Xylem Formation Research Articles

Seasonal Vascular Tissue Formation in Four Boreal Tree Species With a Focus on Callose Deposition in the Phloem

We studied the seasonal dynamics of xylem and phloem formation in four boreal tree species that differed in leaf phenology (evergreen versus winter-deciduous) and wood anatomy (angiosperms versus gymnosperms). We sampled branch cuttings of balsam poplar (Populus balsamifera), trembling aspen (Populus tremuloides), lodgepole pine (Pinus contorta) and Siberian larch (Larix siberica) in bi-weekly intervals from the beginning to the end of the growing season. Cross sections were stained with astra blue and safranin in order to assess the width of the current year’s phloem and to distinguish between the current year’s mature and enlarging/ wall-thickening xylem. After enzymatic clearance of cytoplasmic components, we used scanning electron microscopy to observe seasonal patterns of callose deposition in sieve plates of balsam poplar. The sieve plate pores were not blocked by callose in April (prior to the start of the growing season) and in July. In October, after the end of the growing season, we found only minor callose accumulation, and only in a subset of samples. In three of the four species studied here, phloem formation in spring began before the start of xylogenesis, but the lag was shorter than what was previously reported for temperate environments with longer growing seasons. New xylem cells were first produced in trembling aspen, followed by lodgepole pine, balsam poplar and Siberian larch. Most of the xylem was produced in June and July, and all cells were mature in early September. Phloem production was mostly completed by early August. Balsam poplar had the shortest growing season in terms of cambial activity and leaf presence, suggesting a risk avoidance strategy with regards to frost damage.

Phenology of wood formation in larch (Larix decidua Mill.) trees growing along a 1000-m elevation gradient in the French Southern Alps

Spring temperature increase is the main driver of larch tree wood formation onset along a 1000-m elevation gradient in the Southern Alps, while its cessation is more probably controlled by water stress at the lowest elevation and photoperiod at higher ones. The survival of perennial plants depends on their adaptation to changing environment and specially temperature, which in trees is notably implemented through wood formation process. Our main objective is to understand how the phenology of wood formation is related to environmental factors and to temperature in particular. We monitored the xylogenesis of 60 larch trees, distributed in four stands along an elevation gradient of 1000 m in the French Southern Alps. Cambial activity started around mid-May at the lowest site (1350 m) and around mid-June at the highest one (2300 m), showing a delay of 5.4 days per °C. The onset of wall-thickening and mature phenophases followed the same linear trend with a delay of 5.2 and 3 days per °C, respectively. Phenophase cessations followed a parabolic trend with trees from the lowest site finishing their growth the first, while those from 1700 m finished the last. Our results show that the onset of xylem formation is mainly driven by spring temperature increase, while its cessation is more related to photoperiod, with water shortage being able to hasten it. Future climatic changes will most probably increase growing season length (but not necessarily wood production) and shift upwards the optimal elevation for larch growth in the Southern Alps.

Over-Expression of LaMIR166a Promotes Organs Development in Nicotiana benthamiana

Most studies related to the functions of miR165/166 have used Arabidopsis as the experimental material. However, Nicotiana benthamiana Domin is a more ideal research material as its typical stem structure. Additionally, most of the relevant research has focused on the functions of HD-ZIP III genes. As an upstream regulatory factor, the function of miR165/166 needs to be defined more directly. So far, the function of miR165/166 in N. benthamiana has not yet been examined. The Agrobacterium-mediated genetic transformation method was used to transform LaMIR166a from Larix leptolepis overexpression vector into N. benthamiana leaves, and three transgenic lines were obtained after molecular detection. Observation on the one-month seedlings revealed that overexpression of LaMIR166a markedly enhanced the biomass accumulation of N. benthamiana plants. Additionally, growth parameters including root length and basal diameter, lateral root density, stem ground diameter and stem internode length of transgenic plants were significantly higher than in wild type (WT) plants. Microscopic observation of stem slices and root squashes showed that overexpression of LaMIR166a significantly promoted xylem formation and development, and the growth of root hairs, which showed a bent phenotype. The results of the present study suggest that up-regulation of miR166a promotes the growth and development of N. benthamiana plants.

