Redox-Mediated Endocytosis of a Receptor-Like Kinase during Distal Stem Cell Differentiation Depends on Its Tumor Necrosis Factor Receptor Domain

Recent advances in response to environmental signals during Arabidopsis root development

Plants grow by anchoring their roots in the soil, acquiring essential water and nutrients for growth, and interacting with other signaling factors in the soil. Root systems are crucial for both the basic growth and development of plants and their response to external environmental stimuli. Under different environmental conditions, the configuration of root systems in plants can undergo significant changes, with their strength determining the plant's ability to adapt to the environment. Therefore, understanding the mechanisms by which environmental factors regulate root development is essential for crop root architecture improvement and breeding for stress resistance. This paper summarizes the research progress in genetic regulation of root development of the model plant Arabidopsis thaliana (L.) Heynh. amidst diverse environmental stimuli over the past five years. Specifically, it focuses on the regulatory networks of environmental signals, encompassing light, energy, temperature, water, nutrients, and reactive oxygen species, on root development. Furthermore, it provides prospects for the application of root architecture improvement in crop breeding for stress resistance and nutrient efficiency.

Small Peptides: Orchestrators of Plant Growth and Developmental Processes

Small peptides (SPs), ranging from 5 to 100 amino acids, play integral roles in plants due to their diverse functions. Despite their low abundance and small molecular weight, SPs intricately regulate critical aspects of plant life, including cell division, growth, differentiation, flowering, fruiting, maturation, and stress responses. As vital mediators of intercellular signaling, SPs have garnered significant attention in plant biology research. This comprehensive review delves into SPs' structure, classification, and identification, providing a detailed understanding of their significance. Additionally, we summarize recent findings on the biological functions and signaling pathways of prominent SPs that regulate plant growth and development. This review also offers a perspective on future research directions in peptide signaling pathways.

Hydrogen Peroxide Signaling in the Maintenance of Plant Root Apical Meristem Activity

Hydrogen peroxide (H2O2) is a prevalent reactive oxygen species (ROS) found in cells and takes a central role in plant development and stress adaptation. The root apical meristem (RAM) has evolved strong plasticity to adapt to complex and changing environmental conditions. Recent advances have made great progress in explaining the mechanism of key factors, such as auxin, WUSCHEL-RELATED HOMEOBOX 5 (WOX5), PLETHORA (PLT), SHORTROOT (SHR), and SCARECROW (SCR), in the regulation of RAM activity maintenance. H2O2 functions as an emerging signaling molecule to control the quiescent center (QC) specification and stem cell niche (SCN) activity. Auxin is a key signal for the regulation of RAM maintenance, which largely depends on the formation of auxin regional gradients. H2O2 regulates the auxin gradients by the modulation of intercellular transport. H2O2 also modulates the expression of WOX5, PLTs, SHR, and SCR to maintain RAM activity. The present review is dedicated to summarizing the key factors in the regulation of RAM activity and discussing the signaling transduction of H2O2 in the maintenance of RAM activity. H2O2 is a significant signal for plant development and environmental adaptation.

The NIN-LIKE PROTEIN 7 transcription factor modulates auxin pathways to regulate root cap development in Arabidopsis

The root cap is a small tissue located at the tip of the root with critical functions for root growth. Present in nearly all vascular plants, the root cap protects the root meristem, influences soil penetration, and perceives and transmits environmental signals that are critical for root branching patterns. To perform these functions, the root cap must remain relatively stable in size and must integrate endogenous developmental pathways with environmental signals, yet the mechanism is not clear. We previously showed that low pH conditions altered root cap development, and these changes are mediated by the NIN LIKE PROTEIN 7 (NLP7) transcription factor, a master regulator of nitrate signaling. Here we show that in Arabidopsis NLP7 integrates nitrate signaling with auxin pathways to regulate root cap development. We found that low nitrate conditions promote aberrant release of root cap cells. Nitrate deficiency impacts auxin pathways in the last layer of the root cap, and this is mediated in part by NLP7. Mutations in NLP7 abolish the auxin minimum in the last layer of the root cap and alter root cap expression of the auxin carriers PIN-LIKES 3 (PILS3) and PIN-FORMED 7 (PIN7) as well as transcription factors that regulate PIN expression. Together, our data reveal NLP7 as a link between endogenous auxin pathways and nitrate signaling in the root cap.

