Measurements of resistance to embolism suggest that Cupressus sempervirens has a stem xylem that resists embolism at very negative water potentials, with 50% embolism (P50) at water potentials of approximately -10 MPa. However, field observations in a semi-arid region suggest tree mortality occurs before 10% embolism. To explore the interplay between embolism and plant mortality, we conducted a controlled drought experiment involving two types of CS seedlings: a local seed source (S-type) and a drought-resistant clone propagated from a semi-arid forest (C-type). We measured resistance to embolism, leaf relative water content (RWC), water potential, photosynthesis, electrolyte leakage (EL), plant water loss, leaf hydraulic conductivity, and leaf non-structural carbohydrate (NSC) content during plant dehydration and before rewatering. All measured individuals were monitored for survival or mortality. While the S- and C-types differed in P50, transpiration, and mortality rates, both displayed seedling mortality corresponding to threshold values of 52-55% leaf RWC, 55% and 18.5% percent loss of conductivity (PLC) in the xylem, which corresponds to 48% and 37% average EL values for S and C types, respectively. Although C-type C. sempervirens NSC content increased in response to drought, no differences were observed in NSC content between live and dead seedlings of both types. Our findings do not fully explain tree mortality in the field but they do indicate that loss of membrane integrity occurs before or at xylem water potential, leading to hydraulic failure.

Stem strength plays a crucial role in the growth and development of plants, as well as in their flowering and fruiting. It not only impacts the lodging resistance of crops, but also influences the ornamental value of ornamental plants. Stem development is closely linked to stem strength; however, the roles of the SPL transcription factors in the stem development of herbaceous peony (Paeonia lactiflora Pall.) are not yet fully elucidated. In this study, we obtained and cloned the full-length sequence of PlSPL14, encoding 1085 amino acids. Quantitative real-time PCR (qRT-PCR) analysis revealed that the expression level of PlSPL14 gradually increased with the stem development of P. lactiflora and was significantly expressed in vascular bundles. Subsequently, utilizing the techniques of virus-induced gene silencing (VIGS) and heterologous overexpression in tobacco (Nicotiana tabacum L.), it was determined that PlSPL14-silenced P. lactiflora had a thinner xylem thickness, a decreased stem diameter, and weakened stem strength, while PlSPL14-overexpressing tobacco resulted in a thicker xylem thickness, an increased stem diameter, and enhanced stem strength. Further screening of the interacting proteins of PlSPL14 using a yeast two-hybrid (Y2H) assay revealed an interactive relationship between PlSPL14 and PlSLR1 protein, which acts as a negative regulator of gibberellin (GA). Additionally, the expression level of PlSLR1 gradually decreased during the stem development of P. lactiflora. The above results suggest that PlSPL14 may play a positive regulatory role in stem development and act in the xylem, making it a potential candidate gene for enhancing stem straightness in plants.

Bamboos stand out among other tall plants in being able to generate positive pressure in the xylem at night, pushing water up to the leaves and causing drops to fall from leaf tips as guttation that can amount to a steady nocturnal \'bamboo rain\'. The location and mechanism of nocturnal pressure generation in bamboos are unknown, as are the benefits for the plants. We conducted a study on the tall tropical bamboo species Bambusa oldhamii (giant timber bamboo) growing outdoors in southern California under full irrigation to determine where in the plant the nocturnal pressure is generated, when it rises in the evening, and when it dissipates in the morning. We hypothesized that the build-up of positive pressure would be triggered by the cessation of transpiration-driven sap flow and that resumption of sap flow in the morning would cause the pressure to dissipate. Nocturnal pressure was observed in mature stems and rhizomes, but never in roots. The pressure was episodic and associated with stem swelling and was usually, but not always, higher in rhizomes and basal stems than in stems at greater height. Time series analyses revealed that dry atmospheric conditions were followed by lower nocturnal pressure and rainfall events by higher stem pressure. Nocturnal pressure was unrelated to sap flow and even was generated for a short time in isolated stem pieces placed in water. We conclude that nocturnal pressure in bamboo is not \'root pressure\' but is generated in the pseudo-woody rhizomes and stems. It is unrelated to the presence or absence of sap flow and therefore must be created outside of vessels, such as in phloem, parenchyma, or fibres. It is unlikely to be a drought adaptation and may benefit the plants by maximizing stem water storage for daytime transpiration or by transporting nutrients to the leaves.

