Effects of exogenous 24-epibrassinolide and brassinazole on negative gravitropism and tension wood formation in hybrid poplar (Populus deltoids × Populus nigra)

A COBRA family protein, PtrCOB3, contributes to gelatinous layer formation of tension wood fibers in poplar

Angiosperm trees usually develop tension wood (TW) in response to gravitational stimulation. TW comprises abundant gelatinous (G-) fibers with thick G-layers primarily composed of crystalline cellulose. Understanding the pivotal factors governing G-layer formation in TW fiber remains elusive. This study elucidates the role of a Populus trichocarpa COBRA family protein, PtrCOB3, in the G-layer formation of TW fibers. PtrCOB3 expression was upregulated, and its promoter activity was enhanced during TW formation. Comparative analysis with wild-type trees revealed that ptrcob3 mutants, mediated by Cas9/gRNA gene editing, were incapable of producing G-layers within TW fibers and showed severely impaired stem lift. Fluorescence immunolabeling data revealed a dearth of crystalline cellulose in the tertiary cell wall (TCW) of ptrcob3 TW fibers. The role of PtrCOB3 in G-layer formation is contingent upon its native promoter, as evidenced by the comparative phenotypic assessments of pCOB11::PtrCOB3, pCOB3::PtrCOB3, and pCOB3::PtrCOB11 transgenic lines in the ptrcob3 background. Overexpression of PtrCOB3 under the control of its native promoter expedited G-layer formation within TW fibers. We further identified 3 transcription factors that bind to the PtrCOB3 promoter and positively regulate its transcriptional levels. Alongside the primary TCW synthesis genes, these findings enable the construction of a 2-layer transcriptional regulatory network for the G-layer formation of TW fibers. Overall, this study uncovers mechanistic insight into TW formation, whereby a specific COB protein executes the deposition of cellulose, and consequently, G-layer formation within TW fibers.

Exogenous applications of brassinosteroids promote secondary xylem differentiation in Eucalyptus grandis

Brassinosteroids (BRs) play many pivotal roles in plant growth and development, especially in cell elongation and vascular development. Although its biosynthetic and signal transduction pathway have been well characterized in model plants, their biological roles in Eucalyptus grandis, a major hardwood tree providing fiber and energy worldwide, remain unclear. Here, we treated E. grandis plantlets with 24-epibrassinolide (EBL), the most active BR and/or BR biosynthesis inhibitor brassinazole. We recorded the plant growth and analyzed the cell structure of the root and stem with histochemical methods; then, we performed a secondary growth, BR synthesis, and signaling-related gene expression analysis. The results showed that the BRs dramatically increased the shoot length and diameter, and the exogenous BR increased the xylem area of the stem and root. In this process, EgrBRI1, EgrBZR1, and EgrBZR2 expression were induced by the BR treatment, and the expressions of HD-ZIPIII and cellulose synthase genes were also altered. To further verify the effect of BRs in secondary xylem development in Eucalyptus, we used six-month-old plants as the material and directly applied EBL to the xylem and cambium of the vertical stems. The xylem area, fiber cell length, and cell numbers showed considerable increases. Several key BR-signaling genes, secondary xylem development-related transcription factor genes, and cellulose and lignin biosynthetic genes were also considerably altered. Thus, BR had regulatory roles in secondary xylem development and differentiation via the BR-signaling pathway in this woody plant.

The adjustment of China endemic Heptacodium miconioides Rehd. to temperate zone of Poland

Heptacodium miconioides is an increasingly popular ornamental plant, originally being endemic to China. The late and long flowering determines its ecological and ornamental value in cultivation. The aims of this research were to define and distinguish phenological phases of the development of Heptacodium miconioides in the temperate zone region and identification of anatomical changes within the stem during autumn in relation to phenological phases and climatic conditions. Phenological observations were carried out in Wrocław during 2012-2013, as well as in Warsaw (Poland, 52.6°N, 20.5°E) during 2018-2021. During the last year of research an analysis of the anatomical structure was carried out for young stems that bore flowers that year, as well as older, 2- to 6-year-old ones. The material was collected H1 - 10.09., H2 - 28.09., H3 - 16.10., H4 - 3.11., H5 - 21.11. The width of annual increments in subsequent years was determined; length, width and vessel density in early and latewood for subsequent rings of annual growth was measured, as well as the width of the phloem in 1-6-year-old stems (2016-2021). In the vegetative stage three main stages of development were distinguished (leaf buds have the green tips; full autumn discoloration of leaves; leaves falling). In the generative phase, which lasted on average from August 22^nd to January the 7^th five main phases of development were distinguished (flowering, unripe fruits, ripe fruits, spreading of seeds). Increased average temperature during winter and spring had an effect on the growth pattern: early phenological stages occurred sooner and foliage development lasted 44 days longer. Flowering occurred at a similar date at both observed locations and climatic conditions. This year's shoots flowering on a radial section with axial symmetry, were slightly flattened and in clusters arranged regularly to match the shape. Heptacodium develops 2-6 years old shoots with radial symmetry. The growth ring boundaries are distinct, the wood semi-rings porous, with marked differences in the structure of the primary and secondary shoot. Lignification of tissues before winter ends during late leaf-fall phase. The research indicated the adaptive potential of Heptacodium in response to climatic conditions of temperate zone.

