Interaction between environmental factors affects the accumulation of root proteins in hydroponically grown Eucalyptus globulus (Labill.)

Proteomic analyses unraveling water stress response in two Eucalyptus species originating from contrasting environments for aridity

Eucalyptus are widely cultivated in several regions of the world due to their adaptability to different climatic conditions and amenable to tree breeding programs. With changes in environmental conditions pointing to an increase in aridity in many areas of the globe, the demand for genetic materials that adapt to this situation is required. Therefore, the aim of this work was to identify contrasting differences between two Eucalyptus species under water stress through the identification of differentially abundant proteins. For this, total protein extraction was proceeded from leaves of both species maintained at 40 and 80% of field capacity (FC). The 80% FC water regime was considered as the control and the 40% FC, severe water stress. The proteins were separated by 2-DE with subsequent identification of those differentially abundant by liquid nanocromatography coupled to high resolution MS (Q-Exactive). Comparative proteomics allowed to identify four proteins (ATP synthase gamma and alpha, glutamine synthetase and a vacuolar protein) that were more abundant in drought-tolerant species and simultaneously less abundant or unchanged in the drought- sensitive species, an uncharacterized protein found exclusively in plants under drought stress and also 10 proteins (plastid-lipid, ruBisCO activase, ruBisCO, protease ClpA, transketolase, isoflavone reductase, ferredoxin-NADP reductase, malate dehydrogenase, aminobutyrate transaminase and sedoheptulose-1-bisphosphatase) induced exclusively in the drought-tolerant species in response to water stress. These results suggest that such proteins may play a crucial role as potential markers of water stress tolerance through the identification of species-specific proteins, and future targets for genetic engineering.

Rooting of eucalypt cuttings as a problem-solving oriented model in plant biology

Species of Eucalyptus are some of the most planted trees in the world, providing fiber, cellulose, energy, and wood for construction and furniture in renewable fashion, with the added advantage of fixing large amounts of atmospheric carbon. The efficiency of eucalypts in forestry relies mostly on the clonal propagation of selected genotypes both as pure species and interspecific hybrids. The formation of new roots from cambium tissues at the base of cuttings, referred to as adventitious rooting (AR), is essential for accomplishing clonal propagation successfully. AR is a highly complex, multi-level regulated developmental process, affected by a number of endogenous and environmental factors. In several cases, highly desirable genotypes from an industrial point of view carry along the undesirable trait of difficulty-to-root (recalcitrance). Understanding the bases of this phenotype is needed to identify ways to overcome recalcitrance and allow efficient clonal propagation. Herein, an overview of the state-of-the-art on the basis of AR recalcitrance in eucalypts addressed at various levels of regulation (transcript, protein, metabolite and phenotype), and OMICs techniques is presented. In addition, a focus is also provided on the gaps that need to be filled in order to advance in this strategic biological problem for global forestry industry relying on eucalypts.

Gene expression analysis in Eucalyptus globulus exposed to drought stress in a controlled and a field environment indicates different strategies for short- and longer-term acclimation

Previous knowledge suggested the involvement of specific pathways/proteins that could be identified as potential molecular indicators linked to enhanced drought tolerance in Eucalyptus globulus. Here, we looked for specific variations in key transcripts of two Eucalyptus globulus clones (AL-18 and AL-13) exposed to water deficit and rehydration with two main goals: (i) to check if and how transcripts potentially associated with stress response and protection are modulated in a controlled experiment; and (ii) to verify if the transcript response is robust in a field case study. Our results showed that the controlled experiment induced a severe acute stress that resulted in a strong realignment of gene expression resulting from an overwhelming of physiological adjustments to water limitation. A number of transcripts exhibited altered abundance after the acute water stress: reduction of RuBisCO activase and mitochondrial glycine cleavage system H protein, and increase of isoflavone reductase. Malate dehydrogenase, catalase, dehydration response element B1A and potassium channel GORK showed a different abundance pattern in each clone. The stress in the field was more moderate and chronic and the plants were able to deal with the stress primarily through physiological adjustments resulting in much smaller changes in gene expression. The transcripts of clone AL-18 showed few alterations between irrigated and non-irrigated plants throughout the experiment, while the transcript changes found in clone AL-13 highlighted the impact of early rewatering rather than growing under extended drought typical of a Mediterranean summer. Although a few concurrent responses were found, the results obtained in the field study draw a very distinct picture when compared with the controlled experiment.