In Southeast Asia (SEA) fastidious fungi of the Ceratobasidium genus are associated with proliferation of sprouts and vascular necrosis in cacao and cassava, crops that were introduced from the tropical Americas to this region. Here, we report the isolation and in vitro culture of a Ceratobasidium sp. isolated from cassava with symptoms of witches\' broom disease (CWBD), a devastating disease of this crop in SEA. The genome characterization using a hybrid assembly strategy identifies the fungus as an isolate of the species C. theobromae, the causal agent of vascular streak dieback of cacao in SEA. Both fungi have a genome size > 31 Mb (G+C content 49%), share > 98% nucleotide identity of the Internal Transcribed Spacer (ITS) and > 94% in genes used for species-level identification. Using RNAscope® we traced the pathogen and confirmed its irregular distribution in the xylem and epidermis along the cassava stem, which explains the obtention of healthy planting material from symptom-free parts of a diseased plant. These results are essential for understanding the epidemiology of CWBD, as a basis for disease management including measures to prevent further spread and minimize the risk of introducing C. theobromae via long-distance movement of cassava materials to Africa and the Americas.

Copper (Cu) is an essential micronutrient for humans, but excessive Cu in rice grains causes health risks. Currently, the mechanisms underlying Cu accumulation in rice are unclear. Here, we identified a novel member of the high-affinity copper transporter (Ctr)-like (COPT) protein family in rice, OsCOPT7, which controls Cu accumulation in rice grains. Mutation in the coding sequence of OsCOPT7 (mutant lc1) leads to inhibition of Cu transport through the xylem, contributing to lower Cu concentrations in the grain of lc1. Knockout or modulation of the expression of OsCOPT7 significantly impacts Cu transportation in the xylem and its accumulation in rice grains. OsCOPT7 localizes at the multi-pass membrane in the cell and the gene is expressed in the exodermis and stele cells, facilitating Cu loading into the xylem. OsCOPT7 expression is upregulated under Cu deficiency and in various organs, implying its contribution to Cu distribution within the rice plant. The variable expression pattern of OsCOPT7 suggests that OsCOPT7 expression responds to Cu stress in rice. Moreover, assays reveal that OsCOPT7 expression level is suppressed by the SQUAMOSA promoter-binding protein-like 9 (OsSPL9) and that OsCOPT7 interacts with Antioxidant Protein1 (OsATX1). This study elucidates the involvement of OsCOPT7 in Cu loading into the xylem, its subsequent distribution within the rice plant, and the potential of this protein in reducing the risk of high Cu concentrations in rice grain grown on Cu-contaminated soil.

Esteya vermicola has shown promise as an efficient biological control agent against pine wilt disease, a devastating disease in pine forests caused by the pinewood nematode (PWN, Bursaphelenchus xylophilus). However, the in vivo interactions among E. vermicola, PWN, and pine hosts are less understood, both at the population and molecular levels. In this study, we performed a series of bioassays to investigate E. vermicola colonization patterns in pine xylem and its population responses to PWN invasion in healthy and PWN-induced wilting trees. Our results demonstrated that although E. vermicola exhibits slow growth, its conidia germinate and grew along the pine tracheid, even producing lunate conidia capable of initiating PWN infections within the xylem. Interestingly, while fungal hyphae became undetectable in pine sapling xylem after inoculation, the E. vermicola population increased immediately in response to PWN invasion. Furthermore, we observed a \"leap-frog\" dispersal pattern of fungal colonization in PWN-induced wilting pines, facilitated by the migration of fungal-infected nematodes. Moreover, we explored the molecular mechanisms underlying fungal tolerance to pine defense systems using transcriptomic analysis. Comparative transcriptomics revealed that carbohydrate metabolism and abiotic stress-induced oxidoreductive activities are involved in the fungal tolerance to the pine defense compound β-pinene. This study enhances our understanding of how E. vermicola colonizes and persists within pine xylem, its molecular responses to plant defense compounds, and its population dynamics upon PWN invasion, validating its efficacy as a biocontrol agent against pine wilt disease.

