A DEAD-box protein, AtRH36, is essential for female gametophyte development and is involved in rRNA biogenesis in Arabidopsis

DEAD-box RNA helicases are involved in RNA metabolism, including pre-mRNA splicing, ribosome biogenesis, RNA decay and gene expression. In this study, we identified a homolog of the RH36 gene, AtRH36, which encodes a DEAD-box protein in Arabidopsis thaliana. The gene was expressed ubiquitously throughout the plant. The AtRH36 fused to green fluorescent protein was localized in the nucleus. Homozygosity for the Arabidopsis atrh36 mutants, atrh36-1 and atrh36-2, could not be obtained. Progeny of selfed Arabidopsis atrh36 heterozygote plants were obtained at a heterozygote to wild-type ratio of 1 : 1, which suggested that the AtRH36 gene was involved in gametogenesis. Therefore, we performed a reciprocal cross to determine whether AtRH36 was involved in female gametophyte development. Female gametogenesis was delayed in atrh36-1, and asynchronous development of the female gametophytes was found within a single pistil. Knock-down of AtRH36 gave a pleiotropic phenotype and led to the accumulation of unprocessed 18S pre-rRNA. These results suggest that AtRH36 is essential for mitotic division during female gametogenesis and plays an important role in rRNA biogenesis in Arabidopsis.

Perspectives on leaf dorsoventral polarity

Leaves occur in a vast array of shapes and sizes, with complex diversity contributing to optimization of the principal function of photosynthesis. The program of development from a self-renewing stem cell population to a mature leaf has been of interest to biologists for years. Many genes involved in this process have been identified, particularly in the model eudicot Arabidopsis, so that now we have a greater understanding of mechanisms of stem cell maintenance, cell differentiation and organogenesis. One aspect of leaf development that is of particular interest is the establishment of dorsoventral polarity: the distinct adaxial (upper) and abaxial (lower) sides of the leaf. Early studies postulated conceptual models of how establishment of polarity leads to the development of planar leaves. Studies over the past decade have defined genetic details of this model, and uncovered diverse mechanisms of gene regulation that facilitate development of leaf dorsoventral polarity, including transcriptional regulation, chromatin modification, DNA modification, regulation by short RNAs and translational and post-translational regulation. This review will discuss these regulatory mechanisms in the context of leaf dorsoventrality, and will conclude with unresolved questions and areas of future research.

Application of the gene dosage balance hypothesis to auxin-related ribosomal mutants in Arabidopsis

Several proteomic studies in Arabidopsis have shown the presence of heterogeneous ribosomal populations in different tissues. However, the phenotypic consequences of the imbalance of those ribosomal populations, and the regulatory mechanisms activated to control specific ratios between them, have yet to be evaluated. In our previous report, the phenotypic characterization of the ribosomal protein family L4 (RPL4) in Arabidopsis suggests that the maintenance of proper auxin-regulated developmental responses requires the simultaneous presence of RPL4A- and RPL4D-containing ribosomes. Based on the analysis of the compensatory mechanisms within the RPL4 family proteins in the rpl4a and rpl4d backgrounds, we propose the Gene Dosage Balance Hypothesis (GDBH) as a regulatory mechanism for ribosomal complexes in Arabidopsis. By using the concepts of dosage compensation and hierarchy, GDBH is able to explain the severity and specificity of different ribosomal mutant phenotypes associated with the same ribosomal complex.

Leaving the meristem behind: the genetic and molecular control of leaf patterning and morphogenesis

Leaves, which play an essential role in plant photosynthesis, share common features such as being flat structures, but also show an impressive variability in their sizes and shapes. Following its initiation in the meristems, leaf development is patterned along three polarization axes to establish its basic architecture. This process is further complicated in the case of compound leaves with the formation of new growth axes. Growth and differentiation must be properly coordinated to regulate the size and the flatness of the leaf. This review provides an overview of the genetic and molecular regulatory networks underlying leaf development, with an emphasis on leaf polarity and the comparison of simple and compound leaves.

