Potential Roles of the GRF Transcription Factors in Sorghum Internodes during Post-Reproductive Stages.

Growth-regulating factor (GRF) is a plant-specific family of transcription factors crucial for meristem development and plant growth. Sorghum (Sorghum bicolor L. Moench) is a cereal species widely used for food, feed and fuel. While sorghum stems are important biomass components, the regulation of stem development and the carbohydrate composition of the stem tissues remain largely unknown. Here, we identified 11 SbGRF-encoding genes and found the SbGRF expansion driven by whole-genome duplication events. By comparative analyses of GRFs between rice and sorghum, we demonstrated the divergence of whole-genome duplication (WGD)-derived OsGRFs and SbGRFs. A comparison of SbGRFs' expression profiles supports that the WGD-duplicated OsGRFs and SbGRFs experienced distinct evolutionary trajectories, possibly leading to diverged functions. RNA-seq analysis of the internode tissues identified several SbGRFs involved in internode elongation, maturation and cell wall metabolism. We constructed co-expression networks with the RNA-seq data of sorghum internodes. Network analysis discovered that SbGRF1, 5 and 7 could be involved in the down-regulation of the biosynthesis of cell wall components, while SbGRF4, 6, 8 and 9 could be associated with the regulation of cell wall loosening, reassembly and/or starch biosynthesis. In summary, our genome-wide analysis of SbGRFs reveals the distinct evolutionary trajectories of WGD-derived SbGRF pairs. Importantly, expression analyses highlight previously unknown functions of several SbGRFs in internode elongation, maturation and the potential involvement in the metabolism of the cell wall and starch during post-anthesis stages.

A transcriptional cascade involving BBX22 and HY5 finely regulates both plant height and fruit pigmentation in citrus.

Dwarfing is a pivotal agronomic trait affecting both yield and quality. Citrus species exhibit substantial variation in plant height, among which internode length is a core element. However, the molecular mechanism governing internode elongation remains unclear. Here, we unveiled that the transcriptional cascade consisting of B-BOX DOMAIN PROTEIN 22 (BBX22) and ELONGATED HYPOCOTYL 5 (HY5) finely tunes plant height and internode elongation in citrus. Loss-of-function mutations of BBX22 in an early-flowering citrus (Citrus hindsii "SJG") promoted internode elongation and reduced pigment accumulation, whereas ectopic expression of BBX22 in SJG, sweet orange (C. sinensis), pomelo (C. maxima) or heterologous expression of BBX22 in tomato (Solanum lycopersicum) significantly decreased internode length. Furthermore, exogenous application of gibberellin A3 (GA3) rescued the shortened internode and dwarf phenotype caused by BBX22 overexpression. Additional experiments revealed that BBX22 played a dual role in regulation internode elongation and pigmentation in citrus. On the one hand, it directly bound to and activated the expression of HY5, GA metabolism gene (GA2 OXIDASE 8, GA2ox8), carotenoid biosynthesis gene (PHYTOENE SYNTHASE 1, PSY1) and anthocyanin regulatory gene (Ruby1, a MYB DOMAIN PROTEIN). On the other hand, it acted as a cofactor of HY5, enhancing the ability of HY5 to regulate target genes expression. Together, our results reveal the critical role of the transcriptional cascade consisting of BBX22 and HY5 in controlling internode elongation and pigment accumulation in citrus. Unraveling the crosstalk regulatory mechanism between internode elongation and fruit pigmentation provides key genes for breeding of novel types with both dwarf and health-beneficial fortification in citrus.

GhSBI1, a CUP-SHAPED COTYLEDON 2 homologue, modulates branch internode elongation in cotton.

