Growth Of Plant Organs Research Articles

The Arabidopsis EIN2 restricts organ growth by retarding cell expansion

The growth of plant organ to its characteristic size is a fundamental developmental process, but the mechanism is still poorly understood. Plant hormones play a great role in organ size control by modulating cell division and/or cell expansion. ETHYLENE INSENSITVE 2 (EIN2) was first identified by a genetic screen for ethylene insensitivity and is regarded as a central component of ethylene signaling, but its role in cell growth has not been reported. Here we demonstrate that changed expression of EIN2 led to abnormity of cell expansion by morphological and cytological analyses of EIN2 loss-of-function mutants and the overexpressing transgenic plant. Our findings suggest that EIN2 controls final organ size by restricting cell expansion.

ZEITLUPE positively regulates hypocotyl elongation at warm temperature under light in Arabidopsis thaliana

Hypocotyl cell elongation has been studied as a model to understand how cellular expansion contributes to plant organ growth. Hypocotyl elongation is affected by multiple environmental factors, including light quantity and light quality. Red light inhibits hypocotyl growth via the phytochrome signaling pathways. Proteins of the FLAVIN-BINDING KELCH REPEAT F-BOX 1 / LOV KELCH PROTEIN 2 / ZEITLUPE family are positive regulators of hypocotyl elongation under red light in Arabidopsis. These proteins were suggested to reduce phytochrome-mediated inhibition of hypocotyl elongation. Here, we show that ZEITLUPE also functions as a positive regulator in warmth-induced hypocotyl elongation under light in Arabidopsis.

Simulating leaf growth dynamics through Metropolis-Monte Carlo based energy minimization

Throughout their life span, plants maintain the ability to generate new organs, such as leaves. This is normally done in an orderly way by activating limited groups of dormant cells to divide and grow. It is currently not understood how that process is precisely regulated. We have used the VirtualLeaf framework for plant organ growth modeling to simulate the typical developmental stages of leaves of the model plant Arabidopsis thaliana. For that purpose the Hamiltonian central to the Monte-Carlo based mechanical equilibration of VirtualLeaf was modified. A basic two-dimensional model was defined starting from a rectangular grid with a dynamic phytohormone gradient that spatially instructs the cells in the growing leaf. Our results demonstrate that such a mechanism can indeed reproduce various spatio-temporal characteristics of leaf development and provides clues for further model development.

Physalis floridana Cell Number Regulator1 encodes a cell membrane-anchored modulator of cell cycle and negatively controls fruit size.

Physalis species show a significant variation in berry size; however, the underlying molecular basis is unknown. In this work, we showed that cell division difference in the ovaries might contribute to the ultimate berry size variation within Physalis species, and that mRNA abundance of Physalis floridana Cell Number Regulator1 (PfCNR1), the putative orthologue of the tomato fruit weight 2.2 (FW2.2), was negatively correlated with cell division in the ovaries. Moreover, heterochronic expression variation of the PfCNR1 genes in the ovaries concomitantly correlated with berry weight variation within Physalis species. In transgenic Physalis, multiple organ sizes could be negatively controlled by altering PfCNR1 levels, and cell division instead of cell expansion was primarily affected. PfCNR1 was shown to be anchored in the plasma membrane and to interact with PfAG2 (an AGAMOUS-like protein determining ovary identity). The expression of PfCYCD2;1, a putative orthologue of the mitosis-specific gene CyclinD2;1 in the cell cycle was negatively correlated with the PfCNR1 mRNA levels. PfAG2 was found to selectively bind to the CArG-box in the PfCYCD2;1 promoter and to repress PfCYCD2;1 expression, thus suggesting a PfAG2-mediated pathway for PfCNR1 to regulate cell division. The interaction of PfCNR1 with PfAG2 enhanced the repression of PfCYCD2;1 expression. The nuclear import of PfAG2 was essential in the proposed pathway. Our data provide new insights into the developmental pathways of a cell membrane-anchored protein that modulates cell division and governs organ size determination. This study also sheds light on the link between organ identity and organ growth in plants.