The PXY-CLE41 receptor ligand pair defines a multifunctional pathway that controls the rate and orientation of vascular cell division

Controlling the orientation of cell division is fundamental to the development of complex body plans. This is particularly apparent in plants, where development is determined by differential growth that results solely from changes in cell expansion and orientation of the cell division plane. Despite the fundamental importance of cell division orientation to plant development, the mechanisms regulating this process remain almost completely unknown. During vascular development, the meristematic cambial cells divide down their long axis in a highly orientated manner to generate clear files of cells. The receptor kinase PXY has previously be shown to be essential for this orientation. Here, we demonstrate that the division plane is determined by the interactions of PXY and its peptide ligand, CLE41. PXY is expressed within dividing meristematic cells of the procambium, whereas CLE41 localises to the adjacent phloem cells. Altering the pattern of CLE41 expression leads to a loss of cell division orientation and a dramatic loss of ordered vascular tissue development. By contrast, increasing phloem-specific expression of CLE41 results in more cell divisions, but the orientation of cell division is retained, leading to both increased and well-ordered vascular development. We demonstrate that PXY signalling is down-regulated by CLE41. This feedback mechanism is crucial in integrating the different roles of PXY signalling in controlling xylem differentiation, regulating the rate of vascular cell division and determining the orientation of cell division. Parallels with animal systems indicate that localised signalling from adjacent cells is a general mechanism for defining the plane of cell division.

Signaling sides adaxial-abaxial patterning in leaves

Most leaves are dorsiventrally flattened and develop clearly defined upper and lower surfaces. Light capturing is the specialization of the adaxial or upper surface and the abaxial or lower surface is specialized for gas exchange (Fig. 5.1). This division into adaxial and abaxial domains is also key for the outgrowth of the leaf blade or lamina, which occurs along the boundary between the upper and lower sides. How this polarity is set up is not clear but genetic analysis in a range of species suggests that several highly conserved interlocking pathways are involved. Positional information from the meristem is reinforced by signaling through the epidermal layer as the meristem grows away from the leaf primordium. Opposing ta-siRNA and miRNA gradients help refine distinct adaxial and abaxial sides, and mutual inhibition between the genes expressed on each side stabilizes the boundary. In this review we consider how recent work in a range of species is clarifying our understanding of these processes.

Auxin-mediated ribosomal biogenesis regulates vacuolar trafficking in Arabidopsis

In plants, the mechanisms that regulate the transit of vacuolar soluble proteins containing C-terminal and N-terminal vacuolar sorting determinants (VSDs) to the vacuole are largely unknown. In a screen for Arabidopsis thaliana mutants affected in the trafficking of C-terminal VSD containing proteins, we isolated the ribosomal biogenesis mutant rpl4a characterized by its partial secretion of vacuolar targeted proteins and a plethora of developmental phenotypes derived from its aberrant auxin responses. In this study, we show that ribosomal biogenesis can be directly regulated by auxins and that the exogenous application of auxins to wild-type plants results in vacuolar trafficking defects similar to those observed in rpl4a mutants. We propose that the influence of auxin on ribosomal biogenesis acts as a regulatory mechanism for auxin-mediated developmental processes, and we demonstrate the involvement of this regulatory mechanism in the sorting of vacuolar targeted proteins in Arabidopsis.

The ang3 mutation identified the ribosomal protein gene RPL5B with a role in cell expansion during organ growth

The role of translation in the regulation of higher plant growth and development is not well understood. Mutational analysis is a powerful tool to identify and study the function of genes related to a biological process, such as growth. Here we analyzed functionally the angusta3 (ang3) narrow leaf mutant. The AG3 gene was cloned by fine mapping combined with candidate gene sequencing and it corresponded to the ribosomal protein gene RPL5B. Based on amino acid sequence homology, promoter DNA sequence homology and in silico gene expression analysis, RPL5B was found to be putatively functionally redundant with RPL5A. The morphological analysis of ang3 mutants showed that the leaf lamina area was significantly reduced from the third rosette leaf on, mainly because of decreased width. Cellular analysis of the abaxial epidermal cell layer of the third leaf indicated that the cell number in the mutant was similar to that of the wild type, but the cell size was significantly reduced. We postulate that the reduced cell expansion in the epidermis contributes to the narrow shape of ang3 leaves. Growth was also significantly impaired in hypocotyls and primary roots, hinting at a general role for RPL5B in organ growth, unrelated to dorsiventral axis formation. Comparison of the transcriptome of the shoot apices of the mutant and the wild type revealed a limited number of differentially expressed genes, such as MYB23 and MYB5, of which the lower expression in the ang3 mutant correlated with reduced trichome density. Our data suggest that translation is an important level of control of growth and development in plants.