Branch length is an important plant architecture trait in cotton (Gossypium) breeding. Development of cultivars with short branch has been proposed as a main object to enhance cotton yield potential, because they are suitable for high planting density. Here, we report the molecular cloning and characterization of a semi-dominant quantitative trait locus, Short Branch Internode 1(GhSBI1), which encodes a NAC transcription factor homologous to CUP-SHAPED COTYLEDON 2 (CUC2) and is regulated by microRNA ghr-miR164. We demonstrate that a point mutation found in sbi1 mutants perturbs ghr-miR164-directed regulation of GhSBI1, resulting in an increased expression level of GhSBI1. The sbi1 mutant was sensitive to exogenous gibberellic acid (GA) treatments. Overexpression of GhSBI1 inhibited branch internode elongation and led to the decreased levels of bioactive GAs. In addition, gene knockout analysis showed that GhSBI1 is required for the maintenance of the boundaries of multiple tissues in cotton. Transcriptome analysis revealed that overexpression of GhSBI1 affects the expression of plant hormone signalling-, axillary meristems initiation-, and abiotic stress response-related genes. GhSBI1 interacted with GAIs, the DELLA repressors of GA signalling. GhSBI1 represses expression of GA signalling- and cell elongation-related genes by directly targeting their promoters. Our work thus provides new insights into the molecular mechanisms for branch length and paves the way for the development of elite cultivars with suitable plant architecture in cotton.

PdRabG3f interfered with gibberellin-mediated internode elongation and xylem developing in poplar.

As a member of the small GTPases family, Rab GTPases play a key role in specifying transport pathways in the intracellular membrane trafficking system and are involved in plant growth and development. By quantitative trait locus (QTL) mapping, PdRabG3f was identified as a candidate gene associated with shoot height in a hybrid offspring of Populus deltoides 'Danhong' × Populus simonii 'Tongliao1'. PdRabG3f localized to the nucleus, endoplasmic reticulum and tonoplast and was primarily expressed in the xylem and cambium. Overexpression of PdRabG3f in Populus alba × Populus glandulosa (84 K poplar) had inhibitory effects on vertical and radical growth. In the transgenic lines, there were evident changes in the levels of 15 gibberellin (GA) derivatives, and the application of exogenous GA3 partially restored the phenotypes mediated by GAs deficiency. The interaction between PdRabG3f and RIC4, which was the GA-responsive factor, provided additional explanation for PdRabG3f's inhibitory effect on poplar growth. RNA-seq analysis revealed differentially expressed genes (DEGs) associated with cell wall, xylem, and gibberellin response. PdRabG3f interfering endogenous GAs levels in poplar might involve the participation of MYBs and ultimately affected internode elongation and xylem development. This study provides a potential mechanism for gibberellin-mediated regulation of plant growth through Rab GTPases.

Unveiling the Regulatory Role of miRNAs in Internode Elongation: Integrated Analysis of MicroRNA and mRNA Expression Profiles across Diverse Dwarfing Treatments in Maize (Zea mays L.).

MicroRNAs are crucial regulators of gene expression in maize. However, the mechanisms through which miRNAs control internode elongation remain poorly understood. This study engineered varying levels of internode elongation inhibition, revealing that dwarfing treatments diminished gibberellin levels, curtailed cell longitudinal growth, and slowed the rate of internode elongation. Comprehensive transcriptome and miRNA profiling of the internode elongation zone showed gene expression changes that paralleled the extent of the internode length reduction. We identified 543 genes and 29 miRNAs with significant correlations to internode length, predominantly within families, including miR164 and miR396. By incorporating target gene expression levels, we pinpointed nine miRNA-mRNA pairs that are significantly associated with the regulation of the internode elongation. The inhibitory effects of these miRNAs on their target genes were confirmed through dual-luciferase reporter assays. Overexpression of miR164h in maize resulted in increased internode and cell length, suggesting a novel genetic avenue for manipulating plant stature. These miRNAs may also serve as precise spatiotemporal regulators for in vitro plant development.

Water depth-dependent stem elongation of completely submerged Alternanthera philoxeroides is mediated by intra-internodal growth variations.