Atkinesin-13A modulates cell-wall synthesis and cell expansion in Arabidopsis thaliana via the THESEUS1 pathway.

Growth of plant organs relies on cell proliferation and expansion. While an increasingly detailed picture about the control of cell proliferation is emerging, our knowledge about the control of cell expansion remains more limited. We demonstrate here that the internal-motor kinesin AtKINESIN-13A (AtKIN13A) limits cell expansion and cell size in Arabidopsis thaliana, with loss-of-function atkin13a mutants forming larger petals with larger cells. The homolog, AtKINESIN-13B, also affects cell expansion and double mutants display growth, gametophytic and early embryonic defects, indicating a redundant role of the two genes. AtKIN13A is known to depolymerize microtubules and influence Golgi motility and distribution. Consistent with this function, AtKIN13A interacts genetically with ANGUSTIFOLIA, encoding a regulator of Golgi dynamics. Reduced AtKIN13A activity alters cell wall structure as assessed by Fourier-transformed infrared-spectroscopy and triggers signalling via the THESEUS1-dependent cell-wall integrity pathway, which in turn promotes the excess cell expansion in the atkin13a mutant. Thus, our results indicate that the intracellular activity of AtKIN13A regulates cell expansion and wall architecture via THESEUS1, providing a compelling case of interplay between cell wall integrity sensing and expansion.

Growth rate distribution in the forming lateral root of arabidopsis.

Microscopic observations of lateral roots (LRs) in Arabidopsis thaliana reveal that the cross-sectional shape of the organ changes from its basal to its apical region. The founder cells for LRs are elongated along the parent root axis, and thus from the site of initiation the base of LRs resemble an ellipse. The circumference of the apical part of LRs is usually a circle. The objective of this study was to analyse the characteristics of changes in the growth field of LRs possessing various shapes in their basal regions. The LRs of the wild type (Col-0) and two transgenic arabidopsis lines were analysed. On the basis of measurements of the long and short diameters (DL and DS, respectively) of the ellipse-like figure representing the bases of particular LRs, their asymmetry ratios (DL/DS) were determined. Possible differences between accessions were analysed by applying statistical methods. No significant differences between accessions were detected. Comparisons were therefore made of the maximal, minimal and mean value of the ratio of all the LRs analysed. Taking into consideration the lack of circular symmetry of the basal part, rates of growth were determined at selected points on the surface of LRs by the application of the growth tensor method, a mathematical tool previously applied only to describe organs with rotational symmetry. Maps showing the distribution of growth rates were developed for surfaces of LRs of various asymmetry ratios. The maps of growth rates on the surfaces of LRs having various shapes of the basal part show differences in both the geometry and the manner of growth, thus indicating that the manner of growth of the LR primordium is correlated to its shape. This is the first report of a description of growth of an asymmetric plant organ using the growth tensor method. The mathematical modelling adopted in the study provides new insights into plant organ formation and shape.

Effect of monochromatic UV-A LED irradiation on the growth of tomato seedlings

Effect of 376 nm UV-A LED irradiation on the growth and morphology of tomato seedlings was studied. Tomato seedlings were grown under the 658 nm red LEDs or the red LED supplemented with two irradiation levels of the UV-A. The growth and development of tomato seedlings were significantly enhanced under the red light supplemented with the UV-A. Under the UV-A treatments, the tomato seedlings became more compact, the growth of plant organs was balanced, the leaf area was increased, and the total plant fresh and dry weights were also enhanced. Our findings suggested that the 376 nm UV-A from LEDs had a beneficial effect on the growth and development of tomato seedlings as similarly to the blue light.

Mechanical forces as information: an integrated approach to plant and animal development.

