Genetic ablation of root cap cells in Arabidopsis

A Previously Unknown Maltose Transporter Essential for Starch Degradation in Leaves

A previously unknown maltose transporter is essential for the conversion of starch to sucrose in Arabidopsis leaves at night. The transporter was identified by isolating two allelic mutants with high starch levels and very high maltose, an intermediate of starch breakdown. The mutations affect a gene of previously unknown function, MEX1. We show that MEX1is a maltose transporter that is unrelated to other sugar transporters. The severe mex1 phenotype demonstrates that MEX1is the predominant route of carbohydrate export from chloroplasts at night. Homologous genes in plants including rice and potato indicate that maltose export is of widespread significance.

Diphtheria toxin-mediated cell ablation reveals interregional communication during Arabidopsis seed development

Fertilization of the female gametophyte in angiosperm plants initiates a process of coordinated development of embryo, endosperm, and seed coat that ensures the production of a viable seed. Mutant analysis has suggested that communication between the endosperm and the seed coat is an important determinant in this process. In addition, cell groups within the embryo, derived from the apical and from the basal cell, respectively, after zygote division, concertedly establish a functional root meristem, and cells in the apical region of the embryo are hypothesized to repress cell divisions in the basal cell-derived suspensor. The available evidence for these interregional communication events mostly relies on the analysis of mutant phenotypes in Arabidopsis. To provide independent and direct evidence for communication events, we used conditional domain-specific expression of the diphtheria toxin A chain (DTA) in developing Arabidopsis seeds. By using a collection of cell- or tissue-type-specific promoters, we show that the mGAL4:VP16/UAS two-component gene expression allows reliable spatiotemporal and conditional expression of the GFP:GUS reporter and the DTA gene in the developing embryo and endosperm. Expression of DTA in the protoderm of the embryo proper led to excessive proliferation of suspensor cells, sometimes resulting in the formation of secondary embryos. Endosperm-specific expression of DTA caused complete cessation of seed growth, followed by pattern defects in the embryo and embryo arrest. Taken together, the results presented here substantiate the evidence for and underline the importance of interregional communication in embryo and seed development and demonstrate the usefulness of conditional toxin expression as a method complementary to phenotypic analysis of developmental mutants.

ALTERED RESPONSE TO GRAVITY is a peripheral membrane protein that modulates gravity-induced cytoplasmic alkalinization and lateral auxin transport in plant statocytes

ARG1 (ALTERED RESPONSE TO GRAVITY) is required for normal root and hypocotyl gravitropism. Here, we show that targeting ARG1 to the gravity-perceiving cells of roots or hypocotyls is sufficient to rescue the gravitropic defects in the corresponding organs of arg1-2 null mutants. The cytosolic alkalinization of root cap columella cells that normally occurs very rapidly upon gravistimulation is lacking in arg1-2 mutants. Additionally, vertically grown arg1-2 roots appear to accumulate a greater amount of auxin in an expanded domain of the root cap compared with the wild type, and no detectable lateral auxin gradient develops across mutant root caps in response to gravistimulation. We also demonstrate that ARG1 is a peripheral membrane protein that may share some subcellular compartments in the vesicular trafficking pathway with PIN auxin efflux carriers. These data support our hypothesis that ARG1 is involved early in gravitropic signal transduction within the gravity-perceiving cells, where it influences pH changes and auxin distribution. We propose that ARG1 affects the localization and/or activity of PIN or other proteins involved in lateral auxin transport.

[Mechanism of gravi-sensing and -transduction in gravitropism of higher plants]

In higher plants, some organs such as roots, hypocotyls, and stems, can sense the direction of gravity to regulate their orientation. Gravitropic response is composed of four steps; 1. gravity sensing and conversion of physical stimuli to biochemical signals, 2. intracellular signal transduction in gravity sensing cells, 3. signal transmitting to responding tissues, 4. differential growth of organs. Here we focus on the former two steps. Recent studies using modern technique have gradually unveiled early events and mechanism of gravitropic response. Genetic approach provided evidences that strongly support the classical theory for gravity sensing (step 1). Computational analysis suggested the existence of another gravity sensing mechanism in roots. Spatial and temporal ion imaging in living organs in real time provided information on step 2. In addition, reverse genetic approach suggested asymmetrical intracellular distribution of auxin transporter [correction of transpoter] is a possible link between step 2 and 3. However, molecular basis of the signaling mechanism remains unknown. We believe extensive molecular genetic approach combined with recent techniques cited here shed the light to this ambiguous area of research.

