Blue Light Regulates Secondary Cell Wall Thickening via MYC2/MYC4 Activation of the NST1-Directed Transcriptional Network in Arabidopsis

Cryptochrome-mediated blue light regulates cell lignification via PbbHLH195 activation of the PbNSC in pear fruits

The presence of stone cells in pear fruit, caused by lignified secondary cell walls (SCWs), leads to a grainy texture in the fruit flesh, thereby compromising its overall quality. Lignification is influenced by various environmental signals, including light, however the underlying mechanism are poorly understood. This study reveals that SCW thickening and lignin accumulation in stone cells were regulated by a blue light signal, mediated through the activation of PbNSC by PbbHLH195. The results revealed that the stone cell formation was prompted by supplementary with blue light, with lignin accumulation linked to the upregulation of the NAC STONE CELL PROMOTING FACTOR (PbNSC). PbbHLH195 was identified as a novel molecular hub connecting lignification to blue light signal through its physical interaction with PbCRY1a. The biochemical and functional analysis indicates that PbbHLH195 contributes to stone cell lignification by activating the promoter of PbNSC. Our findings offer novel insights into the mechanisms of lignin biosynthesis in response to blue light, identifying valuable genetic targets for enhancing the fruit quality of pear.

Shade-Induced Regulation of Cell Wall Thickening in Moso Bamboo: A Molecular Pathway Involving XTH Activity

How plants adapt to shade and perform specific morphogenesis is one of the secrets of the kingdom, and unravelling the underlying molecular mechanisms is crucial. Scanning electron microscope results have revealed that a low R/FR ratio (indicating shade conditions) inhibited the cell wall thickness of parenchyma cells and sieve tube cells of Moso bamboo (Phyllostachys edulis). Xyloglucan Endotransglucosylase/hydrolase3 (PheXTH3) was identified as a circadian rhythm gene responsive to a low R/FR ratio by qRT-PCR analysis, and it showed peak activity in the vascular system. PheXTH3 enhanced interfascicular fibre cell wall thickening and lignin accumulation in stable transgenic Arabidopsis thaliana lines. A low R/FR ratio downregulated PheXTH3 expression, subsequently suppressing cell wall thickening in vessel and interfascicular fibre cells. Both Yeast One-Hybrid experiments and Dual-LUC assays revealed that WOX3b1, AP2-39, and XTH3 may form a regulatory pathway. Collectively, we proposed a WOX3b1-AP2-39-XTH3 molecular pathway mediated by the ratio of low R/FR, which may regulate the thickening of cell wall in Moso bamboo.

ZINC FINGER PROTEIN2 suppresses funiculus lignification to ensure seed loading efficiency in Arabidopsis

The plant funiculus anchors the developing seed to the placenta within the inner dorsal pod strands of the silique wall and directly transports nutrients to the seeds. The lignified vasculature critically supports nutrient transport through the funiculus. However, molecular mechanisms underlying lignified secondary cell wall (SCW) biosynthesis in the funiculus remain elusive. Here, we show that the transcription factor ZINC FINGER PROTEIN2 (ZFP2) represses SCW formation in the cortex cells that surround the vasculature. This function is essential for efficient nutrient loading into the seeds. Notably, ZFP2 directly acts on the SCW transcription factor NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1) to repress cortex cell lignification, providing a mechanism of how SCW biosynthesis is restricted to the vasculature of the funiculus to ensure proper seed loading in Arabidopsis.

Unequal Genetic Redundancies Among MYC bHLH Transcription Factors Underlie Seedling Photomorphogenesis in Arabidopsis

Light is one of the most critical ecological cues controlling plant growth and development. Plants have evolved complex mechanisms to cope with fluctuating light signals. In Arabidopsis, bHLH transcription factors MYC2, MYC3, and MYC4 have been shown to play a vital role in protecting plants against herbivory and necrotrophic pathogens. While the role of MYC2 in light-mediated seedling development has been studied in some detail, the role of MYC3 and MYC4 still needs to be discovered. Here, we show that MYC4 negatively regulates seedling photomorphogenesis, while the MYC3 function seems redundant. However, the genetic analysis reveals that MYC3/MYC4 together act as positive regulators of seedling photomorphogenic growth as the myc3myc4 double mutants showed exaggerated hypocotyl growth compared to the myc3 and myc4 single mutants and Col-0. Intriguingly, the loss of MYC2 function in the myc3myc4 double mutant background (myc2myc3myc4) resulted in further enhancement in the hypocotyl growth than myc3myc4 double mutants in WL, BL and FRL, suggesting that MYC2/3/4 together play an essential and positive role in meditating optimal seedling photomorphogenesis. Besides, MYC3/MYC4 genetically and physically interact with HY5 to partially inhibit its function in controlling hypocotyl and photo-pigment accumulation. Moreover, our results suggest that COP1 physically interacts and degrades MYC3 and MYC4 through the 26S proteasomal pathway and controls their response to dark and light for fine-tuning HY5 function and seedling photomorphogenesis.

The role of light in regulating plant growth, development and sugar metabolism: a review

Light provides the necessary energy for plant photosynthesis, which allows plants to produce organic matter and energy conversion, during plant growth and development. Light provides material energy to plants as the basis for cell division and differentiation, chlorophyll synthesis, tissue growth and stomatal movement, and light intensity, photoperiod, and light quality play important roles in these processes. There are several regulatory mechanisms involved in sugar metabolism in plants, and light, as one of the regulatory factors, affects cell wall composition, starch granules, sucrose synthesis, and vascular bundle formation. Similarly, sugar species and genes are affected in the context of light-regulated sugar metabolism. We searched the available databases and found that there are fewer relevant reviews. Therefore, this paper provides a summary of the effects of light on plant growth and development and sugar metabolism, further elaborates on the mechanisms of light effects on plants, and provides some new insights for a better understanding of how plant growth is regulated under different light conditions.

Individual and interactive effects of temperature and blue light on canola growth, lignin biosynthesis and methane emissions

It is now well documented that plants produce methane (CH4) under aerobic conditions. However, the mechanisms of methane production in plants, its potential precursors, and the factors that are involved in the process are not fully understood. Few studies have considered the effects of blue light on methane emissions from plants; however, the combined effects of temperature and blue light have not been studied. We studied the effects of two temperature regimes (22/18 °C and 28/24 °C; 16 h light/8 h dark), and three blue light levels (0, 4, and 8 mW cm-2; 400-500 nm) on the growth, lignin, and methane emissions of canola (Brassica napus). Plants were grown under experimental conditions for three weeks, and then methane, monolignols and other plant traits, including growth, biomass, growth index, photosynthesis, chlorophyll fluorescence, and photosynthetic pigments, were measured. Blue light significantly increased methane emissions, stem height, and growth rate, but decreased stem diameter, leaf number and area, biomass, specific leaf mass, leaf area ratio, shoot/root mass ratio, photosynthetic pigments, sinapyl alcohol, and coniferyl aldehyde. Higher temperature significantly decreased stem diameter, non-photochemical quenching, sinapyl alcohol, and coniferyl aldehyde. Methane emission was negatively correlated with plant dry mass, leaf area per plant, and maximum quantum yield of photosystem II. However, no significant relationships were found between methane and monolignols. In conclusion, plants emitted more methane under stress conditions; however, further studies are required to understand the potential precursors of methane and the mechanism of its synthesis in plants.

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.