Comparing Primary and Secondary Growth of Co-Occurring Deciduous and Evergreen Conifers in an Alpine Habitat

Investigations on primary and secondary growth in co-occurring species will aid in assessment of the physiological adaptation of species and the prediction of forest stand structure dynamics. To explore the correlation and coordination between primary and secondary growth, we monitored the leaf phenology, shoot elongation, and stem growth of co-occurring Larix principis-rupprechtii Mayr. and Picea meyeri Rehd. et Wils. in an alpine habitat, Luya Mountain (North-Central China), during the growing season of 2014. We measured bud development on terminal branches three days per week by direct observations and intra-annual stem xylem formation at weekly intervals by the microcores method. The onset sequence of three organs was the needle, shoot, and stem, without species-specific differences. Needles appeared one month earlier than stem growth in larch, while it was only one week earlier in spruce. The duration of needle growth was the shortest, followed by the shoot and stem. The timing of primary growth (i.e., onset, end, and maximum growth rate) between the two species was asynchronous, but secondary growth was synchronic with the same date of the maximum growth rate occurrence, potentially indicating species competition for resources. Unlike larch, spruce staggered growth peaks among different organs, which may effectively mitigate trees’ internal competition for resources. Soil temperature was positively correlated with both shoot and stem growth in the two species, whereas air temperature and soil water content were positively correlated with needle growth only in larch. Therefore, it can be inferred that the spruce will probably outcompete the larch at cold alpine treeline sites due to its high adaptability to acquiring and allocating resources. These results provide insight into the potential physiological correlation between primary and secondary growth and allow better prediction of future climate change effects on forest ecosystem productivity.

Jasmonic Acid Modulates Xylem Development by Controlling Expression of PIN-FORMED 7

ABSTRACTJasmonic acid (JA) modulates plant development, growth, and responses to stress. Previously, we showed that in Arabidopsis thaliana, JA promotes the formation of extra xylem in roots, and mutant plants unable to express PIN-FORMED 3 (PIN3) and PIN7 formed extra xylem in the absence of exogenous JA. Those results suggested that JA modulates root xylem development by controlling PIN-mediated polar auxin transport. Consistent with this, treatment with an auxin transport inhibitor induced extra xylem formation. Here, we characterized the expression of PIN3 and PIN7 in JA-treated Arabidopsis plants. PIN3 expression was not altered in response to JA; by contrast, PIN7 expression was reduced by JA, which suggested that PIN7 is involved in JA-mediated xylem development. Indeed, overexpressing PIN7 suppressed the formation of extra xylem in response to JA. Based on these results, we propose that JA mediates xylem development by controlling polar auxin transport with PIN7 critically involved in this process.

Asynchronous leaf and cambial phenology in a tree species of the Congo Basin requires space-time conversion of wood traits.

Wood traits are increasingly being used to document tree performance. In the Congo Basin, however, weaker seasonality causes asynchrony of wood traits between trees. Here, we monitor growth and phenology data to date the formation of traits. For two seasons, leaf and cambial phenology were monitored on four Terminalia superba trees (Mayombe) using cameras, cambial pinning and dendrometers. Subsequently, vessel lumen and parenchyma fractions as well as high-resolution isotopes (δ13C/δ18O) were quantified on the formed rings. All traits were dated and related to weather data. We observed between-tree differences in green-up of 45 d, with trees flushing before and after the rainy season. The lag between green-up and onset of xylem formation was 59 ± 21 d. The xylem growing season lasted 159 ± 17 d with between-tree differences of up to 53 d. Synchronized vessel, parenchyma and δ13C profiles were related to each other. Only parenchyma fraction and δ13C were correlated to weather variables, whereas the δ18O pattern showed no trend. Asynchrony of leaf and cambial phenology complicates correct interpretation of environmental information recorded in wood. An integrated approach including high-resolution measurements of growth, stable isotopes and anatomical features allows exact dating of the formation of traits. This methodology offers a means to explore the asynchrony of growth in a rainforest and contribute to understanding this aspect of forest resilience.