Development of enhancer trap lines for functional analysis of the rice genome

Enhancer trapping has provided a powerful strategy for identifying novel genes and regulatory elements. In this study, we adopted an enhancer trap system, consisting of the GAL4/VP16-UAS elements with GUS as the reporter, to generate a trapping population of rice. Currently, 31 443 independent transformants were obtained from two cultivars using Agrobacterium-mediated T-DNA insertion. PCR tests and DNA blot hybridization showed that about 94% of the transformants contained T-DNA insertions. The transformants carried, on average, two copies of the T-DNA, and 42% of the transformants had single-copy insertions. Histochemical assays of approximately 1000 T0 plants revealed various patterns of the reporter gene expression, including expression in only one tissue, and simultaneously in two or more tissues. The expression pattern of the reporter gene in T1 families corresponded well with the T0 plants and segregated in a 3 : 1 Mendelian ratio in majority of the T1 families tested. The frequency of reporter gene expression in the enhancer trap lines was much higher than that in gene trap lines reported previously. Analysis of flanking sequences of T-DNA insertion sites from about 200 transformants showed that almost all the sequences had homology with the sequences in the rice genome databases. Morphologically conspicuous mutations were observed in about 7.5% of the 2679 T1 families that were field-tested, and segregation in more than one-third of the families fit the 3 : 1 ratio. It was concluded that GAL4/VP16-UAS elements provided a useful system for enhancer trap in rice.

A no hydrotropic response root mutant that responds positively to gravitropism in Arabidopsis

For most plants survival depends upon the capacity of root tips to sense and move towards water and other nutrients in the soil. Because land plants cannot escape environmental stress they use developmental solutions to remodel themselves in order to better adapt to the new conditions. The primary site for perception of underground signals is the root cap (RC). Plant roots have positive hydrotropic response and modify their growth direction in search of water. Using a screening system with a water potential gradient, we isolated a no hydrotropic response (nhr) semi-dominant mutant of Arabidopsis that continued to grow downwardly into the medium with the lowest water potential contrary to the positive hydrotropic and negative gravitropic response seen in wild type-roots. The lack of hydrotropic response of nhr1 roots was confirmed in a system with a gradient in air moisture. The root gravitropic response of nhr1 seedlings was significantly faster in comparison with those of wild type. The frequency of the waving pattern in nhr1 roots was increased compared to those of wild type. nhr1 seedlings had abnormal root cap morphogenesis and reduced root growth sensitivity to abscisic acid (ABA) and the polar auxin transport inhibitor N-(1-naphtyl)phtalamic acid (NPA). These results showed that hydrotropism is amenable to genetic analysis and that an ABA signaling pathway participates in sensing water potential gradients through the root cap.

A collection of sequenced and mapped Ds transposon insertion sites in Arabidopsis thaliana

Insertional mutagenesis is a powerful tool for generating knockout mutations that facilitate associating biological functions with as yet uncharacterized open reading frames (ORFs) identified by genomic sequencing or represented in EST databases. We have generated a collection of Dissociation (Ds) transposon lines with insertions on all 5 Arabidopsis chromosomes. Here we report the insertion sites in 260 independent single-transposon lines, derived from four different Ds donor sites. We amplified and determined the genomic sequence flanking each transposon, then mapped its insertion site by identity of the flanking sequences to the corresponding sequence in the Arabidopsis genome database. This constitutes the largest collection of sequence-mapped Ds insertion sites unbiased by selection against the donor site. Insertion site clusters have been identified around three of the four donor sites on chromosomes 1 and 5, as well as near the nucleolus organizers on chromosomes 2 and 4. The distribution of insertions between ORFs and intergenic sequences is roughly proportional to the ratio of genic to intergenic sequence. Within ORFs, insertions cluster near the translational start codon, although we have not detected insertion site selectivity at the nucleotide sequence level. A searchable database of insertion site sequences for the 260 transposon insertion sites is available at http://sgio2.biotec.psu.edu/sr. This and other collections of Arabidopsis lines with sequence-identified transposon insertion sites are a valuable genetic resource for functional genomics studies because the transposon location is precisely known, the transposon can be remobilized to generate revertants, and the Ds insertion can be used to initiate further local mutagenesis.

