Protoxylem Poles Research Articles

The Hierarchy of Protoxylem Groupings in Primary Root and Their Plasticity to Nitrogen Addition in Three Tree Species.

Protoxylem grouping (PG), a classification based on the number of protoxylem poles, is a crucial indicator related to other functional traits in fine roots, affecting growth and survival of individual root. However, within root system, less is known about the arrangement of PG. Moreover, the responses of PG to fertilization are still unclear. Here, we selected three common hardwood species in Northeast China, Juglans mandshurica, Fraxinus mandshurica, and Phellodendron amurense, conducted root pruning and nutrient addition. In this study, we analyzed the PG, morphology, and other anatomy traits of newly formed root branches. The results showed all root length, diameter, and stele, as well as hydraulic conductivity, were significantly positive related to the PG number, and the PG number generally decreased with ascending root developmental order; these patterns were independent of species and fertilization. Additionally, we also found the plasticity of PGs to environmental changes, in terms of the increased frequency of high PG roots after fertilization, significantly in J. mandshurica and F. mandshurica. Therefore, the heterogeneity, hierarchy, and plasticity of individual roots within root system may be widespread in woody plants, which is of great significance to deepen our understanding in root growth and development, as well as the belowground ecological process.

WRKY15 Suppresses Tracheary Element Differentiation Upstream of VND7 During Xylem Formation.

Formation of the vascular cylinder, a structure critical to water and nutrient transport in higher plants, is highly regulated. Here we identify WRKY15 as an important regulator that suppresses tracheary element (TE) differentiation in Arabidopsis (Arabidopsis thaliana). Overexpression of WRKY15 resulted in discontinuous protoxylem vessel files and TEs with reduced spiral wall thickening/lignification. Expression of a dominant-negative WRKY15 variant, WRKY15-EAR, led to extra protoxylem vessels and ectopic TEs with increased spiral wall thickening/lignification. Ectopic TE formation in the root cortex and hypocotyl/leaf epidermis reveals that the suppression of WRKY15 is sufficient to trigger the transdifferentiation of other types of cells to TEs. Expression profiling, RT-qPCR, and reporter analyses revealed that WRKY15 suppresses the expression of VASCULAR-RELATED NAC DOMAIN7 (VND7), a master transcriptional regulator that promotes TE differentiation. We propose that WRKY15 negatively regulates VND7 expression indirectly based on (1) the absence of a W-box in the promoter of VND7 and (2) the observation that WRKY15 and VND7 are expressed in different cells in the vascular cylinder, with WRKY15 expressed in the procambial cells and VND7 in the protoxylem poles of procambium and differentiating TEs. Future research is needed to reveal the details underlying the interaction of WRKY15 and VND7 in plant vascular development.

Adaptive morphoanatomy and ecophysiology of Billbergia euphemiae, a hemiepiphyte Bromeliaceae

Abstract Habitats under distinct selective pressures exert adaptative pressures that can lead individuals of the same species to present different life strategies for their survival. The aim of this study was to analyse morphoanatomical and physiological traits for identification of adaptive ecological strategies related to both terrestrial and epiphytic life phases of Billbergia euphemiae. It was verified that B. euphemiae showed lower height, as well smaller length, width and foliar area in epiphytic phase than in terrestrial phase. Concerning to foliar anatomy, the thicknesses of leaf and water-storage parenchyma were higher in terrestrial phase, as densities of stomata and scales on the abaxial surface were higher in epiphytic phase. About the contents of photosynthetic pigments, only chlorophyll a/b ratio showed differences between life phases. In both habits, plants exhibited roots with absorption hair. In epiphytic phase, roots exhibited higher velamen thickness, smaller outer cortex, higher number of inner cortex cell layers and higher number of protoxylem poles. Thus, B. euphemiae individuals in epiphytic exhibited lots of traits related to water retention, once these plants are not into the ground. Besides, the plasticity observed may contribute for survival of this group in habitats submitted to modifications (e.g., climate change and other variations caused by human interference).