Calcium dynamics of reproductive apple buds during the dormant season in the Western Cape, South Africa

Calcium (Ca) deficiency in apples results in unwanted physical disorders in mature fruit that reduce fruit value. Ca deficiency associated with bitter pit is already prevalent early in the season, as it is initiated a few weeks after full bloom. Information regarding Ca concentration in reproductive buds or bud tissues is lacking, thus we investigated the Ca concentration in reproductive buds of the bitter pit susceptible ‘Braeburn’ and non-susceptible ‘Cripps Pink’ apple cultivars during the dormant season. Wavelength-dispersive x-ray spectroscopy (WD SEM) was used to quantify total Ca concentration of parenchyma and vascular tissues, from June to September, for two seasons (2012 and 2014). Results showed a changing trend in Ca concentration from June to September for total Ca concentration and between cultivars. Total Ca concentration was the highest in ‘Cripps Pink’ during the whole period and related to the non-susceptibly to Ca related disorders. Ca concentration in ‘Cripps Pink’ declined towards September in contrast with the increasing trend of ‘Braeburn’. This may indicate that the total Ca concentration of the buds, and not the change of Ca concentration in buds during winter, is associated with a higher Ca status of fruit on non-susceptible cultivars. The actual Ca concentration of ‘Cripps Pink’ at the end of August was still higher than that of ‘Braeburn’. Results may be explained partly by the time of primary xylem formation in the bud, which happens earlier in ‘Cripps Pink’, as well as the associated increase in Ca translocation, indicating the role of generic differences between cultivars, contributing towards final changes in the inherent Ca concentration at the beginning of the dormant season.

Is size an issue of time? Relationship between the duration of xylem development and cell traits.

Secondary growth is a process related to the formation of new cells that increase in size and wall thickness during xylogenesis. Temporal dynamics of wood formation influence cell traits, in turn affecting cell patterns across the tree ring. We verified the hypothesis that cell diameter and cell wall thickness are positively correlated with the duration of their differentiation phases. Histological sections were produced by microcores to assess the periods of cell differentiation in black spruce [Picea mariana (Mill.) B.S.P.]. Samples were collected weekly between 2002 and 2016 from a total of 50 trees in five sites along a latitudinal gradient in Quebec (Canada). The intra-annual temporal dynamics of cell differentiation were estimated at a daily scale, and the relationships between cell traits and duration of differentiation were fitted using a modified von Bertalanffy growth equation. At all sites, larger cell diameters and cell wall thicknesses were observed in cells that experienced a longer period of differentiation. The relationship was a non-linear, decreasing trend that occasionally resulted in a clear asymptote. Overall, secondary wall deposition lasted longer than cell enlargement. Earlywood cells underwent an enlargement phase that lasted for 12 d on average, while secondary wall thickness lasted 15 d. Enlargement in latewood cells averaged 7 d and secondary wall deposition occurred over an average of 27 d. Cell size across the tree ring is closely connected to the temporal dynamics of cell formation. Similar relationships were observed among the five study sites, indicating shared xylem formation dynamics across the entire latitudinal distribution of the species.The duration of cell differentiation is a key factor involved in cell growth and wall thickening of xylem, thereby determining the spatial variation of cell traits across the tree ring.

Tobacco plants (Nicotiana benthamiana) were influenced by silicon and were not infected by dodder (Cuscuta europaea)