Signals and prepatterns: new insights into organ polarity in plants

The flattening of leaves results from the interaction between upper (adaxial) and lower (abaxial) domains in the developing primordium. These domains are specified by conserved, overlapping genetic pathways involving several distinct transcription factor families and small regulatory RNAs. Polarity determinants employ a series of antagonistic interactions to produce mutually exclusive cell fates whose positioning is likely refined by signaling across the adaxial-abaxial boundary. Signaling candidates include a mobile small RNA-the first positional signal described in adaxial-abaxial polarity. Possible mechanisms to polarize the incipient primordium are discussed, including meristem-derived signaling and a model in which a polarized organogenic zone prepatterns the adaxial-abaxial axis.

Coordination of cell proliferation and cell expansion mediated by ribosome-related processes in the leaves of Arabidopsis thaliana

Co-ordination of cell proliferation and cell expansion is a key regulatory process in leaf-size determination, but its molecular details are unknown. In Arabidopsis thaliana, mutations in a positive regulator of cell proliferation often trigger excessive cell enlargement post-mitotically in leaves. This phenomenon, called compensation syndrome, is seen in the mutant angustifolia3 (an3), which is defective in a transcription co-activator. Such compensation, however, does not occur in response to a decrease in cell number in oligocellula (oli). oli2, oli5 and oli7 did not exhibit compensation and the reduction in cell number in these mutants was moderate. However, when an oli mutation was combined with a different oli mutation to create a double mutant, cell number was further reduced and compensation was induced. Similarly, weak suppression of AN3 expression reduced cell number moderately but did not induce compensation compared with an an3 null mutant. Furthermore, double mutants of either oli2, oli5 or oli7 and an3 showed markedly enhanced compensation. These results suggest that compensation is triggered when cell proliferation regulated by OLI2/OLI5/OLI7 and AN3 is compromised in a threshold-dependent manner. OLI2 encodes a Nop2 homolog in Saccharomyces cerevisiae that is involved in ribosome biogenesis, whereas OLI5 and OLI7 encode ribosome proteins RPL5A and RPL5B, respectively. This suggests that a factor involved in the induction of compensation may be under the dual control of AN3 and a ribosome-related process.

Differential recruitment of WOX transcription factors for lateral development and organ fusion in Petunia and Arabidopsis

Petal fusion in petunia (Petunia x hybrida) results from lateral expansion of the five initially separate petal primordia, forming a ring-like primordium that determines further development. Here, we show that MAEWEST (MAW) and CHORIPETALA SUZANNE (CHSU) are required for petal and carpel fusion, as well as for lateral outgrowth of the leaf blade. Morphological and molecular analysis of maw and maw chsu double mutants suggest that polarity defects along the adaxial/abaxial axis contribute to the observed reduced lateral outgrowth of organ primordia. We show that MAW encodes a member of the WOX (WUSCHEL-related homeobox) transcription factor family and that a partly similar function is redundantly encoded by WOX1 and PRESSED FLOWER (PRS) in Arabidopsis thaliana, indicating a conserved role for MAW/WOX1/PRS genes in regulating lateral organ development. Comparison of petunia maw and Arabidopsis wox1 prs phenotypes suggests differential recruitment of WOX gene function depending on organ type and species. Our comparative data together with previous reports on WOX gene function in different species identify the WOX gene family as highly dynamic and, therefore, an attractive subject for future evo-devo studies.

Leaf development: time to turn over a new leaf?

Molecular cloning of mutations affecting the morphology of plant leaves has proven to be useful for the causal analysis of leaf development. Studies of leaf mutants have produced a wealth of biologically meaningful information on the genes that participate in leaf initiation, leaf polarity specification and maintenance, and leaf expansion and maturation. The availability of collections of gene-indexed insertional mutants, automated platforms for high-throughput imaging, and new morphometry software is making genome-wide leaf phenomics possible and complements classical forward genetics approaches. Large-scale phenomic studies will further our understanding, among others, of two intriguing phenomena that recently reentered the leaf scenario. One is the unexpected relationship between translation and leaf dorsoventrality, recently confirmed by the severe abaxialization of double mutants involving loss-of-function alleles of the developmental selector genes AS1 and AS2 and some genes encoding ribosomal proteins. The second unexplained phenomenon is the compensatory cell enlargement experienced by some leaf mutants, in which a reduced cell number is compensated by their increased cell size compared with the wild type. This compensation suggests that cell cycling and cell enlargement are integrated in leaf primordia via cell-to-cell communication.