Mechanical forces such as tension and compression act throughout growth and development of multicellular organisms. These forces not only affect the size and shape of the cells and tissues but are capable of modifying the expression of genes and the localization of molecular components within the cell, in the plasma membrane, and in the plant cell wall. The magnitude and direction of these physical forces change with cellular and tissue properties such as elasticity. Thus, mechanical forces and the mesoscopic fields that emerge from their local action constitute important sources of positional information. Moreover, physical and biochemical processes interact in non-linear ways during tissue and organ growth in plants and animals. In this review we discuss how such mechanical forces are generated, transmitted, and sensed in these two lineages of multicellular organisms to yield long-range positional information. In order to do so we first outline a potentially common basis for studying patterning and mechanosensing that relies on the structural principle of tensegrity, and discuss how tensegral structures might arise in plants and animals. We then provide some examples of morphogenesis in which mechanical forces appear to act as positional information during development, offering a possible explanation for ubiquitous processes, such as the formation of periodic structures. Such examples, we argue, can be interpreted in terms of tensegral phenomena. Finally, we discuss the hypothesis of mechanically isotropic points as a potentially generic mechanism for the localization and maintenance of stem-cell niches in multicellular organisms. This comparative approach aims to help uncovering generic mechanisms of morphogenesis and thus reach a better understanding of the evolution and development of multicellular phenotypes, focusing on the role of physical forces in these processes.

Effect of T‐DNA insertions on mRNA transcript copy numbers upstream and downstream of the insertion site in Arabidopsis thaliana explored by surface enhanced Raman spectroscopy

We report the effect of a T-DNA insertion on the expression level of mRNA transcripts of the TWISTED DWARF 1 (TWD1) gene upstream and downstream of the T-DNA insertion site in Arabidopsis. A novel approach based on surface-enhanced Raman spectroscopy (SERS) was developed to detect and quantify the effect of a T-DNA insertion on mRNA transcript accumulation at 5'- and 3'-ends of the TWD1 gene. A T-DNA insertion mutant in the TWD1 gene (twd1-2) was chosen to test the sensitivity and the feasibility of the approach. The null mutant of the FK506-like immunophilin protein TWD1 in Arabidopsis shows severe dwarfism and strong disoriented growth of plant organs. A spontaneous arising suppressor allele of twd1-2 called twd-sup displayed an intermediate phenotype between wild type and the knockout phenotype of twd1-2. Both twd1 mutant alleles have identical DNA sequences at the TWD1 locus including the T-DNA insertion in the fourth intron of the TWD1 gene but they show clear variability in the mutant phenotype. We present here the development and application of SERS-based mRNA detection and quantification using the expression of the TWD1 gene in wild type and both mutant alleles. The hallmarks of our SERS approach are a robust and fast assay to detect up to 0.10 fm of target molecules including the ability to omit in vitro transcription and amplification steps after RNA isolation. Instead we perform direct quantification of RNA molecules. This enables us to detect and quantify rare RNA molecules at high levels of precision and sensitivity.

Modeling of spatial variations of growth within apical domes by means of the growth tensor. L Growth specified on dome axis

By using the growth tensor and a natural curvilinear coordinate system for description of the distribution of growth in plant organs, three geometric types of shoot apical domes (parabolic, elliptical and hyperbolic) were modeled. It was assumed that apical dome geometry remains unchanged during growth and that the natural coordinate systems are paraboloidal and prolate spheroidal. Two variants of the displacement velocity fields V were considered. One variant is specified by a constant relative elemental rate of growth along the axis of the dome. The second is specified by a rate increasing proportionally with distance from the geometric focus of the coordinate systems (and the apical dome). The growth tensor was used to calculate spatial variations of growth rates for each variant of each dome type. There is in both variants a clear tendency toward lower growth rates in the distal region of the dome. A basic condition for the existence of a tunica is met.

