Ethylene signaling increases reactive oxygen species accumulation to drive root hair initiation in Arabidopsis.

Root hair initiation is a highly regulated aspect of root development. The plant hormone ethylene and its precursor, 1-amino-cyclopropane-1-carboxylic acid, induce formation and elongation of root hairs. Using confocal microscopy paired with redox biosensors and dyes, we demonstrated that treatments that elevate ethylene levels lead to increased hydrogen peroxide accumulation in hair cells prior to root hair formation. In the ethylene-insensitive receptor mutant, etr1-3, and the signaling double mutant, ein3eil1, the increase in root hair number or reactive oxygen species (ROS) accumulation after ACC and ethylene treatment was lost. Conversely, etr1-7, a constitutive ethylene signaling receptor mutant, has increased root hair formation and ROS accumulation, similar to ethylene-treated Col-0 seedlings. The caprice and werewolf transcription factor mutants have decreased and elevated ROS levels, respectively, which are correlated with levels of root hair initiation. The rhd2-6 mutant, with a defect in the gene encoding the ROS-synthesizing RESPIRATORY BURST OXIDASE HOMOLOG C (RBOHC), and the prx44-2 mutant, which is defective in a class III peroxidase, showed impaired ethylene-dependent ROS synthesis and root hair formation via EIN3EIL1-dependent transcriptional regulation. Together, these results indicate that ethylene increases ROS accumulation through RBOHC and PRX44 to drive root hair formation.

Filling the gaps on root hair development under salt stress and phosphate starvation using current evidence coupled with a meta-analysis approach.

Population expansion is a global issue, especially for food production. Meanwhile, global climate change is damaging our soils, making it difficult for crops to thrive and lowering both production and quality. Poor nutrition and salinity stress affect plant growth and development. Although the impact of individual plant stresses has been studied for decades, the real stress scenario is more complex due to the exposure to multiple stresses at the same time. Here we investigate using existing evidence and a meta-analysis approach to determine molecular linkages between two contemporaneous abiotic stimuli, phosphate (Pi) deficiency and salinity, on a single plant cell model, the root hairs (RHs), which is the first plant cell exposed to them. Understanding how these two stresses work molecularly in RHs may help us build super-adaptable crops and sustainable agriculture in the face of global climate change.

Transcription factor NAC1 activates expression of peptidase-encoding AtCEPs in roots to limit root hair growth.

Plant genomes encode a unique group of papain-type Cysteine EndoPeptidases (CysEPs) containing a KDEL endoplasmic reticulum (ER) retention signal (KDEL-CysEPs or CEPs). CEPs process the cell-wall scaffolding EXTENSIN (EXT) proteins that regulate de novo cell-wall formation and cell expansion. Since CEPs cleave EXTs and EXT-related proteins, acting as cell-wall-weakening agents, they may play a role in cell elongation. The Arabidopsis (Arabidopsis thaliana) genome encodes 3 CEPs (AtCPE1-AtCEP3). Here, we report that the genes encoding these 3 Arabidopsis CEPs are highly expressed in root-hair (RH) cell files. Single mutants have no evident abnormal RH phenotype, but atcep1-3 atcep3-2 and atcep1-3 atcep2-2 double mutants have longer RHs than wild-type (Wt) plants, suggesting that expression of AtCEPs in root trichoblasts restrains polar elongation of the RH. We provide evidence that the transcription factor NAC1 (petunia NAM and Arabidopsis ATAF1, ATAF2, and CUC2) activates AtCEPs expression in roots to limit RH growth. Chromatin immunoprecipitation indicates that NAC1 binds to the promoter of AtCEP1, AtCEP2, and, to a lower extent, AtCEP3 and may directly regulate their expression. Inducible NAC1 overexpression increases AtCEP1 and AtCEP2 transcript levels in roots and leads to reduced RH growth while the loss of function nac1-2 mutation reduces AtCEP1-AtCEP3 gene expression and enhances RH growth. Likewise, expression of a dominant chimeric NAC1-SRDX repressor construct leads to increased RH length. Finally, we show that RH cell walls in the atcep1-3 atcep3-2 double mutant have reduced levels of EXT deposition, suggesting that the defects in RH elongation are linked to alterations in EXT processing and accumulation. Our results support the involvement of AtCEPs in controlling RH polar growth through EXT processing and insolubilization at the cell wall.