A semi-dominant mutation in the ribosomal protein L10 gene suppresses the dwarf phenotype of the acl5 mutant in Arabidopsis thaliana

Disruption of the Arabidopsis thaliana ACAULIS5 (ACL5) gene, which has recently been shown to encode thermospermine synthase, results in a severe dwarf phenotype. A previous study showed that sac51-d, a dominant suppressor mutant of acl5-1, has a premature termination codon in an upstream open reading frame (ORF) of SAC51, which encodes a putative transcription factor, and suggested the involvement of upstream ORF-mediated translational control in ACL5-dependent stem elongation. Here we report the identification of a gene responsible for sac52-d, another semi-dominant suppressor mutant of acl5-1. SAC52 encodes ribosomal protein L10 (RPL10A), which is highly conserved among eukaryotes and implicated in translational regulation. Transformation of acl5-1 mutants with a genomic fragment containing the sac52-d allele rescued the dwarf phenotype of acl5-1. GUS reporter activity under the control of a SAC51 promoter with its upstream ORF was higher in acl5-1 sac52-d than in acl5-1, suggesting that suppression of the acl5-1 phenotype by sac52-d is attributable, in part, to enhanced translation of certain transcripts including SAC51. We also found that a T-DNA insertion allele of SAC52/RPL10A causes lethality in the female gametophyte.

A model for the 26S proteasome and ribosome actions in leaf polarity formation

Leaf morphogenesis requires the establishment of adaxial-abaxial polarity in emerging leaf primordia, and a number of genes participating in this process have been identified in recent years. We previously reported that the 26S proteasome is important in specifying the leaf adaxial fate. More recently, two papers from separate researches showed that several genes encoding ribosomal large subunit proteins also play an important role in leaf adaxial-abaxial patterning. Here we show that plants with a single mutation in the genes encoding either 26S proteasome subunits or ribosomal proteins shared similar abnormalities in some leaves, with an outgrowth formed on the distal part of the leaf abaxial side. Plants harboring these 26S proteasome or ribosome mutations in combination with an additional mutation asymmetric leaves1 or 2 (as1 or as2) demonstrated severely defective leaves, and the phenotypes of these double mutants were very similar. Because activities of the 26S proteasome and ribosome both affect the level of functional proteins, the recent findings suggest that a previously unrecognized regulation, the protein level regulation, is critical in normal leaf patterning. A regulatory model for the 26S proteasome and ribosome actions in leaf patterning is discussed.

Transcript profiling demonstrates absence of dosage compensation in Arabidopsis following loss of a single RPL23a paralog

Translation of nucleus-encoded messages in plants is conducted by the cytoplasmic ribosome, an enzyme that is comprised of two RNA/protein subunits. In Arabidopsis thaliana, the 81 different ribosomal proteins (r-proteins) of the cytosolic ribosome belong to gene families with multiple expressed members. Given that ribosomes generally contain only one copy of each r-protein, regulatory mechanisms must exist to ensure their stoichiometric accumulation. These mechanisms must be dynamic, allowing for adjustments to ribosome biogenesis to fulfill biological requirements for protein synthesis during development, and following stress induction of global changes in gene expression. In this study, we investigated whether r-protein paralogs are feedback regulated at the transcript level by obtaining a T-DNA knockout of one member, RPL23aB, from the two-member RPL23a family. Expression of the lone functional paralog in this line, RPL23aA, was compared to the expression of both paralogs in wildtype plants under non-stressed, low temperature-, and high light stresses. RPL23aA expression was not upregulated in RPL23aB knockouts to compensate for paralog-loss, and consequently knockouts showed reduced total abundance of RPL23a transcripts. However, no phenotype developed in RPL23aB knockouts, suggesting that this paralog is dispensable under experimental conditions examined, or that compensation by RPL23aA may occur post-transcriptionally. Patterns of RPL23aA and RPL23aB transcript accumulation in wildtype plants suggest that paralogs respond coordinately to developmental and stress stimuli.

New clues to organ size control in plants

Plant growth has unparalleled importance for human civilization, yet we are only starting to gain an understanding of its mechanisms. The growth rate and final size of plant organs is determined by both genetic constraints and environmental factors. Regulatory inputs act at two control points: on proliferation; and on the transition between proliferation and differentiation. Cell-autonomous and short-range growth signals act within meristematic domains, whereas diffusible signals from differentiated parts to proliferating cells provide measures of geometry and size and channel environmental inputs.