control plant cell osmolarity were also shown by olive plants under these stresses. Taken together, these findings suggest that the Chétoui variety is moderately sensitive to both drought and salt stress, although it has greater ability to tolerate water depletion.

Poplar woody taproot under bending stress: the asymmetric response of the convex and concave sides

Background and Aims Progress has been made in understanding the physiological and molecular basis of root response to mechanical stress, especially in the model plant Arabidopsis thaliana, in which bending causes the initiation of lateral root primordia toward the convex side of the bent root. In the case of woody roots, it has been reported that mechanical stress induces an asymmetric distribution of lateral roots and reaction wood formation, but the mechanisms underlying these responses are largely unknown. In the present work, the hypothesis was tested that bending could determine an asymmetric response in the two sides of the main root axis as cells are stretched on the convex side and compressed on the concave side. Methods Woody taproots of 20 seedlings were bent to an angle of 90° using a steel net. Changes in the anatomy, lignin and phytohormone content and proteome expression in the two sides of the bent root were analysed; anatomical changes, including dissimilarities and similarities to those found in poplar bent woody stem, were also considered. Key Results Compression forces at the concave side of poplar root induced the formation of reaction wood which presented a high lignin content and was associated with the induction of cambium cell activity. Auxin seemed to be the main hormone triggering lignin deposition and cell wall strengthening in the concave sides. Abscisic acid appeared to function in the water stress response induced by xylem structures and/or osmotic alterations in the compression sides, whereas gibberellins may control cell elongation and gravitropisms. Conclusions Poplar root reaction wood showed characteristics different from those produced in bent stem. Besides providing biomechanical functions, a bent root ensures water uptake and transport in the deforming condition induced by tension and compression forces by two different strategies: an increase in xylem thickness in the compressed side, and lateral root formation in the tension side.

MicroRNAs expression patterns in the response of poplar woody root to bending stress

The paper reports for the first time, in poplar woody root, the expression of five mechanically-responsive miRNAs. The observed highly complex expression pattern of these miRNAs in the bent root suggest that their expression is not only regulated by tension and compression forces highlighting their role in several important processes, i.e., lateral root formation, lignin deposition, and response to bending stress. Mechanical stress is one of the major abiotic stresses significantly affecting plant stability, growth, survival, and reproduction. Plants have developed complex machineries to detect mechanical perturbations and to improve their anchorage. MicroRNAs (miRNAs), small non-coding RNAs (18-24 nucleotides long), have been shown to regulate various stress-responsive genes, proteins and transcription factors, and play a crucial role in counteracting adverse conditions. Several mechanical stress-responsive miRNAs have been identified in the stem of Populus trichocarpa plants subjected to bending stress. However, despite the pivotal role of woody roots in plant anchorage, molecular mechanisms regulating poplar woody root responses to mechanical stress have still been little investigated. In the present paper, we investigate the spatial and temporal expression pattern of five mechanically-responsive miRNAs in three regions of bent poplar woody taproot and unstressed controls by quantitative RT-PCR analysis. Alignment of the cloned and sequenced amplified fragments confirmed that their nucleotide sequences are homologous to the mechanically-responsive miRNAs identified in bent poplar stem. Computational analysis identified putative target genes for each miRNA in the poplar genome. Additional miRNA target sites were found in several mechanical stress-related factors previously identified in poplar root and a subset of these was further analyzed for expression at the mRNA or protein level. Integrating the results of miRNAs expression patterns and target gene functions with our previous morphological and proteomic data, we concluded that the five miRNAs play crucial regulatory roles in reaction woody formation and lateral root development in mechanically-stressed poplar taproot.

Effect of short-term cadmium stress on Populus nigra L. detached leaves

Pollution by toxic metals, accumulating into soils as result of human activities, is a worldwide major concern in industrial countries. Plants exhibit different degrees of tolerance to heavy metals, as a consequence of their ability to exclude or accumulate them in particular tissues, organs or sub-cellular compartments. Molecular information about cellular processes affected by heavy metals is still largely incomplete. As a fast-growing, highly tolerant perennial plant species, poplar has become a model for environmental stress response investigations. To study the short-term effects of cadmium accumulation in leaves, we analyzed photosystem II (PSII) quantum yield, hydrogen peroxide (H2O2) generation, hormone levels variation, as well as proteome profile alteration of 50μM CdSO4 vacuum-infiltrated poplar (Populus nigra L.) detached leaves. Cadmium management brought about an early and sustained production of hydrogen peroxide, an increase of abscisic acid, ethylene and gibberellins content, as well as a decrease in cytokinins and auxin levels, whereas photosynthetic electron transport was unaffected. Proteomic analysis revealed that twenty-one proteins were differentially induced in cadmium-treated leaves. Identification of fifteen polypeptides allowed to ascertain that most of them were involved in stress response while the remaining ones were involved in photosynthetic carbon metabolism and energy production.

