Xylem Fibers Research Articles

Simplicia quality analysis and phytochemical screening of red bracts of Kepok banana flower (Musa paradisiaca L.)

Background: The Kepok banana plant (Musa paradisiaca L.) has been processed into food preparations, empirically used as medicine, and as a cosmetic for hair and skin care. Kepok banana flowers have bracts and male flowers contain nutritional content, namely carbohydrates, protein, low fat, high fiber and also minerals such as phosphorus, iron and calcium) as well as vitamins A, B1 and vitamin C. To determine the potential of the red bracts of Kepok banana flowers as an efficacious ingredient in medical and cosmetic preparations, simplicia characteristics testing was carried out to determine the quality of simplicial and phytochemical screening to determine secondary metabolite compounds in plant. Objective: The aim of this research was to identify the characteristics of simplicia and screen phytochemicals in the red bracts of Kepok banana flowers (Musa paradisiaca L.). Method: This research was carried out experimentally in the Pharmaceutical and Chemical Technology laboratory, Faculty of Pharmacy, Lincoln University College Malaysia. The red bracts of the Kepok banana flower (Musa paradisiaca L.) were dried by drying, protected from direct sunlight. Extract of the red bracts of the Kepok banana flower (Musa paradisiaca L.) was obtained using the Ultrasonic Assisted Extraction (UAE) method with 70% ethanol solvent. Results: Microscopic results of powder and fresh simplicia of red bracts of Kepok banana flowers (Musa paradisiaca L.) were obtained, epidermis fragments, xylem fibers, sclerenchyma fibers, anomocytic type stomata, endocarp and essential oil. The results of the simplicia quality inspection showed that the water soluble essence content was 37.15%, the ethanol soluble essence content was 16.57%, the water content was 5.31%, the total ash content was 9.04%. and the acid insoluble ash content was 2.42%. Phytochemical screening of the red bractea extract of Kepok banana flowers (Musa paradisiaca L.) obtained alkaloids, phenolics, flavonoids, tannins and triterpenoids/steroids. Conclusion: The results of the quality analysis of the simplicia red bracts of the Kepok banana flower (Musa paradisiaca L.) met the simplicia quality standards and the results of the phytochemical screening examination of the red bractea extract of the Kepok banana flower (Musa paradisiaca L.) contained alkaloids, phenolics, flavonoids, tannins and triterpenoids/steroids compounds.

Spatiotemporal bifurcation of HY5-mediated blue-light signaling regulates wood development during secondary growth

Plants have evolved photoreceptors to optimize their development during primary growth, including germination, hypocotyl elongation, cotyledon opening, and root growth, allowing them to adapt to challenging light conditions. The light signaling transduction pathway during seedling establishment has been extensively studied, but little molecular evidence is available for light-regulated secondary growth, and how light regulates cambium-derived tissue production remains largely unexplored. Here, we show that CRYPTOCHROME (CRY)-dependent blue light signaling and the subsequent attenuation of ELONGATED HYPOCOTYL 5 (HY5) movement to hypocotyls are key inducers of xylem fiber differentiation in Arabidopsis thaliana. Using grafted chimeric plants and hypocotyl-specific transcriptome sequencing of light signaling mutants under controlled light conditions, we demonstrate that the perception of blue light by CRYs in shoots drives secondary cell wall (SCW) deposition at xylem fiber cells during the secondary growth of hypocotyls. We propose that HY5 is a blue light-responsive mobile protein that inhibits xylem fiber formation via direct transcriptional repression of NAC SECONDARY WALL THICKENING PROMOTING 3 (NST3). CRYs retain HY5 in the nucleus, impede its long-distance transport from leaf to hypocotyl, and they initiate NST3-driven SCW gene expression, thereby triggering xylem fiber production. Our findings shed light on the long-range CRYs-HY5-NST3 signaling cascade that shapes xylem fiber development, highlighting the activity of HY5 as a transcriptional repressor during secondary growth.