Exploring the puzzle of Reactive Oxygen Species (ROS) acting on root hair cells.

Reactive oxygen species (ROS) are essential signaling molecules that enable cells to respond rapidly to a range of stimuli. The capacity of plants to recognize various stressors, incorporate a variety of environmental inputs, and initiate stress-response networks depends on ROS. Plants develop resilience and defensive systems as a result of these processes. Root hairs (RHs) are central components of the root biology since they increase the surface area of the root, anchor it in the soil, increase its ability to absorb water and nutrients, and foster interactions between microorganisms. In this review, we specifically focused on RHs cells and we highlighted the identification of ROS receptors, important new regulatory hubs that connect ROS production, transport, and signaling in the context of two hormonal pathways (auxin and ethylene) and under low temperature environmental input related to nutrients. As ROS plays a crucial role in regulating cell elongation rates, RHs are rapidly gaining traction as a very valuable single plant cell model for investigating ROS homeostasis and signaling. These promising findings might soon aid in the development of plants and roots that are more resilient to environmental stressors.

Arabidopsis pollen prolyl-hydroxylases P4H4/6 are relevant for correct hydroxylation and secretion of LRX11 in pollen tubes.

Major constituents of the plant cell walls are structural proteins that belong to the hydroxyproline-rich glycoprotein (HRGP) family. Leucine-rich repeat extensin (LRXs) proteins contain a leucine-rich domain and a C-terminal domain with repetitive Ser-Pro(3-5) motifs that are potentially to be O-glycosylated. It has been demonstrated that pollen-specific LRX8-11 from Arabidopsis thaliana are necessary to maintain the integrity of the pollen tube cell wall during polarized growth. In HRGP including classical extensins (EXTs) and likely in LRXs, proline residues are converted to hydroxyproline by prolyl-4-hydroxylases (P4Hs), thus defining novel O-glycosylation sites. In this context, we aimed to determine whether hydroxylation and subsequent O-glycosylation of Arabidopsis pollen LRXs are necessary for their proper function and cell wall localization in pollen tubes. We hypothesized that pollen-expressed P4H4 and P4H6 catalyze the hydroxylation of the proline units present in Ser-Pro3-5 motifs of LRX8-LRX11. Here, we show the p4h4-1 p4h6-1 double mutant exhibits a reduction in pollen germination rates and a slight reduction in pollen tube length. Pollen germination is also inhibited by P4Hs inhibitors, suggesting that prolyl hydroxylation is required for pollen tube development. Plants expressing pLRX11::LRX11-GFP in the p4h4-1 p4h6-1 background show partial re-localization of LRX11-GFP from the pollen tube tip apoplast to the cytoplasm. Finally, IP-MS-MS analysis revealed a decrease in oxidized prolines (hydroxyprolines) in LRX11-GFP in the p4h4-1 p4h6-1 background compared to lrx11 plants expressing pLRX11::LRX11-GFP. Taken together, these results suggest P4H4 and P4H6 are required for pollen germination and for proper hydroxylation of LRX11 necessary for its localization at the cell wall of pollen tubes.

Cell surface receptor kinase FERONIA linked to nutrient sensor TORC signaling controls root hair growth at low temperature linked to low nitrate in Arabidopsis thaliana.

Root hairs (RH) are excellent model systems for studying cell size and polarity since they elongate several hundred-fold their original size. Their tip growth is determined both by intrinsic and environmental signals. Although nutrient availability and temperature are key factors for a sustained plant growth, the molecular mechanisms underlying their sensing and downstream signaling pathways remain unclear. We use genetics to address the roles of the cell surface receptor kinase FERONIA (FER) and the nutrient sensing TOR Complex 1 (TORC) in RH growth. We identified that low temperature (10°C) triggers a strong RH elongation response in Arabidopsis thaliana involving FER and TORC. We found that FER is required to perceive limited nutrient availability caused by low temperature. FERONIA interacts with and activates TORC-downstream components to trigger RH growth. In addition, the small GTPase Rho of plants 2 (ROP2) is also involved in this RH growth response linking FER and TOR. We also found that limited nitrogen nutrient availability can mimic the RH growth response at 10°C in a NRT1.1-dependent manner. These results uncover a molecular mechanism by which a central hub composed by FER-ROP2-TORC is involved in the control of RH elongation under low temperature and nitrogen deficiency.