Phosphate availability alters architecture and causes changes in hormone sensitivity in the Arabidopsis root system

The postembryonic developmental program of the plant root system is plastic and allows changes in root architecture to adapt to environmental conditions such as water and nutrient availability. Among essential nutrients, phosphorus (P) often limits plant productivity because of its low mobility in soil. Therefore, the architecture of the root system may determine the capacity of the plant to acquire this nutrient. We studied the effect of P availability on the development of the root system in Arabidopsis. We found that at P-limiting conditions (<50 microM), the Arabidopsis root system undergoes major architectural changes in terms of lateral root number, lateral root density, and primary root length. Treatment with auxins and auxin antagonists indicate that these changes are related to an increase in auxin sensitivity in the roots of P-deprived Arabidopsis seedlings. It was also found that the axr1-3, axr2-1, and axr4-1 Arabidopsis mutants have normal responses to low P availability conditions, whereas the iaa28-1 mutant shows resistance to the stimulatory effects of low P on root hair and lateral root formation. Analysis of ethylene signaling mutants and treatments with 1-aminocyclopropane-1-carboxylic acid showed that ethylene does not promote lateral root formation under P deprivation. These results suggest that in Arabidopsis, auxin sensitivity may play a fundamental role in the modifications of root architecture by P availability.

Expression of the AtGH3a gene, an Arabidopsis homologue of the soybean GH3 gene, is regulated by phytochrome B

Light is one of the most crucial environmental cues for plants. Phytochrome, one of the major photoreceptors of plants, regulates expression of many genes. We screened for Arabidopsis promoter trap lines that exhibited light-dependent reporter gene expression. Molecular analysis of one such line revealed that a reporter gene was inserted near an Arabidopsis homologue of the soybean GH3 gene, AtGH3a. We performed RNA gel blot analysis to further characterize the response of the AtGH3a gene to light. In response to the end-of-day far-red light treatment, the expression increased substantially. Analysis of the phyB-deficient mutant indicated that this light response is under the control of phytochrome B. The expression was also induced by exogenous auxin. Furthermore, the light response was substantially reduced in the auxin-related axr2 mutant. Taken together, it is suggested that phytochrome B regulates the expression of genes by altering the levels of auxin.

Root gravitropism: an experimental tool to investigate basic cellular and molecular processes underlying mechanosensing and signal transmission in plants

The ability of plant organs to use gravity as a guide for growth, named gravitropism, has been recognized for over two centuries. This growth response to the environment contributes significantly to the upward growth of shoots and the downward growth of roots commonly observed throughout the plant kingdom. Root gravitropism has received a great deal of attention because there is a physical separation between the primary site for gravity sensing, located in the root cap, and the site of differential growth response, located in the elongation zones (EZs). Hence, this system allows identification and characterization of different phases of gravitropism, including gravity perception, signal transduction, signal transmission, and curvature response. Recent studies support some aspects of an old model for gravity sensing, which postulates that root-cap columellar amyloplasts constitute the susceptors for gravity perception. Such studies have also allowed the identification of several molecules that appear to function as second messengers in gravity signal transduction and of potential signal transducers. Auxin has been implicated as a probable component of the signal that carries the gravitropic information between the gravity-sensing cap and the gravity-responding EZs. This has allowed the identification and characterization of important molecular processes underlying auxin transport and response in plants. New molecular models can be elaborated to explain how the gravity signal transduction pathway might regulate the polarity of auxin transport in roots. Further studies are required to test these models, as well as to study the molecular mechanisms underlying a poorly characterized phase of gravitropism that is independent of an auxin gradient.