Physiological and Transcriptomic Analysis provide Molecular Insight into 24-Epibrassinolide mediated Cr(VI)-Toxicity Tolerance in Pepper Plants

The ever-increasing industrial activities over the decades have generated high toxic metals such as chromium (Cr) that hampers plant growth and development. To counter Cr-toxicity, plants have evolved complex defensive systems including hormonal crosstalk with various signaling pathways. 24-epibrassinolide (24-EBR) lowers oxidative stress and alleviates Cr(VI)-toxicity in plants. In this study, the concealed BR-mediated influences on Cr(VI)-stress tolerance were explored by transcriptome analysis in the Capsicum annuum. Results revealed a linkage between plant development under Cr(VI)-stress and the mitigating effect of 24-epibrassinolide and brassinazole. Growth inhibition, chlorophyll degradation, and a significant rise of malondialdehyde (MDA) were observed after 40 mg/L Cr(VI) treatment in Brz supplemented seedlings, whereas 24-EBR supplemented seedlings exhibited commendatory effect. Comparative transcriptome analysis showed that the expression levels of 6687 genes changed (3846 up-regulated and 2841 downregulated) under Cr(VI)-stress with Brz supplementation. Whereas the expression levels of only 1872 genes changed under Cr(VI)-stress with 24-EBR supplementation (1223 up-regulated and 649 downregulated). The functional categories of the differentially expressed genes (DEGs) by gene ontology (GO) revealed that drug transport, defense responses, and drug catabolic process were the considerable enrichments between 24-EBR and Brz supplemented seedlings under Cr(VI)-stress. Furthermore, auxin signaling, glutathione metabolism, ABC transporters, MAPK pathway, and 36 heavy metal-related genes were significantly differentially expressed components between Cr(VI)-stress, 24-EBR, and Brz supplemented seedlings. Overall, our data demonstrate that employing 24-EBR can commendably act as a growth stimulant in plants subjected to Cr(VI)-stress by modulating the physiological and defense regulatory system.

HD-Zip III Gene Family: Identification and Expression Profiles during Leaf Vein Development in Soybean

Leaf veins constitute the transport network for water and photosynthetic assimilates in vascular plants. The class III homeodomain-leucine zipper (HD-Zip III) gene family is central to the regulation of vascular development. In this research, we performed an overall analysis of the HD-Zip III genes in soybean (Glycine max L. Merr.). Our analysis included the phylogeny, conservation domains and cis-elements in the promoters of these genes. We used the quantitative reverse transcription-polymerase chain reaction to characterize the expression patterns of HD-Zip III genes in leaf vein development and analyze the effects of exogenous hormone treatments. In this study, twelve HD-Zip III genes were identified from the soybean genome and named. All soybean HD-Zip III proteins contained four highly conserved domains. GmHB15-L-1 transcripts showed steadily increasing accumulation during all stages of leaf vein development and were highly expressed in cambium cells. GmREV-L-1 and GmHB14-L-2 had nearly identical expression patterns in soybean leaf vein tissues. GmREV-L-1 and GmHB14-L-2 transcripts remained at stable high levels at all xylem developmental stages. GmREV-L-1 and GmHB14-L-2 were expressed at high levels in the vascular cambium and xylem cells. Overall, GmHB15-L-1 may be an essential regulator that is responsible for the formation or maintenance of soybean vein cambial cells. GmREV-L-1 and GmHB14-L-2 were correlated with xylem differentiation in soybean leaf veins. This study will pave the way for identifying the molecular mechanism of leaf vein development.

Bouncing back stronger: Diversity, structure, and molecular regulation of gelatinous fiber development

Gelatinous fibers (G-fibers) are specialized contractile cells found in a diversity of vascular plant tissues, where they provide mechanical support and/or facilitate plant mobility. G-fibers are distinct from typical fibers by the presence of an innermost thickened G-layer, comprised mainly of axially oriented cellulose microfibrils. Despite the disparate developmental origins-tension wood fibers from the vascular cambium or primary phloem fibers from the procambium-G-fiber development, composition, and molecular signatures are remarkably similar; however, important distinctions do exist. Here, we synthesize current knowledge of the phylogenetic diversity, compositional makeup, and the molecular profiles that characterize G-fiber development and highlight open questions for future investigation.