Eggplant NAC domain transcription factor SmNST1 as an activator promotes secondary cell wall thickening

NAC-domain transcription factors (TFs) are plant-specific transcriptional regulators playing crucial roles in plant secondary cell wall (SCW) biosynthesis. SCW is important for plant growth and development, maintaining plant morphology, providing rigid support, ensuring material transportation and participating in plant stress responses as a protective barrier. However, the molecular mechanisms underlying SCW in eggplant have not been thoroughly explored. In this study, the NAC domain TFs SmNST1 and SmNST2 were cloned from the eggplant line 'Sanyue qie'. SmNST1 and SmNST2 expression levels were the highest in the roots and stems. Subcellular localization analysis showed that they were localized in the cell membrane and nucleus. Their overexpression in transgenic tobacco showed that SmNST1 promotes SCW thickening. The expression of a set of SCW biosynthetic genes for cellulose, xylan and lignin, which regulate SCW formation, was increased in transgenic tobacco. Bimolecular fluorescence and luciferase complementation assays showed that SmNST1 interacted with SmNST2 in vivo. Yeast one-hybrid, electrophoretic mobility shift assay (EMSA) and Dual-luciferase reporter assays showed that SmMYB26 directly bound to the SmNST1 promoter and acted as an activator. SmNST1 and SmNST2 interact with the SmMYB108 promoter and repress SmMYB108 expression. Altogether, we showed that SmNST1 positively regulates SCW formation, improving our understanding of SCW biosynthesis transcriptional regulation.

C2H2 zinc finger protein PagIDD15A regulates secondary wall thickening and lignin biosynthesis in poplar

Wood production is largely determined by the activity of cambial cell proliferation, and the secondary cell wall (SCW) thickening of xylem cells determines the wood property. In this study, we identified an INDETERMINATE DOMAIN (IDD) type C2H2 zinc finger transcription factor PagIDD15A as a regulator of wood formation in Populus alba × Populus glandulosa. Downregulation of PagIDD15A expression by RNA interference (RNAi) inhibited xylem development and xylem cell secondary wall thickening. RNA-seq analysis showed that PagPAL1, PagCCR2 and PagCCoAOMT1 were downregulated in the differentiating xylem of the PagIDD15A-RNAi transgenic plants, showing that PagIDD15A may regulate SCW biosynthesis through inhibiting lignin biosynthesis. The downregulation of PagVND6-B2, PagMYB10 and PagMYC4 and upregulation of PagWRKY12 in the differentiating xylem of RNAi transgenic plants suggest that PagIDD15A may also regulate these transcription factor (TF) genes to affect SCW thickening. RT-qPCR analysis in the phloem-cambium of RNAi transgenic demonstrates that PagIDD15A may regulate the expression of the genes associated with cell proliferation, including, PagSHR (SHORTROOT), PagSCR (SCARECROW), PagCYCD3;1 (CYCLIN D3;1) and PagSMR4 (SIAMESE-RELATED4), to affect the cambial activity. This study provides the knowledge of the IDD-type C2H2 zinc finger protein in regulating wood formation.

Advances in functional studies of plant MYC transcription factors

Myelocytomatosis (MYC) transcription factors (TFs) belong to the basic helix-loop-helix (bHLH) family in plants and play a central role in governing a wide range of physiological processes. These processes encompass plant growth, development, adaptation to biotic and abiotic stresses, as well as secondary metabolism. In recent decades, significant strides have been made in comprehending the multifaceted regulatory functions of MYCs. This advancement has been achieved through the cloning of MYCs and the characterization of plants with MYC deficiencies or overexpression, employing comprehensive genome-wide 'omics' and protein-protein interaction technologies. MYCs act as pivotal components in integrating signals from various phytohormones' transcriptional regulators to orchestrate genome-wide transcriptional reprogramming. In this review, we have compiled current research on the role of MYCs as molecular switches that modulate signal transduction pathways mediated by phytohormones and phytochromes. This comprehensive overview allows us to address lingering questions regarding the interplay of signals in response to environmental cues and developmental shift. It also sheds light on the potential implications for enhancing plant resistance to diverse biotic and abiotic stresses through genetic improvements achieved by plant breeding and synthetic biology efforts.

The regulatory mechanism of rapid lignification for timely anther dehiscence

Anther dehiscence is a crucial event in plant reproduction, tightly regulated and dependent on the lignification of the anther endothecium. In this study, we investigated the rapid lignification process that ensures timely anther dehiscence in Arabidopsis. Our findings reveal that endothecium lignification can be divided into two distinct phases. During Phase I, lignin precursors are synthesized without polymerization, while Phase II involves simultaneous synthesis of lignin precursors and polymerization. The transcription factors MYB26, NST1/2, and ARF17 specifically regulate the pathway responsible for the synthesis and polymerization of lignin monomers in Phase II. MYB26-NST1/2 is the key regulatory pathway responsible for endothecium lignification, while ARF17 facilitates this process by interacting with MYB26. Interestingly, our results demonstrate that the lignification of the endothecium, which occurs within approximately 26 h, is much faster than that of the vascular tissue. These findings provide valuable insights into the regulation mechanism of rapid lignification in the endothecium, which enables timely anther dehiscence and successful pollen release during plant reproduction.

Stem Cells and Differentiation in Vascular Tissues

Plant vascular tissues are crucial for the long-distance transport of water, nutrients, and a multitude of signal molecules throughout the plant body and, therefore, central to plant growth and development. The intricate development of vascular tissues is orchestrated by unique populations of dedicated stem cells integrating endogenous as well as environmental cues. This review summarizes our current understanding of vascular-related stem cell biology and of vascular tissue differentiation. We present an overview of the molecular and cellular mechanisms governing the maintenance and fate determination of vascular stem cells and highlight the interplay between intrinsic and external cues. In this context, we emphasize the role of transcription factors, hormonal signaling, and epigenetic modifications. We also discuss emerging technologies and the large repertoire of cell types associated with vascular tissues, which have the potential to provide unprecedented insights into cellular specialization and anatomical adaptations to distinct ecological niches.

Blue light photoreceptor cryptochrome 1 promotes wood formation and anthocyanin biosynthesis in Populus

Blue light photoreceptor cryptochrome 1 (CRY1) in herbaceous plants plays crucial roles in various developmental processes, including cotyledon expansion, hypocotyl elongation and anthocyanin biosynthesis. However, the function of CRY1 in perennial trees is unclear. In this study, we identified two ortholog genes of CRY1 (PagCRY1a and PagCRY1b) from Populus, which displayed high sequence similarity to Arabidopsis CRY1. Overexpression of PagCRY1 substantially inhibited plant growth and promoted secondary xylem development in Populus, while CRISPR/Cas9-mediated knockout of PagCRY1 enhanced plant growth and delayed secondary xylem development. Moreover, overexpression of PagCRY1 dramatically increased anthocyanin accumulation. The further analysis supported that PagCRY1 functions specifically in response to blue light. Taken together, our results demonstrated that modulating the expression of blue light photoreceptor CRY1 ortholog gene in Populus could significantly influence plant biomass production and the process of wood formation, laying a foundation for further investigating the light-regulated tree growth.

Genome-wide identification of ZmMYC2 binding sites and target genes in maize

BACKGROUND

Jasmonate (JA) is the important phytohormone to regulate plant growth and adaption to stress signals. MYC2, an bHLH transcription factor, is the master regulator of JA signaling. Although MYC2 in maize has been identified, its function remains to be clarified.

RESULTS

To understand the function and regulatory mechanism of MYC2 in maize, the joint analysis of DAP-seq and RNA-seq is conducted to identify the binding sites and target genes of ZmMYC2. A total of 3183 genes are detected both in DAP-seq and RNA-seq data, potentially as the directly regulating genes of ZmMYC2. These genes are involved in various biological processes including plant growth and stress response. Besides the classic cis-elements like the G-box and E-box that are bound by MYC2, some new motifs are also revealed to be recognized by ZmMYC2, such as nGCATGCAnn, AAAAAAAA, CACGTGCGTGCG. The binding sites of many ZmMYC2 regulating genes are identified by IGV-sRNA.

CONCLUSIONS

All together, abundant target genes of ZmMYC2 are characterized with their binding sites, providing the basis to construct the regulatory network of ZmMYC2 and better understanding for JA signaling in maize.