Proteomic analysis of Populus × euramericana (clone I-214) roots to identify key factors involved in zinc stress response

Contamination of soil and water by heavy metals has become a widespread problem; environmental pollution by high zinc (Zn) concentration occurs frequently. Although poplar (Populus spp.) has been identified as suitable for phytoremediation approaches, its response to high Zn concentrations are still not clearly understood. For this reason, we investigated the effects of Zn in Populus×euramericana clone I-214 roots by proteomic analysis. Comparative experiments were conducted on rooted woody cuttings grown in nutrient solutions containing 1mM (treatment) or 1μM (control) Zn concentrations. A gel-based proteomic approach coupled with morphological and chemical analysis was used to identify differentially represented proteins in treated roots and to investigate the effect of Zn treatment on the poplar root system. Data shows that Zn was accumulated preferentially in roots, that the antioxidant system, the carbohydrate/energy and amino acid metabolisms were the main pathways modulated by Zn excess, and that mitochondria and vacuoles were the cellular organelles predominately affected by Zn stress. A coordination between cell death and proliferation/growth seems to occur under this condition to counteract the Zn-induced damage.

Temporal analysis of poplar woody root response to bending stress

Temperate perennial woody plants use different environmental signals to coordinate their growth and development in relation to seasonal changes. Preliminary evidences suggest that, even during dormancy, plants maintain effective metabolic activities and molecular mechanisms ensuring them an eventual recording of mechanical loads during winter times. Despite their great importance for productivity and survival, plant biology investigations have poorly characterized the root growth cycle and its response to environmental stresses. In this study, we describe the proteomic changes occurring over the time in poplar root either in the absence or in response to a bending stress; corresponding expression of cell cycle regulator and auxin transporter genes was also evaluated by reverse transcription polymerase chain reaction analysis. Our results confirm previous evidences on the effect of the bending stress on the anticipation of root growth resumption, providing additional insights on a temporal modulation of various plant metabolic processes involved in dormancy break, growth resumption and stress response in the bent root; these events seem related to the differential compression and tension force distribution occurring over the plant taproot.

A genomics approach to deciphering lignin biosynthesis in switchgrass

It is necessary to overcome recalcitrance of the biomass to saccharification (sugar release) to make switchgrass (Panicum virgatum) economically viable as a feedstock for liquid biofuels. Lignin content correlates negatively with sugar release efficiency in switchgrass, but selecting the right gene candidates for engineering lignin biosynthesis in this tetraploid outcrossing species is not straightforward. To assist this endeavor, we have used an inducible switchgrass cell suspension system for studying lignin biosynthesis in response to exogenous brassinolide. By applying a combination of protein sequence phylogeny with whole-genome microarray analyses of induced cell cultures and developing stem internode sections, we have generated a list of candidate monolignol biosynthetic genes for switchgrass. Several genes that were strongly supported through our bioinformatics analysis as involved in lignin biosynthesis were confirmed by gene silencing studies, in which lignin levels were reduced as a result of targeting a single gene. However, candidate genes encoding enzymes involved in the early steps of the currently accepted monolignol biosynthesis pathway in dicots may have functionally redundant paralogues in switchgrass and therefore require further evaluation. This work provides a blueprint and resources for the systematic genome-wide study of the monolignol pathway in switchgrass, as well as other C4 monocot species.

Identification, characterization of an AP2/ERF transcription factor that promotes adventitious, lateral root formation in Populus

Using activation tagging in Populus, we have identified five mutant lines showing changes in their adventitious rooting. Among the affected lines, three showed increased and two decreased adventitious rooting. We have positioned the tag in the mutant lines via recovering genomic sequences flanking the left-hand border of the activation tagging vector and validated the transcriptional activation of the proximal genes. We further characterized one line in which the cause of the observed rooting phenotype was up-regulation of a gene encoding a transcription factor of the AP2/ERF family of unknown function (PtaERF003). We show, through retransformation, that this gene has a positive effect on both adventitious and lateral root proliferation. Comparative expression analyses show that the phenotype does not result from ectopic expression but rather up-regulation of the native expression pattern of the gene. PtaERF003 function is linked to auxin signal transduction pathway, as suggested by the rapid induction and accentuated phenotypes of the transgenic plants in presence of the hormone. Upregulation of PtaERF003 led to most significant metabolic changes in the shoot suggesting of a broader regulatory role of the gene that is not restricted to root growth and development. Our study shows that dominant tagging approaches in poplar can successfully identify novel molecular factors controlling adventitious and lateral root formation in woody plants. Such discoveries can lead to technologies that can increase root proliferation and, thus, have significant economic and environmental benefits.