The effect of silicon (Si) on tobacco (Nicotiana benthamiana) development and dodder (Cuscuta europaea) – tobacco interaction were studied. Three Si application approaches were tested: tobacco seed priming (2.5 mM Si and 5 mM Si; 2.5S, 5S), watering tobacco plants with Si solution (2.5 mM Si and 5 mM Si; 2.5W, 5W) and foliar application (1 mM Si and 2.5 mM Si; 1F, 2.5F). Dodder was not able to infect the host plant in almost all Si treatments. Only in the control and 2.5W treatments was dodder able to infect its host. A significant increase in all observed antioxidant enzymes activities (POX, CAT and SOD) occurred in the plants of 2.5W treatment after infection in comparison with the uninfected 2.5W treatment and control plants, which indicated the importance of antioxidant enzymes activities in the plant parasite – host interaction. Resistance of Si treated plants to dodder could have been due to the changes in the cell wall properties of the epidermis and cortex where activity of POX was confirmed histochemically.The growth and development of tobacco shoots were evaluated after four and eight weeks of cultivation in the individual Si treatments. The development of shoots was enhanced after eight weeks of cultivation in the 2.5S, 5S, 2.5W and 5W treatments in comparison with the control treatment. However, a negative effect of Si was observed in 1F and 2.5F treatments. In the majority of cases, the plants treated with Si had decreased chlorophyll content when compared to control, except for chl a in 5W plants after 8 weeks of cultivation. Contrary to this, carotenoids increased in all Si treated plants after eight weeks cultivation in comparison with the control. The secondary xylem formation in tobacco was enhanced after 4 and 8 weeks cultivation in shoots of plants receiving the 2.5S, 5S, 2.5W and 5W treartments. The cambium was the most active in producing secondary xylem in the 2.5S treatment. Protein profile and antioxidant enzymes activities (POX, CAT and SOD) were altered by Si treatment. After 8 weeks of cultivation, activities of POX were significantly decreased in 2.5S, 5S, 2.5W and 5W in comparison with control. Catalase was decreased in 2.5S, 5S and 5W in comparison with the control, however, 1F and 2.5F treatments had significantly increased CAT and SOD activities. The specific activity of POX was confirmed histochemically in Si treated plants in the cell walls of several stem tissues like the epidermis, cortex and pith. A small amount of H2O2 was detected in leaves in the control and Si treated plants. The amount of O2− decreased in all treatments with time. The highest Si concentration in the plants (almost 800 mg . kg−1 d. w.) was detected in the 2.5W, 5W treatments.

Growth rate rather than growing season length determines wood biomass in dry environments

A number of studies have suggested that growing season length determines carbon sequestration of forest ecosystems. Given the possibility that drought-induced growth decline will be caused by a prolonged growing season under a warming climate, we investigated the effect of growth rate and duration of xylem production on annual wood biomass in drought-prone environments. We analyzed the intra-annual dynamics of wood formation in Qilian junipers (Juniperus przewalskii) from the semi-arid north-eastern Tibetan Plateau, China (2009–2014) and in ponderosa pine (Pinus ponderosa) from the hyperarid Mojave Desert in Nevada, USA (2015–2016). Most variability in the number of xylem cells (Ncell) was related to growth rate (rm) rather than duration of cell production (Dcell). At the Tibetan sites, 69.9% and 54.7% of variability in Ncell was attributable to rm for the lower and upper treeline, respectively. Within the Mojave Desert site, 53.9% of the variability in Ncell was related to rm. The growth rate in the Tibetan Plateau forest is affected by minimum temperature and precipitation. Thus, rm is a primary control on wood biomass in conifer species of semi-arid forests. Under warmer and drier conditions, a longer growing season will not benefit xylem formation of conifers, and in turn warming-induced drought could limit carbon sequestration by reducing the rate of cell production.

Annual Variations in Norway Spruce Xylem Studied Using Infrared Micro-spectroscopy

In temperate environments, ring width, cell size and cell wall thickness within the xylem of trees are known to be affected by climate conditions. Less is known about the effect of climate conditions on the chemical characteristics of the xylem, which are important for the susceptibility of the tissue towards fungal infections as well as for the degradability of the material within the forest ecosystem. We explored the use of infrared microspectroscopy to investigate the possible effects of temperature and drought on the relative amount of cell wall biopolymers, i.e. the ratios between cellulose, hemicellulose and lignin in the earlywood xylem cell walls of Norway spruce (Picea abies (L.) Karst.) in temperate forests. Drought and warm temperatures were significantly correlated to the hemicellulose to lignin ratio of the earlywood formed the following year, perhaps due to a reduced amount of stored resources being available for xylem formation.

