The single Marchantia polymorpha FERONIA homolog reveals an ancestral role in regulating cellular expansion and integrity.

Plant cells are surrounded by a cell wall, a rigid structure that is not only important for cell and organ shape, but is also crucial for intercellular communication and interactions with the environment. In the flowering plant Arabidopsis thaliana, the 17 members of the Catharanthus roseus RLK1-like (CrRLK1L) receptor kinase family are involved in a multitude of physiological and developmental processes, making it difficult to assess their primary or ancestral function. To reduce genetic complexity, we characterized the single CrRLK1L gene of Marchantia polymorpha, MpFERONIA (MpFER). Plants with reduced MpFER levels show defects in vegetative development, i.e. rhizoid formation and cell expansion, and have reduced male fertility. In contrast, cell integrity and morphogenesis of the gametophyte are severely affected in Mpfer null mutants and MpFER overexpression lines. Thus, we conclude that the CrRLK1L gene family originated from a single gene with an ancestral function in cell expansion and the maintenance of cellular integrity. During land plant evolution, this ancestral gene diversified to fulfill a multitude of specialized physiological and developmental roles in the formation of both gametophytic and sporophytic structures essential to the life cycle of flowering plants.

MicroRNA miR396 Regulates the Switch between Stem Cells and Transit-Amplifying Cells in Arabidopsis Roots.

To ensure an adequate organ mass, the daughters of stem cells progress through a transit-amplifying phase displaying rapid cell division cycles before differentiating. Here, we show that Arabidopsis thaliana microRNA miR396 regulates the transition of root stem cells into transit-amplifying cells by interacting with GROWTH-REGULATING FACTORs (GRFs). The GRFs are expressed in transit-amplifying cells but are excluded from the stem cells through inhibition by miR396. Inactivation of the GRFs increases the meristem size and induces periclinal formative divisions in transit-amplifying cells. The GRFs repress PLETHORA (PLT) genes, regulating their spatial expression gradient. Conversely, PLT activates MIR396 in the stem cells to repress the GRFs. We identified a pathway regulated by GRF transcription factors that represses stem cell-promoting genes in actively proliferating cells, which is essential for the progression of the cell cycle and the orientation of the cell division plane. If unchecked, the expression of the GRFs in the stem cell niche suppresses formative cell divisions and distorts the organization of the quiescent center. We propose that the interactions identified here between miR396 and GRF and PLT transcription factors are necessary to establish the boundary between the stem cell niche and the transit-amplifying region.

An update on cell surface proteins containing extensin-motifs.

In recent years it has become clear that there are several molecular links that interconnect the plant cell surface continuum, which is highly important in many biological processes such as plant growth, development, and interaction with the environment. The plant cell surface continuum can be defined as the space that contains and interlinks the cell wall, plasma membrane and cytoskeleton compartments. In this review, we provide an updated view of cell surface proteins that include modular domains with an extensin (EXT)-motif followed by a cytoplasmic kinase-like domain, known as PERKs (for proline-rich extensin-like receptor kinases); with an EXT-motif and an actin binding domain, known as formins; and with extracellular hybrid-EXTs. We focus our attention on the EXT-motifs with the short sequence Ser-Pro(3-5), which is found in several different protein contexts within the same extracellular space, highlighting a putative conserved structural and functional role. A closer understanding of the dynamic regulation of plant cell surface continuum and its relationship with the downstream signalling cascade is a crucial forthcoming challenge.

Post-transcriptional control of GRF transcription factors by microRNA miR396 and GIF co-activator affects leaf size and longevity.

The growth-regulating factors (GRFs) are plant-specific transcription factors. They form complexes with GRF-interacting factors (GIFs), a small family of transcriptional co-activators. In Arabidopsis thaliana, seven out of the nine GRFs are controlled by microRNA miR396. Analysis of Arabidopsis plants carrying a GRF3 allele insensitive to miR396 revealed a strong boost in the number of cells in leaves, which was further enhanced synergistically by an additional increase of GIF1 levels. Genetic experiments revealed that GRF3 can still increase cell number in gif1 mutants, albeit to a much lesser extent. Genome-wide transcript profiling indicated that the simultaneous increase of GRF3 and GIF1 levels causes additional effects in gene expression compared to either of the transgenes alone. We observed that GIF1 interacts in vivo with GRF3, as well as with chromatin-remodeling complexes, providing a mechanistic explanation for the synergistic activities of a GRF3-GIF1 complex. Interestingly, we found that, in addition to the leaf size, the GRF system also affects the organ longevity. Genetic and molecular analysis revealed that the functions of GRFs in leaf growth and senescence can be uncoupled, demonstrating that the miR396-GRF-GIF network impinges on different stages of leaf development. Our results integrate the post-transcriptional control of the GRF transcription factors with the progression of leaf development.

