Spliceosomal snRNAs, the Essential Players in pre-mRNA Processing in Eukaryotic Nucleus: From Biogenesis to Functions and Spatiotemporal Characteristics.

Spliceosomal small nuclear RNAs (snRNAs) are a fundamental class of non-coding small RNAs abundant in the nucleoplasm of eukaryotic cells, playing a crucial role in splicing precursor messenger RNAs (pre-mRNAs). They are transcribed by DNA-dependent RNA polymerase II (Pol II) or III (Pol III), and undergo subsequent processing and 3' end cleavage to become mature snRNAs. Numerous protein factors are involved in the transcription initiation, elongation, termination, splicing, cellular localization, and terminal modification processes of snRNAs. The transcription and processing of snRNAs are regulated spatiotemporally by various mechanisms, and the homeostatic balance of snRNAs within cells is of great significance for the growth and development of organisms. snRNAs assemble with specific accessory proteins to form small nuclear ribonucleoprotein particles (snRNPs) that are the basal components of spliceosomes responsible for pre-mRNA maturation. This article provides an overview of the biological functions, biosynthesis, terminal structure, and tissue-specific regulation of snRNAs.

COBL7 is required for stomatal formation via regulation of cellulose deposition in Arabidopsis.

As a key regulator of plant photosynthesis, water use efficiency and immunity, stomata are specialized cellular structures that adopt defined shapes. However, our knowledge about the genetic players of stomatal pore formation and stomatal morphogenesis remains limited. Forward genetic screening, positional cloning, confocal and electron microscopy, physiological and pharmacological assays were employed for isolation and characterization of mutants and genes. We identified a mutant, dsm1, with impaired cytokinesis and deformed stomata. DSM1 is highly expressed in guard mother cells and guard cells, and encodes COBRA-LIKE 7 (COBL7), a plant-specific glycosylphosphatidylinositol (GPI)-anchored protein. COBRA-LIKE 7 and its closest homologue, COBL8, are first enriched on the forming cell plates during cytokinesis, and then their subcellular distribution and abundance change are correlated with the progressive stages of stomatal pore formation. Both COBL7 and COBL8 possess an ability to bind cellulose. Perturbing the expression of COBL7 and COBL8 leads to a decrease in cellulose content and inhibition of stomatal pore development. Moreover, we found that COBL7, COBL8 and CSLD5 have synergistic effects on stomatal development and plant growth. Our findings reveal that COBL7 plays a predominant and functionally redundant role with COBL8 in stomatal formation through regulating cellulose deposition and ventral wall modification in Arabidopsis.

The Arabidopsis Rab protein RABC1 affects stomatal development by regulating lipid droplet dynamics.

Lipid droplets (LDs) are evolutionarily conserved organelles that serve as hubs of cellular lipid and energy metabolism in virtually all organisms. Mobilization of LDs is important in light-induced stomatal opening. However, whether and how LDs are involved in stomatal development remains unknown. We show here that Arabidopsis thaliana LIPID DROPLETS AND STOMATA 1 (LDS1)/RABC1 (At1g43890) encodes a member of the Rab GTPase family that is involved in regulating LD dynamics and stomatal morphogenesis. The expression of RABC1 is coordinated with the different phases of stomatal development. RABC1 targets to the surface of LDs in response to oleic acid application in a RABC1GEF1-dependent manner. RABC1 physically interacts with SEIPIN2/3, two orthologues of mammalian seipin, which function in the formation of LDs. Disruption of RABC1, RABC1GEF1, or SEIPIN2/3 resulted in aberrantly large LDs, severe defects in guard cell vacuole morphology, and stomatal function. In conclusion, these findings reveal an aspect of LD function and uncover a role for lipid metabolism in stomatal development in plants.

COBL9 and COBL7 synergistically regulate root hair tip growth via controlling apical cellulose deposition.

Root hairs are cylindrical extensions of root epidermal cells that are important for the acquisition of water and minerals, interactions between plant and microbes. The deposition of cell wall materials in the tip enables root hairs to maintain elongation constantly. To date, our knowledge of the regulators that connect the architecture of cell wall and the root hair development remains very limited. Here, we demonstrated that COBL9 and COBL7, two genes of COBRA-Like family in Arabidopsis as well as their counterparts in rice, OsBC1L1 and OsBC1L8, regulate root hair growth. Single mutant cobl9, double mutants cobl7 cobl9 and double mutants osbc1l1 osbc1l8 all displayed prematurely terminated root hair elongation, though at varying levels. COBL7-YFP and COBL9-YFP accumulate prominently in the growing tips of newly emerged root hairs. Furthermore, cobl9, cobl7 cobl9 and osbc1l1 osbc1l8 mutants were defective in the enrichment of cellulose in the tips of the growing root hairs. We also discovered that overexpression of COBL9 could promote root hair elongation and salinity tolerance. Taken together, these results provide compelling evidence that the polarized COBL7 and COBL9 in the tip of the emerging root hairs have conserved roles in regulating root hair development and stress adaptation in dicots and monocots.