Do woody vines use gelatinous fibers to climb?

Many plant movements are facilitated by contractile cells called gelatinous fibers (G-fibers), but how G-fibers function in the climbing movements of woody vines remains underexplored. In this Insight, we compare the presence and distribution of G-fibers in the stems of stem-twiners, which wrap around supports, with non-stem-twiners, which attach to supports via tendrils or adventitious roots. An examination of 164 species spanning the vascular plant phylogeny reveals that G-fibers are common in stem-twiners but scarce in non-stem-twiners, suggesting that G-fibers are preferentially formed in the organ responsible for movement. When present, G-fibers are in the xylem, phloem, pericycle, and/or cortex. We discuss the hypothesis that G-fibers are foundational to plant movement and highlight research opportunities concerning G-fiber development and function.

Exogenous Brassinosteroid Facilitates Xylem Development in Pinus massoniana Seedlings

Brassinosteroids (BRs) are known to be essential regulators for wood formation in herbaceous plants and poplar, but their roles in secondary growth and xylem development are still not well-defined, especially in pines. Here, we treated Pinus massoniana seedlings with different concentrations of exogenous BRs, and assayed the effects on plant growth, xylem development, endogenous phytohormone contents and gene expression within stems. Application of exogenous BR resulted in improving development of xylem more than phloem, and promoting xylem development in a dosage-dependent manner in a certain concentration rage. Endogenous hormone determination showed that BR may interact with other phytohormones in regulating xylem development. RNA-seq analysis revealed that some conventional phenylpropanoid biosynthesis- or lignin synthesis-related genes were downregulated, but the lignin content was elevated, suggesting that new lignin synthesis pathways or other cell wall components should be activated by BR treatment in P. massoniana. The results presented here reveal the foundational role of BRs in regulating plant secondary growth, and provide the basis for understanding molecular mechanisms of xylem development in P. massoniana.

Genome-wide analysis of lncRNA and mRNA expression and endogenous hormone regulation during tension wood formation in Catalpa bungei

BACKGROUND

Phytohormones are the key factors regulating vascular development in plants, and they are also involved in tension wood (TW) formation. Although the theory of hormone distribution in TW formation is widely supported, the effects of endogenous hormones on TW formation have not yet been assessed. In this study, TW formation was induced in Catalpa bungei by artificial bending. The phytohormone content of TW, opposite wood (OW) and normal wood (NW) was determined using liquid chromatography-mass spectrometry (LC-MS), and transcriptome sequencing was performed. The hormone content and related gene expression data were comprehensively analyzed.

RESULTS

The results of analyses of the plant hormone contents indicated significantly higher levels of cis-zeatin (cZ), indoleacetic acid (IAA) and abscisic acid (ABA) in TW than in OW. Genes involved in the IAA and ABA synthesis pathways, such as ALDH (evm.

MODEL

group5.1511) and UGT (evm.

MODEL

scaffold36.20), were significantly upregulated in TW. and the expression levels of ARF (evm.

MODEL

group5.1332), A-ARR (evm.

MODEL

group0.1600), and TCH4 (evm.

MODEL

group2.745), which participate in IAA, cZ and Brassinolide (BR) signal transduction, were significantly increased in TW. In particular, ARF expression may be regulated by long noncoding RNAs (lncRNAs) and the HD-ZIP transcription factor ATHB-15.

CONCLUSIONS

We constructed a multiple hormone-mediated network of C. bungei TW formation based on hormone levels and transcriptional expression profiles were identified during TW formation.

Brassinosteroid regulation of wood formation in poplar

Brassinosteroids have been implicated in the differentiation of vascular cell types in herbaceous plants, but their roles during secondary growth and wood formation are not well defined. Here we pharmacologically and genetically manipulated brassinosteroid levels in poplar trees and assayed the effects on secondary growth and wood formation, and on gene expression within stems. Elevated brassinosteroid levels resulted in increases in secondary growth and tension wood formation, while inhibition of brassinosteroid synthesis resulted in decreased growth and secondary vascular differentiation. Analysis of gene expression showed that brassinosteroid action is positively associated with genes involved in cell differentiation and cell-wall biosynthesis. The results presented here show that brassinosteroids play a foundational role in the regulation of secondary growth and wood formation, in part through the regulation of cell differentiation and secondary cell wall biosynthesis.