Root Anatomical Structure of <i>Jatropha curcas</i> Seedlings—A Short Report

Jatropha curcas has been expected as a biodiesel plant which can be grown in degraded lands. The structure of roots at the seedling stage, in particular cell wall modification in exodermis and endodermis, was microscopically observed. In addition, it was discussed if the first four peripheral roots that emerge from the base of the primary root (taproot) are lateral roots or adventitious roots. The primary root and the first-order lateral roots formed diarchy stele, in which two protoxylem poles present in primary xylem of root. Consequently, the first four peripheral roots cannot be lateral roots, but should be adventitious roots formed at the base of hypocotyl. In both the primary and first-order lateral roots, exodermis and endodermis formed highly lignified cell walls. Moreover, the exodermal and endodermal cell walls formed Casparian strips, which could be observed without special staining by fluorescent dye under ultraviolet microscopy. Such cell-wall modification in root exodermis and endodermis may play an important role for J. curcas under soil stresses in degraded lands.

OCORRÊNCIA DE RAÍZES GEMÍFERAS EM <i>Tabebuia roseoalba </i> (Ridl.) Sandwith (BIGNONIACEAE, LAMIALES)

A espécie Tabebuia roseoalba, presente na flora nativa do cerrado, apresenta propagação vegetativa em ambiente natural. A presença de sistemas subterrâneos produzindo gemas em espécies arbóreas está relacionada com a sobrevivência em condições de distúrbios ambientais. Assim, o objetivo deste trabalho foi analisar e caracterizar anatomicamente o sistema subterrâneo produtor de gemas caulinares nessa espécie. O material selecionado (raízes plagiotrópicas) foi cortado em fragmentos de aproximadamente dez centímetros de comprimento, lavados com água e plantados em recipientes plásticos contendo vermiculita. Os fragmentos foram mantidos em câmara termostática durante sessenta dias para induzir as gemas radiculares. Posteriormente, o material foi seccionado à mão e corado com safranina e azul de metileno para o exame microscópico. Os resultados indicaram que o sistema subterrâneo analisado possui um arranjo característico de raízes com o polo do protoxilema localizado mais externamente (protoxilema exarco), no qual o centro da estrutura é composto por xilema. As características observadas indicam que as gemas são reparativas, cuja formação está relacionada com algum distúrbio no sistema subterrâneo.

Heterorhizy and fine root architecture of rabbiteye blueberry (Vaccinium virgatum) softwood-cuttings.

All fine root systems consist of individual fine roots. Individual roots have morphological, anatomical, and functional heterogeneity (heterorhizy). Heterorhizy plays crucial roles in plant ecosystems. However, in many species, the heterorhizy and fine root system architecture based on individual root units are unclear. This study investigated heterorhizy along the root system architecture of Vaccinium virgatum Ait (rabbiteye blueberry) softwood-cuttings (propagated from annual shoots in growing season) using protoxylem groups (PGs), a classification according to the number of protoxylem poles, as an indicator of individual root traits. Individual roots of rabbiteye blueberry varied from monarch to heptarch. The frequency of roots with larger number of PGs decreased but those with smaller number of PGs increased from adventitious roots toward lateral roots with different branching levels. This architecture were stable among cultivars, collecting position of the cuttings, or indole acetic acids treatment. Individual root sizes and secondary growth were positively correlated with the PGs. These results indicate that branching itself strongly and broadly controls individual root traits. The individual roots were classified into two types: monarch and diarch roots with small size and lacking secondary growth (thought to be hair roots in core Ericaceae) and triarch or more PG roots with large size and showing secondary growth. These heterogeneous individual roots responded differently to the experimental factors. In particular, elongation of the large roots significantly contributed to increased total root length. These results mean that heterorhizic plasticity is a determinant of root system development and heterorhizic variation exists even under practical cutting condition. In conclusion, we demonstrated heterorhizy of rabbieye blueberry cuttings based on the strong relationships of PG, individual root morphology and growth potential, and root system architecture. This study also supports strong connection between root morphology and functional roles intermediated by the PG.