Blue light regulated lignin and cellulose content of soybean petioles and stems under low light intensity

To improve light harvest and plant structural support under low light intensity, it is useful to investigate the effects of different ratios of blue light on petiole and stem growth. Two true leaves of soybean seedlings were exposed to a total light intensity of 200μmolm-2 s-1 , presented as either white light or three levels of blue light (40μmolm-2 s-1 , 67μmolm-2 s-1 and 100μmolm-2 s-1 ) for 15days. Soybean petioles under the low blue light treatment upregulated expression of genes relating to lignin metabolism, enhancing lignin content compared with the white light treatment. The low blue light treatment had high petiole length, increased plant height and improved petiole strength arising from high lignin content, thus significantly increasing leaf dry weight relative to the white light treatment. Compared with white light, the treatment with the highest blue light ratio reduced plant height and enhanced plant support through increased cellulose and hemicellulose content in the stem. Under low light intensity, 20% blue light enhanced petiole length and strength to improve photosynthate biomass; whereas 50% blue light lowered plants' centre of gravity, preventing lodging and conserving carbohydrate allocation.

Deciphering the intricate hierarchical gene regulatory network: unraveling multi-level regulation and modifications driving secondary cell wall formation

Wood quality is predominantly determined by the amount and the composition of secondary cell walls (SCWs). Consequently, unraveling the molecular regulatory mechanisms governing SCW formation is of paramount importance for genetic engineering aimed at enhancing wood properties. Although SCW formation is known to be governed by a hierarchical gene regulatory network (HGRN), our understanding of how a HGRN operates and regulates the formation of heterogeneous SCWs for plant development and adaption to ever-changing environment remains limited. In this review, we examined the HGRNs governing SCW formation and highlighted the significant key differences between herbaceous Arabidopsis and woody plant poplar. We clarified many confusions in existing literatures regarding the HGRNs and their orthologous gene names and functions. Additionally, we revealed many network motifs including feed-forward loops, feed-back loops, and negative and positive autoregulation in the HGRNs. We also conducted a thorough review of post-transcriptional and post-translational aspects, protein-protein interactions, and epigenetic modifications of the HGRNs. Furthermore, we summarized how the HGRNs respond to environmental factors and cues, influencing SCW biosynthesis through regulatory cascades, including many regulatory chains, wiring regulations, and network motifs. Finally, we highlighted the future research directions for gaining a further understanding of molecular regulatory mechanisms underlying SCW formation.

Jasmonate activates secondary cell wall biosynthesis through MYC2-MYB46 module

Formation of secondary cell wall (SCW) is tightly regulated spatiotemporally by various developmental and environmental signals. Successful fine-tuning of the trade-off between SCW biosynthesis and stress responses requires a better understanding of how plant growth is regulated under environmental stress conditions. However, the current understanding of the interplay between environmental signaling and SCW formation is limited. The lipid-derived plant hormone jasmonate (JA) and its derivatives are important signaling components involved in various physiological processes including plant growth, development, and abiotic/biotic stress responses. Recent studies suggest that JA is involved in SCW formation but the signaling pathway has not been studied for how JA regulates SCW formation. We tested this hypothesis using the transcription factor MYB46, a master switch for SCW biosynthesis, and JA treatments. Both the transcript and protein levels of MYB46, a master switch for SCW formation, were significantly increased by JA treatment, resulting in the upregulation of SCW biosynthesis. We then show that this JA-induced upregulation of MYB46 is mediated by MYC2, a central regulator of JA signaling, which binds to the promoter of MYB46. We conclude that this MYC2-MYB46 module is a key component of the plant response to JA in SCW formation.

Cotton GhNAC4 promotes drought tolerance by regulating secondary cell wall biosynthesis and ribosomal protein homeostasis

Drought has a severe impact on the quality and yield of cotton. Deciphering the key genes related to drought tolerance is important for understanding the regulation mechanism of drought stress and breeding drought-tolerant cotton cultivars. Several studies have demonstrated that NAC transcription factors are crucial in the regulation of drought stress, however, the related functional mechanisms are still largely unexplored. Here, we identified that NAC transcription factor GhNAC4 positively regulated drought stress tolerance in cotton. The expression of GhNAC4 was significantly induced by abiotic stress and plant hormones. Silencing of GhNAC4 distinctly impaired the resistance to drought stress and overexpressing GhNAC4 in cotton significantly enhanced the stress tolerance. RNA-seq analysis revealed that overexpression of GhNAC4 enriched the expression of genes associated with the biosynthesis of secondary cell walls and ribosomal proteins. We confirmed that GhNAC4 positively activated the expressions of GhNST1, a master regulator reported previously in secondary cell wall formation, and two ribosomal protein-encoding genes GhRPL12 and GhRPL18p, by directly binding to their promoter regions. Overexpression of GhNAC4 promoted the expression of downstream genes associated with the secondary wall biosynthesis, resulting in enhancing secondary wall deposition in the roots, and silencing of GhRPL12 and GhRPL18p significantly impaired the resistance to drought stress. Taken together, our study reveals a novel pathway mediated by GhNAC4 that promotes secondary cell wall biosynthesis to strengthen secondary wall development and regulates the expression of ribosomal protein-encoding genes to maintain translation stability, which ultimately enhances drought tolerance in cotton.

RtNAC055 promotes drought tolerance via a stomatal closure pathway linked to methyl jasmonate/hydrogen peroxide signaling in Reaumuria trigyna

The stomata regulate CO2 uptake and efficient water usage, thereby promoting drought stress tolerance. NAC proteins (NAM, ATAF1/2, and CUC2) participate in plant reactions following drought stress, but the molecular mechanisms underlying NAC-mediated regulation of stomatal movement are unclear. In this study, a novel NAC gene from Reaumuria trigyna, RtNAC055, was found to enhance drought tolerance via a stomatal closure pathway. It was regulated by RtMYC2 and integrated with jasmonic acid signaling and was predominantly expressed in stomata and root. The suppression of RtNAC055 could improve jasmonic acid and H2O2 production and increase the drought tolerance of transgenic R. trigyna callus. Ectopic expression of RtNAC055 in the Arabidopsis atnac055 mutant rescued its drought-sensitive phenotype by decreasing stomatal aperture. Under drought stress, overexpression of RtNAC055 in poplar promoted ROS (H2O2) accumulation in stomata, which accelerated stomatal closure and maintained a high photosynthetic rate. Drought upregulated the expression of PtRbohD/F, PtP5CS2, and PtDREB1.1, as well as antioxidant enzyme activities in heterologous expression poplars. RtNAC055 promoted H2O2 production in guard cells by directly binding to the promoter of RtRbohE, thus regulating stomatal closure. The stress-related genes RtDREB1.1/P5CS1 were directly regulated by RtNAC055. These results indicate that RtNAC055 regulates stomatal closure by maintaining the balance between the antioxidant system and H2O2 level, reducing the transpiration rate and water loss, and improving photosynthetic efficiency and drought resistance.

Light spectra manipulation stimulates growth, specialized metabolites and nutritional quality in Anethum graveolens

Light-Emitting Diodes (LED) play a major role in manipulating light spectra that helps in regulating the growth and specialized metabolite synthesis relevant to the plant defence system. In this study, we assessed photosynthetic performance, phytonutrients, and anatomical variations of an aromatic herb Anethum graveolens (also known as dill), grown under various combinations of LED lights viz. red (100R:0B), red:blue (50R:50B); blue (0R:100B) and warm white (WW, served as control). Exposure to 0R:100B LED lights led to the tallest stem height, whereas, the number of leaves were highest under 50R:50B LED lights. The photosynthetic performance was observed to be highest under 50R:50B LED lights. HPLC analysis revealed chlorogenic acid and rosmarinic acid as the major phenolic compounds accumulated under different spectral irradiations. The highest chlorogenic acid content was observed in 50R:50B LED treated dill plants, while 100R:0B light showed the highest accumulation of rosmarinic acid. Dill plants grown under 50R:50B light displayed a relatively higher content of volatile compounds including, myristicin (phenylpropene), psi-limonene, and α-phellandrene (monoterpenoids). Expression analyses of candidate genes of phenylpropanoid and monoterpenoid biosynthetic pathways showed good correlations with the enhanced phenolic compounds and monoterpenes detected under appropriate light treatments. Further, the stem anatomy revealed higher vascularization under the influence of 0R:100B LED lights, whereas, intense histochemical localization of specialized metabolites could be correlated with enhanced accumulation of phenolic compounds and terpenoids observed in this study. Taken together, these studies suggest that proper combinations of blue and red spectra of light could play important role to augment the growth and phytochemical characteristics of dill, thus improving its value addition in the food industry.