Functional specialization of the plant miR396 regulatory network through distinct microRNA-target interactions.

MicroRNAs (miRNAs) are ∼21 nt small RNAs that regulate gene expression in animals and plants. They can be grouped into families comprising different genes encoding similar or identical mature miRNAs. Several miRNA families are deeply conserved in plant lineages and regulate key aspects of plant development, hormone signaling, and stress response. The ancient miRNA miR396 regulates conserved targets belonging to the GROWTH-REGULATING FACTOR (GRF) family of transcription factors, which are known to control cell proliferation in Arabidopsis leaves. In this work, we characterized the regulation of an additional target for miR396, the transcription factor bHLH74, that is necessary for Arabidopsis normal development. bHLH74 homologs with a miR396 target site could only be detected in the sister families Brassicaceae and Cleomaceae. Still, bHLH74 repression by miR396 is required for margin and vein pattern formation of Arabidopsis leaves. MiR396 contributes to the spatio-temporal regulation of GRF and bHLH74 expression during leaf development. Furthermore, a survey of miR396 sequences in different species showed variations in the 5' portion of the miRNA, a region known to be important for miRNA activity. Analysis of different miR396 variants in Arabidopsis thaliana revealed that they have an enhanced activity toward GRF transcription factors. The interaction between the GRF target site and miR396 has a bulge between positions 7 and 8 of the miRNA. Our data indicate that such bulge modulates the strength of the miR396-mediated repression and that this modulation is essential to shape the precise spatio-temporal pattern of GRF2 expression. The results show that ancient miRNAs can regulate conserved targets with varied efficiency in different species, and we further propose that they could acquire new targets whose control might also be biologically relevant.

Control of cell proliferation in Arabidopsis thaliana by microRNA miR396.

Cell proliferation is an important determinant of plant form, but little is known about how developmental programs control cell division. Here, we describe the role of microRNA miR396 in the coordination of cell proliferation in Arabidopsis leaves. In leaf primordia, miR396 is expressed at low levels that steadily increase during organ development. We found that miR396 antagonizes the expression pattern of its targets, the GROWTH-REGULATING FACTOR (GRF) transcription factors. miR396 accumulates preferentially in the distal part of young developing leaves, restricting the expression of GRF2 to the proximal part of the organ. This, in turn, coincides with the activity of the cell proliferation marker CYCLINB1;1. We show that miR396 attenuates cell proliferation in developing leaves, through the repression of GRF activity and a decrease in the expression of cell cycle genes. We observed that the balance between miR396 and the GRFs controls the final number of cells in leaves. Furthermore, overexpression of miR396 in a mutant lacking GRF-INTERACTING FACTOR 1 severely compromises the shoot meristem. We found that miR396 is expressed at low levels throughout the meristem, overlapping with the expression of its target, GRF2. In addition, we show that miR396 can regulate cell proliferation and the size of the meristem. Arabidopsis plants with an increased activity of the transcription factor TCP4, which reduces cell proliferation in leaves, have higher miR396 and lower GRF levels. These results implicate miR396 as a significant module in the regulation of cell proliferation in plants.

Proline-rich extensin-like receptor kinases PERK5 and PERK12 are involved in pollen tube growth.

Proline-rich extensin-like receptor kinases (PERKs) belong to the hydroxyproline-rich glycoprotein (HRGP) superfamily known to be involved in many plant developmental processes. Here, we characterized two pollen-expressed PERKs from Arabidopsis thaliana, PERK5 and PERK12. Pollen tube growth was impaired in single and double perk5-1 perk12-1 loss of function mutants, with an impact on seed production. When the segregation was analysed, a male gametophytic defect was found, indicating that perk5-1 and perk12-1 mutants carry deficient pollen transmission. Furthermore, perk5-1 perk12-1 displayed an excessive accumulation of pectins and cellulose at the cell wall of the pollen tubes. Our results indicate that PERK5 and PERK12 are necessary for proper pollen tube growth, highlighting their role in cell wall assembly and reactive oxygen species homeostasis.

The BES1/BZR1-family transcription factor MpBES1 regulates cell division and differentiation in Marchantia polymorpha.