High saturation photocurrent THz waveguide-type MUTC-photodiodes reaching mW output power within the WR3.4 band.

In this paper, we report on waveguide-type modified uni-traveling-carrier photodiodes (MUTC-PDs) providing a record high output power level for non-resonant photodiodes in the WR3.4 band. Indium phosphide (InP) based waveguide-type 1.55 µm MUTC-PDs have been fabricated and characterized thoroughly. Maximum output powers of -0.6 dBm and -2.7 dBm were achieved at 240 GHz and 280 GHz, respectively. This has been accomplished by an optimized layer structure and doping profile design that takes transient carrier dynamics into account. An energy-balance model has been developed to study and optimize carrier transport at high optical input intensities. The advantageous THz capabilities of the optimized MUTC layer structure are confirmed by experiments revealing a transit time limited cutoff frequency of 249 GHz and a saturation photocurrent beyond 20 mA in the WR3.4 band. The responsivity for a 16 µm long waveguide-type THz MUTC-PD is found to be 0.25 A/W. In addition, bow-tie antenna integrated waveguide-type MUTC-PDs are fabricated and reported to operate up to 0.7 THz above a received power of -40 dBm.

New molecular components that regulates the transcriptional hub in root hairs: coupling environmental signals to endogenous hormones to coordinate growth.

Root hairs (RH) have become an important model system for studying plant growth and how plants modulate their growth in response to cell-intrinsic and environmental stimuli. Here, we will discuss recent advances in our understanding of the molecular mechanisms underlying the growth of Arabidopsis thaliana RH in the interface between responses to environmental cues (e.g. nutrients such as nitrates, phosphate and microorganism) and hormonal stimuli (e.g. auxin). RH growth is under the control of several transcription factors that are also under strong regulation at different levels. In this review we highlight recent new discoveries along these transcriptional pathways that may increase our capacity to enhance nutrient uptake by the roots in the context of abiotic stresses. We used text-mining capacities of the PlantConnectome database to generate the most updated view of RH growth in these complex biological contexts.

Terahertz photodiode integration with multi-octave-bandwidth dielectric rod waveguide probe.

Photonic integrated circuits play a vital role in enabling terahertz (THz) applications that require multi-octave bandwidth. Prior research has been limited in bandwidth due to rectangular waveguide (WRs) interconnects, which can only support single octave at low loss. To overcome this fundamental limitation, we exploit the ultra-wideband (UWB) near-field coupling between planar waveguides and silicon (Si)-based subwavelength dielectric rod waveguides (DRWs) to interconnect THz bandwidth uni-traveling-carrier photodiodes (UTC-PDs) at 0.08-1.03 THz. In a proof-of-concept experiment, the on-chip integrated UTC-PDs demonstrate a UWB operation from 0.1 THz to 0.4 THz. Furthermore, by employing Si DRWs as probes, multi-octave device-under-test characterization of UTC-PDs integrated with UWB transition is enabled with only one DRW probe. The proposed UWB interconnect technology is distinct from previously used WR-based ground-signal-ground probes or quasi-optical free-space coupling since it can provide multi-octave bandwidth and enable on-chip THz circuit integration.

Sphenoid sinus mucocele causing ptosis with pupil-spared ophthalmoplegia: a hint on carotid artery doppler ultrasound.