Complete submergence, especially deep submergence, poses a serious threat to the growth and survival of plants. One study previously showed that Alternanthera philoxeroides (a herbaceous perennial plant) submerged at depth of 2 m presented fast stem elongation and reduced stem elongation as water depth increased. In the present study, we aimed to figure out from the morphological and anatomical perspective how the differential growth response of the plant to water depth was achieved. We investigated the elongation of different stem parts and the relationship of stem elongation to cell size and number in A. philoxeroides by conducting experiments using a series of submergence depths (0 m, 2 m, 5 m, and 9 m). The results showed that, in comparison with unsubmerged plants, completely submerged plants exhibited enhanced elongation at depths of 2 m and 5 m but suppressed elongation at depth of 9 m in immature stem internodes, and displayed very little elongation in mature stem internodes at any depths. The stem growth of A. philoxeroides at any submergence depth was chiefly caused by the elongation of the basal parts of immature internodes. The elongation of the basal parts of immature internodes was highly correlated to both cell proliferation and cell enlargement, but the elongation of the middle and upper parts of immature internodes correlated nearly only with cell enlargement. This study provided new information on the growth responses of A. philoxeroides to heterogeneous submergence environments and deepened our understanding of the growth performance of terrestrial plants in habitats prone to deep floods.

Molecular mechanism of internode elongation in rice.

Rice plants that form ventilated tissues, such as aerenchyma in the leaves, stems, and roots, allow for growth in waterlogged conditions (paddy fields), but they cannot breathe and drown in flooded environments where the whole plant body is submerged. However, deepwater rice plants grown in flood-prone areas of Southeast Asia survive in prolonged flooded environments by taking in air through an elongated stem (internode) and leaves that emerge above the water surface, even if the water level is several meters high and flooding continues for several months. Although it has been known that plant hormones, such as ethylene and gibberellins, promote internode elongation in deepwater rice plants, the genes that control rapid internode elongation during submergence have not been identified. We recently identified several genes responsible for the quantitative trait loci involved in internode elongation in deepwater rice. Identification of the the genes revealed a molecular gene network from ethylene to gibberellins in which internode elongation is promoted by novel ethylene-responsive factors and enhances gibberellin responsiveness at the internode. In addition, elucidation of the molecular mechanism of internode elongation in deepwater rice will help our understanding of the internode elongation mechanism in normal paddy rice and contribute to improving crops through the regulation of internode elongation.

Identification and expression profiles of xylogen-like arabinogalactan protein (XYLP) gene family in Phyllostachys edulis in different developmental tissues and under various abiotic stresses.

Xylogen-like arabinogalactan protein (XYLP) is an atypical lipid transport protein. In this study, 23 Phyllostachys edulis XYLPs were identified, and their proteins contain characteristic structures of AGP and nsLTP domain. All PeXYLPs can be divided into four clades, and their genes were unevenly distributed on 11 chromosome scaffolds. Collinear analysis revealed that segmental duplication was the main driver for PeXYLP family expansion. The cis-acting elements presented in the promoter are involved in various regulations of PeXYLPs expression. G.O. annotation revealed that PeXYLPs are mainly interested in lipid transport and synthesis and primarily function at the plasma membrane. Transcriptome analysis revealed that PeXYLPs were spatiotemporally expressed and displayed significant variability during various tissue development. Besides that, some PeXYLPs also respond to multiple phytohormones and abiotic stresses. By semi-quantitative RT-PCR, the response of some PeXYLPs to MeJA was confirmed, and the proteins were shown to localize to the plasma membrane mainly. WGCNA in defined regions of fast-growing bamboo shoots revealed that 5 PeXYLPs in 4 gene co-expression modules showed a positive module-trait relationship with three fast-growing regions. This systematic analysis of the PeXYLP family will provide a foundation for further insight into the functions of individual PeXYLP in a specific tissue or organ development, phytohormone perception, and stress responses in the future.

Single-base methylome analysis reveals dynamic changes of genome-wide DNA methylation associated with rapid stem growth of woody bamboos.

MAIN CONCLUSION: CG and CHG methylation levels in the rapid shoot growth stages (ST2-ST4) of woody bamboos were obviously decreased, which might regulate the internode elongation during rapid shoot growth, while CHH methylation was strongly associated with shoot developmental time or age. DNA methylation plays a critical role in the regulation of plant growth and development. Woody bamboos have a unique trait of rapid stem growth resulted from internode elongation at the shooting period. However, it is still unclear whether DNA methylation significantly controls the bamboo rapid stem growth. Here we present whole-genome DNA methylation profiles of the paleotropical woody bamboo Bonia amplexicaulis at five newly defined stages of shoot growth, named ST1-ST5. We found that CG and CHG methylation levels in the rapid shoot growth stages (ST2-ST4) were significantly lower than in the incubation (ST1) and plateau stages (ST5). The changes in methylation levels mainly occurred in flanking regions of genes and gene body regions, and 23647 differentially methylated regions (DMRs) were identified between ST1 and rapid shoot growth stages (ST2-ST4). Combined with transcriptome analysis, we found that DMR-related genes enriched in the auxin and jasmonic acid (JA) signal transduction, and other pathways closely related to plant growth. Intriguingly, CHH methylation was not involved in the rapid shoot growth, but strongly associated with shoot developmental time by gradually accumulating in transposable elements (TEs) regions. Overall, our results reveal the importance of DNA methylation in regulating the bamboo rapid shoot growth and suggest a role of DNA methylation associated with development time or age in woody bamboos.