CCT39-COMT1/BGLU18-2 module promotes lignin biosynthesis in poplar

Lignin is a crucial constituent of cell walls and plays a pivotal role in plant growth and development. However, the transcriptional regulatory network governing lignin biosynthesis is not fully understood. In this study, we observed that PpnCCT39 overexpression resulted in greener stems, larger basal diameters, and increased stem dry weight. Additionally, the secondary xylem of lines overexpressing PpnCCT39 was wider, had larger xylem fiber cell areas, and thicker cell walls, compared to those of wild-type plants. Furthermore, PpnCCT39 overexpression led to elevated lignin content and enhanced the rigidity of secondary cell walls. RNA-seq and ChIP-seq association analyses identified 826 potential regulatory target genes of PpnCCT39 that were upregulated and expressed in 1-month-old PpnCCT39 overexpression lines. Gene enrichment analyses revealed enrichment in pathways related to cell wall formation, xylem and phloem development, and the phenylpropanoid pathway. Two genes involved in lignin biosynthesis, PagCOMT1 and PagBGLU18–2, exhibited significantly increased expression in stems of lines overexpressing PpnCCT39, as demonstrated by high FPKM values and RT-qPCR results. Further investigations using yeast one-hybrid, dual-luciferase assays, and electrophoretic mobility shift assays demonstrated that PpnCCT39 directly activates the transcription of PagCOMT1 and PagBGLU18–2, thereby promoting lignin biosynthesis. This study elucidated the transcriptional regulatory mechanism of PpnCCT39 in poplars and revealed its role in activating the expression of key lignin biosynthesis genes. PpnCCT39 facilitates lignin biosynthesis and secondary growth processes, offering a novel theoretical framework for modulating lignin biosynthesis and enhancing timber yield through molecular design.

Investigating the existence of an osmotic barrier between xylem fibres and vessels in sugar maple (Acer saccharum) using microCT.

Sugar maples (Acer saccharum Marshall) develop elevated stem pressures in springtime through the compression and expansion of gas bubbles present within xylem fibres. The stability of this gas within the fibres is hypothesised to be due to the elevated sugar concentration of maple sap and the presence of an osmotic barrier between fibres and vessels. Without this osmotic barrier gas bubbles are predicted to dissolve rapidly. In this work we investigated the existence of this osmotic barrier. We quantified the fraction of the xylem occupied by gas-filled fibres using synchrotron based microCT. After imaging fresh stem segments we perfused them with either a 2% sucrose solution or water, imaging again following perfusion. In this way we directly observed how total gas present in the fibres changed when an osmotic pressure difference should be present, with the 2% sucrose solution, and when it is absent, with the water. Following a first round of perfusion we perfused stem segments with the other perfusate, repeating this multiple times to observe how switching perfusates affected gas-filled fibres. We found that perfusing stem segments with water resulted in a significant reduction in the xylem fibre gas, but perfusing stem segments with a sucrose solution did not significantly reduce the gas in the fibres. These results support the hypothesis that an osmotic barrier exists between fibres and vessels.

PagNAC2a promotes phloem fiber development by regulating PagATL2 in poplar

Phloem fiber is a key component of phloem tissue and is involved in supporting its structural integrity. NAC domain transcription factors are master switches that regulate secondary cell wall (SCW) biosynthesis in xylem fibers, but the mechanism by which NACs regulate phloem fiber development remains unexplored. Here, a NAC2-like gene in poplar, PagNAC2a, was shown to be involved in phloem fiber differentiation. qRT-PCR and GUS staining revealed that PagNAC2a was specifically expressed in the phloem zone of poplar stems. The overexpression of PagNAC2a in poplar increased plant biomass by increasing plant height, stem diameter, and leaf area. Stem anatomy analysis revealed that overexpression of PagNAC2a resulted in enhanced phloem fiber differentiation and cell wall deposition. In addition, PagNAC2a directly upregulated the expression of PagATL2, a gene involved in phloem development, as revealed by yeast one hybrid (Y1H) and electrophoretic mobility shift assay (EMSA) assays. Overall, we proposed that the PagNAC2a was a positive regulator of phloem fiber development in poplar, and these results provided insights into the molecular mechanisms involved in the differentiation of phloem fibers.