Variability studies of allochthonous stone pine (Pinus pinea L.) plantations in Chile through nut protein profiling

Stone pine (Pinus pinea) is characterized by low differentiation of growth parameters, high phenotypic plasticity and low genetic variability; detecting its diversity in introduced Chilean populations is therefore relevant for conservation and breeding programs. Here, variability among allochthonous Stone pine populations in Chile was explored using electrophoresis-based proteomic analysis of pine nuts. Cones from 30 populations distributed along a climatic gradient in Chile were surveyed and sampled, and proteins were extracted from seed flour using the TCA-acetone precipitation protocol. Extracts were subjected to SDS-PAGE and 2-DE for protein resolution, gel images captured, and spot or bands intensity quantified and subjected to statistical analysis (ANOVA, unsupervised Hierarchical Analysis Clustering and PLS regression). Protein yield ranged among populations from 161.7 (North populations) to 298.7 (South populations) mg/g dry weight. A total of 50 bands were resolved by SDS-PAGE in the 6.5-200 kDa Mr. range, of which 17 showed quantitative or qualitative differences, with 12 proteins identified. Pine nut extracts from the most distant populations were analyzed by 2-DE and a total of 129 differential spots were observed, out of which 13 were proposed as putative protein markers of variability. Out of the 129 spots, 118 proteins were identified after MALDI-TOF/TOF analysis. Identified proteins were classified into two principal categories: reserve and stress related. We provide the first protein map of P. pinea nuts. The use of a proteomic approach was useful to detect variability of Stone pine across three Chilean macrozones, with correlations between protein profiles and geoclimatic parameters, suggesting a new approach to study the variability of this species.

BIOLOGICAL SIGNIFICANCE

This study presents the first protein map of Stone pine nuts, relevant for the advancement of protein characterization in pine nuts. Putative protein markers are proposed, evidencing that a proteomic approach may be useful to detect variability of Stone pine across Chilean macrozones, suggesting a new approach to study the variability of this species, which may also be extrapolated to other forest fruit species.

Physiological, biochemical, and proteome profiling reveals key pathways underlying the drought stress responses of Hippophae rhamnoides

The effects of drought on plant growth and development are occurring as a result of climate change and the growing scarcity of water resources. Hippophae rhamnoides has been exploited for soil and water conservation for many years. However, the outstanding drought-resistance mechanisms possessed by this species remain unclear. The protein, physiological, and biochemical responses to medium and severe drought stresses in H. rhamnoides seedlings are analyzed. Linear decreases in photosynthesis rate, transpiration rate, and the content of indole acetic acid in roots, as well as a linear increase in the contents of abscisic acid, superoxide dismutase, glutathione reductase, and zeatin riboside in leaves are observed as water potential decreased. At the same time, cell membrane permeability, malondialdehyde, stomatal conductance, water use efficiency, and contents of zeatin riboside in roots and indole acetic acid in leaves showed nonconsistent changes. DIGE and MS/MS analysis identified 51 differently expressed protein spots in leaves with functions related to epigenetic modification and PTM in addition to normal metabolism, photosynthesis, signal transduction, antioxidative systems, and responses to stimuli. This study provides new insights into the responses and adaptations in this drought-resistant species and may benefit future agricultural production.

Eucalyptus urograndis stem proteome is responsive to short-term cold stress

Eucalyptus urograndis is a hybrid eucalyptus of major economic importance to the Brazilian pulp and paper industry. Although widely used in forest nurseries around the country, little is known about the biochemical changes imposed by environmental stress in this species. In this study, we evaluated the changes in the stem proteome after short-term stimulation by exposure to low temperature. Using two-dimensional gel electrophoresis coupled to high-resolution mass spectrometry-based protein identification, 12 proteins were found to be differentially regulated and successfully identified after stringent database searches against a protein database from a closely related species (Eucalyptus grandis). The identification of these proteins indicated that the E. urograndis stem proteome responded quickly to low temperature, mostly by down-regulating specific proteins involved in energy metabolism, protein synthesis and signaling. The results of this study represent the first step in understanding the molecular and biochemical responses of E. urograndis to thermal stress.

Hormonal dynamics during recovery from drought in two Eucalyptus globulus genotypes: from root to leaf

Drought is a limiting environmental stress that represents a growing constraint to the forestry sector. Eucalyptus globulus is a widely planted coppice species, which capacity to cope with water deficit has already been described. However, the capacity of this species to recover is still poorly understood. In this study, we aimed to investigate the changes in abscisic acid (ABA), ABA-glucose ester (ABA-GE) and jasmonic acid (JA) content in leaves, xylem sap and roots of two genotypes (AL-10 and AL-18) during rewatering (2 h, 4 h, 24 h, and 168 h), after a drought stress period (0 h). We wished to clarify the role of these hormones in the recovery from drought and to determine whether these hormonal relations were related to specific genotype metabolisms. Our results showed that drought caused an increased in ABA and ABA-GE levels in all analysed plant parts, while JA content decreased in leaves, increased in xylem sap and did not change in roots. Some of these responses were genotype specific. During rewatering, ABA and ABA-GE content decreased in both genotypes and all plant parts, but at different time scales, and JA levels did not greatly change. Again, the genotypes responded differently. Altogether, our results characterised the response pattern of clone AL-10 as more responsive and defended that leaf should be used in preliminary screening methods of stress tolerance. The hormonal dynamics were related to the previously documented responses of these genotypes and sustain further physiological and molecular studies of water stress in this and other tree species.