Genome-wide identification and analysis of growth regulating factor genes in Brachypodium distachyon: in silico approaches

Growth-regulating factor (GRF) genes may play important roles for regulating growth and development in different plant tissues and organs. Here we report the first genome-wide analysis of the GRF gene family in Brachypodium. We performed in silico comparative analysis of GRF genes, including their structure, duplication in the genome, conserved motifs, and phylogenetic relationship. At the end of the study, 10 BdGRF genes were identified. The highest number of GRF genes was identified on chromosome 1 with 5 members, whereas the least number of genes (only 1 member) was found on chromosomes 2, 4, and 5. Of those, a single segmental duplication was observed in the Brachypodium genome. Average exon and intron numbers were determined as 3 and 4, respectively. Motif analysis showed that WRC and QLQ residues were consistent in all GRF protein sequences. Gene Ontology terms showed that 10 BdGRF proteins grouped in the same biological function, biological process, and cellular component groups. In addition, we compared the new BdGRF proteins with the other monocot and dicot GRF proteins sequences. Phylogenetic analysis revealed that GRF proteins of monocot and dicot species were clustered together in a joined tree; in particular, the monocot species (Brachypodium, maize, and rice) were grouped into the same cluster with high bootstrap values. We assume that the results of this study will provide molecular insights about GRF proteins in grass species.

Numerical Analysis of Steady State Patterns in Cell-Based Auxin Transport Models

Cell-based models that describe the pattern formation and the flow of chemicals in plant organs are important building blocks in a multiscale simulation of a whole plant. An example of an important mechanism is the transport of the hormone auxin throughout the plant's organs because it is closely related to the growth characteristics of roots, shoots and leaves. Based on experimental evidence, a number of cell-based auxin transport models were developed. Due to the intercellular transport of chemicals, these models are complex dynamical systems with a large set of endogenous and exogenous parameters. The models share underlying mathematicalprinciples w.r.t. steady state pattern formation which plays a central role in the growth and development of plant organs. This calls for a uniform computational approach. In our research we focus on computer simulations of general cell-based transport models and more specically we use numerical bifurcation analysis to study the steady state patterns. It indicates how the stability ofpatterns is lost or gained as the system parameters change. Bifurcation analysis of ODEs is widespread in biology and various numerical tools produce bifurcation diagrams. However, these automatic tools do not work for large scale problems, such as biological patterning. Indeed, realistic simulations of large tissues that take multiple interacting chemicals into account give rise to very large and sparse systems of coupled ODEs. We analyze recent large scale transport models with new mathematical and computational tools that enable quantitative prediction of the bifurcations that appear at the macroscopic level in these models. This allows us to predict the patterns and self-similar solutions that appear during organ growth and to see how their stability changes as endogenous parameters are modified or as externally applied changes are enforced. We use these methods to compare the model output with observed data such as the auxin distribution and venation patterns in leaves in order to get a better understanding of the processes that regulate organ development in plants.

Synthesis of Very-Long-Chain Fatty Acids in the Epidermis Controls Plant Organ Growth by Restricting Cell Proliferation

Author SummaryThe epidermis functions as an important interface with the environment, but in plants it is also essential for establishing and maintaining the primary plant body. Recent studies have shown that the epidermis participates in both driving and restricting plant growth via inter-cell-layer communication. However, it remains an open question as to whether the epidermis can send signals to internal plant tissues to control cell proliferation during development. Here we report that the synthesis of very-long-chain fatty acids (VLCFAs) in the epidermis is essential for the proper control of cell proliferation in the plant Arabidopsis thaliana. We find that defects in VLCFA synthesis cause cells in the vasculature or in the rib zone of shoot apices to overproliferate. When VLCFA levels decrease, we observe that the synthesis of the phytohormone cytokinin increases in the vasculature. We also find that when cytokinin is degraded by the expression of cytokinin oxidase in the vasculature, enhanced cell proliferation in internal tissues is suppressed, indicating that VLCFA synthesis in the epidermis is required to suppress cytokinin biosynthesis and thus cell overproliferation. Our results demonstrate that shoot growth is controlled by interactions between the surface (epidermis) and the axis (vasculature) of the plant body, and highlight a role for VLCFAs in this interaction.