OsCIP1, a secreted protein, binds to and stabilizes OsCR4 to promote aleurone layer development, seed germination and early seedling growth in rice

The receptor kinase CRINKLY 4 (CR4) and its orthologs are known for their essential roles in cell differentiation and their shuttling between plasma membrane and cytoplasmic vesicles, a unique feature tied to their extracellular domain. However, the extracellular regulators of CR4 have been little known. Here we identified an OsCR4 Interacting Protein 1 (OsCIP1) (also named as OsLTPL36 in rice) by a yeast two-hybrid screen using the extracellular domain of OsCR4 (OsCR4E) as bait. OsCIP1/OsLTPL36 harbors a signal peptide and is localized to the outer surface of the plasma membrane. It interacted with the TNFR subdomain of OsCR4, causing an increase in OsCR4 recycling to the plasma membrane. oscip1, in which OsCR4 protein was decreased, exhibited thinner aleurone layer, late germination and delayed growth; while OsCIP1-overexpressing plants, in which OsCR4 protein was increased, displayed enhanced growth at the early seedling stage. OsCIP1 was cleaved between W61 and Q62, and the resulting C-terminal half exhibited a greater affinity for OsCR4E than did its precursor. Abolishing this cleavage site compromises OsCIP1's ability to promote seedling growth. Our results provide valuable clues for the regulation of CR4 activity and its functions in aleurone layer cell differentiation by a secreted small protein in rice.

Reciprocal Regulation of Shh Trafficking and H2O2 Levels via a Noncanonical BOC-Rac1 Pathway

Among molecules that bridge environment, cell metabolism, and cell signaling, hydrogen peroxide (H₂O₂) recently appeared as an emerging but central player. Its level depends on cell metabolism and environment and was recently shown to play key roles during embryogenesis, contrasting with its long-established role in disease progression. We decided to explore whether the secreted morphogen Sonic hedgehog (Shh), known to be essential in a variety of biological processes ranging from embryonic development to adult tissue homeostasis and cancers, was part of these interactions. Here, we report that H₂O₂ levels control key steps of Shh delivery in cell culture: increased levels reduce primary secretion, stimulate endocytosis and accelerate delivery to recipient cells; in addition, physiological in vivo modulation of H₂O₂ levels changes Shh distribution and tissue patterning. Moreover, a feedback loop exists in which Shh trafficking controls H₂O₂ synthesis via a non-canonical BOC-Rac1 pathway, leading to cytoneme growth. Our findings reveal that Shh directly impacts its own distribution, thus providing a molecular explanation for the robustness of morphogenesis to both environmental insults and individual variability.

Arabidopsis U-box E3 ubiquitin ligase PUB11 negatively regulates drought tolerance by degrading the receptor-like protein kinases LRR1 and KIN7

Both plant receptor-like protein kinases (RLKs) and ubiquitin-mediated proteolysis play crucial roles in plant responses to drought stress. However, the mechanism by which E3 ubiquitin ligases modulate RLKs is poorly understood. In this study, we showed that Arabidopsis PLANT U-BOX PROTEIN 11 (PUB11), an E3 ubiquitin ligase, negatively regulates abscisic acid (ABA)-mediated drought responses. PUB11 interacts with and ubiquitinates two receptor-like protein kinases, LEUCINE RICH REPEAT PROTEIN 1 (LRR1) and KINASE 7 (KIN7), and mediates their degradation during plant responses to drought stress in vitro and in vivo. pub11 mutants were more tolerant, whereas lrr1 and kin7 mutants were more sensitive, to drought stress than the wild type. Genetic analyses show that the pub11 lrr1 kin7 triple mutant exhibited similar drought sensitivity as the lrr1 kin7 double mutant, placing PUB11 upstream of the two RLKs. Abscisic acid and drought treatment promoted the accumulation of PUB11, which likely accelerates LRR1 and KIN7 degradation. Together, our results reveal that PUB11 negatively regulates plant responses to drought stress by destabilizing the LRR1 and KIN7 RLKs.

Salicylic acid promotes quiescent center cell division through ROS accumulation and down-regulation of PLT1, PLT2, and WOX5