SNORKEL Genes Relating to Flood Tolerance Were Pseudogenized in Normal Cultivated Rice.

SNORKEL1 (SK1) and SNORKEL2 (SK2) are ethylene responsive factors that regulate the internode elongation of deepwater rice in response to submergence. We previously reported that normal cultivated rice lacks SK genes because the Chromosome 12 region containing SK genes was deleted from its genome. However, no study has analyzed how the genome defect occurred in that region by comparing normal cultivated rice and deepwater rice. In this study, comparison of the sequence of the end of Chromosome 12, which contains SK genes, between normal and deepwater rice showed that complicated genome changes such as insertions, deletions, inversions, substitutions, and translocation occurred frequently in this region. In addition to SK1 and SK2 of deepwater rice, gene prediction analysis identified four genes containing AP2/ERF domains in normal cultivated rice and six in deepwater rice; we called these genes SK-LIKE (SKL) genes. SKs and SKLs were present in close proximity to each other, and the SKLs in normal cultivated rice were in tandem. These predicted genes belong to the same AP2/ERF subfamily and were separated into four types: SK1, SK2, SKL3, and SKL4. Sequence comparison indicated that normal cultivated rice possesses a gene with high homology to SK2, which we named SKL1. However, none of the predicted SKLs except for SKL3s were expressed during submergence. Although SKL3s were expressed in both normal and deepwater rice, normal rice does not undergo internode elongation, suggesting that its expression does not contribute to internode elongation. Plants overexpressing SKL1, which showed the most homology to SK2, underwent internode elongation similar to plants overexpressing SK1 and SK2 under normal growth conditions. A yeast one-hybrid assay showed that the C-end of SKL1 has transcription activity, as do the C-ends of SK1 and SK2. Our results suggested that SKLs were derived via gene duplication, but were not expressed and pseudogenized in normal cultivated rice during sequence evolution.

OsARP6 Is Involved in Internode Elongation by Regulating Cell-Cycle-Related Genes.

The SWR1 complex (SWR1-C) is important for the deposition of histone variant H2A.Z into chromatin to regulate gene expression. Characterization of SWR1-C subunits in Arabidopsis thaliana has revealed their role in variety of developmental processes. Oryza sativa actin related protein 6 (OsARP6) is a subunit of rice SWR1-C. Its role in rice plant development is unknown. Here, we examined the subcellular localization, expression patterns, and loss of function phenotypes for this protein and found that OsARP6 is a nuclear localized protein, and is broadly expressed. OsARP6 interacted with OsPIE1, a central ATPase subunit of rice SWR1-C. The osarp6 knockout mutants displayed pleiotropic phenotypic alterations in vegetative and reproductive traits, including semi-dwarf phenotype, lower tillers number, short leaf length, changes in spikelet morphology, and seed abortion. Microscopic thin sectioning of the top internode revealed that the dwarf phenotype of osarp6 was due to reduced number of cells rather than reduced cell length. The altered transcript level of genes involved in cell division suggested that OsARP6 affects cell cycle regulation. In addition, H2A.Z levels were reduced at the promoters and transcription start sites (TSS) of the regulated genes in osarp6 plants. Together, these results suggest that OsARP6 is involved in rice plant development, and H2A.Z deposition.

Pretreatment of seeds with hydrogen peroxide improves deep-sowing tolerance of wheat seedlings.