Root development and structure in seedlings of Ginkgo biloba.

The popular, highly recognizable, well-known gymnosperm, Ginkgo biloba, was studied to document selected developmental features, which are little known in its primary root system from root tips to cotyledonary node following seed germination. Using seedlings grown in soil, vermiculite, or a mixture, we examined sections at various distances from the root cap to capture a developmental sequence of anatomical structures by using standard brightfield, epifluorescence, and confocal microscopic techniques. The vascular cylinder is usually a diarch stele, although modified diarchy and triarchy are found. Between exarch protoxylem poles, metaxylem usually develops into a complete disc, except near the transition region, which has irregularly arranged tracheary cells. The disc of primary xylem undergoes secondary growth on its metaxylem flanks with many tracheids added radially within a few weeks. Production of fibers in secondary phloem also accompanies secondary growth. In the cortex, endodermis produces Casparian bands early in development and continues into the upper transition region. Phi cells with phi-thickenings (bands of lignified walls) of a layer of inner cortex are often evident before endodermis, and then adjoining, additional layers of cortex develop phi cells; phi cells do not occur in the upper transition region or stem. An exodermis is produced early in root development and is continuous into the transition region and cotyledonary node. Seedling root axes of Ginkgo biloba are more complex than the literature suggests, and our findings contribute to our knowledge of root structure of this ancient gymnosperm.

Possible role of glutamine synthetase of the prokaryotic type (GSI-like) in nitrogen signaling in Medicago truncatula

Genes containing domains related to glutamine synthetase of the prokaryotic type (GSI-like) are widespread in higher plants, but their function is currently unknown. To gain insights into the possible role of GSI-like proteins, we characterized the GSI-like gene family of Medicago truncatula and investigated the functionality of the encoded proteins. M. truncatula contains two-expressed GSI-like genes, MtGSIa and MtGSIb, encoding polypeptides of 454 and 453 amino acids, respectively. Heterologous complementation assays of a bacterial glnA mutant indicate that the proteins are not catalytically functional for glutamine synthesis. Gene expression was investigated by qRT-PCR and western blot analysis in different organs of the plant and under different nitrogen (N) regimes, revealing that both genes are preferentially expressed in roots and root nodules, and that their expression is influenced by the N-status of the plant. Analysis of transgenic plants expressing MtGSI-like-promoter-gusA fusion, indicate that the two genes are strongly expressed in the root pericycle, and interestingly, the expression is enhanced at the sites of nodule emergence being particularly strong in specific cells located in front of the protoxylem poles. Taken together, the results presented here support a role of GSI-like proteins in N sensing and/or signaling, probably operating at the interface between perception of the N-status and the developmental processes underlying both root nodule and lateral root formation. This study indicates that GSI-like genes may represent a novel class of molecular players of the N-mediated signaling events.

Morpho-anatomical characteristics conferring drought tolerance in roots of sugar cane genotypes (Saccharum L., Poaceae)

Sugar cane is a major product of Brazilian agriculture, providing about R$ 56 billion of exports per year. Morphological and anatomical analyses enable the identification of plants, whose genotype is still unknown, and increase our knowledge of organs adaptations, such as roots, to different environment conditions. This study aimed to describe the morpho-anatomy of roots of six sugar cane genotypes. Plants were obtained by insertion of stalks in 3L containers filled with coconut fiber substrate. They were held for 7 days in a germination chamber (BOD MA 403) under a 16-h photoperiod with a light intensity of 38 μmol m−2 s−1 and a constant temperature of 35 °C. Daily irrigation was performed, keeping the substrate close to saturation. After this period, plants were transferred to a greenhouse and grew under approximately 116 μmol m−2 s−1 of light intensity at 30 °C and 80 % relative humidity, during 45 days of daily irrigation. For anatomical characterization, 20 roots from stalks and tillers of five plants per genotype were fixed in FAA 50 % and preserved in alcohol 70 %. For each root, the median region was selected for freehand-sectioning, using a razor blade. The results confirmed that anatomical features might be useful for cultivar identification. RB867515 cultivar showed roots with anatomical features that provide drought resistance, including a reduced number of cortical layers. The roots of SP801816 cultivar had more cortical layers and protoxylem poles in stalks and tillers, indicating a reduced drought resistance.