Brassinosteroids (BRs) play essential roles in growth and development in seed plants;1 disturbances in BR homeostasis lead to altered mitotic activity in meristems2,3 and organ boundaries4,5 and to changes in meristem determinacy.6 An intricate signaling cascade linking the perception of BRs at the plasma membrane to the regulation of master transcriptional regulators belonging to the BEH, for BES1 homologues, family7 has been described in great detail in model angiosperms. Homologs of these transcription factors are present in streptophyte algae and in land plant lineages where BR signaling or function is absent or has not yet been characterized. The genome of the bryophyte Marchantia polymorpha does not encode for BR receptors but includes one close ortholog of Arabidopsis thaliana BRI1-EMS-SUPPRESSOR 1 (AtBES1)8 and Arabidopsis thaliana BRASSINAZOLE-RESISTANT 1 (AtBZR1),9 MpBES1. Altered levels of MpBES1 severely compromised cell division and differentiation, resulting in stunted thalli that failed to differentiate adult tissues and reproductive organs. The transcriptome of Mpbes1 knockout plants revealed a significant overlap with homologous functions controlled by AtBES1 and AtBZR1, suggesting that members of this gene family share a subset of common targets. Indeed, MpBES1 behaved as a gain-of-function substitute of AtBES1/AtBZR1 when expressed in Arabidopsis, probably because it mediates conserved functions but evades the regulatory mechanisms that native counterparts are subject to. Our results show that this family of transcription factors plays an ancestral role in the control of cell division and differentiation in plants and that BR signaling likely co-opted this function and imposed additional regulatory checkpoints upon it.

To preserve or to destroy, that is the question: the role of the cell wall integrity pathway in pollen tube growth.

In plants, cell-shape is defined by the cell wall, a complex network of polymers located outside the plasma membrane. During cell growth, cell wall properties have to be adjusted, assuring cell expansion without compromising cell integrity. Plasma membrane-located receptors sense cell wall properties, transducing extracellular signals into intracellular cascades through the cell wall integrity (CWI) pathway that, in turn, leads to adjustments in the regulation and composition of the cell wall. Using pollen tube growth as a single celled model system, we describe the importance of RAPID ALKALINIZATION FACTOR (RALF) peptides as sensors of cell wall integrity. RALF peptides can mediate the communication between cell wall components and plasma membrane-localized receptor-like kinases (RLKs) of the CrRLK1L family. The subsequent activation of intracellular pathways regulates H+, Ca2+, and ROS levels in the cell and apoplast, thereby modulating cell wall integrity. Interestingly, the RALF-CrRLK1L module and some of the components working up- and downstream of the RLK is conserved in many other developmental and physiological signaling processes.

Arabidopsis pollen extensins LRX are required for cell wall integrity during pollen tube growth.

Proper cell wall assembly is crucial during pollen tube growth. Leucine-rich repeat extensins (LRXs) are extracellular glycoproteins which belong to the hydroxyproline-rich glycoprotein (HRGP) family. They contain a conserved N-terminal leucine-rich repeat (LRR) domain and a highly variable C-terminal extensin domain. Here, we characterized four LRX proteins (LRX8 through LRX11) from pollen of Arabidopsis thaliana. To investigate the role of LRX8-LRX11 in pollen germination and pollen tube growth, multiple T-DNA lrx mutants were obtained. The lrx mutants display abnormal pollen tubes with an irregular deposition of callose and pectin. They also show serious alterations in pollen germination and segregation ratio. Our results suggest that LRXs are involved in ensuring proper cell wall assembly during pollen tube growth.

Genome polymorphisms and gene differential expression in a 'back-and-forth' ploidy-altered series of weeping lovegrass (Eragrostis curvula).

Molecular markers were used to analyze the genomic structure of an euploid series of Eragrostis curvula, obtained after a tetraploid dihaploidization procedure followed by chromosome re-doubling with colchicine. Considerable levels of genome polymorphisms were detected between lines. Curiously, a significant number of molecular markers showed a revertant behavior following the successive changes of ploidy, suggesting that genome alterations were specific and conferred genetic structures characteristic of a given ploidy level. Genuine reversion was confirmed by sequencing. Cluster analysis demonstrated grouping of tetraploids while the diploid was more distantly related with respect to the rest of the plants. Polymorphic revertant sequences involved mostly non-coding regions, although some of them displayed sequence homology to known genes. A revertant sequence corresponding to a P-type adenosine triphosphatase was found to be differentially represented in cDNA libraries obtained from the diploid and a colchiploid, but was not found expressed in the original tetraploid. Transcriptome profiling of inflorescence followed by real-time polymerase chain reaction validation showed 0.34% polymorphic bands between apomictic tetraploid and sexual diploid plants. Several of the polymorphic sequences corresponded to known genes. Possible correlation between the results observed here and a recently reported genome-wide non-Mendelian inheritance mechanism in Arabidopsis thaliana are discussed.

RALF4/19 peptides interact with LRX proteins to control pollen tube growth in Arabidopsis.