Drought is a prevalent natural factor limiting crop production in arid regions across the world. To overcome this limitation, seeds are sown much deeper to boost germination by soil moisture produced by underground water. Seed pretreatment can effectively induce deep-sowing tolerance in plants. In the present study, we evaluated whether H2O2 pretreatment of seeds can initiate metabolic changes and lead to improved deep-sowing tolerance in wheat. Pretreatment with 0.05 µM H2O2 promoted first internode elongation by 13% in the deep-sowing tolerant wheat cultivar "Tir" and by 32% in the sensitive cultivar "Kiraç-66" under deep-sowing conditions, whereas internode elongation was inhibited by diphenyleneiodonium chloride. In contrast to Tir seedlings, H2O2 levels in the first internode of Kiraç-66 seedlings increased under deep-sowing condition in the H2O2-treated group compared to controls. Moreover, these seedlings had significantly lower catalase (CAT), peroxidase (POX), and ascorbate peroxidase (APX) activities but higher NADPH oxidase (NOX) and superoxide dismutase (SOD) activities under the same conditions, which consequently induced greater H2O2 accumulation. Contrary to Tir, both total glutathione and glutathione S-transferase (GST) activity decreased in Kiraç-66 after deep-sowing at 10 cm. However, H2O2 treatment increased the total glutathione amounts and the activities of glutathione-related enzymes (except GST and GPX) in the first internode of Kiraç-66. Taken together, these data support that H2O2 acts as a signaling molecule in the activation of antioxidant enzymes (specifically NOX, SOD, and CAT), regulation of both glutathione-related enzymes and total glutathione content, and upregulation of the cell wall-loosening protein gene TaEXPB23.

Nitrogen and Stem Development: A Puzzle Still to Be Solved.

High crop yields are generally associated with high nitrogen (N) fertilizer rates. A growing tendency that is urgently demanding the adoption of precision technologies that manage N more efficiently, combined with the advances of crop genetics to meet the needs of sustainable farm systems. Among the plant traits, stem architecture has been of paramount importance to enhance harvest index in the cereal crops. Nonetheless, the reduced stature also brought undesirable effect, such as poor N-uptake, which has led to the overuse of N fertilizer. Therefore, a better understanding of how N signals modulate the initial and late stages of stem development might uncover novel semi-dwarf alleles without pleiotropic effects. Our attempt here is to review the most recent advances on this topic.

FLOWERING LOCUS T2 Promotes Shoot Apex Development and Restricts Internode Elongation via the 13-Hydroxylation Gibberellin Biosynthesis Pathway in Poplar.

The adaptation and survival of boreal and temperate perennials relies on the precise demarcation of the growing season. Seasonal growth and development are defined by day length and temperature signals. Under long-day conditions in spring, poplar FLOWERING LOCUS T2 (FT2) systemically induces shoot growth. In contrast, FT2 downregulation induced by autumnal short days triggers growth cessation and bud set. However, the molecular role of FT2 in local and long-range signaling is not entirely understood. In this study, the CRISPR/Cas9 editing tool was used to generate FT2 loss of function lines of hybrid poplar. Results indicate that FT2 is essential to promote shoot apex development and restrict internode elongation under conditions of long days. The application of bioactive gibberellins (GAs) to apical buds in FT2 loss of function lines was able to rescue bud set. Expression analysis of GA sensing and metabolic genes and hormone quantification revealed that FT2 boosts the 13-hydroxylation branch of the GA biosynthesis pathway in the shoot apex. Paclobutrazol treatment of WT leaves led to limited internode growth in the stem elongation zone. In mature leaves, FT2 was found to control the GA 13-hydroxylation pathway by increasing GA2ox1 and reducing GA3ox2 expression, causing reduced GA1 levels. We here show that in poplar, the FT2 signal promotes shoot apex development and restricts internode elongation through the GA 13-hydroxylation pathway.

Physiological Mechanism of Internode Bending Growth After the Excision of Shoot Sheath in Fargesia yunnanensis and Its Implications for Understanding the Rapid Growth of Bamboos.