GmZPR3d Interacts with GmHD-ZIP III Proteins and Regulates Soybean Root and Nodule Vascular Development.

Fabaceans produce two major classes of symbiotic nodules: the indeterminate type characterized by a persistent meristem, and the determinate type that lacks a persistent meristem. The class III homeodomain leucine zipper (HD-ZIP III) transcription factor family influence development of multiple lateral organs and meristem maintenance, but their role in determinate nodule development is not known. HD-ZIP III protein activity is post-translationally regulated by members of the small leucine zipper protein (ZPR) family in arabidopsis. We characterized the ZPR gene family in soybean and evaluated their ability to interact with two key members of GmHD-ZIP III family through yeast two-hybrid assays. GmZPR3d displayed the strongest interaction with GmHD-ZIP III-2 among the different pairs evaluated. GmHD-ZIP III-1, -2, and GmZPR3d showed overlapping expression patterns in the root stele and in nodule parenchyma tissues. Over-expression of GmZPR3d resulted in ectopic root secondary xylem formation, and enhanced expression of vessel-specific master switch genes in soybean. The nodules in ZPR3d over-expressing roots were larger in size, had a relatively larger central zone and displayed increased nodule vascular branching. The results from this study point to a key role for GmZPR3d in soybean root and nodule development.

Auxin-mediated Aux/IAA-ARF-HB signaling cascade regulates secondary xylem development in Populus.

Wood development is strictly regulated by various phytohormones and auxin plays a central regulatory role in this process. However, how the auxin signaling is transducted in developing secondary xylem during wood formation in tree species remains unclear. Here, we identified an Aux/INDOLE-3-ACETIC ACID 9 (IAA9)-AUXIN RESPONSE FACTOR 5 (ARF5) module in Populus tomentosa as a key mediator of auxin signaling to control early developing xylem development. PtoIAA9, a canonical Aux/IAA gene, is predominantly expressed in vascular cambium and developing secondary xylem and induced by exogenous auxin. Overexpression of PtoIAA9m encoding a stabilized IAA9 protein significantly represses secondary xylem development in transgenic poplar. We further showed that PtoIAA9 interacts with PtoARF5 homologs via the C-terminal III/IV domains. The truncated PtoARF5.1 protein without the III/IV domains rescued defective phenotypes caused by PtoIAA9m. Expression analysis showed that the PtoIAA9-PtoARF5 module regulated the expression of genes associated with secondary vascular development in PtoIAA9m- and PtoARF5.1-overexpressing plants. Furthermore, PtoARF5.1 could bind to the promoters of two Class III homeodomain-leucine zipper (HD-ZIP III) genes, PtoHB7 and PtoHB8, to modulate secondary xylem formation. Taken together, our results suggest that the Aux/IAA9-ARF5 module is required for auxin signaling to regulate wood formation via orchestrating the expression of HD-ZIP III transcription factors in poplar.

Contrasting strategies of xylem formation between black spruce and balsam fir in Quebec, Canada.

Present-day global warming is occurring faster at higher elevations. Although there is much information regarding the divergent responses of tree growth to climate change, the altitudinal patterns of species-specific xylogenesis remains poorly understood. We investigated the xylogenesis of balsam fir (Abies balsamea Mill.) and black spruce (Picea mariana Mill. B.S.P.) at two elevations in Quebec (Canada). The number of enlarging and mature cells of the developing tree ring were counted on microcores collected weekly between 2011 and 2014. At the lower site, the growth pattern and duration of xylogenesis were similar between species. No difference in responses to temperature and solar radiation between species was observed. At the higher site, however, cell production was higher and lasted longer in balsam fir than black spruce. Furthermore, the xylem growth of balsam fir had a stronger response to temperature and solar radiation than black spruce. These findings demonstrate the contrasting strategies of wood formation of the two species, with black spruce being more conservative than balsam fir. Our study provides evidence that sympatric species can have species-specific growth dynamics and site-specific responses to the local environment. Predictions of tree growth under a changing environment require the incorporation of species-specific growth strategies.

