Differential Interaction between RAC/ROP-GTPases and RIC-Effectors: A Network Hub for Broader Signal Transduction in Pollen Tubes

To date knowledge about plant RAC/ROP-GTPase effectors and downstream targets is still limited. This work aims on elucidation of related signal transduction networks involved in pollen tube growth. Yeast two-hybrid and Pull Down methodology were used to identify and characterize hitherto unknown components of RAC-related protein complexes from Nicotiana tabacum. Nt-RIC11pt specifically interacts with diverse active tobacco RAC-GTPases, and it is particularly significant, that their binding affinity is differential, thus implicating a multifaceted role in an interconnected RIC-RAC network. Moreover, Y2H-screening for Nt-RIC11pt targets identified Nt-CAR4, which is phylogenetically assigned to a multifaceted family of novel unusual GTPase activating proteins (GAP). It is argued that scaffold Nt-RIC11pt connects active Nt-RAC3 with membrane-bound Nt-CAR4, thus relaying GAP-activity. Quantitative RT-PCR demonstrates Nt-RIC11pt is primarily expressed in pollen and YFP-fusion proteins show homogeneous cytoplasmic localization in growing tubes, what builds the prerequisite for a proposed role in broader signal transduction. By synoptically integrating experimental data, bioinformatic sequence comparison, phylogenetic analyses, and detailed literature review, this study hypothesizes a concept in which RIC-effectors collectively constitute a multifaceted network hub linking diverse GTPase-dependent processes.

Exocytosis and endocytosis: coordinating and fine-tuning the polar tip growth domain in pollen tubes

Pollen tubes rapidly elongate, penetrate, and navigate through multiple female tissues to reach ovules for sperm delivery by utilizing a specialized form of polar growth known as tip growth. This process requires a battery of cellular activities differentially occurring at the apical growing region of the plasma membrane (PM), such as the differential cellular signaling involving calcium (Ca2+), phospholipids, and ROP-type Rho GTPases, fluctuation of ions and pH, exocytosis and endocytosis, and cell wall construction and remodeling. There is an emerging understanding of how at least some of these activities are coordinated and/or interconnected. The apical active ROP modulates exocytosis to the cell apex for PM and cell wall expansion differentially occurring at the tip. The differentiation of the cell wall involves at least the preferential distribution of deformable pectin polymers to the apex and non-deformable pectin polymers to the shank of pollen tubes, facilitating the apical cell expansion driven by high internal turgor pressure. Recent studies have generated inroads into how the ROP GTPase-based intracellular signaling is coordinated spatiotemporally with the external wall mechanics to maintain the tubular cell shape and how the apical cell wall mechanics are regulated to allow rapid tip growth while maintaining the cell wall integrity under the turgor pressure. Evidence suggests that exocytosis and endocytosis play crucial but distinct roles in this spatiotemporal coordination. In this review, we summarize recent advances in the regulation and coordination of the differential pectin distribution and the apical domain of active ROP by exocytosis and endocytosis in pollen tubes.

The Rho-family GTPase OsRac1 controls rice grain size and yield by regulating cell division

Grain size is a key factor for determining grain yield in crops and is a target trait for both domestication and breeding, yet the mechanisms underlying the regulation of grain size are largely unclear. Here we show that the grain size and yield of rice (Oryza sativa) is positively regulated by ROP GTPase (Rho-like GTPase from plants), a versatile molecular switch modulating plant growth, development, and responses to the environment. Overexpression of rice OsRac1ROP not only increases cell numbers, resulting in a larger spikelet hull, but also accelerates grain filling rate, causing greater grain width and weight. As a result, OsRac1 overexpression improves grain yield in O. sativa by nearly 16%. In contrast, down-regulation or deletion of OsRac1 causes the opposite effects. RNA-seq and cell cycle analyses suggest that OsRac1 promotes cell division. Interestingly, OsRac1 interacts with and regulates the phosphorylation level of OsMAPK6, which is known to regulate cell division and grain size in rice. Thus, our findings suggest OsRac1 modulates rice grain size and yield by influencing cell division. This study provides insights into the molecular mechanisms underlying the control of rice grain size and suggests that OsRac1 could serve as a potential target gene for breeding high-yield crops.