Use of anatomical root markers for species identification in Catasetum (Orchidaceae) at the Portal da Amazônia region, MT, Brazil

Orchidaceae is one of the largest botanical families, with approximately 780 genera. Among the genera of this family, Catasetum currently comprises 166 species. The aim of this study was to characterize the root anatomy of eight Catasetum species, verifying adaptations related to epiphytic habit and looking for features that could contribute to the vegetative identification of such species. The species studied were collected at the Portal da Amazônia region, Mato Grosso state, Brazil. The roots were fixed in FAA 50, cut freehand, and stained with astra blue/fuchsin. Illustrations were obtained with a digital camera mounted on a photomicroscope. The roots of examined species shared most of the anatomical characteristics observed in other species of the Catasetum genus, and many of them have adaptations to the epiphytic habit, such as presence of secondary thickening in the velamen cell walls, exodermis, cortex, and medulla. Some specific features were recognized as having taxonomic application, such as composition of the thickening of velamen cell walls, ornamentation of absorbent root-hair walls, presence of tilosomes, composition and thickening of the cortical cell walls, presence of mycorrhizae, endodermal cell wall thickening, the number of protoxylem poles, and composition and thickening of the central area of the vascular cylinder. These traits are important anatomical markers to separate the species within the genus and to generate a dichotomous identification key for Catasetum. Thus, providing a useful tool for taxonomists of this group

Morphoanatomical and Biochemical Changes in the Roots of Rice Plants Induced by Plant Growth-Promoting Microorganisms

The goal of the present study was to characterize anatomical and biochemical changes in rice plant roots in response to seed treatment with rhizobacteria (Burkholderia pyrrocinia(R-46) +Pseudomonas fluorescens(R-55)) andTrichoderma asperellum(Ta: mixture of strains T-06, T-09, T-12, and T-52). The experimental design was completely randomized, with six treatments (R-46, R-55, R-46 + R-55, Ta+ R-46 + R-55, Ta, and control) and ten replicates. Treatments Ta and R-46 + R-55 increased the root length and diameter as well as the cortex expansion and induced a 2% expansion of the aerenchymal space. Treatments Ta and R-46 increased the vascular cylinder diameter. The number of protoxylem poles and metaxylem vessel elements was increased by R-46 and R-55. The total phenol content increased with treatments Ta, R-46 + R-55, R-46, and R-55, and all the treatments increased the flavonoid content. The lignin content increased with the Ta and R-55 treatments. All the root architecture modifications resulting from the interaction between seedlings and bioagents (rhizobacteria andTrichodermaspp.) observed in the present study favored the root plasticity of rice seedlings.

Morfología y anatomía de ejes caulinares, licofilos y esporangios de Phlegmariurus phylicifolius: un aporte a la sistemática de las Lycopodiaceae neotropicales