The AP2/ERF transcription factor PtoERF15 confers drought tolerance via JA-mediated signaling in Populus

Drought stress is one of the major limiting factors for the growth and development of perennial trees. Xylem vessels act as the center of water conduction in woody species, but the underlying mechanism of its development and morphogenesis under water-deficient conditions remains elucidation. Here, we identified and characterized an osmotic stress-induced ETHYLENE RESPONSE FACTOR 15 (PtoERF15) and its target, PtoMYC2b, which was involved in mediating vessel size, density, and cell wall thickness in response to drought in Populus tomentosa. PtoERF15 is preferentially expressed in differentiating xylem of poplar stems. Overexpression of PtoERF15 contributed to stem water potential maintaining, thus promoting drought tolerance. RNA-Seq and biochemical analysis further revealed that PtoERF15 directly regulated PtoMYC2b, encoding a switch of JA signaling pathway. Additionally, our findings verify that three sets of homologous genes from NAC (NAM, ATAF1/2, and CUC2) gene family: PtoSND1-A1/A2, PtoVND7-1/7-2, and PtoNAC118/120, as the targets of PtoMYC2b, are involved in the regulation of vessel morphology in poplar. Collectively, our study provides molecular evidence for the involvement of the PtoERF15-PtoMYC2b transcription cascade in maintaining stem water potential through the regulation of xylem vessel development, ultimately improving drought tolerance in poplar.

Alternative Splicing for Leucanthemella linearis NST1 Contributes to Variable Abiotic Stress Resistance in Transgenic Tobacco

Leucanthemella linearis is a marsh plant in the family Compositae. It has good water and moisture resistance and ornamental properties, which makes it one of the important materials for chrysanthemum breeding and genetic improvement. The NST1 (NAC secondary wall enhancement factor 1) gene is associated with the thickening of the secondary walls of fiber cells in the plant ducts and the secondary xylem and plays an important role in plant stress resistance. In this study, two variable spliceosomes of the NST1 gene were identified from a chrysanthemum plant by using bioinformatics, qRT-PCR, transgene, and paraffin section methods to explore the molecular mechanism of the variable splicing of NST1 under abiotic stress. The results show that only three amino acids were found to be different between the two LlNST1 variants. After being treated with salt, drought, and low temperatures, analysis of the expression levels of the LlNST1 and LlNST1.1 genes in Ll showed that LlNST1.1 could respond to low temperatures and salt stress and had a weak response to drought stress. However, the expression level of LlNST1 under the three treatments was lower than that of LlNST1.1. LlNST1 transgenic tobacco showed increased saline-alkali resistance and low-temperature resistance at the seedling stage. LlNST1.1 transgenic tobacco also showed enhanced saline-alkali resistance and drought resistance at the seedling stage. In conclusion, the functions of the two variable spliceosomes of the NST1 gene are very different under abiotic stress. Therefore, this study verified the function of the variable spliceosome of NST1 and improved the stress resistance of the chrysanthemum plant under examination by regulating the expression of the NST protein, which lays a material foundation for the improvement of plant stress resistance materials and has important significance for the study of the resistance of chrysanthemum plants to abiotic stress.

Jasmonic acid regulates lignin deposition in poplar through JAZ5-MYB/NAC interaction

Jasmonic acid (JA) is a phytohormone involved in plant defense, growth, and development, etc. However, the regulatory mechanisms underlying JA-mediated lignin deposition and secondary cell wall (SCW) formation remain poorly understood. In this study, we found that JA can inhibit lignin deposition and SCW thickening in poplar trees through exogenous MeJA treatment and observation of the phenotypes of a JA synthesis mutant, opdat1. Hence, we identified a JA signal inhibitor PtoJAZ5, belonging to the TIFY gene family, which is involved in the regulation of secondary vascular development of Populus tomentosa. RT-qPCR and GUS staining revealed that PtoJAZ5 was highly expressed in poplar stems, particularly in developing xylem. Overexpression of PtoJAZ5 inhibited SCW thickening and down-regulated the expression of SCW biosynthesis-related genes. Further biochemical analysis showed that PtoJAZ5 interacted with multiple SCW switches NAC/MYB transcription factors, including MYB3 and WND6A, through yeast two-hybrid and bimolecular fluorescent complementation experiments. Transcriptional activation assays demonstrated that MYB3-PtoJAZ5 and WND6A-PtoJAZ5 complexes regulated the expression of lignin synthetic genes. Our results suggest that PtoJAZ5 plays a negative role in JA-induced lignin deposition and SCW thickening in poplar and provide new insights into the molecular mechanisms underlying JA-mediated regulation of SCW formation.

Characterization of chromatin accessibility and gene expression reveal the key genes involved in cotton fiber elongation

Cotton (Gossypium hirsutum L.) is an important economic crop, and cotton fiber is one of the longest plant cells, which provides an ideal model for the study of cell elongation and secondary cell wall synthesis. Cotton fiber length is regulated by a variety of transcription factors (TF) and their target genes; however, the mechanism of fiber elongation mediated by transcriptional regulatory networks is still unclear to a large extent. Here, we used a comparative assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) assay and RNA-seq analysis to identify fiber elongation transcription factors and genes using the short-fiber mutant ligon linless-2 (Li2 ) and wild type (WT). A total of 499 differential target genes were identified and GO analysis shows that differential genes are mainly involved in plant secondary wall synthesis and microtubule-binding processes. Analysis of the genomic regions preferentially accessible (Peak) has identified a number of overrepresented TF-binding motifs, highlighting sets of TFs that are important for cotton fiber development. Using ATAC-seq and RNA-seq data, we have constructed a functional regulatory network of each TF regulatory target gene and also the network pattern of TF regulating differential target genes. Further, to obtain the genes related to fiber length, the differential target genes were combined with FLGWAS data to identify the genes highly related to fiber length. Our work provides new insights into cotton fiber elongation.

Genome-Wide Identification and Expression Analysis of Dendrocalamus farinosus CCoAOMT Gene Family and the Role of DfCCoAOMT14 Involved in Lignin Synthesis

As the main component of plant cell walls, lignin can not only provide mechanical strength and physical defense for plants, but can also be an important indicator affecting the properties and quality of wood and bamboo. Dendrocalamus farinosus is an important economic bamboo species for both shoots and timber in southwest China, with the advantages of fast growth, high yield and slender fiber. Caffeoyl-coenzyme A-O-methyltransferase (CCoAOMT) is a key rate-limiting enzyme in the lignin biosynthesis pathway, but little is known about it in D. farinosus. Here, a total of 17 DfCCoAOMT genes were identified based on the D. farinosus whole genome. DfCCoAOMT1/14/15/16 were homologs of AtCCoAOMT1. DfCCoAOMT6/9/14/15/16 were highly expressed in stems of D. farinosus; this is consistent with the trend of lignin accumulation during bamboo shoot elongation, especially DfCCoAOMT14. The analysis of promoter cis-acting elements suggested that DfCCoAOMTs might be important for photosynthesis, ABA/MeJA responses, drought stress and lignin synthesis. We then confirmed that the expression levels of DfCCoAOMT2/5/6/8/9/14/15 were regulated by ABA/MeJA signaling. In addition, overexpression of DfCCoAOMT14 in transgenic plants significantly increased the lignin content, xylem thickness and drought resistance of plants. Our findings revealed that DfCCoAOMT14 can be a candidate gene that is involved in the drought response and lignin synthesis pathway in plants, which could contribute to the genetic improvement of many important traits in D. farinosus and other species.