The physiological function of bamboo shoot sheaths is still unclear. In the present study, we investigated the anatomical and physiological influences of bamboo shoot sheaths on internode elongation by longitudinally striping parts of sheaths. The internodes would bend toward the bare sides during night. The results showed that amounts of water leaked at the cut of shoot sheaths during night, which impeded the increase of water, water pressure and assimilate transport rates, and decreased starch and soluble sugar catabolism in the bare side of the internodes. A higher level of water pressure and sugar metabolism increased the vacuole expansion and promoted the cell expansion in the outer sides as compared to the bare sides. The bending growth of internodes was mainly due to the significant differences in cell expansion, which was led by the difference in water pressure and sugar hydrolysis levels between the inner and outer sides. Bamboo internode elongation mainly relied on the increase of water pressure and soluble sugar concentration. Shoot sheaths played an important role in the rapid growth of bamboo shoots as a controller in water and assimilate transportation. This study gave a new insight into understanding the rapid growth mechanism of bamboo plants.

OsbHLH073 Negatively Regulates Internode Elongation and Plant Height by Modulating GA Homeostasis in Rice.

Internode elongation is one of the key agronomic traits determining a plant's height and biomass. However, our understanding of the molecular mechanisms controlling internode elongation is still limited in crop plant species. Here, we report the functional identification of an atypical basic helix-loop-helix transcription factor (OsbHLH073) through gain-of-function studies using overexpression (OsbHLH073-OX) and activation tagging (osbhlh073-D) lines of rice. The expression of OsbHLH073 was significantly increased in the osbhlh073-D line. The phenotype of osbhlh073-D showed semi-dwarfism due to deficient elongation of the first internode and poor panicle exsertion. Transgenic lines overexpressing OsbHLH073 confirmed the phenotype of the osbhlh073-D line. Exogenous gibberellic acid (GA3) treatment recovered the semi-dwarf phenotype of osbhlh073-D plants at the seedling stage. In addition, quantitative expression analysis of genes involving in GA biosynthetic and signaling pathway revealed that the transcripts of rice ent-kaurene oxidases 1 and 2 (OsKO1 and OsKO2) encoding the GA biosynthetic enzyme were significantly downregulated in osbhlh073-D and OsbHLH073-OX lines. Yeast two-hybrid and localization assays showed that the OsbHLH073 protein is a nuclear localized-transcriptional activator. We report that OsbHLH073 participates in regulating plant height, internode elongation, and panicle exsertion by regulating GA biosynthesis associated with the OsKO1 and OsKO2 genes.

The acid invertase gene family is involved in internode elongation in Phyllostachys heterocycla cv. pubescens.

Acid invertases (INVs) play a pivotal role in both vegetative and reproductive growth of plants. However, their possible functions in fast-growing plants such as bamboo are largely unknown. Here, we report the molecular characterization of acid INVs in Phyllostachys heterocycla cv. pubescens, a fast-growing bamboo species commercially grown worldwide. Nine acid INVs (PhINVs), including seven cell wall INVs (PhCWINV1, PhCWINV2, PhCWINV3, PhCWINV4, PhCWINV5, PhCWINV6 and PhCWINV7) and two vacuolar INVs (PhVINV11 and PhVINV12) were isolated. Bioinformatic analyses demonstrated that they all share high amino acid identity with other INVs from different plant species and contain the motifs typically conserved in acid INV. Enzyme activity assays revealed a significantly higher INV activity in the fast-growing tissues, such as the elongating internodes of stems. Detailed quantitative reverse-transcription PCR analyses showed various expression patterns of PhINVs at different developmental stages of the elongating stems. With the exception of PhCWINV6, all PhINVs were ubiquitously expressed in a developmental-specific manner. Further studies in Arabidopsis exhibited that constitutive expression of PhCWINV1, PhCWINV4 or PhCWINV7 increased the biomass production of transgenic plants, as indicated by augmented plant heights and shoot dry weights than the wild-type plants. All these results suggest that acid INVs play a crucial role in the internode elongation of P. heterocycla cv. pubescens and would provide valuable information for the dissection of their exact biological functions in the fast growth of bamboo.

Symplasmic phloem unloading and post-phloem transport during bamboo internode elongation.