OBJECTIVE: Sphenoid sinuses mucocele (SSM) is an uncommon cause of orbital apex syndrome (OAS). Diagnosis of neurological complications in SSM might be delayed when the expansion of mucocele beyond the sinuses is not evident in conventional sinuses imaging. METHODS: We present a case of a 76-years old man with spared-pupil ophthalmoplegia associated with ptosis caused by a unilateral left SSM in which internal carotid artery Doppler ultrasound showed distal sub-occlusion waves pattern. RESULTS: Sinus occupation was noted in the magnetic resonance imaging (MRI) and was further evaluated in computed tomography (CT) scan and MR angiography. Nor CT or MR angiography showed clear evidence of neighboring structures compression. Doppler ultrasound of internal carotid showed high-resistance waveforms and decreased wave velocities helping diagnosis. Structures compression was confirmed intra-operatively and the patient was discharged asymptomatic after sphenoid sinus drainage. CONCLUSION: In this first report of carotid Doppler ultrasound findings in a patient with a neurological presentation of a sphenoid sinus mucocele, a high-resistance waveform of the internal carotid may help differentiate uncomplicated sinusitis from invasive mucocele.

In-Depth Evaluation of Saliency Maps for Interpreting Convolutional Neural Network Decisions in the Diagnosis of Glaucoma Based on Fundus Imaging.

Glaucoma, a leading cause of blindness, damages the optic nerve, making early diagnosis challenging due to no initial symptoms. Fundus eye images taken with a non-mydriatic retinograph help diagnose glaucoma by revealing structural changes, including the optic disc and cup. This research aims to thoroughly analyze saliency maps in interpreting convolutional neural network decisions for diagnosing glaucoma from fundus images. These maps highlight the most influential image regions guiding the network's decisions. Various network architectures were trained and tested on 739 optic nerve head images, with nine saliency methods used. Some other popular datasets were also used for further validation. The results reveal disparities among saliency maps, with some consensus between the folds corresponding to the same architecture. Concerning the significance of optic disc sectors, there is generally a lack of agreement with standard medical criteria. The background, nasal, and temporal sectors emerge as particularly influential for neural network decisions, showing a likelihood of being the most relevant ranging from 14.55% to 28.16% on average across all evaluated datasets. We can conclude that saliency maps are usually difficult to interpret and even the areas indicated as the most relevant can be very unintuitive. Therefore, its usefulness as an explanatory tool may be compromised, at least in problems such as the one addressed in this study, where the features defining the model prediction are generally not consistently reflected in relevant regions of the saliency maps, and they even cannot always be related to those used as medical standards.

Gaussian processes retrieval of crop traits in Google Earth Engine based on Sentinel-2 top-of-atmosphere data.

The unprecedented availability of optical satellite data in cloud-based computing platforms, such as Google Earth Engine (GEE), opens new possibilities to develop crop trait retrieval models from the local to the planetary scale. Hybrid retrieval models are of interest to run in these platforms as they combine the advantages of physically- based radiative transfer models (RTM) with the flexibility of machine learning regression algorithms. Previous research with GEE primarily relied on processing bottom-of-atmosphere (BOA) reflectance data, which requires atmospheric correction. In the present study, we implemented hybrid models directly into GEE for processing Sentinel-2 (S2) Level-1C (L1C) top-of-atmosphere (TOA) reflectance data into crop traits. To achieve this, a training dataset was generated using the leaf-canopy RTM PROSAIL in combination with the atmospheric model 6SV. Gaussian process regression (GPR) retrieval models were then established for eight essential crop traits namely leaf chlorophyll content, leaf water content, leaf dry matter content, fractional vegetation cover, leaf area index (LAI), and upscaled leaf variables (i.e., canopy chlorophyll content, canopy water content and canopy dry matter content). An important pre-requisite for implementation into GEE is that the models are sufficiently light in order to facilitate efficient and fast processing. Successful reduction of the training dataset by 78% was achieved using the active learning technique Euclidean distance-based diversity (EBD). With the EBD-GPR models, highly accurate validation results of LAI and upscaled leaf variables were obtained against in situ field data from the validation study site Munich-North-Isar (MNI), with normalized root mean square errors (NRMSE) from 6% to 13%. Using an independent validation dataset of similar crop types (Italian Grosseto test site), the retrieval models showed moderate to good performances for canopy-level variables, with NRMSE ranging from 14% to 50%, but failed for the leaf-level estimates. Obtained maps over the MNI site were further compared against Sentinel-2 Level 2 Prototype Processor (SL2P) vegetation estimates generated from the ESA Sentinels' Application Platform (SNAP) Biophysical Processor, proving high consistency of both retrievals (R 2 from 0.80 to 0.94). Finally, thanks to the seamless GEE processing capability, the TOA-based mapping was applied over the entirety of Germany at 20 m spatial resolution including information about prediction uncertainty. The obtained maps provided confidence of the developed EBD-GPR retrieval models for integration in the GEE framework and national scale mapping from S2-L1C imagery. In summary, the proposed retrieval workflow demonstrates the possibility of routine processing of S2 TOA data into crop traits maps at any place on Earth as required for operational agricultural applications.