The Regulatory Roles of MYC TFs in Plant Stamen Development

The stamen, as the male reproductive organ of flowering plants, plays a critical role in completing the life cycle of plants. MYC transcription factors are members of the bHLH IIIE subgroup and participate in a number of plant biological processes. In recent decades, a number of studies have confirmed that MYC transcription factors actively participate in the regulation of stamen development and have a critical impact on plant fertility. In this review, we summarized how MYC transcription factors play a role in regulating secondary thickening of the anther endothecium, the development and degradation of the tapetum, stomatal differentiation, and the dehydration of the anther epidermis. With regard to anther physiological metabolism, MYC transcription factors control dehydrin synthesis, ion and water transport, and carbohydrate metabolism to influence pollen viability. Additionally, MYCs participate in the JA signal transduction pathway, where they directly or indirectly control the development of stamens through the ET-JA, GA-JA, and ABA-JA pathways. By identifying the functions of MYCs during plant stamen development, it will help us to obtain a more comprehensive understanding not only on the molecular functions of this TF family but also the mechanisms underlying stamen development.

Identification of nsLTP family in Chinese white pear (Pyrus bretschneideri) reveals its potential roles in russet skin formation

MAIN CONCLUSION

Identification of PbLTP genes in pear and functional characterization of PbLTP4 in the transport of suberin monomers of russet skin formation. Non-specific lipid-transfer protein (nsLTP) is an abundant and diverse alkaline small molecule protein in the plant kingdom with complex and diverse biophysiological functions, such as transfer of phospholipids, reproductive development, pathogen defence and abiotic stress response. Up to now, only a tiny fraction of nsLTPs have been functionally identified, and the distribution of nsLTPs in pear (Pyrus bretschneideri) (PbLTPs) has not been fully characterized. In this study, the genome-wide analysis of the nsLTP gene family in the pear genome identified 67 PbLTP proteins, which could be divided into six types (1, 2, C, D, E, and G). Similar intron/exon structural patterns were observed in the same type, strongly supporting their close evolutionary relationship. In addition, PbLTP4 was highly expressed in russet pear skin compared with green skin, which was located in the plasma membrane. Coexpression network analysis showed that PbLTP4 closely related to suberin biosynthetic genes. The biological function of PbLTP4 in promoting suberification has been demonstrated by overexpression in Arabidopsis. Identification of suberin monomers showed that PbLTP4 promotes suberification by regulating 9,12-octadecadienoic acid and hexadecanoic acid transport. These results provide helpful insights into the characteristics of PbLTP genes and their biological function in the transport of suberin monomers of russet skin formation.

The RLCK-VND6 module coordinates secondary cell wall formation and adaptive growth in rice

The orderly deposition of secondary cell wall (SCW) in plants is implicated in various biological programs and is precisely controlled. Although many positive and negative regulators of SCW have been documented, the molecular mechanisms underlying SCW formation coordinated with distinct cellular physiological processes during plant adaptive growth remain largely unclear. Here, we report the identification of Cellulose Synthase co-expressed Kinase1 (CSK1), which encodes a receptor-like cytoplasmic kinase, as a negative regulator of SCW formation and its signaling cascade in rice. Transcriptome deep sequencing of developing internodes and genome-wide co-expression assays revealed that CSK1 is co-expressed with cellulose synthase genes and is responsive to various stress stimuli. The increased SCW thickness and vigorous vessel transport in csk1 indicate that CSK1 functions as a negative regulator of SCW biosynthesis. Through observation of green fluorescent protein-tagged CSK1 in rice protoplasts and stable transgenic plants, we found that CSK1 is localized in the nucleus and cytoplasm adjacent to the plasma membrane. Biochemical and molecular assays demonstrated that CSK1 phosphorylates VASCULAR-RELATED NAC-DOMAIN 6 (VND6), a master SCW-associated transcription factor, in the nucleus, which reduces the transcription of a suite of SCW-related genes, thereby attenuating SCW accumulation. Consistently, genetic analyses show that CSK1 functions upstream of VND6 in regulating SCW formation. Interestingly, our physiological analyses revealed that CSK1 and VND6 are involved in abscisic acid-mediated regulation of cell growth and SCW deposition. Taken together, these results indicate that the CSK1-VND6 module is an important component of the SCW biosynthesis machinery, which coordinates SCW accumulation and adaptive growth in rice. Our study not only identifies a new regulator of SCW biosynthesis but also reveals a fine-tuned mechanism for precise control of SCW deposition, offering tools for rationally tailoring agronomic traits.

Exploring the metabolic basis of growth/defense trade-offs in complex environments with Nicotiana attenuata plants cosilenced in NaMYC2a/b expression

In response to challenges from herbivores and competitors, plants use fitness-limiting resources to produce (auto)toxic defenses. Jasmonate signaling, mediated by MYC2 transcription factors (TF), is thought to reconfigure metabolism to minimize these formal costs of defense and optimize fitness in complex environments. To study the context-dependence of this metabolic reconfiguration, we cosilenced NaMYC2a/b by RNAi in Nicotiana attenuata and phenotyped plants in the field and increasingly realistic glasshouse setups with competitors and mobile herbivores. NaMYC2a/b had normal phytohormonal responses, and higher growth and fitness in herbivore-reduced environments, but were devastated in high herbivore-load environments in the field due to diminished accumulations of specialized metabolites. In setups with competitors and mobile herbivores, irMYC2a/b plants had lower fitness than empty vector (EV) in single-genotype setups but increased fitness in mixed-genotype setups. Correlational analyses of metabolic, resistance, and growth traits revealed the expected defense/growth associations for most sectors of primary and specialized metabolism. Notable exceptions were some HGL-DTGs and phenolamides that differed between single-genotype and mixed-genotype setups, consistent with expectations of a blurred functional trichotomy of metabolites. MYC2 TFs mediate the reconfiguration of primary and specialized metabolic sectors to allow plants to optimize their fitness in complex environments.

CcNAC1 by Transcriptome Analysis Is Involved in Sudan Grass Secondary Cell Wall Formation as a Positive Regulator

Sudan grass is a high-quality forage of sorghum. The degree of lignification of Sudan grass is the main factor affecting its digestibility in ruminants such as cattle and sheep. Almost all lignocellulose in Sudan grass is stored in the secondary cell wall, but the mechanism and synthesis of the secondary cell wall in Sudan grass is still unclear. In order to study the mechanism of secondary cell wall synthesis in Sudan grass, we used an in vitro induction system of Sudan grass secondary cell wall. Through transcriptome sequencing, it was found that the NAC transcription factor CcNAC1 gene was related to the synthesis of the Sudan grass secondary cell wall. This study further generated CcNAC1 overexpression lines of Arabidopsis to study CcNAC1 gene function in secondary cell wall synthesis. It was shown that the overexpression of the CcNAC1 gene can significantly increase lignin content in Arabidopsis lines. Through subcellular localization analysis, CcNAC1 genes could be expressed in the nucleus of a plant. In addition, we used yeast two-hybrid screening to find 26 proteins interacting with CcNAC1. GO and KEGG analysis showed that CcNAC1 relates to the metabolic pathways and biosynthesis of secondary metabolites. In summary, the synthesis of secondary cell wall of Sudan grass can be regulated by CcNAC1.