Identification of BpEXP family genes and functional characterization of the BpEXPA1 gene in the stems development of Betula platyphylla

Expansins (EXPs) are unique plant cell wall proteins with the ability to induce cell wall expansion and play potential roles in xylem development. In the present study, a total of 25 BpEXP genes were identified in Betula platyphylla. Results of bioinformatics analysis described that BpEXP gene family was highly conserved in the process of evolution. All these genes were clustered into four groups, EXPA (Expansin A), EXPB (Expansin B), EXLA (Expansin-like A) and EXLB (Expansin-like B), according to phylogenetic analysis and BpEXPA1 was highly homologous to PttEXP1 and PttEXP2. The results of RT-qPCR showed that BpEXPA1 was expressed higher in stems and preferentially expressed in the first internodes, followed by apical buds and the third internodes, promoter expression analysis with GUS assay demonstrated that it was expressed in developing xylem, suggesting that BpEXPA1 might be involved in the development of the primary stems of birch. Overexpression of BpEXPA1 can promote cortex cell expansion and then enlarge the cortex cell area and layer, however inhibit the secondary cell wall deposition and result in the thinner cell wall and larger lumens of xylem fiber in transgenic plants. This study will provide information for investigating the regulation mechanism of BpEXP family genes and gene resources for birch genetics improvement.

EVALUATION OF PROPERTIES OF WOOD PLASTIC COMPOSITES MADE FROM SEVEN TYPES OF LIGNOCELLULOSIC FIBERS

This article aims to investigate the characteristics of wood plastic composites (WPC) prepared from polyethylene (PE) reinforced with lignocellulosic fibers derived from the xylem and bark of Masson pine, fir, cypress, as well as from Moso bamboo. The surface polarity and elemental composition of fibers were determined through contact angle measurements and X-ray photoelectron spectroscopy (XPS). The lignocellulosic fiber/PE composites were manufactured through hot-pressing technique, and their water absorption, mechanical properties, and mildew resistance were evaluated. The results revealed that the surface free energy of xylem fibers was higher than that of bark fibers among the three conifer species. XPS analysis showed that the O/C ratio of bark was consistently lower than that of xylem fiber. Among the three conifers, the Masson pine bark had the lowest O/C ratio (22.25%), while its xylem fibers had the highest ratio of 41.64%. WPC made with bark fibers had better water resistance. Additionally, the composites reinforced with xylem fibers showed superior static bending strength, impact strength, and mildew-resistant properties as compared to the composites reinforced with bark fibers. WPC made from bamboo fibers exhibited the best water resistance, with a water absorption rate and thickness swelling rate of 1.83% and 1.42%, respectively. They also had the highest static bending strength, elastic modulus, and impact strength, at 41.31 MPa, 3.82 GPa, and 10.24 kJ/m2, respectively. The WPC made from fir xylem fibers showed the most effective mildew resistance, with the smallest damage (0.50).

PlLAC15 Facilitates Syringyl Lignin Deposition to Enhance Stem Strength in Herbaceous Peony

Stems are prone to bending or lodging due to inadequate stem strength, which seriously reduces the cut-flower ornamental quality of herbaceous peony (Paeonia lactiflora Pall.). Plant LACCASE (LAC), a copper-containing polyphenol oxidase, has been shown to participate in the polymerization process of monolignols; however, the role of LAC in regulating the stem strength of P. lactiflora remains unclear. Here, the full-length cDNA of PlLAC15, which demonstrated a positive association with stem strength, was isolated. It consisted of 1790 nucleotides, encoding 565 amino acids that had four typical laccase copper ion-binding domains. Moreover, PlLAC15 was highly expressed in the stem, and its expression level gradually significantly increased during stem development. Furthermore, PlLAC15 was found to be localized specifically to the cell wall, and its recombinant protein exhibited laccase activity. Additionally, the role of PlLAC15 in regulating the stem strength of P. lactiflora was confirmed by transgenic studies. When PlLAC15 was overexpressed in tobacco, stem strength increased by more than 50%, S-lignin was significantly deposited, and the lignification degree of stem xylem fiber cells increased. These results suggested that PlLAC15 facilitated S-lignin deposition to enhance stem strength in P. lactiflora, which would provide precious information that benefits future exploration of stem bending or lodging resistance in plants.