Class III Peroxidases in Response to Multiple Abiotic Stresses in Arabidopsis thaliana Pyrenean Populations.

Class III peroxidases constitute a plant-specific multigene family, where 73 genes have been identified in Arabidopsis thaliana. These genes are members of the reactive oxygen species (ROS) regulatory network in the whole plant, but more importantly, at the root level. In response to abiotic stresses such as cold, heat, and salinity, their expression is significantly modified. To learn more about their transcriptional regulation, an integrative phenotypic, genomic, and transcriptomic study was executed on the roots of A. thaliana Pyrenean populations. Initially, the root phenotyping highlighted 3 Pyrenean populations to be tolerant to cold (Eaux), heat (Herr), and salt (Grip) stresses. Then, the RNA-seq analyses on these three populations, in addition to Col-0, displayed variations in CIII Prxs expression under stressful treatments and between different genotypes. Consequently, several CIII Prxs were particularly upregulated in the tolerant populations, suggesting novel and specific roles of these genes in plant tolerance against abiotic stresses.

Class III Peroxidases PRX01, PRX44, and PRX73 Control Root Hair Growth in Arabidopsis thaliana.

Root hair cells are important sensors of soil conditions. They grow towards and absorb water-soluble nutrients. This fast and oscillatory growth is mediated by continuous remodeling of the cell wall. Root hair cell walls contain polysaccharides and hydroxyproline-rich glycoproteins, including extensins (EXTs). Class-III peroxidases (PRXs) are secreted into the apoplastic space and are thought to trigger either cell wall loosening or polymerization of cell wall components, such as Tyr-mediated assembly of EXT networks (EXT-PRXs). The precise role of these EXT-PRXs is unknown. Using genetic, biochemical, and modeling approaches, we identified and characterized three root-hair-specific putative EXT-PRXs, PRX01, PRX44, and PRX73. prx01,44,73 triple mutation and PRX44 and PRX73 overexpression had opposite effects on root hair growth, peroxidase activity, and ROS production, with a clear impact on cell wall thickness. We use an EXT fluorescent reporter with contrasting levels of cell wall insolubilization in prx01,44,73 and PRX44-overexpressing background plants. In this study, we propose that PRX01, PRX44, and PRX73 control EXT-mediated cell wall properties during polar expansion of root hair cells.

Morphometric and molecular characterization of Kudoa encrasicoli n. sp. (Myxozoa: Myxosporea) from the European anchovy, Engraulis encrasicolus (L.) (Clupeiformes: Engraulidae).

