The phosphoinositide signature guides the final step of plant cytokinesis.

Plant cytokinesis, which fundamentally differs from that in animals, requires the outward expansion of a plasma membrane precursor named the cell plate. How the transition from a cell plate to a plasma membrane occurs remains poorly understood. Here, we report that the acquisition of plasma membrane identity occurs through lateral patterning of the phosphatidylinositol 4,5-bisphosphate PI(4,5)P2 at the newly formed cell plate membrane. There, the phosphoinositide phosphatase SAC9 emerges as a key regulator, colocalizing with and regulating the function of the microtubule-associated protein MAP65-3 at the cell plate leading zone. In sac9-3 mutant, the polar distribution of PI(4,5)P2 at the cell plate is altered, leading to ectopic recruitment of the cytokinesis apparatus and formation of an additional cell plate insertion site. We propose that at the cell plate, SAC9 drives the depletion of PI(4,5)P2, which acts as a polar cue to spatially separate cell plate expansion from the acquisition of plasma membrane identity during final step of cytokinesis.

Maintaining asymmetry in cell division.

Promoting asymmetric division through microtubule dynamics establishes cell fate.

Connecting the plant cytoskeleton to the cell surface via the phosphoinositides.

Plants have developed fine-tuned cellular mechanisms to respond to a variety of intracellular and extracellular signals. These responses often necessitate the rearrangement of the plant cytoskeleton to modulate cell shape and/or to guide vesicle trafficking. At the cell periphery, both actin filaments and microtubules associate with the plasma membrane that acts as an integrator of the intrinsic and extrinsic environments. At this membrane, acidic phospholipids such as phosphatidic acid, and phosphoinositides contribute to the selection of peripheral proteins and thereby regulate the organization and dynamic of the actin and microtubules. After recognition of the importance of phosphatidic acid on cytoskeleton dynamics and rearrangement, it became apparent that the other lipids might play a specific role in shaping the cytoskeleton. This review focuses on the emerging role of the phosphatidylinositol 4,5-bisphosphate for the regulation of the peripherical cytoskeleton during cellular processes such as cytokinesis, polar growth, biotic and abiotic responses.

SCARECROW-LIKE28 modulates organ growth in Arabidopsis by controlling mitotic cell cycle exit, endoreplication, and cell expansion dynamics.

The processes that contribute to plant organ morphogenesis are spatial-temporally organized. Within the meristem, mitosis produces new cells that subsequently engage in cell expansion and differentiation programs. The latter is frequently accompanied by endoreplication, being an alternative cell cycle that replicates the DNA without nuclear division, causing a stepwise increase in somatic ploidy. Here, we show that the Arabidopsis SCL28 transcription factor promotes organ growth by modulating cell expansion dynamics in both root and leaf cells. Gene expression studies indicated that SCL28 regulates members of the SIAMESE/SIAMESE-RELATED (SIM/SMR) family, encoding cyclin-dependent kinase inhibitors with a role in promoting mitotic cell cycle (MCC) exit and endoreplication, both in response to developmental and environmental cues. Consistent with this role, mutants in SCL28 displayed reduced endoreplication, both in roots and leaves. We also found evidence indicating that SCL28 co-expresses with and regulates genes related to the biogenesis, assembly, and remodeling of the cytoskeleton and cell wall. Our results suggest that SCL28 controls, not only cell proliferation as reported previously but also cell expansion and differentiation by promoting MCC exit and endoreplication and by modulating aspects of the biogenesis, assembly, and remodeling of the cytoskeleton and cell wall.

Potent inhibition of TCP transcription factors by miR319 ensures proper root growth in Arabidopsis.

KEY MESSAGE: Proper root growth depends on the clearance of TCP transcripts from the root apical meristem by microRNA miR319. The evolutionarily conserved microRNA miR319 regulates genes encoding TCP transcription factors in angiosperms. The miR319-TCP module controls cell proliferation and differentiation in leaves and other aerial organs. The current model sustains that miR319 quantitatively tunes TCP activity during leaf growth and development, ultimately affecting its size. In this work we studied how this module participates in Arabidopsis root development. We found that misregulation of TCP activity through impairment of miR319 binding decreased root meristem size and root length. Cellular and molecular analyses revealed that high TCP activity affects cell number and cyclin expression but not mature cell length, indicating that, in roots, unchecking the expression of miR319-regulated TCPs significantly affects cell proliferation. Conversely, tcp multiple mutants showed no obvious effect on root growth, but strong defects in leaf morphogenesis. Therefore, in contrast to the quantitative regulation of the TCPs by miR319 in leaves, our data suggest that miR319 clears TCP transcripts from root cells. Hence, we provide new insights into the functions of the miR319-TCP regulatory system in Arabidopsis development, highlighting a different modus operandi for its action mechanism in roots and shoots.

The Arabidopsis GRAS-type SCL28 transcription factor controls the mitotic cell cycle and division plane orientation.

