The bHLH transcription factor gene AtUPB1 regulates growth by mediating cell cycle progression in Arabidopsis

OsRR26, a type-B response regulator, modulates salinity tolerance in rice via phytohormone-mediated ROS accumulation in roots and influencing reproductive development

MAIN CONCLUSION

OsRR26 is a cytokinin-responsive response regulator that promotes phytohormone-mediated ROS accumulation in rice roots, regulates seedling growth, spikelet fertility, awn development, represses NADPH oxidases, and negatively affects salinity tolerance. Plant two-component systems (TCS) play a pivotal role in phytohormone signaling, stress responses, and circadian rhythm. However, a significant knowledge gap exists regarding TCS in rice. In this study, we utilized a functional genomics approach to elucidate the role of OsRR26, a type-B response regulator in rice. Our results demonstrate that OsRR26 is responsive to cytokinin, ABA, and salinity stress, serving as the ortholog of Arabidopsis ARR11. OsRR26 primarily localizes to the nucleus and plays a crucial role in seedling growth, spikelet fertility, and the suppression of awn development. Exogenous application of cytokinin led to distinct patterns of reactive oxygen species (ROS) accumulation in the roots of both WT and transgenic plants (OsRR26OE and OsRR26KD), indicating the potential involvement of OsRR26 in cytokinin-mediated ROS signaling in roots. The application of exogenous ABA resulted in varied cellular compartmentalization of ROS between the WT and transgenic lines. Stress tolerance assays of these plants revealed that OsRR26 functions as a negative regulator of salinity stress tolerance across different developmental stages in rice. Physiological and biochemical analyses unveiled that the knockdown of OsRR26 enhances salinity tolerance, characterized by improved chlorophyll retention and the accumulation of soluble sugars, K+ content, and amino acids, particularly proline.

The N-terminal α2 helix element is critical for the activity of the rice transcription factor MYC2

Jasmonates (JAs) are a class of phytohormones that play a crucial role in plant growth, development, and environmental stress responses. Central to JA signaling are the MYC2-type transcription factors, as they activate the expression of JA-responsive genes. We previously used CRISPR-Cas9-based genome editing to engineer rice OsMYC2 and yielded a mutant (myc2-5) with a single amino acid (aa) deletion (75I) outside the known functional domains of the protein. This myc2-5 mutant also showed some JA-deficient phenotypes, promoting us to investigate how 75I deletion affects JA responses. The mutation is found in the α2 helix element at the N-terminal of OsMYC2. The deletion of 75I in OsMYC2 rendered plants deficient in most of the JA responses, including root growth, leaf senescence, spikelet development, and resistance to pathogens and herbivores. Biochemical assays revealed that the 75I deletion markedly reduced OsMYC2 protein accumulation, subsequently diminishing its transcriptional activity. However, the deletion did not influence the protein's subcellular localization, DNA-binding capability, or its interactions with JAZ transcriptional repressors and the Mediator complex subunit MED25. Additionally, the screening of seven other deletions in the α2 helix further reinforces the importance of this protein element. Our results highlight the significance of the α2 helix in the N-terminus for OsMYC2's functionality, primarily through modulating its protein levels. This insight expands our knowledge of JA signaling and opens new avenues for research into the yet-to-be-explored domains of the MYC2 protein, with the potential to tailor JA responses in rice and other plant species.

Basic helix-loop-helix transcription factor PxbHLH02 enhances drought tolerance in Populus (Populus simonii × P. nigra)

The basic helix-loop-helix (bHLH) transcription factors (TFs) are involved in plant morphogenesis and various abiotic and biotic stress responses. However, further exploration is required of drought-responsive bHLH family members and their detailed regulatory mechanisms in Populus. Two bHLH TF genes, PxbHLH01/02, were identified in Populus simonii × P. nigra and cloned. The aim of this study was to examine the role of bHLH TFs in drought tolerance in P. simonii × P. nigra. The results showed that the amino acid sequences of the two genes were homologous to Arabidopsis thaliana UPBEAT1 (AtUPB1) and overexpression of PxbHLH01/02 restored normal root length in the AtUPB1 insertional mutant (upb1-1). The PxbHLH01/02 gene promoter activity analysis suggested that they were involved in stress responses and hormone signaling. Furthermore, Arabidopsis transgenic lines overexpressing PxbHLH01/02 exhibited higher stress tolerance compared with the wild-type. Populus simonii × P. nigra overexpressing PxbHLH02 increased drought tolerance and exhibited higher superoxide dismutase and peroxidase activities, lower H2O2 and malondialdehyde content, and lower relative conductivity. The results of transcriptome sequencing (RNA-seq) and quantitative real-time PCR suggested that the response of PxbHLH02 to drought stress was related to abscisic acid (ABA) signal transduction. Overall, the findings of this study suggest that PxbHLH02 from P. simonii × P. nigra functions as a positive regulator of drought stress responses by regulating stomatal aperture and promoting ABA signal transduction.