Arabidopsis thaliana: A Model for the Study of Root and Shoot Gravitropism

For most plants, shoots grow upward and roots grow downward. These growth patterns illustrate the ability for plant organs to guide their growth at a specified angle from the gravity vector (gravitropism). They allow shoots to grow upward toward light, where they can photosynthesize, and roots to grow downward into the soil, where they can anchor the plant as well as take up water and mineral ions.Gravitropism involves several steps organized in a specific response pathway. These include the perception of a gravistimulus (reorientation within the gravity field), the transduction of this mechanical stimulus into a physiological signal, the transmission of this signal from the site of sensing to the site of response, and a curvature-response which allows the organ tip to resume growth at a predefined set angle from the gravity vector.The primary sites for gravity sensing are located in the cap for roots, and in the endodermis for shoots. The curvature response occurs in the elongation zones for each organ. Upon gravistimulation, a gradient of auxin appears to be generated across the stimulated organ, and be transmitted to the site of response where it promotes a differential growth response. Therefore, while the gravity-induced auxin gradient has to be transmitted from the cap to the elongation zones in roots, there is no need for a longitudinal transport in shoots, as sites for gravity sensing and response overlap in this organ.A combination of molecular genetics, physiology, biochemistry and cell biology, coupled with the utilization of Arabidopsis thaliana as a model system, have recently allowed the identification of a number of molecules involved in the regulation of each phase of gravitropism in shoots and roots of higher plants. In this review, we attempt to summarize the results of these experiments, and we conclude by comparing the molecular and physiological mechanisms that underlie gravitropism in these organs.

ABBREVIATIONS

GSPA: gravitational set point angle; IAA: indole-3-acetic acid; NAA: 1-naphthalene acetic acid; NPA: 1-N-naphthylphthalamic acid; 2,4-D: 2,4-dichlorphenoxy acetic acid; TIBA: 2,3,5-triiodobenzoic acid.

Transactivation of BARNASE under the AtLTP1 promoter affects the basal pole of the embryo and shoot development of the adult plant in Arabidopsis

Genetically controlled expression of a toxin provides a tool to remove a specific structure and consequently study its role during a developmental process. The availability of many tissue-specific promoters is a good argument for the development of such a strategy in plants. We have developed a conditional system for targeted toxin expression and demonstrated its use for generating embryo phenotypes that can bring valuable information about signalling during embryogenesis. The BARNASE gene was expressed in the Arabidopsis embryo under the control of two promoters, one from the cyclin AtCYCB1 gene and one from the AtLTP1 gene (Lipid Transfer Protein 1). One-hundred percent seed abortion was obtained with the cyclin promoter. Surprisingly however, the embryos displayed a range of lethal phenotypes instead of a single arrested stage as expected from this promoter. We also show that BARNASE expression under the control of the AtLTP1 promoter affects the basal pole of the globular embryo. Together with reporter expression studies, this result suggests a role of the epidermis in controlling the development of the lower tier of the embryo. This defect was not embryo-lethal and we show that the seedlings displayed a severe shoot phenotype correlated to epidermal defects. Therefore, the epidermis does not play an active role during organogenesis in seedlings but is important for the postgermination development of a viable plant.

The PINOID protein kinase regulates organ development in Arabidopsis by enhancing polar auxin transport

Arabidopsis pinoid mutants show a strong phenotypic resemblance to the pin-formed mutant that is disrupted in polar auxin transport. The PINOID gene was recently cloned and found to encode a protein-serine/threonine kinase. Here we show that the PINOID gene is inducible by auxin and that the protein kinase is present in the primordia of cotyledons, leaves and floral organs and in vascular tissue in developing organs or proximal to meristems. Overexpression of PINOID under the control of the constitutive CaMV 35S promoter (35S::PID) resulted in phenotypes also observed in mutants with altered sensitivity to or transport of auxin. A remarkable characteristic of high expressing 35S::PID seedlings was a frequent collapse of the primary root meristem. This event triggered lateral root formation, a process that was initially inhibited in these seedlings. Both meristem organisation and growth of the primary root were rescued when seedlings were grown in the presence of polar auxin transport inhibitors, such as naphthylphtalamic acid (NPA). Moreover, ectopic expression of PINOID cDNA under control of the epidermis-specific LTP1 promoter provided further evidence for the NPA-sensitive action of PINOID. The results presented here indicate that PINOID functions as a positive regulator of polar auxin transport. We propose that PINOID is involved in the fine-tuning of polar auxin transport during organ formation in response to local auxin concentrations.