Effect of girdling on phloem and xylem formation in a range of gymnosperms and angiosperms

Summary Girdling is often used as an experimental method to study source/sink controls of cambial growth. However, the phloem responses to girdling have not been well investigated. The aim of this research was to characterize the anatomical changes in the phloem and xylem of different species following trunk girdling. Different species of gymnosperm (Abies sibirica, Pinus sylvestris) and angiosperm (Alnus incana, Populus tremula) woody plants were selected. We girdled trunks during active growth and sampled tissues at two levels (1 cm and 35 cm) above the girdle at the end of the growing season. General responses to girdling were recorded for the studied species, such as increased phloem increments, parenchymatization of conducting tissues, reduction in the size of conducting elements, and an increase in the size of axial parenchyma cells in the phloem and xylem. We observed the suppression of xylogenesis in 3 out of 4 species. Differences in the structure of conducting tissues were found, which are due to species differences in the initial tissue structure. In gymnosperms, noticeable differences in the number of resin ducts in the xylem were observed between control and girdled trees. In angiosperms, we found the formation of cells with thickened cell walls in girdled trees (i.e., the formation of phloem and xylem fibers with thickened cell walls in aspen and sclereids in the phloem of alder). Based on the literature data, the observed responses may be due to both the wounding effect and the influence of the high sugar content above the girdle.

Genome-Wide Methylation Landscape Uncovers the Role of DNA Methylation in Ramie (Boehmeria nivea L.) Bast Fiber Growth

Ramie is one of the most important fiber crops in China, with fibers extracted from stem barks having been used as textile materials for thousands of years. DNA methylation is an important epigenetic modification involved in plant growth and development. However, the role of methylation in ramie fiber growth remains poorly understood. In the present study, we investigated the DNA methylation landscape of the nuclear genome in bark sections taken from the top (TPS) and the middle (MPS) of the stems of ramie plants, which represent different stages of fiber growth, using whole-genome bisulfite sequencing. We detected 7,709,555 and 8,508,326 5-methylcytosines in the TPS and MPS genomes, respectively. The distribution of methylation across three sequence contexts, CG, CHG, and CHH, varied greatly among gene elements, with methylation at CHH being the most prevalent. Comparison of methylation levels between the TPS and MPS genomes revealed 23.162 Mb of differentially methylated genomic regions, encompassing 9485 genes. Among these differentially methylated genes, 841 exhibited altered expression in the MPS genome. Notably, an SND2 ortholog Bni05G006779 showed a negative correlation between its expression and methylation levels. Overexpression of Bni05G006779 in Arabidopsis dramatically increased the number of xylem fibers and the secondary wall thickness of the fibers in the stems of transgenic plants. These findings provide important insights into the involvement of DNA methylation in regulating ramie fiber growth.

STUDY ON ANATOMICAL CHARACTERIZATION OFCLERODENDRUM QUADRILOCULARE (BLANCO) MERR

Clerodendrum quadriloculare (Blanco) Merr., commonly called as the bronze-leaved Clerodendrum, shooting star, or Starburst bush, is native to New Guinea and Philippines. It coming under the family Lamiaceae.Traditionally, C. quadriloculare is used in Filipino folk medicine. Despite its traditional medicinal use, systematic anatomical studies on the plant are limited. Investigating its anatomical featuresare crucial for authenticating traditional claims, preventing adulteration, and distinguishing sibling species. Microscopic examination of C. quadrilocularereveals distinct superficial and anatomical characteristics. The leaf anatomy shows lower lamina comprises starch grains. The leaves exhibit anomocytic stomata. Numerous glandular dots are present on the epidermal peel. The midrib shows a thick cuticular epidermis with glandular hairs. The medulla features vascular bundles, with the outer phloem region having fibrous bands. The pith consists of small prismatic crystals. The petioles anatomy includes a thick-walled epidermis and numerous glandular trichomes. The vascular bundleswith brachysclereids as bundle caps. The stems transverse section reveals an outer periderm with lenticels. The outer edge of the secondary phloem contains fibers. The xylem vessels are linearly arranged and uni-bi seriate, with the presence of xylem parenchyma and fibers. The pith is thick-walled, with primary bundles intruding into it. The wood highlights tracheids and large vessel elements, with lateral vessel walls featuring bordered pits. The bark is well-deposited with starch granules. In conclusion, the anatomical study of C. quadriloculare provides a comprehensive understanding of its internal structure, contributing to the scientific validation of its traditional medicinal uses. The findings highlight the plants complex histological features, which are essential for its identification, potential medicinal applications, and conservation. This research underscores the importance of anatomical profiling in authenticating traditional knowledge and protecting valuable plant species from adulteration and misidentification.