To study the source and content change of oridonin in the ice ribbons, the contents of oridonin in the ice ribbons and bleeding sap of Isodon rubescens at different times were determined with RP-HPLC. The paraffin sectioning and electron microscopy imaging were performed to study the transport channel of oridonin in the stem. The results showed that there were abundant xylem rays and perfect pit pairs in the secondary xylem of I. rubescens stems. The oridonin content in the ice ribbons of I. rubescens stems was lower than that in the stem of I. rubescens and even decreased over time. The contents of oridonin in the bleeding sap of I. rubescens stems was equal to that in second-day ice ribbons and was lower than that in first-day ice ribbons. The water in the ice ribbons of I. rubescens stems originated from water absorbed by the roots from soil. This water was transported from the roots of I. rubescens to the stem and then transferred through efficient lateral conducting tissues to the surface of the stem. The oridonin in the phloem and cortex of I. rubescens stems dissolves in water originating from the soil and freezes in the form of ice ribbons below 0 °C.

The plasticity of the xylem and its associated hydraulic properties play crucial roles in plant acclimation to environmental changes, with vessel diameter (Dv) being the most functionally prominent trait. While the effects of external environmental factors on xylem formation and Dv are not fully understood, the endogenous hormones indole-3-acetic acid (IAA) and abscisic acid (ABA) are known to play significant signalling roles under stress conditions. This study investigates how these hormones impact Dv under various environmental changes. Experiments were conducted in maize plants subjected to drought, soil salinity, and high CO2 concentration treatments. We found that drought and soil salinity significantly reduced Dv at the same stem internode, while an elevated CO2 concentration can mitigate this decrease in Dv. Remarkably, significant negative correlations were observed between Dv and the contents of IAA and ABA when considering the different treatments. Moreover, appropriate foliar application of either IAA or ABA on well-watered and stressed plants led to a decrease in Dv, while the application of corresponding inhibitors resulted in an increase in Dv. This finding underscores the causal relationship between Dv and the levels of both IAA and ABA, offering a promising approach to manipulating xylem vessel size.

The biosynthesis of cellulose, lignin, and hemicelluloses in plant secondary cell walls (SCWs) is regulated by a hierarchical transcriptional regulatory network. This network features orthologous transcription factors shared between poplar and Arabidopsis, highlighting a foundational similarity in their genetic regulation. However, knowledge on the discrepant behavior of the transcriptional-level molecular regulatory mechanisms between poplar and Arabidopsis remains limited. In this study, we investigated the function of PagMYB128 during wood formation and found it had broader impacts on SCW formation compared to its Arabidopsis ortholog, AtMYB103. Transgenic poplar trees overexpressing PagMYB128 exhibited significantly enhanced xylem development, with fiber cells and vessels displaying thicker walls, and an increase in the levels of cellulose, lignin, and hemicelluloses in the wood. In contrast, plants with dominant repression of PagMYB128 demonstrated the opposite phenotypes. RNA sequencing and reverse transcription - quantitative polymerase chain reaction showed that PagMYB128 could activate SCW biosynthetic gene expression, and chromatin immunoprecipitation along with yeast one-hybrid, and effector-reporter assays showed this regulation was direct. Further analysis revealed that PagSND1 (SECONDARY WALL-ASSOCIATED NAC-DOMAIN PROTEIN1) directly regulates PagMYB128 but not cell wall metabolic genes, highlighting the pivotal role of PagMYB128 in the SND1-driven regulatory network for wood development, thereby creating a feedforward loop in SCW biosynthesis.

Tension wood is a specialized xylem tissue associated with gravitropism in angiosperm trees. However, few regulators of tension wood formation have been identified. The molecular mechanisms underpinning tension wood formation remain elusive. Here, we report that a Populus KNOTTED-like homeobox gene, PagKNAT2/6b, is involved in tension wood formation and gravity response. Transgenic poplar plants overexpressing PagKNAT2/6b displayed more sensitive gravitropism than controls, as indicated by increased stem curvature. Microscopic examination revealed greater abundance of fibre cells with a gelatinous cell wall layer (G-layer) and asymmetric growth of secondary xylem in PagKNAT2/6b overexpression lines. Conversely, PagKNAT2/6b dominant repression plants exhibited decreased tension wood formation and reduced response to gravity stimulation. Moreover, sensitivity to gravity stimulation showed a negative relationship with development stage. Expression of genes related to growth and senescence was affected in PagKNAT2/6b transgenic plants. More importantly, transcription activation and electrophoretic mobility shift assays suggested that PagKNAT2/6b promotes the expression of cytokinin metabolism genes. Consistently, cytokinin content was increased in PagKNAT2/6b overexpression plants. Therefore, PagKNAT2/6b is involved in gravitropism and tension wood formation, likely via modulation of cytokinin metabolism.