Growing season and radial growth predicted for Fagus sylvatica under climate change

Climate scenarios for Slovenia suggest an increase in the mean annual temperature by 2 °C over the next six decades, associated with changes in the seasonal distribution of precipitation. European beech is an ecologically and economically important forest species in Europe, so it is important to understand the influence of changing conditions on its phenology and productivity for the upcoming years. We hypothesise that the ongoing warming and reduction in precipitation during the growing season will shorten the period of xylem development, thus limiting beech growth in the next decades. Xylem formation was monitored weekly from 2008 to 2016 at two sites in Slovenia. Onset and cessation of cell enlargement and secondary wall formation, as well as xylem growth, are used to evaluate climate-growth relationships by means of partial least squares regression and to predict xylem formation phenology and annual xylem increments under climate change scenarios. A positive correlation of spring phenological phases with March–May temperatures is found. In contrast, autumn phenological phases show a negative correlation with August and September temperatures, while high temperatures at the beginning of the year delay growth cessation. According to the selected climate change scenarios, phenological phases may advance by 2 days decade-1 in spring and delay by 1.5 days decade-1 in autumn. The duration of the growing season may increase by 20 days over the next six decades, resulting in 38 to 83% wider xylem increments. The growth of beech is expected to increase under a warming climate in the sites characterised by abundant water availability.

Early identification of graft compatibility through histological studies in mango

Three polyembryonic mango rootstocks namely, Nekkare, Olour and Vellaikolumban grafted with Alphonso were used to investigate grafting success and histological evaluation of graft union. The observations were recorded at 07, 14 and 21 days after grafting (DAG). The results revealed that softwood grafting of Alphonso on to Nekkare rootstock showed significantly higher vegetative growth related parameters for graft union than the rest of rootstocks (Olour and Vellaikolumban). The Olour and Vellaikolumban grafted with Alphonso showed moderate and lesser graft compatibility, respectively. The graft union compatibility was confirmed through histological studies. It was observed on 07th DAG that the tissue was juxtapose, initiated callus formation, merged union in Vellaikolumban, Olour and Nekkare rootstocks with Alphonso, respectively. Similarly, on 14thDAG, continuous proliferation of callus in Alphonso/Olour, numerous separation areas in Alphonso/Vellaikolumban were observed, while Alphonso//Nekkare had enough callus and which trigger the progressive development of xylem and phloem cells. Furthermore, on 21st DAG, graft union between Alphonso and Nekkare had better connection, vascular bundles differentiation, annual thickenings, xylem formation and xylem strands as compared to Olour and Vellaikolumban rootstocks. Histological observations and growth attributes confirmed that Alphonso grafted on Nekkare rootstock showed high graft compatibility than the rest of rootstocks, hence it can be utilized for the production of more mango saplings.

From xylogenesis to tree rings: wood traits to investigate tree response to environmental changes

ABSTRACTIt is noteworthy that the largest part of global vegetation biomass depends on a thin layer of cells: the vascular cambium. Understanding the wood formation processes and relationships with environmental factors is a crucial and timely research question requiring interdisciplinary efforts, also to upscale the information gained and to evaluate implications for tree growth and forest productivity.We provide an overview of wood formation processes up to tree-ring development, bearing in mind that the combined action of intrinsic factors and environmental drivers determines the anatomical traits of a tree ring formed at a specific time and position within the tree’s architecture. After briefly reviewing intrinsic factors, we focus attention on environmental drivers highlighting how a correct interpretation of environmental signals in tree rings must be grounded in a deep knowledge of xylogenesis and consequent wood anatomical traits. We provide guidelines on novel methods and approaches recently developed to study xylem formation. We refer to existing literature on established techniques for retrospective analyses in tree-ring series of anatomical and isotopic traits, to assess long-term ecophysiological responses to environmental variations, also giving advice on possible bias because of interand within-tree variability.Finally, we highlight that, once the temporal axis of intra-annual tree-ring variability of xylem traits is established by xylogenesis analysis, a multidisciplinary approach linking classical dendro-ecology, wood functional traits (dendro-anatomy) and eco-physiology (here focusing on dendro-isotopes) allows a better interpretation of past environmental events hidden in tree rings, and more reliable forecasts of wood growth in response to climate change.