MKKK20 works as an upstream triple-kinase of MKK3-MPK6-MYC2 module in Arabidopsis seedling development

The mitogen-activated protein kinase (MAPK) cascade is involved in several signal transduction processes in eukaryotes. Here, we report a mechanistic function of MAP kinase kinase kinase 20 (MKKK20) in light signal transduction pathways. We show that MKKK20 acts as a negative regulator of photomorphogenic growth at various wavelengths of light. MKKK20 not only regulates the expression of light signaling pathway regulatory genes but also gets regulated by the same pathway genes. The atmyc2 mkkk20 double mutant analysis shows that MYC2 works downstream to MKKK20 in the regulation of photomorphogenic growth. MYC2 directly binds to the promoter of MKKK20 to modulate its expression. The protein-protein interaction study indicates that MKKK20 physically interacts with MYC2, and this interaction likely suppresses the MYC2-mediated promotion of MKKK20 expression. Further, the protein phosphorylation studies demonstrate that MKKK20 works as the upstream kinase of MKK3-MPK6-MYC2 module in photomorphogenesis.

Blue Light Regulates Cell Wall Structure and Carbohydrate Metabolism of Soybean Hypocotyl

Soybean stem elongation and thickening are related to cell wall composition. Plant morphogenesis can be influenced by blue light, which can regulate cell wall structure and composition, and affect stem growth and development. Here, using proteomics and metabolomics, differentially expressed proteins and metabolites of hypocotyls grown in the dark and under blue light were studied to clarify the effects of blue light on the cell wall structure and carbohydrate metabolism pathway of soybean hypocotyls. Results showed that 1120 differential proteins were upregulated and 797 differential proteins were downregulated under blue light treatment, while 63 differential metabolites were upregulated and 36 differential metabolites were downregulated. Blue light promoted the establishment of cell wall structure and composition by regulating the expression of both the enzymes and metabolites related to cell wall structural composition and nonstructural carbohydrates. Thus, under blue light, the cross-sectional area of the hypocotyl and xylem were larger, the longitudinal length of pith cells was smaller, elongation of the soybean hypocotyl was inhibited, and diameter was increased.

A Phytochrome B-PIF4-MYC2/MYC4 module inhibits secondary cell wall thickening in response to shaded light

Secondary cell walls (SCWs) in stem cells provide mechanical strength and structural support for growth. SCW thickening varies under different light conditions. Our previous study revealed that blue light enhances SCW thickening through the redundant function of MYC2 and MYC4 directed by CRYPTOCHROME1 (CRY1) signaling in fiber cells of the Arabidopsis inflorescence stem. In this study, we find that the Arabidopsis PHYTOCHROME B mutant phyB displays thinner SCWs in stem fibers, but thicker SCWs are deposited in the PHYTOCHROME INTERACTING FACTOR (PIF) quadruple mutant pif1pif3pif4pif5 (pifq). The shaded light condition with a low ratio of red to far-red light inhibits stem SCW thickening. PIF4 interacts with MYC2 and MYC4 to affect their localization in nuclei, and this interaction results in inhibition of the MYCs' transactivation activity on the NST1 promoter. Genetic evidence shows that regulation of SCW thickening by PIFs is dependent on MYC2/MYC4 function. Together, the results of this study reveal a PHYB-PIF4-MYC2/MYC4 module that inhibits SCW thickening in fiber cells of the Arabidopsis stem.

The physiological and transcriptomic study of secondary growth in Neolamarckia cadamba stimulated by the ethylene precursor ACC

Though many biological roles of ethylene have been investigated intensively, the molecular mechanism of ethylene's action in woody plants remains unclear. In this study, we investigated the effects of exogenous 1-aminocyclopropane-1-carboxylic acid (ACC), the precursor of ethylene, on the growth of Neolamarckia cadamba seedlings, a fast-growing tropical tree. After 14 days of ACC treatment, the plants showed a reduced physiological morphology while stem diameter increased; however, this did not occur after the addition of 1-MCP. Meanwhile, the lignin content of N. cadamba also increased. Transcriptome analysis revealed that the expression of the ethylene biosynthesis and signaling genes ACC oxidase (ACO) and ethylene insensitive 3 (EIN3) were up-regulated mainly at the 6th hour and the 3rd day of the ACC treatment, respectively. The transcription levels of transcription factors, mainly in the basic helix-loop-helix (bHLH), ethylene response factor (ERF), WRKY and v-myb avian myeloblastosis viral oncogene homolog (MYB) families, involved in the ethylene signaling and secondary growth also increased significantly. Furthermore, in accordance to the increased lignification of the stem, the transcriptional level of key enzymes in the phenylalanine pathway were elevated after the ACC treatment. Our results revealed the physiological and molecular mechanisms underlying the secondary growth stimulated by exogenous ACC treatment on N. cadamba seedlings.

Vacuolar H+ -ATPase subunit VAB3 regulates cell growth and ion homeostasis in Arabidopsis

Vacuolar H⁺ -ATPase (V-ATPase) has diverse functions related to plant development and growth. It creates the turgor pressure that drives cell growth by generating the energy needed for the active transport of solutes across the tonoplast. V-ATPase is a large protein complex made up of multiheteromeric subunits, some of which have unknown functions. In this study, a forward genetics-based strategy was employed to identify the vab3 mutant, which displayed resistance to isoxaben, a cellulose synthase inhibitor that could induce excessive transverse cell expansion. Map-based cloning and genetic complementary assays demonstrated that V-ATPase B subunit 3 (VAB3) is associated with the observed insensitivity of the mutant to isoxaben. Analysis of the vab3 mutant revealed defective ionic homeostasis and hypersensitivity to salt stress. Treatment with a V-ATPase inhibitor exacerbated ionic tolerance and cell elongation defects in the vab3 mutant. Notably, exogenous low-dose Ca²⁺ or Na⁺ could partially restore isoxaben resistance of the vab3 mutant, suggesting a relationship between VAB3-regulated cell growth and ion homeostasis. Taken together, the results of this study suggest that the V-ATPase subunit VAB3 is required for cell growth and ion homeostasis in Arabidopsis.

WAKL8 Regulates Arabidopsis Stem Secondary Wall Development

Wall-associated kinases/kinase-likes (WAKs/WAKLs) are plant cell surface sensors. A variety of studies have revealed the important functions of WAKs/WAKLs in regulating cell expansion and defense in cells with primary cell walls. Less is known about their roles during the development of the secondary cell walls (SCWs) that are present in xylem vessel (XV) and interfascicular fiber (IF) cells. In this study, we used RNA-seq data to screen Arabidopsis thaliana WAKs/WAKLs members that may be involved in SCW development and identified WAKL8 as a candidate. We obtained T-DNA insertion mutants wakl8-1 (inserted at the promoter region) and wakl8-2 (inserted at the first exon) and compared the phenotypes to wild-type (WT) plants. Decreased WAKL8 transcript levels in stems were found in the wakl8-2 mutant plants, and the phenotypes observed included reduced stem length and thinner walls in XV and IFs compared with those in the WT plants. Cell wall analysis showed no significant changes in the crystalline cellulose or lignin content in mutant stems compared with those in the WT. We found that WAKL8 had alternative spliced versions predicted to have only extracellular regions, which may interfere with the function of the full-length version of WAKL8. Our results suggest WAKL8 can regulate SCW thickening in Arabidopsis stems.

Lignin biosynthesis and accumulation in response to abiotic stresses in woody plants

Woody plants have to experience various abiotic stresses due to their immobility and perennial characteristics. However, woody plants have evolved a series of specific regulation pathways in physiological and molecular mechanisms to deal with adverse environments. Compared with herbaceous plants, perennial woody plants have the advantages of developed roots and hard stems, and increased secondary xylem, which can strengthen the vascular system of the plants. The lignification process involves the lignin deposition on the cell wall by oxidation and polymerization of lignin monomer, which plays an important role in abiotic stress tolerance. This review focuses on recent progress in the biosynthesis, content, and accumulation of lignin in response to various abiotic stresses in plants. The role of transcription factors is also discussed in regulating lignin biosynthesis to enhance abiotic stress tolerance via changing cell wall lignification. Although woody plants shared similar lignin biosynthesis mechanisms with herbaceous plants, the temporal and spatial expression and stress response profiles of lignin biosynthetic genes provide the basis for the differences in stress tolerance of various species. An in-depth understanding of the role of lignin in the abiotic stress tolerance of woody plants will lay the foundation for the next step in tree resistance breeding through genetic engineering.