Ethylene signaling and regulation in plant growth and stress responses

Gaseous phytohormone ethylene affects many aspects of plant growth and development. The ethylene signaling pathway starts when ethylene binds to its receptors. Since the cloning of the first ethylene receptor ETR1 from Arabidopsis, a large number of studies have steadily improved our understanding of the receptors and downstream components in ethylene signal transduction pathway. This article reviews the regulation of ethylene receptors, signal transduction, and the posttranscriptional modulation of downstream components. Functional roles and importance of the ethylene signaling components in plant growth and stress responses are also discussed. Cross-reactions of ethylene with auxin and other phytohormones in plant organ growth will be analyzed. The studies of ethylene signaling in plant growth, development, and stress responses in the past decade greatly advanced our knowledge of how plants respond to endogenous signals and environmental factors.

Vertex-element models for anisotropic growth of elongated plant organs.

New tools are required to address the challenge of relating plant hormone levels, hormone responses, wall biochemistry and wall mechanical properties to organ-scale growth. Current vertex-based models (applied in other contexts) can be unsuitable for simulating the growth of elongated organs such as roots because of the large aspect ratio of the cells, and these models fail to capture the mechanical properties of cell walls in sufficient detail. We describe a vertex-element model which resolves individual cells and includes anisotropic non-linear viscoelastic mechanical properties of cell walls and cell division whilst still being computationally efficient. We show that detailed consideration of the cell walls in the plane of a 2D simulation is necessary when cells have large aspect ratio, such as those in the root elongation zone of Arabidopsis thaliana, in order to avoid anomalous transverse swelling. We explore how differences in the mechanical properties of cells across an organ can result in bending and how cellulose microfibril orientation affects macroscale growth. We also demonstrate that the model can be used to simulate growth on realistic geometries, for example that of the primary root apex, using moderate computational resources. The model shows how macroscopic root shape can be sensitive to fine-scale cellular geometries.

Cell expansion and microtubule behavior in ray floret petals of Gerbera hybrida: Responses to light and gibberellic acid

It is still unclear how light and gibberellins are integrated to regulate petal size. Here, we report that light improves both the length and the width of the ray floret petals in G. hybrid, but GA(3) promotes only the petal length. It is also revealed that the control of the petal size by light and GA(3) depends on modulating the cell size, which is governed by the behavior of cortical microtubule.Light and gibberellins are important regulators of plant organ growth. However, little is known about their roles in petal size determination. Here, we report how light and gibberellic acid (GA(3)) signals are integrated to regulate the ray floret (Rf) size in Gerbera hybrida. The inflorescences of G. hybrida at stages 1.5 were cultivated in vitro for 9 d followed by the determination of the Rf petal size. Results demonstrated that the light signal significantly enhanced both the length and the width of Rf petals, but GA(3) promoted only the petal length. Moreover, GA(3) displayed a synergistic positive effect on the length but an antagonistic effect on the width with the light signal. Measurements of the petal cells revealed that the cell size, not the cell number, exhibited a dominant contribution to the petal size in response to light and GA(3) signals. Furthermore, light and GA(3) signals not only induced an obvious reorientation of cortical microtubules (MTs) into transverse arrays but also promoted the recovery of the MT lengths in petal cells following oryzalin (an MT depolymerizing agent) treatment. Importantly, disruption of the MT lengths and arrays by oryzalin could inhibit the cell expansion and the petal enlargement induced by light or/and GA(3) signals. Taken together, it is concluded that the control of the petal size by light and GA(3) signals mainly depends on modulating the cell size and, moreover, the organization of the cortical MTs plays a crucial role in the control of the cell size and hence the Rf petal growth.