Wavy grain in sycamore maple (Acer pseudoplatanus) — a structural analysis of xylem and phloem

Summary Wavy grain (fiddle back) in trees has attracted the interest of both science and industry due to its aesthetic and economic importance. As the underlying causes of this particular structural feature in trees remain uncertain, this study investigates the possibility of non-invasively detecting wavy grain patterns within tree bark tissue of sycamore maple (Acer pseudoplatanus). Wavy grain, characterized by alternating light and dark stripes on the wood surface, is highly valued in the production of musical instruments. This research explores the potential of employing transmission light microscopy and microcomputer tomography (μCT) to examine the internal structure of phloem tissue to inspect whether the wavy grain structure found in the xylem continues in the bark. The investigation of phloem tissue proved challenging due to its irregular cellular arrangement. Nonetheless, slight phloem fibre deviations were observed in individual trees. However, no definite connection was found between the periodic oscillations of the xylem fibre tissue and the structure of the phloem in a given area. Variability in wavy grain expression among the xylem of the trees posed an additional challenge, with amplitudes and wavelengths differing across specimens. The application of μCT offered advantages over traditional sample preparation, enabling the study of the anatomical structure across larger areas and multiple depths without altering the specimen. This study sheds light on the potential of employing advanced imaging techniques to detect wavy grain patterns, providing insights that could benefit industries reliant on high-quality wood, such as musical instrument crafting and forestry.

The effect of nitrogen source and levels on hybrid aspen tree physiology and wood formation.

Nitrogen can be taken up by trees in the form of nitrate, ammonium and amino acids, but the influence of the different forms on tree growth and development is poorly understood in angiosperm species like Populus. We studied the effects of both organic and inorganic forms of nitrogen on growth and wood formation of hybrid aspen trees in experimental conditions that allowed growth under four distinct steady-state nitrogen levels. Increased nitrogen availability had a positive influence on biomass accumulation and the radial dimensions of both xylem vessels and fibers, and a negative influence on wood density. An optimal level of nitrogen availability was identified where increases in biomass accumulation outweighed decreases in wood density. None of these responses depended on the source of nitrogen except for shoot biomass accumulation, which was stimulated more by treatments complemented with nitrate than by ammonium alone or the organic source arginine. The most striking difference between the nitrogen sources was the effect on lignin composition, whereby the abundance of H-type lignin increased only in the presence of nitrate. The differential effect of nitrate is possibly related to the well-known role of nitrate as a signaling compound. RNA-sequencing revealed that while the lignin-biosynthetic genes did not significantly (FDR <0.01) respond to added NO3 - , the expression of several laccases, catalysing lignin polymerization, was dependent on N-availability. These results reveal a unique role of nitrate in wood formation and contribute to the knowledge basis for decision-making in utilizing hybrid aspen as a bioresource.

The Role of Tubulin and Thaumatin Genes and Osmotic Factors in Salinity Tolerance of Tomato Plants

Soil salinity is a drastic abiotic factor that affects many physiological processes and whole plants’ activities, as well as up- and down-regulating gene expression. Studying the effect of salinity on tubulin and thaumatin relative gene expression as DNA markers for salinity stress in tomato plants is a scarcely studied topic. Tubulin regulates and plays an important role in the immunolocalization of xylem and phloem fibers in stems and additionally maintains the concept of heavy microtubule contribution during cellulose microfibril confession in secondary cell walls under abiotic and biotic stresses. Like tubulin, thaumatin-like proteins are concomitant with plant defense responses against both biotic and abiotic stresses. The expression of the thaumatin gene can be meaningfully induced after plants' exposure to either drought, freezing, or salinity stresses. Thus, the present investigation was conducted to study the impact of different salinity levels (0, 75, 100, and 120 mM NaCl) on the tomato plants’ growth, osmotic adjustment, and relative gene expression of both antioxidant and salt tolerance genes. As the salt concentration intensified, the fresh and dry weight of the shoots and the roots reduced significantly, accompanied by a reduction in chlorophyll a and carotenoids. On the other hand, salinity stress significantly decreased the level of osmotica (e.g., soluble sugars and soluble proteins) in tomato tissues compared to non-saline-grown plants, while a significant accumulation of free amino acids was recorded. At the molecular level, it was observed that the relative expression of the polyphenol oxidase, peroxidase, and thaumatin genes was high at the level of 100 mM NaCl, but it was suppressed at 120 mM NaCl. In contrast, salinity down-regulated tubulin gene expression in stressed tomato plants relative to controls, revealing various mechanisms that instigated salinity tolerance, which is concentration-dependent. The study recommended the importance of amino acids as osmotica as well as the relative expressions of PPO, peroxidase, and thaumatin genes in conferring salt tolerance at low to moderate salt levels.