The European anchovy represents the main fisheries for countries in the Mediterranean and Black Sea basins. The skeletal muscle of 13 of 48 (27.1%) Engraulis encrasicolus (L.) specimens from North East Atlantic waters (FAO 27.8.c) was found infected with interfibrillar elongated plasmodia (130-980 µm in length) containing mature myxospores belonging to the genus Kudoa Meglitsch, 1947. No flesh softening was found associated with infection. Fresh myxospores were 10.8 ± 0.7 (9.1-12.3) µm in width 1, 11.3 ± 0.9 (9.5-13.4) µm in width 2, 6.7 ± 0.4 (5.8-7.4) µm in thickness, and 6.9 ± 0.5 (5.8-7.5) µm in length. They were almost stellate in apical view having three pointed-edged shell valves bearing three small polar capsules equal in size 5.0 ± 0.3 (4.4-5.4) µm long and 2.4 ± 0.2 (2.0-3.0) µm wide, and one rounded- to rarely bluntly pointed-edged shell valve bearing a large and particularly wide polar capsule 6.8 ± 0.4 (5.9-7.6) µm long and 4.1 ± 0.2 (3.6-4.4) µm wide. Morphological and morphometrical comparisons between these myxospores and those of Kudoa thyrsites (Gilchrist, 1923) from the clupeid Sardina pilchardus (Walbaum) (North East Atlantic waters, FAO 27.9.a), with which exhibited a similarity of 98.9% and 96.2% using SSU and LSU rDNA sequences, respectively, support the creation of Kudoa encrasicoli n. sp. Morphometrical analysis of the polar capsules of flattened myxospores is suggested as a useful approach to differentiate phylogenetically related kudoids with stellate or almost stellate myxospores bearing four polar capsules.

A Stringent-Response-Defective Bradyrhizobium diazoefficiens Strain Does Not Activate the Type 3 Secretion System, Elicits an Early Plant Defense Response, and Circumvents NH4NO3-Induced Inhibition of Nodulation.

When subjected to nutritional stress, bacteria modify their amino acid metabolism and cell division activities by means of the stringent response, which is controlled by the Rsh protein in alphaproteobacteria. An important group of alphaproteobacteria are the rhizobia, which fix atmospheric N2 in symbiosis with legume plants. Although nutritional stress is common for rhizobia while infecting legume roots, the stringent response has scarcely been studied in this group of soil bacteria. In this report, we obtained a mutant with a kanamycin resistance insertion in the rsh gene of Bradyrhizobium diazoefficiens, the N2-fixing symbiont of soybean. This mutant was defective for type 3 secretion system induction, plant defense suppression at early root infection, and nodulation competition. Furthermore, the mutant produced smaller nodules, although with normal morphology, which led to lower plant biomass production. Soybean (Glycine max) genes GmRIC1 and GmRIC2, involved in autoregulation of nodulation, were upregulated in plants inoculated with the mutant under the N-free condition. In addition, when plants were inoculated in the presence of 10 mM NH4NO3, the mutant produced nodules containing bacteroids, and GmRIC1 and GmRIC2 were downregulated. The rsh mutant released more auxin to the culture supernatant than the wild type, which might in part explain its symbiotic behavior in the presence of combined N. These results indicate that the B. diazoefficiens stringent response integrates into the plant defense suppression and regulation of nodulation circuits in soybean, perhaps mediated by the type 3 secretion system.IMPORTANCE The symbiotic N2 fixation carried out between prokaryotic rhizobia and legume plants performs a substantial contribution to the N cycle in the biosphere. This symbiotic association is initiated when rhizobia infect and penetrate the root hairs, which is followed by the growth and development of root nodules, within which the infective rhizobia are established and protected. Thus, the nodule environment allows the expression and function of the enzyme complex that catalyzes N2 fixation. However, during early infection, the rhizobia find a harsh environment while penetrating the root hairs. To cope with this nuisance, the rhizobia mount a stress response known as the stringent response. In turn, the plant regulates nodulation in response to the presence of alternative sources of combined N in the surrounding medium. Control of these processes is crucial for a successful symbiosis, and here we show how the rhizobial stringent response may modulate plant defense suppression and the networks of regulation of nodulation.

The tip of the iceberg: emerging roles of TORC1, and its regulatory functions in plant cells.

Target of Rapamycin (TOR) is an evolutionarily conserved protein kinase that plays a central role in coordinating cell growth with light availability, the diurnal cycle, energy availability, and hormonal pathways. TOR Complex 1 (TORC1) controls cell proliferation, growth, metabolism, and defense in plants. Sugar availability is the main signal for activation of TOR in plants, as it also is in mammals and yeast. Specific regulators of the TOR kinase pathway in plants are inorganic compounds in the form of major nutrients in the soils, and light inputs via their impact on autotrophic metabolism. The lack of TOR is embryo-lethal in plants, whilst dysregulation of TOR signaling causes major alterations in growth and development. TOR exerts control as a regulator of protein translation via the action of proteins such as S6K, RPS6, and TAP46. Phytohormones are central players in the downstream systemic physiological TOR effects. TOR has recently been attributed to have roles in the control of DNA methylation, in the abundance of mRNA splicing variants, and in the variety of regulatory lncRNAs and miRNAs. In this review, we summarize recent discoveries in the plant TOR signaling pathway in the context of our current knowledge of mammalian and yeast cells, and highlight the most important gaps in our understanding of plants that need to be addressed in the future.