Cryptochrome-mediated blue-light signal contributes to lignin biosynthesis in stone cells in pear fruit

Light environment is an indispensable factor that regulates multitudinous developmental processes during the whole life cycle of plants, including fruit development. Stone cells which negatively influence pear fruit quality because of their strongly lignified cell wall are also affected by light, however, how light qualities influence lignin biosynthesis in pear remains unclear. Here, the calli of European pear (Pyrus communis L.) treated with different lights were used to explore the changes in phenotype, lignin content, and H₂O₂ content, coupled with RNA-Seq and quantitative real-time PCR (qRT-PCR) to investigate the possible regulation pathway of light on lignin biosynthesis in stone cells. Results showed that blue light increased the expression of lignin structure genes and promoted lignin accumulation. Besides, four blue light receptors cryptochromes (CRYs) were identified in white pear, named PbCRY1a (Pbr024556.1), PbCRY1b (Pbr001636.3), PbCRY2a (Pbr023037.1), and PbCRY2b (Pbr002655.4). qRT-PCR analysis showed that PbCRY1a is highly expressed in cultivars with a high content of stone cells. Furthermore, the molecular function of PbCRY1a on stone cell formation in pear fruit was demonstrated by genetic transformation of pear calli and Agrobacterium-mediated transient overexpression in pear fruitlets. Co-expression network analyses with RNA-seq data showed that 8 MYB and 5 NAC genes were classified into different co-expression clusters with lignin biosynthesis genes under blue light conditions. These results indicate that CRY-mediated blue-light signal plays an important role in cell wall lignification and promotes the formation of stone cells in pear by regulating downstream genes.

Insights into the Molecular Regulation of Lignin Content in Triploid Poplar Leaves

After polyploidization, plants usually undergo some morphological and physiological changes, including the lignin content of polyploids usually becoming lower than that of diploids. However, the regulatory mechanism of the variation of lignin content in polyploid plants remains unclear. Therefore, in this research, we used full-sib poplar triploids and diploids to explore the molecular regulatory basis of lignin content in poplar triploid leaves through the determination of lignin content, the observation of xylem cells, and transcriptome sequencing. The results showed that the lignin content of triploid leaves was significantly lower than that of diploid leaves. The xylem cells of triploid leaves were significantly larger than those of diploids. Transcriptome sequencing data show that most lignin biosynthesis genes were significantly downregulated, and genes related to cell growth were mostly upregulated in triploid leaves compared with diploid leaves. In addition, co-expression network analysis showed that several transcription factors might be involved in the regulation of lignin biosynthesis. Consequently, the altered expression of genes related to lignin might lead to the reduced lignin content in triploids. These results provide a theoretical basis for further exploring the molecular mechanism of the variation of polyploid lignin content and the utilization of polyploid lignocellulosic resources.

Simple, Fast and Efficient Methods for Analysing the Structural, Ultrastructural and Cellular Components of the Cell Wall

Plant cell walls are a fundamental component of plant biology and play an essential role in plant growth and development. The metabolic components of the cell wall can be investigated in a fast, simple, and highly efficient manner using various and distinct microscopy techniques. Here, we report implementing a flowchart to analyse tobacco plants’ structural, ultrastructural, and metabolic components supplemented with far-red light. In addition, biochemical components, such as lignin, cellulose, phenolic compounds, and reducing sugars, present in the plant cell walls were quantified using light, fluorescence, and electron microscopy. Our data were generated from samples prepared via tissue fixation, incorporation in resins, and slicing using microtomes. Moreover, we have used routine staining and contrast techniques to characterise plant cell walls. Here, we describe several protocols that use classic and modern techniques as well as qualitative and quantitative analytical methods to study cell walls, enabling the plant research community to understand and select the most suitable methods for the microscopic analysis of metabolic components. Finally, we discuss specific ideas aimed at new students of plant anatomy and microscopy. This research not only described the structural, ultrastructural, and metabolic components of the plant cell wall, but also explained the strategies for understanding cellular development.

FLA11 and FLA12 glycoproteins fine-tune stem secondary wall properties in response to mechanical stresses

Secondary cell walls (SCWs) in stem xylem vessel and fibre cells enable plants to withstand the enormous compressive forces associated with upright growth. It remains unclear if xylem vessel and fibre cells can directly sense mechanical stimuli and modify their SCW during development. We provide evidence that Arabidopsis SCW-specific Fasciclin-Like Arabinogalactan-proteins 11 (FLA11) and 12 (FLA12) are possible cell surface sensors regulating SCW development in response to mechanical stimuli. Plants overexpressing FLA11 (OE-FLA11) showed earlier SCW development compared to the wild-type (WT) and altered SCW properties that phenocopy WT plants under compression stress. By contrast, OE-FLA12 stems showed higher cellulose content compared to WT plants, similar to plants experiencing tensile stress. fla11, OE-FLA11, fla12, and OE-FLA12 plants showed altered SCW responses to mechanical stress compared to the WT. Quantitative polymerase chain reaction (qPCR) and RNA-seq analysis revealed the up-regulation of genes and pathways involved in stress responses and SCW synthesis and regulation. Analysis of OE-FLA11 nst1 nst3 plants suggests that FLA11 regulation of SCWs is reliant on classical transcriptional networks. Our data support the involvement of FLA11 and FLA12 in SCW sensing complexes to fine-tune both the initiation of SCW development and the balance of lignin and cellulose synthesis/deposition in SCWs during development and in response to mechanical stimuli.

Multi-layered Regulation of Plant Cell Wall Thickening

Plants need to develop thickened cell walls with appropriate localization through precise regulation during the process of growth and development in order to support their body weight and to build long distance transportation systems. Wall thickening is achieved through a multitude of regulatory networks in various tissues under changeable environments. In this mini-review, we summarize current understanding of the regulatory pathways and mechanisms involved in cell wall thickening. Regulation of cell wall thickening is not only mechanistically essential to understand the plant structure accretion but also has applicable significance to plant cell wall biomass utilization.

Structural Alteration of Rice Pectin Affects Cell Wall Mechanical Strength and Pathogenicity of the Rice Blast Fungus Under Weak Light Conditions

Pectin, a component of the plant cell wall, is involved in cell adhesion and environmental adaptations. We generated OsPG-FOX rice lines with little pectin due to overexpression of the gene encoding a pectin-degrading enzyme [polygalacturonase (PG)]. Overexpression of OsPG2 in rice under weak light conditions increased the activity of PG, which increased the degradation of pectin in the cell wall, thereby reducing adhesion. Under weak light conditions, the overexpression of OsPG decreased the pectin content and cell adhesion, resulting in abnormally large intercellular gaps and facilitating invasion by the rice blast fungus. OsPG2-FOX plants had weaker mechanical properties and greater sensitivity to biotic stresses than wild-type (WT) plants. However, the expression levels of disease resistance genes in non-infected leaves of OsPG2-FOX were more than twice as high as those of the WT and the intensity of disease symptoms was reduced, compared with the WT. Under normal light conditions, overexpression of OsPG2 decreased the pectin content, but did not affect cell adhesion and sensitivity to biotic stresses. Therefore, PG plays a role in regulating intercellular adhesion and the response to biotic stresses in rice.