Phlegmariurus is the only genus of Lycopodiaceae with the species grouped in 22 informal groups. Species level relationships within Phlegmariurus are poorly understood and their circumscriptions require a thorough molecular and morphological review. A detailed study of morphology and anatomy of caulinar axes, lycophylls and sporangia of Phlegmariurus phylicifolius was carried out in order to contribute to the elucidation of species circumscription in the informal group Phlegmariurus phlegmaria. Small pieces of caulinar axes bearing trophophylls, sporophylls and sporangia were fixed, dehydrated, Histowax (paraffin) embedded, sectioned in a rotatory microtome, and stained using the common Safranin O-Fast Green technique; handmade cross sections were also made and stained with the same technique. P. phylicifolius includes slender, pendulous plants up to 40cm long. Shoots heterophyllous, in the basal divisions ca. 10-20(-25) mm in diameter including the trophophylls, then abruptly constricted to (1-) 1.5-2(-2.5) mm in diameter including the imbricate, reduced sporophylls. Trophophylls are borne in alternating whorls of three, or decussate, subdecussate, or alternate, widely spaced in alternate leaved caulinar axes portions, perpendicular to the caulinar axes to falcately ascending, lanceolate to linear-lanceolate, with flat to slightly revolute margins. Each lycophyll is supplied by a single central vascular bundle, connected to a protoxylem pole in the stele. At the site of leaf-trace departure, no leaf (lycophyll) gap is present. Caulinar axes excluding leaves 0.7-1.2 mm thick at the base, upward tapering to ca. 0.5 mm. Caulinar axes present unistratified epidermis and endodermis, the cortex is characterized by the presence of a trabecular structure of lisigenous origin formed in the parenchimatous tissue next to the endodermis. The vascular tissue occupies the central part of the caulinar axes, forming a plectostele ofsubradiate organization, with five poles ofprotoxylem. The epidermal cells present sinuous anticlinal walls; invaginations in the inner side of external periclinal wall of the epidermal cells could be probably adaptive morphological feature of a water deficient environment. Leaves of constricted terminal divi- sions are decussate, or subdecussate, continuously or discontinuously sporangiate, appressed, abaxially rounded to carinate, widely lanceolate to widely ovate or subcordate, acute to mucronate or cuspidate, shorter than the sporangia. Each sporangium originates from a group of epidermal cells, axilar to the sporophylls. The cell walls of epidermal cell of the sporangia are Huperzioideae type. The morphological studies of trophophylls contribute to confirm the differences between P. phylicifolius and P. subulatus.

Legume nodulation

Legume nodulation

Change of Cotyledon Number in Red Leaf Beet (Beta vulgarisL. var. ciclaL) and Altered Vascular Bundles of Tricotyledonous Seedling

Seedlings of monocotyledon, tricotyledons and tetracotyledons were compared to those of dicotyledonous wild type of red leaf beet (Beta vulgaris L. var cicla L). These seedlings were progenies of the mother plant that was transformed using floral dip method, but detected no ectopic expression of the target gene. Paraffin sections of the shoot tip showed that the shoot apical meristem (SAM) in the tricotyledonous seedlings changed to trilateral compared to long oval meristem in the wild type; meanwhile, three leaf primordia appeared alternately with cotyledonous primordia in tricotyledonous seedlings compared to two leaf primordia of the wild type, which suggests the direct role of cotyledons in determining where from the primordia of the first leaves arise. Paraffin sections and cleared tissues of hypocotyls and roots showed two lines of protoxylem poles in hypocotyl of wild type compared to three in tricotyledonous seedling. These observations suggested the correlated patterns of vascular and cotyledon number.

In vitro organogenesis from root culture segments of Bixa orellana L. (Bixaceae)

The organogenic potential of root explants derived from cultured seedlings of Bixa orellana L. (annatto) was investigated in response to different incubation conditions and either 4.44 μM 6-benzyladenine, 4.54 μM Thidiazuron, or 4.56 μM Zeatin. Explants cultured in liquid media with agitation generally showed better development of adventitious buds versus explants cultured on semi-solid media. The most adventitious buds developed from explants cultured in liquid media under a 16-h photoperiod. Use of Zeatin and Thidiazuron promoted the development of more adventitious buds than 6-benzyladenine but morphological abnormalities among regenerating shoots and plants were observed. Fewer adventitious buds developed from explants cultured in liquid media supplemented with 6-benzyladenine, but the buds gave rise to the highest percentage of morphologically normal regenerated plants. Histological analyses showed that adventitious buds originated from cell proliferation within the pericycle, opposite the protoxylem poles of the explant. Seedling root tissue is useful for in vitro propagation of B. orellana.