Autocrine regulation of root hair size by the RALF-FERONIA-RSL4 signaling pathway.

Root hair (RH) size has vital physiological implications, since it influences the surface area of the root and thus the ability of the plant to absorb water and nutrients from the soil. Arabidopsis ROOT HAIR DEFECTIVE 6-LIKE 4 (RSL4), a bHLH transcription factor, controls the expression of hundreds of RH genes, and RSL4 expression itself can trigger ectopic RH growth. Recent studies reveal an autocrine mechanism governing plant RH cell growth in which the extracellular peptide RAPID ALKALINIZATION FACTOR 1 (RALF1) and receptor FERONIA (FER) act as a central hub between the cell surface and downstream signaling events. RALF1-FER promotes the phosphorylation of eIF4E1. Then, phosphorylated eIF4E1 further regulates the synthesis of RH proteins, including RSL4, to promote RH growth. High levels of RSL4 exert a negative feedback on RALF1 expression via directly binding to the RALF1 gene promoter, slowing RH growth and determining final RH cell size.

A cell surface arabinogalactan-peptide influences root hair cell fate.

Root hairs (RHs) develop from specialized epidermal trichoblast cells, whereas epidermal cells that lack RHs are known as atrichoblasts. The mechanism controlling RH cell fate is only partially understood. RH cell fate is regulated by a transcription factor complex that promotes the expression of the homeodomain protein GLABRA 2 (GL2), which blocks RH development by inhibiting ROOT HAIR DEFECTIVE 6 (RHD6). Suppression of GL2 expression activates RHD6, a series of downstream TFs including ROOT HAIR DEFECTIVE 6 LIKE-4 (RSL4) and their target genes, and causes epidermal cells to develop into RHs. Brassinosteroids (BRs) influence RH cell fate. In the absence of BRs, phosphorylated BIN2 (a Type-II GSK3-like kinase) inhibits a protein complex that regulates GL2 expression. Perturbation of the arabinogalactan peptide (AGP21) in Arabidopsis thaliana triggers aberrant RH development, similar to that observed in plants with defective BR signaling. We reveal that an O-glycosylated AGP21 peptide, which is positively regulated by BZR1, a transcription factor activated by BR signaling, affects RH cell fate by altering GL2 expression in a BIN2-dependent manner. Changes in cell surface AGP disrupts BR responses and inhibits the downstream effect of BIN2 on the RH repressor GL2 in root epidermis.

Influence of cell wall polymers and their modifying enzymes during plant-aphid interactions.

Aphids are a major issue for commercial crops. These pests drain phloem nutrients and transmit ~50% of the known insect-borne viral diseases. During aphid feeding, trophic structures called stylets advance toward the phloem intercellularly, disrupting cell wall polymers. It is thought that cell wall-modifying enzymes (CWMEs) present in aphid saliva facilitate stylet penetration through this intercellular polymer network. Additionally, different studies have demonstrated that host settling preference, feeding behavior, and colony performance of aphids are influenced by modulating the CWME expression levels in host plants. CWMEs have been described as critical defensive elements for plants, but also as a key virulence factor for plant pathogens. However, whether CWMEs are elements of the plant defense mechanisms or the aphid infestation process remains unclear. Therefore, in order to better consider the function of CWMEs and cell wall-derived damage-associated molecular patterns (DAMPs) during plant-aphid interactions, the present review integrates different hypotheses, perspectives, and experimental evidence in the field of plant-aphid interactions and discusses similarities to other well-characterized models such as the fungi-plant pathosystems from the host and the attacker perspectives.