Barley ROP-INTERACTIVE PARTNER-a organizes into RAC1- and MICROTUBULE-ASSOCIATED ROP-GTPASE ACTIVATING PROTEIN 1-dependent membrane domains

Small ROP (also called RAC) GTPases are key factors in polar cell development and in interaction with the environment. ROP-Interactive Partner (RIP) proteins are predicted scaffold or ROP-effector proteins, which function downstream of activated GTP-loaded ROP proteins in establishing membrane heterogeneity and cellular organization. Grass ROP proteins function in cell polarity, resistance and susceptibility to fungal pathogens but grass RIP proteins are little understood.We found that the barley (Hordeum vulgare L.) RIPa protein can interact with barley ROPs in yeast. Fluorescent-tagged RIPa, when co-expressed with the constitutively activated ROP protein CA RAC1, accumulates at the cell periphery or plasma membrane. Additionally, RIPa, locates into membrane domains, which are laterally restricted by microtubules, when co-expressed with RAC1 and MICROTUBULE-ASSOCIATED ROP-GTPASE ACTIVATING PROTEIN 1. Both structural integrity of MICROTUBULE-ASSOCIATED ROP-GTPASE ACTIVATING PROTEIN 1 and microtubule stability are key to maintenance of RIPa-labeled membrane domains. In this context, RIPa also accumulates at the interface of barley and invading hyphae of the powdery mildew fungus Blumeria graminis f.sp. hordei.Data suggest that barley RIPa interacts with barley ROPs and specifies RAC1 activity-associated membrane domains with potential signaling capacity. Lateral diffusion of this RAC1 signaling capacity is restricted the resulting membrane heterogeneity requires intact microtubules and MICROTUBULE-ASSOCIATED ROP-GTPASE ACTIVATING PROTEIN 1. Focal accumulation of RIPa at sites of fungal attack may indicate locally restricted ROP activity at sites of fungal invasion.

FERONIA’s sensing of cell wall pectin activates ROP GTPase signaling in Arabidopsis

ABSTRACTPlant cells need to monitor the cell wall dynamic to control the wall homeostasis required for a myriad of processes in plants, but the mechanisms underpinning cell wall sensing and signaling in regulating these processes remain largely elusive. Here, we demonstrate that receptor-like kinase FERONIA senses the cell wall pectin polymer to directly activate the ROP6 GTPase signaling pathway that regulates the formation of the cell shape in the Arabidopsis leaf epidermis. The extracellular malectin domain of FER directly interacts with de-methylesterified pectin in vivo and in vitro. Both loss-of-FER mutations and defects in the pectin biosynthesis and de-methylesterification caused changes in pavement cell shape and ROP6 signaling. FER is required for the activation of ROP6 by de-methylesterified pectin, and physically and genetically interacts with the ROP6 activator, RopGEF14. Thus, our findings elucidate a cell wall sensing and signaling mechanism that connects the cell wall to cellular morphogenesis via the cell surface receptor FER.

A Barley Powdery Mildew Fungus Non-Autonomous Retrotransposon Encodes a Peptide that Supports Penetration Success on Barley

ABSTRACTPlant immunity is overcome by pathogens by the means of secreted effectors. Host effector targets might be proteins acting in pathogen defense or serve demands of the pathogen. The barley ROP GTPase HvRACB is involved in entry of the powdery mildew fungusBlumeria graminisf.sp.hordei (Bgh)into barley epidermal cells. We found that HvRACB interacts with theROP-interactive peptide 1 (ROPIP1) that is encoded on the active non-long terminal repeat retroelement Eg-R1 ofBgh. Over-expression of ROPIP1 in barley epidermal cells and host-induced post-transcriptional gene silencing (HIGS) ofROPIP1suggested that ROPIP1 is involved in virulence ofBgh. Bimolecular fluorescence complementation and co-localization supported that ROPIP1 can interact with activated HvRACB in planta. We show that ROPIP1 is expressed byBghon barley and translocated into the cytoplasm of infected barley cells. ROPIP1 is recruited to microtubules upon co-expression of MICROTUBULE ASSOCIATED ROP GTPase ACTIVATING PROTEIN (HvMAGAP1) and can destabilize cortical microtubules.BghROPIP might target HvRACB and manipulate host cell microtubule organization for facilitated host cell entry. Data suggest a possible neo-functionalization of retroelement-derived transcripts for the evolution of a pathogen virulence effector.