Classical Ethylene Insensitive Mutants of the Arabidopsis EIN2 Orthologue Lack the Expected ‘hypernodulation’ Response in Lotus japonicusF

Three independent ethylene insensitive mutants were selected from an EMS- mutagenized population of Lotus japonicus MG-20 (Miyakojima). The mutants, called 'Enigma', were mutated in the LjEIN2a gene from Lotus chromosome 1, sharing significant homology with Arabidopsis EIN2 (ethylene-insensitive2). All three alleles showed classical ethylene insensitivity phenotypes (e.g., Triple Response), but lacked the increased nodulation phenotype commonly associated with ethylene insensitivity. Indeed, all showed a marginal reduction in nodule number per plant, a phenotype that is enigmatic to sickle, an ethylene-insensitive EIN2 mutant in Medicago truncatula. In contrast to wild type, but similar to an ETR1-1 ethylene ethylene-insensitive transgenic of L. japonicus, enigma mutants formed nodules in between the protoxylem poles, demonstrating the influence of ethylene on radial positioning. Suppression of nodule numbers by nitrate and colonisation by mycorrhizal fungi in the enigma-1 mutant were indistinguishable from the wild-type MG-20. However, reflecting endogenous ethylene feedback, the enigma-1 mutant released more than twice the wild-type amount of ethylene. enigma-1 had a moderate reduction in growth, greater root mass (and lateral root formation), delayed flowering and ripening, smaller pods and seeds. Expression analysis of ethylene-regulated genes, such as ETR1, NRL1 (neverripe-like 1), and EIL3 in shoots and roots of enigma-1 and MG-20 illustrated that the ethylene-insensitive mutation strongly affected transcriptional responses in the root. These mutants open the possibility that EIN2 in L. japonicus, a determinate nodulating legume, acts in a more complex fashion possibly through the presence of a duplicated copy of LjEIN2.

Anatomia de raízes de nove espécies de Bromeliaceae (Poales) da região amazônica do estado de Mato Grosso, Brasil

Este estudo busca caracterizar raízes de Bromeliaceae: Aechmea bromeliifolia, A. castelnavii, A. mertensii (Bromelioideae), Dyckia duckei, D. paraensis, D. racemosa (Pitcairnoideae), Tillandsia adpressiflora, T. didistachae e T. paraensis (Tillandsioideae) ocorrentes nas regiões amazônicas (Mato Grosso-MT), procurando levantar caracteres de valor taxonômico e significado ecológico. As espécies estudadas são epífitas e suas raízes se caracterizam por apresentar velame pluriestratificado, córtex diferenciado, endoderme e periciclo unisseriados, cilindro vascular poliarco e medula com células de paredes espessadas. Estruturas anatômicas como: número de camadas e tipo de espessamento das paredes das células do velame, tipo de espessamento de parede da exoderme e endoderme, presença de idioblastos contendo cristais e número de pólos de protoxilema agrupam as espécies nos diferentes gêneros e subfamílias. A presença de canais de mucilagem no córtex de A. castelnavii relatados pela primeira vez na literatura para Bromeliaceae é caráter diagnóstico. As raízes de Dyckia e Tillandsia apresentam maior número de caracteres comuns, representando maior similaridade entre Pitcairnioideae e Tillandsioideae. Raízes com velame, exoderme com células de paredes espessadas constituindo uma camada mecânica, canais de mucilagem, lacunas de ar no córtex interno e idioblastos com cristais são estruturas adaptativas ao hábito epifítico.