Arabidopsis PIZZA Has the Capacity to Acylate Brassinosteroids

Brassinosteroids (BRs) affect a wide range of developmental processes in plants and compromised production or signalling of BRs causes severe growth defects. To identify new regulators of plant organ growth, we searched the Arabidopsis FOX (Full-length cDNA Over-eXpressor gene) collection for mutants with altered organ size and isolated two overexpression lines that display typical BR deficient dwarf phenotypes. The phenotype of these lines, caused by an overexpression of a putative acyltransferase gene PIZZA (PIZ), was partly rescued by supplying exogenous brassinolide (BL) and castasterone (CS), indicating that endogenous BR levels are rate-limiting for the growth of PIZ overexpression lines. Our transcript analysis further showed that PIZ overexpression leads to an elevated expression of genes involved in BR biosynthesis and a reduced expression of BR inactivating hydroxylases, a transcriptional response typical to low BR levels. Taking the advantage of relatively high endogenous BR accumulation in a mild bri1-301 background, we found that overexpression of PIZ results in moderately reduced levels of BL and CS and a strong reduction of typhasterol (TY) and 6-deoxocastasterone (6-deoxoCS), suggesting a role of PIZ in BR metabolism. We tested a set of potential substrates in vitro for heterologously expressed PIZ and confirmed its acyltransferase activity with BL, CS and TY. The PIZ gene is expressed in various tissues but as reported for other genes involved in BR metabolism, the loss-of-function mutants did not display obvious growth phenotypes under standard growth conditions. Together, our data suggest that PIZ can modify BRs by acylation and that these properties might help modulating endogenous BR levels in Arabidopsis.

Hierarchical Helical Order in the Twisted Growth of Plant Organs

The molecular and cellular basis of left-right asymmetry in plant morphogenesis is a fundamental issue in biology. A rapidly elongating root or hypocotyl of twisting mutants of Arabidopsis thaliana exhibits a helical growth with a handedness opposite to that of the underlying cortical microtubule arrays in epidermal cells. However, how such a hierarchical helical order emerges is currently unknown. We propose a model for investigating macroscopic chiral asymmetry in Arabidopsis mutants. Our elastic model suggests that the helical pattern observed is a direct consequence of the simultaneous presence of anisotropic growth and tilting of cortical microtubule arrays. We predict that the root helical pitch angle is a function of the microtubule helical angle and elastic moduli of the tissues. The proposed model is versatile and is potentially important for other biological systems ranging from protein fibrous structures to tree trunks.

Growth and maintenance costs of leaves and roots in two populations of Quercus ilex native to distinct substrates

This work tests the hypothesis that growth and maintenance costs of plant organs are higher in more stressful soils. Two populations of Quercus ilex L were selected in the southern Iberian Peninsula, these growing in similar climates but different soil types, namely a brown well-developed soil on slate rock, and a stressful lithosol on gypsum rock. In both localities, growth and maintenance respiration were measured in undetached young and mature leaves (trees under natural conditions) and fine roots (hydroponically grown seedling). Young leaves of the two populations displayed an almost identical growth cost (1.53 g glucose g-1). The maintenance cost was higher in the young (40.2 vs. 25.3 mg glucose g-1 day-1; P < 0.05) and in the mature (7.64 vs. 4.33 mg glucose g-1 day-1; P < 0.001) leaves of individuals growing in gypsum soils. The growth cost of fine roots was the same in both populations (1.18 g glucose g-1) while the maintenance cost was higher in the Gypsum population (8.95 vs. 7.39 mg glucose g-1 day-1; P < 0.01). The results show for first time that the cost of organ maintenance may be related to the degree of soil stress in their native habitats.

Obtaining transgenic tobacco plants expressing conserved regions of the AINTEGUMENTA gene in antisense orientation

The AINTEGUMENTA gene encodes a transcription factor involved in the regulation of growth of both reproductive and vegetative plant organs. Two conserved regions of this gene were amplified and cloned in antisense orientation. The produced genetic constructs were used to obtain transgenic tobacco plants with the AINTEGUMENTA gene expression downregulated. Over half of the transgenic plants were characterized by smaller leaves, stems, and flowers with their shapes and symmetry not changed. The transgenic plants ceased to grow earlier, flowered later, and had fewer flowers than controls. The main reason for smaller organs was shown to be the lower cell number in an organ with cell sizes unchanged. The genetic constructs can be used to obtain transgenic plants of various species with smaller organs.