Tissue-specific directionality of cellulose synthase complex movement inferred from cellulose microfibril polarity in secondary cell walls of Arabidopsis

In plant cells, cellulose synthase complexes (CSCs) are nanoscale machines that synthesize and extrude crystalline cellulose microfibrils (CMFs) into the apoplast where CMFs are assembled with other matrix polymers into specific structures. We report the tissue-specific directionality of CSC movements of the xylem and interfascicular fiber walls of Arabidopsis stems, inferred from the polarity of CMFs determined using vibrational sum frequency generation spectroscopy. CMFs in xylems are deposited in an unidirectionally biased pattern with their alignment axes tilted about 25° off the stem axis, while interfascicular fibers are bidirectional and highly aligned along the longitudinal axis of the stem. These structures are compatible with the design of fiber-reinforced composites for tubular conduit and support pillar, respectively, suggesting that during cell development, CSC movement is regulated to produce wall structures optimized for cell-specific functions.

Integrated microRNA and whole-transcriptome sequencing reveals the involvement of small and long non-coding RNAs in the fiber growth of ramie plant

BackgroundMicroRNAs (miRNAs) and long non-coding RNAs (lncRNAs) are the two main types of non-coding RNAs that play crucial roles in plant growth and development. However, their specific roles in the fiber growth of ramie plant (Boehmeria nivea L. Gaud) remain largely unknown.MethodsIn this study, we performed miRNA and whole-transcriptome sequencing of two stem bark sections exhibiting different fiber growth stages to determine the expression profiles of miRNAs, lncRNAs, and protein-encoding genes.ResultsAmong the identified 378 miRNAs and 6,839 lncRNAs, 88 miRNAs and 1,288 lncRNAs exhibited differential expression. Bioinformatics analysis revealed that 29 and 228 differentially expressed protein-encoding genes were targeted by differentially expressed miRNAs and lncRNAs, respectively, constituting eight putative competing endogenous RNA networks. lncR00022274 exhibited downregulated expression in barks with growing fibers. It also had an antisense overlap with the MYB gene, BntWG10016451, whose overexpression drastically increased the xylem fiber number and secondary wall thickness of fibers in the stems of transgenic Arabidopsis, suggesting the potential association of lncR00022274-BntWG10016451 expression with fiber growth.ConclusionsThese findings provide insights into the roles of ncRNAs in the regulation of fiber growth in ramie, which can be used for the biotechnological improvement of its fiber yield and quality in the future.

Drought-induced fiber water release and xylem embolism susceptibility of intact balsam poplar saplings.

Balsam poplar (Populus balsamifera L.) is a widespread tree species in North America with significant ecological and economic value. However, little is known about the susceptibility of saplings to drought-induced embolism and its link to water release from surrounding xylem fibers. Questions remain regarding localized mechanisms that contribute to the survival of saplings in vivo of this species under drought. Using X-ray micro-computed tomography on intact saplings of genotypes Gillam-5 and Carnduff-9, we found that functional vessels are embedded in a matrix of water-filled fibers under well-watered conditions in both genotypes. However, water-depleted fibers started to appear under moderate drought stress while vessels remained water-filled in both genotypes. Drought-induced xylem embolism susceptibility was comparable between genotypes, and a greater frequency of smallerdiameter vessels in GIL-5 did not increase embolism resistance in this genotype. Despite having smaller vessels and a total vessel number that was comparable to CAR-9, stomatal conductance was generally higher in GIL-5 compared to CAR-9. In conclusion, our in vivo data on intact saplings indicate that differences in embolism susceptibility are negligible between GIL-5 and CAR-9, and that fiber water release should be considered as a mechanism that contributes to the maintenance of vessel functional status in saplings of balsam poplar experiencing their first drought event.