The D-type cyclin CYCD4;1 modulates lateral root density in Arabidopsis by affecting the basal meristem region

Root cell division occurs primarily in the apical meristem, from which cells are displaced into the basal meristem, where division decreases and cell length increases before the final differentiation zone. The organization of the root in concentric files implies coordinated division and differentiation of cell types, including the xylem pole pericycle cells, which uniquely can resume division to initiate lateral roots (LR). Here, we show that D-type cyclin CYCD4;1 is expressed in meristematic pericycle protoxylem poles and is required for normal LR density. Cycd4;1 mutants also show a displacement of the apical/basal meristem boundary in the pericycle and longer pericycle basal meristem cells, whereas other cell layers and overall meristem size and root growth are unaffected. Auxin is proposed to separately prepattern and stimulate LR initiation. Stimulation is unimpaired in cycd4;1, suggesting CYCD4;1 requirement for normal spacing but not initiation. Both pericycle cell length and LR density phenotypes of cycd4;1 are rescued by low concentrations of applied auxin, suggesting that the basal meristem has a role in determining LR density. We further show CYCD4;1 is rate-limiting for sucrose-dependent LR formation, since CYCD4;1 expression is sucrose-dependent and wild-type roots fully phenocopy cycd4;1 in sucrose absence. We conclude that CYCD4;1 links meristem pericycle cell behavior to LR density consistent with a basal meristem prepatterning model and that D-type cyclins can confer division potential of defined cell types through cell-specific expression patterns.

ANATOMY OF THE TRANSITION REGION OF Vigna radiata (L.) Wilczek. (MUNG BEAN).

Seedlings of Vigna radiata (L.) Wilczek (Mung bean) were examined by light microscopy to study vascular development through the transition region. The beginning of vascular transition took place about one centimeter below the soil surface by the appearance of a pith between the large central vessels. At the higher level, pith parenchyma increased and the vessels are separated into four groups. The protoxylem maintains its peripheral position, but the metaxylem instead of differentiating toward the center, diverges laterally from the protoxylem. At successively higher levels, the hypocotyl axis widens and the distances between the protoxylem poles as well as the divergence of each xylem arms increased. The reorientation of primary xylem took place gradually upward. The length of the transition region from a point where the axis is exarch and protostele to a point where the xylem form approximately endarch condition was about two centimeter. Thus, most stages of vascular transition in mung bean seedling occur in the lower portion of the hypocotyl, and the reorientation of vascular tissue from exarch radial to the endarch collateral arrangement may be completed in the upper portion of the hypocotyl. The behavior of vascular cambium activity and the difference in maturation degree at the different levels of the hypocotyl indicating that the upper portion of mung bean hypocotyl is older than the lower one. Adventitious root primordia were recorded at the basal portion of hypocotyl and their development was associated with the activity of vascular cambium.

Microarray analysis of developing flax hypocotyls identifies novel transcripts correlated with specific stages of phloem fibre differentiation.

Hypocotyls are a commonly used model to study primary growth in plants, since post-germinative hypocotyls increase in size by cell elongation rather than cell division. Flax hypocotyls produce phloem fibres in bundles one to two cell layers thick, parallel to the protoxylem poles of the stele. Cell wall deposition within these cells occurs rapidly at a well-defined stage of development. The aim was to identify transcripts associated with distinct stages of hypocotyl and phloem fibre development. Stages of flax hypocotyl development were defined by analysing hypocotyl length in relation to fibre secondary wall deposition. Selected stages of development were used in microarray analyses to identify transcripts involved in the transition from elongation to secondary cell wall deposition in fibres. Expression of specific genes was confirmed by qRT-PCR and by enzymatic assays. Genes enriched in the elongation phase included transcripts related to cell-wall modification or primary-wall deposition. Transcripts specifically enriched at the transition between elongation and secondary wall deposition included beta-galactosidase and arabinogalactan proteins. Later stages of wall development showed an increase in secondary metabolism-related transcripts, chitinases and glycosyl hydrolases including KORRIGAN. Microarray analysis also identified groups of transcription factors enriched at one or more stages of fibre development. Subsequent analysis of a differentially expressed beta-galactosidase confirmed that the post-elongation increase in beta-galactosidase enzyme activity was localized to phloem fibres. Transcripts were identified associated with specific stages of hypocotyl development, in which phloem fibre cells were undergoing thickening of secondary walls. Temporal and spatial regulation of beta-galactosidase activity suggests a role for this enzyme in remodelling of flax bast fibre cell walls during secondary cell wall deposition.