Spring precipitation effects on formation of first row of earlywood vessels in Quercus variabilis at Qinling Mountain (China)

Under drier spring conditions, Q. variabilis might slow the FEV growth rate but will not produce narrower FEVs. FEV size might be a trade-off between hydraulic efficiency and hydraulic safety. Wood anatomical features can encode tree reactions to fluctuating environmental conditions. The first row of earlywood vessels (FEV) has proven to be very promising with respect to the inter-annual climate variability. However, knowledge of how intra-annual FEV formation reflects climate is still limited. We observed wood formation of Quercus variabilis in a warm temperate–subtropical climate transition at Qinling Mountain, China. Using micro-cores, growth of six trees was monitored at weekly/biweekly intervals February–December in 2015 and 2016, of which 2016 had a drier spring. Cambial activity and xylem cell differentiation were documented by microscopic observation. The FEV diameter and ring width were measured. The onset of xylem growth, which significantly differed between the years, occurred 8 days earlier in 2016, in mid-March. However, the completion of FEV was synchronized in mid-April over the 2 years. This delayed completion of FEV in 2016, when precipitation increased and Standardized Precipitation Evapotranspiration Index changed from a negative to positive value. The cessation of xylem formation was 3 weeks earlier during the drought year (September 5, 2016) compared to the moist year (September 29, 2015) because of the drought conditions in late summer. This finding was supported by evidence that a positive and significant correlation between precipitation and xylem growth was found only in 2016. The ring width differed significantly between the years, but the FEV diameter did not. Our results suggest that the drought-induced delayed formation of FEV and earlier cessation could be a strategy for oak trees to adapt to water stress.

Auxin (IAA) and soluble carbohydrate seasonal dynamics monitored during xylogenesis and phloemogenesis in Scots pine

The metabolic activity of phytohormones and the accumulation of carbohydrates affect the reactivation of the cambial zone and the radial increment of woody plants. We aimed to monitor the dynamics of indole-3-acetic acid (IAA) concentration and amounts of soluble carbohydrates during xylem and phloem formation of one growing season (2015). Six sample trees of Scots pine (Pinus sylvestris L.), aged 80 years on average, growing in the Sobešice research site (404 m a.s.l.) in the Czech Republic were selected. We obtained microcore samples at weekly intervals by the Trephor tool method for cell formation analysis and spectrophotometric determination of IAA and soluble carbohydrate contents. We found that time of the highest concentration of IAA (last week of April) coincided with time of the maximum number of cells in the cambial zone and highest expansion of the cell enlargement stage. When the IAA concentration was too low to be measured, latewood tracheids started to form, and late phloem sieve cell formation ceased. The highest concentration of soluble carbohydrates was 200.40 ± 21.6 µg GLU per sample (May 14). This coincided with the fastest weekly xylem cell increment. This research shows that IAA and soluble carbohydrate dynamics directly affects xylem and phloem formation.

Natural variation at XND1 impacts root hydraulics and trade-off for stress responses in Arabidopsis

Soil water uptake by roots is a key component of plant performance and adaptation to adverse environments. Here, we use a genome-wide association analysis to identify the XYLEM NAC DOMAIN 1 (XND1) transcription factor as a negative regulator of Arabidopsis root hydraulic conductivity (Lpr). The distinct functionalities of a series of natural XND1 variants and a single nucleotide polymorphism that determines XND1 translation efficiency demonstrate the significance of XND1 natural variation at species-wide level. Phenotyping of xnd1 mutants and natural XND1 variants show that XND1 modulates Lpr through action on xylem formation and potential indirect effects on aquaporin function and that it diminishes drought stress tolerance. XND1 also mediates the inhibition of xylem formation by the bacterial elicitor flagellin and counteracts plant infection by the root pathogen Ralstonia solanacearum. Thus, genetic variation at XND1, and xylem differentiation contribute to resolving the major trade-off between abiotic and biotic stress resistance in Arabidopsis.