Water stress and recovery in the performance of two Eucalyptus globulus clones: physiological and biochemical profiles

Eucalyptus plantations are among the most productive forest stands in Portugal and Spain, being mostly used for pulp production and, more recently, as an energy crop. However, the region's Mediterranean climate, with characteristic severe summer drought, negatively affects eucalypt growth and increases mortality. Although the physiological response to water shortage is well characterized for this species, evidence about the plants' recovery ability remains scarce. In order to assess the physiological and biochemical response of Eucalyptus globulus during the recovery phase, two genotypes (AL-18 and AL-10) were submitted to a 3-week water stress period at two different intensities (18 and 25% of field capacity), followed by 1 week of rewatering. Recovery was assessed 1 day and 1 week after rehydration. Drought reduced height, biomass, water potential, NPQ and gas exchange in both genotypes. Contrarily, the levels of pigments, chlorophyll fluorescence parameters (F(v) /F(m) and (φPSII)), MDA and ABA increased. During recovery, the physiological and biochemical profile of stressed plants showed a similar trend: they experienced reversion of altered traits (MDA, ABA, E, g(s), pigments), while other parameters did not recover ((φPSII), NPQ). Furthermore, an overcompensation of CO(2) assimilation was achieved 1 week after rehydration, which was accompanied by greater growth and re-establishment of oxidative balance. Both genotypes were tolerant to the tested conditions, although clonal differences were found. AL-10 was more productive and showed a more rapid and dynamic response to rehydration (namely in carotenoid content, (φPSII) and NPQ) compared to clone AL-18.

Proteomic plasticity of two Eucalyptus genotypes under contrasted water regimes in the field

Water deficit affects tree growth and limits wood production. In an attempt to identify the molecular triggers of adaptation mechanisms to water deficit in Eucalyptus, we investigated protein expression patterns of two ecophysiologically contrasted Eucalyptus genotypes. They were grown in the field in either natural conditions or irrigated for 7 weeks during the dry season in the Republic of Congo. At the phenotypic level, genotype (G), treatment (T) and/or G × T interaction effects were observed for above- and below-ground biomass-related traits. At the molecular level, changes in protein abundance were recorded in leaves (acidic pH 4-7, and basic pH 7-11, proteomes) and stems (acidic proteome) using two-dimensional gel electrophoresis (2-DE). One third of the detected protein spots displayed significant G, T and/or G × T effects, and 158 of them were identified by tandem mass spectrometry (LC-MS/MS) analysis. Thus, several proteins whose molecular plasticity was genetically controlled (i.e. G × T effect) were revealed, highlighting adaptive mechanisms to water deficit specific to each genotype, namely cell wall modification, cell detoxification and osmoregulation. Transcript abundances corresponding to G × T proteins were also investigated by quantitative RT-PCR. These proteins represent relevant targets to improve drought resistance in this ecologically and economically important forest tree genus.

Proteomics research on forest trees, the most recalcitrant and orphan plant species

The contribution of proteomics to the knowledge of forest tree (the most recalcitrant and almost forgotten plant species) biology is being reviewed and discussed, based on the author's own research work and papers published up to November 2010. This review is organized in four introductory sections starting with the definition of forest trees (1), the description of the environmental and economic importance (2) and its derived current priorities and research lines for breeding and conservation (3) including forest tree genomics (4). These precede the main body of this review: a general overview to proteomics (5) for introducing the forest tree proteomics section (6). Proteomics, defined as scientific discipline or experimental approach, it will be discussed both from a conceptual and methodological point of view, commenting on realities, challenges and limitations. Proteomics research in woody plants is limited to a reduced number of genera, including Pinus, Picea, Populus, Eucalyptus, and Fagus, mainly using first-generation approaches, e.g., those based on two-dimensional electrophoresis coupled to mass spectrometry. This area joins the own limitations of the technique and the difficulty and recalcitrance of the plant species as an experimental system. Furthermore, it contributes to a deeper knowledge of some biological processes, namely growth, development, organogenesis, and responses to stresses, as it is also used in the characterization and cataloguing of natural populations and biodiversity (proteotyping) and in assisting breeding programmes.