PagSWEET17a Mediates Sugar Transport in Root and Affects Drought Tolerance in Populus alba × P. glandulosa

Sugars are the principal carbon and energy sources and serve as osmotic regulators and radical scavengers, thus playing an important role in plant responses to drought. Sugar transporters steering the distribution of sugar are vital players involved in tolerance to drought. Sugars Will Eventually be Exported Transporters (SWEETs) facilitate both the influx and efflux of mono- and/or disaccharides and control both inter and intracellular distribution of sugars. PagSWEET17a in Populus alba × P. glandulosa is one of four orthologous genes of AtSWEET17 in Arabidopsis. Unlike AtSWEET17, which is a vacuolar localized transporter, PagSWEET17a is localized to the endoplasmic reticulum (ER). Here, the role of PagSWEET17a in poplar responses to drought stress was investigated. PagSWEET17a was specifically expressed in cambium cells in younger root but mainly located in phloem fibers, and xylem vessels and fiber cells in the root undergoing secondary growth. Loss of PagSWEET17a inhibited the growth of roots in poplar seedlings and led to a decreased tolerance to drought. Analysis of sugar profiles revealed that accumulations of predominant sugars were significantly suppressed in both knockout (KO) mutant lines under drought. PagSWEET17a might contribute to poplar tolerance to drought by promoting drought-induced root expansion and diminishing oxidative damages caused by the stress.

A Pectate Lyase Gene Plays a Critical Role in Xylem Vascular Development in Arabidopsis.

As a major component of the plant primary cell wall, structure changes in pectin may affect the formation of the secondary cell wall and lead to serious consequences on plant growth and development. Pectin-modifying enzymes including pectate lyase-like proteins (PLLs) participate in the remodeling of pectin during organogenesis, especially during fruit ripening. In this study, we used Arabidopsis as a model system to identify critical PLL genes that are of particular importance for vascular development. Four PLL genes, named AtPLL15, AtPLL16, AtPLL19, and AtPLL26, were identified for xylem-specific expression. A knock-out T-DNA mutant of AtPLL16 displayed an increased amount of pectin, soluble sugar, and acid-soluble lignin (ASL). Interestingly, the atpll16 mutant exhibited an irregular xylem phenotype, accompanied by disordered xylem ray cells and an absence of interfascicular phloem fibers. The xylem fiber cell walls in the atpll16 mutant were thicker than those of the wild type. On the contrary, AtPLL16 overexpression resulted in expansion of the phloem and a dramatic change in the xylem-to-phloem ratios. Altogether, our data suggest that AtPLL16 as a pectate lyase plays an important role during vascular development in Arabidopsis.

Xylem development and phloem conductivity in relation to the stem mechanical strength of Paeonia lactiflora

The quality of cut Paeonia lactiflora flowers is limited by their low stem mechanical strength, but the underlying mechanism of this low strength is poorly understood. In this study, two P. lactiflora cultivars with distinct stem mechanical strengths (Chui Touhong with low stem mechanical strength and Da Fugui with high stem mechanical strength) were used as test materials. The xylem development was examined at the cellular level, and the phloem conductivity was analyzed by evaluating phloem geometry. The results showed that the secondary cell wall formation of the xylem of Chui Touhong was affected primarily in fiber cells but was affected little in vessel cells. The formation of the secondary cell walls in the xylem fiber cells of Chui Touhong was delayed, resulting in longer and thinner fiber cells with a lack of cellulose and S-lignin in the secondary cell walls. Moreover, the phloem conductivity of Chui Touhong was lower than that of Da Fugui, and more callose was accumulated in the lateral walls of the phloem sieve elements of Chui Touhong. Consequently, the delayed deposition of the secondary cell walls of the xylem fiber cells was the main factor leading to the low stem mechanical strength of Chui Touhong, and the low stem mechanical strength was closely related to the low conductivity of sieve tubes and extensive callose accumulation in the phloem. These findings provide a new perspective on enhancing P. lactiflora stem mechanical strength by targeting single cell level, and lay the foundation for future works on the correlation between phloem long-distance transport and stem mechanical strength.