OsBC1L1 and OsBC1L8 function in stomatal development in rice.

Stomata that are bordered by pairs of guard cells are specialized for regulating gas exchange and transpiration in plants. The stomatal morphology of grass is unique, characterized by two dumbbell-shaped guard cells flanked by two lateral subsidiary cells. This morphology and developmental pattern enable grass stomata to respond to environmental signals efficiently. In this study, we demonstrated that knockout either OsBC1L1 or OsBC1L8, two close homologs of OsBC1L family causes no discernible defects in rice stomatal development, however, the double knockout mutant osbc1l1 osbc1l8 exhibits excess stomatal production and stomatal clustering. OsBC1L1 overexpression also causes abnormal stomatal patterning in rice. Moreover, osbc1l1 osbc1l8 has many defective stomata complexes with only one subsidiary cell. The expression of OsSPCH2 and OsFAMA, two genes key to stomatal development is both down-regulated in osbc1l1 osbc1l8. In contrast, overexpressing OsBC1L1 suppresses only the expression of OsSPCH2. Both OsBC1L1 and OsBC1L8 could be detected to be localized at the cell plate and plasma membrane during cell division of guard mother cells and subsidiary mother cells. Taken together, these results suggest that OsBC1L1 and OsBC1L8 play essential roles in the development of rice stomatal complex likely through their involvement in cell reproduction.

BIG regulates stomatal immunity and jasmonate production in Arabidopsis.

Plants have evolved an array of responses that provide them with protection from attack by microorganisms and other predators. Many of these mechanisms depend upon interactions between the plant hormones jasmonate (JA) and ethylene (ET). However, the molecular basis of these interactions is insufficiently understood. Gene expression and physiological assays with mutants were performed to investigate the role of Arabidopsis BIG gene in stress responses. BIG transcription is downregulated by methyl JA (MeJA), necrotrophic infection or mechanical injury. BIG deficiency promotes JA-dependent gene induction, increases JA production but restricts the accumulation of both ET and salicylic acid. JA-induced anthocyanin accumulation and chlorophyll degradation are enhanced and stomatal immunity is impaired by BIG disruption. Bacteria- and lipopolysaccaride (LPS)-induced stomatal closure is reduced in BIG gene mutants, which are hyper-susceptible to microbial pathogens with different lifestyles, but these mutants are less attractive to phytophagous insects. Our results indicate that BIG negatively and positively regulate the MYC2 and ERF1 arms of the JA signalling pathway. BIG warrants recognition as a new and distinct regulator that regulates JA responses, the synergistic interactions of JA and ET, and other hormonal interactions that reconcile the growth and defense dilemma in Arabidopsis.

Sphingosine kinase AtSPHK1 functions in fumonisin B1-triggered cell death in Arabidopsis.

The fungal toxin Fumonisin B1 (FB1) is a strong inducer to trigger plant hypersensitive responses (HR) along with increased long chain bases (LCB) and long chain base phosphates (LCBP) contents, though the regulatory mechanism of FB1 action and how the LCB/LCBP signalling cassette functions during the process is still not fully understood. Here, we report sphingosine kinase 1 (SPHK1) as a key factor in FB1-induced HR by modulating the salicylic acid (SA) pathway and reactive oxygen species (ROS) accumulation in Arabidopsis thaliana. Overexpression of SPHK1 increases the FB1-induced accumulations of ROS and SA. The double mutant that simultaneously overexpresses SPHK1 and suppresses the SPPASE or DPL1, two enzymes are mainly responsible for Phyto-sphingosine-1-phosphate (Phyto-S1P) removal, showed enhanced susceptibility to FB1 killing and FB1-induced SA activation than the plants overexpress SPHK1 alone. Exogenous sphingosine-1-phosphate (S1P) can modulate the transcription of the SA-responsive marker gene PR1 in a concentration-dependent biphasic manner. Suppression of SPHK1 decreases SA production whereas promotes jasmonic acid (JA) biosynthesis in response to FB1 applications. Our findings indicate a role of SPHK1 in modulating FB1-triggered cell death via SA and JA pathway interactions.