Gene expression is reconfigured rapidly during the cell cycle to execute the cellular functions specific to each phase. Studies conducted with synchronized plant cell suspension cultures have identified hundreds of genes with periodic expression patterns across the phases of the cell cycle, but these results may differ from expression occurring in the context of intact organs. Here, we describe the use of fluorescence-activated cell sorting to analyze the gene expression profile of G2/M cells in the growing root. To this end, we isolated cells expressing the early mitosis cell cycle marker CYCLINB1;1-GFP from Arabidopsis root tips. Transcriptome analysis of these cells allowed identification of hundreds of genes whose expression is reduced or enriched in G2/M cells, including many not previously reported from cell suspension cultures. From this dataset, we identified SCL28, a transcription factor belonging to the GRAS family, whose messenger RNA accumulates to the highest levels in G2/M and is regulated by MYB3R transcription factors. Functional analysis indicates that SCL28 promotes progression through G2/M and modulates the selection of cell division planes.

Analysis of Expression Gradients of Developmental Regulators in Arabidopsis thaliana Roots.

The regulatory mechanisms involved in plant development include many signals, some of them acting as graded positional cues regulating gene expression in a concentration-dependent manner. These regulatory molecules, that can be considered similar to animal morphogens, control cell behavior in developing organs. A suitable experimental approach to study expression gradients in plants is quantitative laser scanning confocal microscopy (LSCM) using Arabidopsis thaliana root tips as a model system. In this chapter, we outline a detailed method for image acquisition using LSCM, including detailed microscope settings and image analysis using FIJI as software platform.

Robust increase of leaf size by Arabidopsis thaliana GRF3-like transcription factors under different growth conditions.

An increase in crop yield is essential to reassure food security to meet the accelerating global demand. Several genetic modifications can increase organ size, which in turn might boost crop yield. Still, only in a few cases their performance has been evaluated under stress conditions. MicroRNA miR396 repress the expression of GROWTH-REGULATING FACTOR (GRF) genes that codes for transcription factors that promote organ growth. Here, we show that both Arabidopsis thaliana At-GRF2 and At-GRF3 genes resistant to miR396 activity (rGRF2 and rGRF3) increased organ size, but only rGRF3 can produce this effect without causing morphological defects. Furthermore, introduction of At-rGRF3 in Brassica oleracea can increase organ size, and when At-rGRF3 homologs from soybean and rice are introduced in Arabidopsis, leaf size is also increased. This suggests that regulation of GRF3 activity by miR396 is important for organ growth in a broad range of species. Plants harboring rGRF3 have larger leaves also under drought stress, a condition that stimulates miR396 accumulation. These plants also showed an increase in the resistance to virulent bacteria, suggesting that the size increment promoted by rGRF3 occurs without an obvious cost on plant defenses. Our findings indicate that rGRF3 can increase plant organ size under both normal and stress conditions and is a valuable tool for biotechnological applications.

Comparative proteomic and metabolomic studies between Prunus persica genotypes resistant and susceptible to Taphrina deformans suggest a molecular basis of resistance.

The worldwide-distributed leaf peach curl disease is caused by the biotroph Taphrina deformans. To characterize the plant-fungus interaction, resistant and susceptible Prunus persica genotypes grown in the orchard were studied. Asymptomatic leaves were tested for fungal presence. In all resistant leaves analyzed the fungus was not detected. Conversely, leaves from the susceptible genotype were categorized according to the presence or absence of the pathogen. Comparative metabolomic analysis disclosed the metabolite composition associated with resistant and susceptible interactions, and of compounds involved in fungal growth inhibition such as chlorogenic acid, whose in vitro antifungal activity was verified in this work. Differential proteome studies revealed that chloroplasts are important site of plant defense responses against T. deformans. Members of the Bet-v1-like family protein differentially responded to the pathogen. Extracellular pathogenesis-related proteins, evaluated by qRT-PCR, and an enone oxidoreductase are constitutively present in leaves of resistant trees and could be related to fungal resistance. This study is a global view of the changes in the metabolome, proteome and transcripts related to plant defense in naturally infected leaves of susceptible plants during the asymptomatic stage. Additionally, it provides clues to the successful molecular mechanisms operating in resistant plants, which neither develop the disease nor harbor the pathogen.

Unravelling early events in the Taphrina deformans-Prunus persica interaction: an insight into the differential responses in resistant and susceptible genotypes.

Leaf peach curl is a devastating disease affecting leaves, flowers and fruits, caused by the dimorphic fungus Taphrina deformans. To gain insight into the mechanisms of fungus pathogenesis and plant responses, leaves of a resistant and two susceptible Prunus persica genotypes were inoculated with blastospores (yeast), and the infection was monitored during 120 h post inoculation (h.p.i.). Fungal dimorphism to the filamentous form and induction of reactive oxygen species (ROS), callose synthesis, cell death and defence compound production were observed independently of the genotype. Fungal load significantly decreased after 120 h.p.i. in the resistant genotype, while the pathogen tended to grow in the susceptible genotypes. Metabolic profiling revealed a biphasic re-programming of plant tissue in susceptible genotypes, with an initial stage co-incident with the yeast form of the fungus and a second when the hypha is developed. Transcriptional analysis of PRs and plant hormone-related genes indicated that pathogenesis-related (PR) proteins are involved in P. persica defence responses against T. deformans and that salicylic acid is induced in the resistant genotype. Conducted experiments allowed the elucidation of common and differential responses in susceptible versus resistant genotypes and thus allow us to construct a picture of early events during T. deformans infection.