The communication of changes in the extracellular matrix to the interior of the cell is crucial for a cell's function. The extracellular peptides of the RAPID ALKALINIZATION FACTOR (RALF) family have been identified as ligands of receptor-like kinases of the CrRLK1L subclass, but the exact mechanism of their perception is unclear. We found that Arabidopsis RALF4 and RALF19 redundantly regulate pollen tube integrity and growth, and that their function depends on pollen-expressed proteins of the LEUCINE-RICH REPEAT EXTENSIN (LRX) family, which play a role in cell wall development but whose mode of action is not understood. The LRX proteins interact with RALFs, monitoring cell wall changes, which are communicated to the interior of the pollen tube via the CrRLK1L pathway to sustain normal growth.

StRemorin1.3 hampers Potato virus X TGBp1 ability to increase plasmodesmata permeability, but does not interfere with its silencing suppressor activity.

The Triple Gene Block 1 (TGBp1) protein encoded by the Potato virus X is a multifunctional protein that acts as a suppressor of RNA silencing or facilitates the passage of virus from cell to cell by promoting the plasmodesmata opening. We previously showed that the membrane raft protein StRemorin1.3 is able to impair PVX infection. Here, we show that overexpressed StRemorin1.3 does not impair the silencing suppressor activity of TGBp1, but affects its ability to increase plasmodesmata permeability. A similar effect on plasmodesmata permeability was observed with other movement proteins, suggesting that REM is a general regulator of plasmodesmal size exclusion limit. These results add to our knowledge of the mechanisms underlying the StREM1.3 role in virus infection.

Dynamics of chromatin accessibility and gene regulation by MADS-domain transcription factors in flower development.

BACKGROUND: Development of eukaryotic organisms is controlled by transcription factors that trigger specific and global changes in gene expression programs. In plants, MADS-domain transcription factors act as master regulators of developmental switches and organ specification. However, the mechanisms by which these factors dynamically regulate the expression of their target genes at different developmental stages are still poorly understood. RESULTS: We characterized the relationship of chromatin accessibility, gene expression, and DNA binding of two MADS-domain proteins at different stages of Arabidopsis flower development. Dynamic changes in APETALA1 and SEPALLATA3 DNA binding correlated with changes in gene expression, and many of the target genes could be associated with the developmental stage in which they are transcriptionally controlled. We also observe dynamic changes in chromatin accessibility during flower development. Remarkably, DNA binding of APETALA1 and SEPALLATA3 is largely independent of the accessibility status of their binding regions and it can precede increases in DNA accessibility. These results suggest that APETALA1 and SEPALLATA3 may modulate chromatin accessibility, thereby facilitating access of other transcriptional regulators to their target genes. CONCLUSIONS: Our findings indicate that different homeotic factors regulate partly overlapping, yet also distinctive sets of target genes in a partly stage-specific fashion. By combining the information from DNA-binding and gene expression data, we are able to propose models of stage-specific regulatory interactions, thereby addressing dynamics of regulatory networks throughout flower development. Furthermore, MADS-domain TFs may regulate gene expression by alternative strategies, one of which is modulation of chromatin accessibility.

MicroRNA miR396 and RDR6 synergistically regulate leaf development.

The microRNA (miRNA) miR396 regulates GROWTH-REGULATING FACTORs (GRFs), a plant specific family of transcription factors. Overexpression of miR396 causes a decrease in the GRFs that has been shown to affect cell proliferation in the meristem and developing leaves. To bring further insights into the function of the miR396 regulatory network we performed a mutant enhancer screen of a stable Arabidopsis transgenic line expressing 35S:miR396b, which has a reduction in leaf size. From this screen we recovered several mutants enhancing this phenotype and displaying organs with lotus- or needle-like shape. Analysis of these plants revealed mutations in as2 and rdr6. While 35S:miR396b in an as2 context generated organs with lotus-like shape, the overexpression of the miRNA in an rdr6 mutant background caused more important developmental defects, including pin-like organs and lobed leaves. Combination of miR396 overexpressors, and rdr6 and as2 mutants show additional organ defects, suggesting that the three pathways act in concert. Genetic interactions during leaf development were observed in a similar way between miR396 overexpression and mutants in RDR6, SGS3 or AGO7, which are known to participate in trans-acting siRNA (ta-siRNA) biogenesis. Furthermore, we found that miR396 can cause lotus- and pin-like organs per se, once a certain expression threshold is overcome. In good agreement, mutants accumulating high levels of TCP4, which induces miR396, interacted with the AS1/AS2 pathway to generate lotus-like organs. The results indicate that the miR396 regulatory network and the ta-siRNA biogenesis pathway synergistically interact during leaf development and morphogenesis.