Cell lineage, cell signaling and the control of plant morphogenesis

It is clear that cell-cell signaling is critical for the development of both root and shoot structures. Recently, several of the key gene products required for intercellular signaling have been defined, and the developmental processes regulated by cell-cell interactions are beginning to be elucidated. Surprisingly, these results suggest that the mechanisms by which plant cells communicate with each other may be quite distinct from those used in animal systems.

Auxin transport promotes Arabidopsis lateral root initiation

Lateral root development in Arabidopsis provides a model for the study of hormonal signals that regulate postembryonic organogenesis in higher plants. Lateral roots originate from pairs of pericycle cells, in several cell files positioned opposite the xylem pole, that initiate a series of asymmetric, transverse divisions. The auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) arrests lateral root development by blocking the first transverse division(s). We investigated the basis of NPA action by using a cell-specific reporter to demonstrate that xylem pole pericycle cells retain their identity in the presence of the auxin transport inhibitor. However, NPA causes indoleacetic acid (IAA) to accumulate in the root apex while reducing levels in basal tissues critical for lateral root initiation. This pattern of IAA redistribution is consistent with NPA blocking basipetal IAA movement from the root tip. Characterization of lateral root development in the shoot meristemless1 mutant demonstrates that root basipetal and leaf acropetal auxin transport activities are required during the initiation and emergence phases, respectively, of lateral root development.

MADS-box gene evolution beyond flowers: expression in pollen, endosperm, guard cells, roots and trichomes

MADS-box genes encode transcriptional regulators involved in diverse aspects of plant development. Here we describe the cloning and mRNA spatio-temporal expression patterns of five new MADS-box genes from Arabidopsis: AGL16, AGL18, AGL19, AGL27 and AGL31. These genes will probably become important molecular tools for both evolutionary and functional analyses of vegetative structures. We mapped our data and previous expression patterns onto a new MADS-box phylogeny. These analyses suggest that the evolution of the MADS-box family has involved a rapid and simultaneous functional diversification in vegetative as well as reproductive structures. The hypothetical ancestral genes had broader expression patterns than more derived ones, which have been co-opted for putative specialized functions as suggested by their expression patterns. AGL27 and AGL31, which are closely related to the recently described flowering-time gene FLC (previously AGL25), are expressed in most plant tissues. AGL19 is specifically expressed in the outer layers of the root meristem (lateral root cap and epidermis) and in the central cylinder cells of mature roots. AGL18, which is most similar in sequence to the embryo-expressed AGL15 gene, is expressed in the endosperm and in developing male and female gametophytes, suggesting a role for AGL18 that is distinct from previously characterized MADS-box genes. Finally, AGL16 RNA accumulates in leaf guard cells and trichomes. Our new phylogeny reveals seven new monophyletic clades of MADS-box sequences not specific to flowers, suggesting that complex regulatory networks involving several MADS-box genes, similar to those that control flower development, underlie development of vegetative structures.

Short root mutant of Lotus japonicus with a dramatically altered symbiotic phenotype

Legume plants carefully control the extent of nodulation in response to rhizobial infection. To examine the mechanism underlying this process we conducted a detailed analysis of the Lotus japonicus hypernodulating mutants, har1-1, 2 and 3 that define a new locus, HYPERNODULATION ABERRANT ROOT FORMATION (Har1), involved in root and symbiotic development. Mutations in the Har1 locus alter root architecture by inhibiting root elongation, diminishing root diameter and stimulating lateral root initiation. At the cellular level these developmental alterations are associated with changes in the position and duration of root cell growth and result in a premature differentiation of har1-1 mutant root. No significant differences between har1-1 mutant and wild-type plants were detected with respect to root growth responses to 1-aminocyclopropane1-carboxylic acid, the immediate precursor of ethylene, and auxin; however, cytokinin in the presence of AVG (aminoetoxyvinylglycine) was found to stimulate root elongation of the har1-1 mutant but not the wild-type. After inoculation with Mesorhizobium loti, the har1 mutant lines display an unusual hypernodulation (HNR) response, characterized by unrestricted nodulation (hypernodulation), and a concomitant drastic inhibition of root and shoot growth. These observations implicate a role for the Har1 locus in both symbiotic and non-symbiotic development of L. japonicus, and suggest that regulatory processes controlling nodule organogenesis and nodule number are integrated in an overall mechanism governing root growth and development.