In traditional opinions, no radial transportation was considered to occur in the bamboo internodes but was usually considered to occur in the nodes. Few studies have involved the phloem unloading and post-phloem transport pathways in the rapid elongating bamboo shoots. Our observations indicated a symplastic pathway in phloem unloading and post-unloading pathways in the culms of Fargesiayunnanensis Hsueh et Yi, based on a 5,6-carboxyfluorescein diacetate tracing experiment. Significant lignification and suberinization in fiber and parenchyma cell walls in maturing internodes blocked the apoplastic transport. Assimilates were transported out of the vascular bundles in four directions in the inner zones but in two directions in the outer zones via the continuum of parenchyma cells. In transverse sections, assimilates were outward transported from the inner zones to the outer zones. Assimilates transport velocities varied with time, with the highest values at 0):00 h, which were affected by water transport. The assimilate transport from the adult culms to the young shoots also varied with the developmental degree of bamboo shoots, with the highest transport velocities in the rapidly elongating internodes. The localization of sucrose, glucose, starch grains and the related enzymes reconfirmed that the parenchyma cells in and around the vascular bundles constituted a symplastic pathway for the radial transport of sugars and were the main sites for sugar metabolism. The parenchyma cells functioned as the 'rays' for the radial transport in and between vascular bundles in bamboo internodes. These results systematically revealed the transport mechanism of assimilate and water in the elongating bamboo shoots.

TDIF overexpression in poplars retards internodal elongation and enhances leaf venation through interaction with other phytohormones.

As a member of the CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION-related (CLE) peptide family, tracheary element differentiation inhibitory factor (TDIF) plays crucial roles in vascular meristem maintenance by promoting cell proliferation and inhibiting xylem cell differentiation. In Populus trichocarpa, six TDIF-encoding genes are all expressed in vascular tissues, and in Arabidopsis PtTDIFpro:GUS lines, the expression driven by PtTDIF promoters were predominantly detected in stem vascular bundles, initiating leaves and leaf veins. Although exogenous application of two poplar TDIF peptides did not evidently affect the shoot growth in vitro, overexpression of PtTDIF genes in hybrid poplar severely retarded the internodal elongation by upregulating the expression of GA2ox and GA20ox genes and thus decreasing the level of endogenous gibberellins (GAs), which phenotypic defect could be rescued by exogenously applied GA3. In addition, TDIF overexpression unexpectedly induced a more complex venation pattern in poplar leaves, which was underpinned by the elevated expression of WOX4 and WOX13 genes. Upon TDIF treatment, the DR5:GUS poplar leaves revealed a higher GUS activity and in TDIF-overexpressing leaves, the transcript abundances of several PIN-FORMED (PIN) genes, especially that of PIN1, were increased, which implied an integration of TDIF and auxin in mediating this process. Collectively, data of this work presented novel activities of TDIF involved in internode elongation and leaf vein formation, thus revealing the divergent functions of TDIF in perennial tree species from those in annual herbaceous Arabidopsis.

Ethephon-regulated maize internode elongation associated with modulating auxin and gibberellin signal to alter cell wall biosynthesis and modification.

Ethephon efficiently regulates plant growth to modulate the maize (Zea mays L.) stalk strength and yield potential, yet there is little information on how ethylene governs a specific cellular response for altering internode elongation. Here, the internode elongation kinetics, cell morphological and physiological properties and transcript expression patterns were investigated in the ethephon-treated elongating internode. Ethephon decreased the internode elongation rate, shortened the effective elongation duration, and advanced the growth process. Ethephon regulated the expression patterns of expansin and secondary cell wall-associated cellulose synthase genes to alter cell size. Moreover, ethephon increased the activities and transcripts level of phenylalanine ammonia-lyase and peroxidase, which contributed to lignin accumulation. Otherwise, ethephon-boosted ethylene evolution activated ethylene signal and increased ZmGA2ox3 and ZmGA2ox10 transcript levels while down-regulating ZmPIN1a, ZmPIN4 and ZmGA3ox1 transcript levels, which led to lower accumulation of gibberellins and auxin. In addition, transcriptome profiles confirmed previous results and identified several transcription factors that are involved in the ethephon-modulated transcriptional regulation of cell wall biosynthesis and modification and responses to ethylene, gibberellins and auxin. These results indicated that ethylene-modulated auxin and gibberellins signaling mediated the transcriptional operation of cell wall modification to regulate cell elongation in the ethephon-treated maize internode.