Salicylic acid (SA) plays a crucial role in plant immunity. However, its function in plant development is poorly understood. The quiescent center (QC), which maintains columella stem cells (CSCs) in the root apical meristem and typically exhibits low levels of cell division, is critical for root growth and development. Here, we show that the Arabidopsis thaliana SA overaccumulation mutant constitutively activated cell death 1 (cad1), which exhibits increased cell division in the QC, is rescued by additional mutations in genes encoding the SA biosynthetic enzyme SALICYLIC ACID INDUCTION DEFFICIENT2 (SID2) or the SA receptor NONEXPRESSER OF PR GENES1 (NPR1), indicating that QC cell division in the cad1 mutant is promoted by the NPR1-dependent SA signaling pathway. The application of exogenous SA also promoted QC cell division in wild-type plants in a dose-dependent manner and largely suppressed the expression of genes involved in QC maintenance, including those encoding the APETALA2 (AP2) transcription factors PLETHORA1 (PLT1) and PLT2, as well as the homeodomain transcription factor WUSCHEL-RELATED HOMEOBOX5 (WOX5). Moreover, we showed that SA promotes reactive oxygen species (ROS) production, which is necessary for the QC cell division phenotype in the cad1 mutant. These results provide insight into the function of SA in QC maintenance.

Receptor-like Kinases in Root Development: Current Progress and Future Directions

Cell-to-cell and cell-to-environment communications are critical to the growth and development of plants. Cell surface-localized receptor-like kinases (RLKs) are mainly involved in sensing various extracellular signals to initiate their corresponding cellular responses. As important vegetative organs for higher plants to adapt to a terrestrial living situation, roots play a critical role for the survival of plants. It has been demonstrated that RLKs control many biological processes during root growth and development. In this review, we summarize several key regulatory processes during Arabidopsis root development in which RLKs play critical roles. We also put forward a number of relevant questions that are required to be explored in future studies.

Emerging mechanisms to fine-tune receptor kinase signaling specificity

Organisms need to constantly inform their cellular machinery about the biochemical and physical status of their surroundings to adapt and thrive. While some external signals are also sensed intracellularly, a considerable share of external information is registered already at the plasma membrane (PM). Receptor kinases (RKs) are crucial for plant cells to integrate such cues from the environment, from microbes, or from other cells to coordinate their physiological response and their development. Early studies on RK signaling depicted the path from external signal to internal response in a linear fashion, but recent findings show that these cellular information highways are highly interconnected and pass signals through molecular intersections. In this review, we first discuss how individual RKs simultaneously contribute to the transduction and deconvolution of a multitude of signals by controlled assembly into diverse RK complexes, exemplified by FERONIA signaling versatility. We then elaborate on how cells can exert highly localized control over the assembly, interaction and composition of such complexes in order to attain essential cellular output specificity.

MINI SEED 2 (MIS2) Encodes a Receptor-like Kinase that Controls Grain Size and Shape in Rice

BACKGROUND

Grain size is a key agronomic trait that is directly associated with grain yield in rice. Although several genes related to grain size in rice have been identified, our understanding of the mechanism of grain development is still limited.

RESULTS

In this study, we reported the characterization of a novel seed size mutant mini seed 2 (mis2), in which the grain showed reduced length, width and thickness along with wrinkled surface. Microscopic analysis revealed that the spikelet epidermal cell size was reduced but the cell number was increased in the mis2 mutant, suggesting that MIS2 controls grain size by coordinately regulating epidermal cell size and cell number. Map-based cloning revealed that MIS2 encodes a receptor-like kinase CRINKLY4 (CR4) which showed the highest expression in developing panicles. The MIS2 protein is localized primarily on the plasma membrane along with the endosome. However, the Arg258Gln mutation located in extracellular domain in the mis2 mutant disturbed its subcellular localization. Additionally, three major haplotypes of MIS2 were identified in the japonica, indica and aus rice cultivars. The 18-bp InDel (insertion and deletion) in the 5'-UTR (untranslated region) caused different expression level of MIS2 in haplotypes.

CONCLUSIONS

We reported a key role of OsCR4 in controlling grain size and shape by coordinately regulating epidermal cell size and cell number. The Arg258 in the extracellular seven-repeat domain is essential for the correct subcellular behavior and function of the OsCR4 protein.

The roles of endomembrane trafficking in plant abiotic stress responses

Endomembrane trafficking is a fundamental cellular process in all eukaryotic cells and its regulatory mechanisms have been extensively studied. In plants, the endomembrane trafficking system needs to be constantly adjusted to adapt to the ever-changing environment. Evidence has accumulated supporting the idea that endomembrane trafficking is tightly linked to stress signaling pathways to meet the demands of rapid changes in cellular processes and to ensure the correct delivery of stress-related cargo molecules. However, the underlying mechanisms remain unknown. In this review, we summarize the recent findings on the functional roles of both secretory trafficking and endocytic trafficking in different types of abiotic stresses. We also highlight and discuss the unique properties of specific regulatory molecules beyond their conventional functions in endosomal trafficking during plant growth under stress conditions.