Identification and Characterization of Secondary Wall-Associated NAC Genes and Their Involvement in Hormonal Responses in Tobacco (Nicotiana tabacum)

Secondary wall-associated NAC (SWN) genes are a subgroup of NAC (NAM, ATAF, and CUC) transcription factors (TF) that play a key role in regulating secondary cell wall biosynthesis in plants. However, this gene family has not been systematically characterized, and their potential roles in response to hormones are unknown in Nicotiana tabacum. In this study, a total of 40 SWN genes, of which 12 from Nicotiana tomentosiformis, 13 from Nicotiana sylvestris, and 15 from Nicotiana tabacum, were successfully identified. The 15 SWNs from Nicotiana tabacum were further classified into three groups, namely, vascular-related NAC domain genes (NtVNDs), NAC secondary wall thickening promoting factor genes (NtNSTs), and secondary wall-associated NAC domain genes (NtSNDs). The protein characteristic, gene structure, and chromosomal location of 15 NtSWNs (also named Nt1 to Nt15) were also analyzed. The NtVND and NtNST group genes had five conserved subdomains in their N-terminal regions and a motif (LP[Q/x] L[E/x] S[P/A]) in their diverged C- terminal regions. Some hormones, dark and low-temperature related cis-acting elements, were significantly enriched in the promoters of NtSWN genes. A comprehensive expression profile analysis revealed that Nt4 and Nt12 might play a role in vein development. Others might be important for stem development. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) revealed that in the NtNST group, genes such as Nt7, Nt8, and Nt13 were more sensitive than the genes in NtVND and NtSND groups under abiotic stress conditions. A transactivation assay further suggested that Nt7, Nt8, and Nt13 showed a significant transactivation activity. Overall, SWN genes were finally identified and characterized in diploid and tetraploid tobacco, revealing new insights into their evolution, variation, and homology relationships. Transcriptome, cis-acting element, qRT-PCR, and transactivation assay analysis indicated the roles in hormonal and stress responses, which provided further resources in molecular mechanism and genetic improvement.

A gene that underwent adaptive evolution, LAC2 (LACCASE), in Populus euphratica improves drought tolerance by improving water transport capacity

Drought severely limits plant development and growth; accordingly, plants have evolved strategies to prevent water loss and adapt to water deficit conditions. However, experimental cases that corroborate these evolutionary processes are limited. The LACCASEs (LACs) family is involved in various plant development and growth processes. Here, we performed an evolutionary analysis of LACs from Populus euphratica and characterized the functions of LACs in Arabidopsis and poplar. The results showed that in PeuLACs, multiple gene duplications led to apparent functional redundancy as the result of various selective pressures. Among them, PeuLAC2 underwent strong positive selection. Heterologous expression analyses showed that the overexpression of PeuLAC2 alters the xylem structure of plants, including thickening the secondary cell wall (SCW) and increasing the fiber cell length and stem tensile strength. Altogether, these changes improve the water transport capacity of plants. The analysis of the physiological experimental results showed that PeuLAC2-OE lines exhibited a stronger antioxidant response and greater drought tolerance than WT. Three genes screened by transcriptome analysis, NAC025, BG1, and UGT, that are associated with SCW synthesis and drought stress were all upregulated in the PeuLAC2-OE lines, implying that the overexpression of PeuLAC2 thickened the SCW, improved the water transport capacity of the plant, and further enhanced its drought tolerance. Our study highlights that genes that have undergone adaptive evolution may participate in the development of adaptive traits in P. euphratica and that PeuLAC2 could be a candidate gene for molecular genetic breeding in trees.

A small molecule inhibits cell elongation by modulating cell wall polysaccharide composition in Arabidopsis

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This study identified a small molecule for modification of cell wall composition.

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The molecule can be used as a tool to study cell wall remodeling during plant growth.

The plant primary cell wall is comprised of pectin, cellulose and hemicelluloses, whose dynamic interactions play essential roles in plant cell elongation. Through a chemical genetics screening, we identified a small molecule, named cell wall modulator (CWM), which disrupted cell growth and deformed cell shape in etiolated Arabidopsis hypocotyl. A pectin defective mutant qua2, identified from screening an Arabidopsis EMS mutant library, showed a reduced sensitivity to CWM treatment. On the other hand, pectinase treatment suppressed the CWM induced phenotype. Furthermore, cellulose content was decreased in response to CWM treatment, while the cellulose synthesis mutants ixr1 and ixr2 were hypersensitive to CWM. Together, the study identified a small molecule CWM that induced a modification of the cell wall in elongating cells, likely through interfering with pectin modification. This molecule may be used as a tool to study cell wall remodeling during plant growth.

Anthocyanins are Key Regulators of Drought Stress Tolerance in Tobacco

Abiotic stresses will be one of the major challenges for worldwide food supply in the near future. Therefore, it is important to understand the physiological mechanisms that mediate plant responses to abiotic stresses. When subjected to UV, salinity or drought stress, plants accumulate specialized metabolites that are often correlated with their ability to cope with the stress. Among them, anthocyanins are the most studied intermediates of the phenylpropanoid pathway. However, their role in plant response to abiotic stresses is still under discussion. To better understand the effects of anthocyanins on plant physiology and morphogenesis, and their implications on drought stress tolerance, we used transgenic tobacco plants (AN1), which over-accumulated anthocyanins in all tissues. AN1 plants showed an altered phenotype in terms of leaf gas exchanges, leaf morphology, anatomy and metabolic profile, which conferred them with a higher drought tolerance compared to the wild-type plants. These results provide important insights for understanding the functional reason for anthocyanin accumulation in plants under stress.

MYC2-Activated TRICHOME BIREFRINGENCE-LIKE37 Acetylates Cell Walls and Enhances Herbivore Resistance

O-Acetylation of polysaccharides predominantly modifies plant cell walls by changing the physicochemical properties and, consequently, the structure and function of the cell wall. Expression regulation and specific function of cell wall-acetylating enzymes remain to be fully understood. In this report, we cloned a previously identified stunted growth mutant named sucrose uncoupled1 (sun1) in Arabidopsis (Arabidopsis thaliana). SUN1 encodes a member of the TRICHOME BIREFRINGEN-LIKE family, AtTBL37 AtTBL37 is highly expressed in fast-growing plant tissues and encodes a Golgi apparatus-localized protein that regulates secondary cell wall thickening and acetylation. In sun1, jasmonate signaling and expression of downstream chemical defense genes, including VEGETATIVE STORAGE PROTEIN1 and BRANCHED-CHAIN AMINOTRANSFERASE4, are increased but, unexpectedly, sun1 is more susceptible to insect feeding. The central transcription factor in jasmonate signaling, MYC2, binds to and induces AtTBL37 expression. MYC2 also promotes the expression of many other TBLs Moreover, MYC activity enhances cell wall acetylation. Overexpression of AtTBL37 in the myc2-2 background reduces herbivore feeding. Our study highlights the role of O-acetylation in controlling plant cell wall properties, plant development, and herbivore defense.

PbMC1a/1b regulates lignification during stone cell development in pear (Pyrus bretschneideri) fruit

Programmed cell death (PCD) and secondary cell wall (SCW) thickening in pear fruit are accompanied by the deposition of cellulose and lignin to form stone cells. Metacaspase is an important protease for development, tissue renewal and PCD. The understanding of the molecular mechanism whereby pear (Pyrus) metacaspase promotes PCD and cell wall lignification is still limited. In this study, the Metacaspases gene family (PbMCs) from P. bretschneideri was identified. PbMC1a/1b was associated with lignin deposition and stone cell formation by physiological data, semiquantitative real-time polymerase chain reaction (RT-PCR) and quantitative RT-PCR (qRT-PCR). Relative to wild-type (WT) Arabidopsis, the overexpression of PbMC1a/1b increased lignin deposition and delayed growth, thickened the cell walls of vessels, xylary fibers and interfascicular fibers, and increased the expression of lignin biosynthetic genes. Yeast two-hybrid (Y2H), bimolecular fluorescence complementation (BiFC) and GST pull-down assays indicated that the PbMC1a/1b protein physically interacted with PbRD21. Simultaneously, the transient expression of PbMC1a/1b and PbRD21 led to significant changes in the expression of genes and lignin contents in pear fruits and flesh calli. These results indicate that PbMC1a/1b plays an important role in cell wall lignification, possibly by interacting with PbRD21 to increase the mRNA levels of some lignin synthesis-associated genes and promote the formation of stone cells in pear fruit.