Integrative analysis of physiology, biochemistry and transcriptome reveals the mechanism of leaf size formation in Chinese cabbage (Brassica rapa L. ssp. pekinensis)

Introduction

The leaf, the main product organ, is an essential factor in determining the Chinese cabbage growth, yield and quality.

Methods

To explore the regulatory mechanism of leaf size development of Chinese cabbage, we investigated the leaf size difference between two high-generation inbred lines of Chinese cabbage, Y2 (large leaf) and Y7 (small leaf). Furtherly, the transcriptome and cis-acting elements analyses were conducted.

Results and Discussion

According to our results, Y2 exhibited a higher growth rate than Y7 during the whole growth stage. In addition, the significant higher leaf number was observed in Y2 than in Y7. There was no significant difference in the number of epidermal cells and guard cells per square millimeter between Y2 and Y7 leaves. It indicated that cell numbers caused the difference in leaf size. The measurement of phytohormone content confirmed that GA1 and GA3 mainly play essential roles in the early stage of leaf growth, and IPA and ABA were in the whole leaf growth period in regulating the cell proliferation difference between Y2 and Y7. Transcriptome analysis revealed that cyclins BraA09g010980.3C (CYCB) and BraA10g027420.3C (CYCD) were mainly responsible for the leaf size difference between Y2 and Y7 Chinese cabbage. Further, we revealed that the transcription factors BraA09gMYB47 and BraA06gMYB88 played critical roles in the difference of leaf size between Y2 and Y7 through the regulation of cell proliferation.

Conclusion

This observation not only offers essential insights into understanding the regulation mechanism of leaf development, also provides a promising breeding strategy to improve Chinese cabbage yield.

Brassinosteroid signaling restricts root lignification by antagonizing SHORT-ROOT function in Arabidopsis

Cell wall lignification is a key step in forming functional endodermis and protoxylem (PX) in plant roots. Lignified casparian strips (CS) in endodermis and tracheary elements of PX are essential for selective absorption and transport of water and nutrients. Although multiple key regulators of CS and PX have been identified, the spatial information that drives the developmental shift to root lignification remains unknown. Here, we found that brassinosteroid (BR) signaling plays a key role in inhibiting root lignification in the root elongation zone. The inhibitory activity of BR signaling occurs partially through the direct binding of BRASSINAZOLE-RESISTANT 1 (BZR1) to SHORT-ROOT (SHR), repressing the SHR-mediated activation of downstream genes that are involved in root lignification. Upon entering the mature root zone, BR signaling declines rapidly, which releases SHR activity and initiates root lignification. Our results provide a mechanistic view of the developmental transition to cell wall lignification in Arabidopsis thaliana roots.

An angiosperm NLR Atlas reveals that NLR gene reduction is associated with ecological specialization and signal transduction component deletion

Nucleotide-binding leucine-rich-repeat (NLR) genes comprise the largest family of plant disease-resistance genes. Angiosperm NLR genes are phylogenetically divided into the TNL, CNL, and RNL subclasses. NLR copy numbers and subclass composition vary tremendously across angiosperm genomes. However, the evolutionary associations between genomic NLR content and ecological adaptation, or between NLR content and signal transduction components, are poorly characterized because of limited genome availability. In this study, we established an angiosperm NLR atlas (ANNA, https://biobigdata.nju.edu.cn/ANNA/) that includes NLR genes from over 300 angiosperm genomes. Using ANNA, we revealed that NLR copy numbers differ up to 66-fold among closely related species owing to rapid gene loss and gain. Interestingly, NLR contraction was associated with adaptations to aquatic, parasitic, and carnivorous lifestyles. The convergent NLR reduction in aquatic plants resembles the lack of NLR expansion during the long-term evolution of green algae before the colonization of land. A co-evolutionary pattern between NLR subclasses and plant immune pathway components was also identified, suggesting that immune pathway deficiencies may drive TNL loss. Finally, we identified a conserved TNL lineage that may function independently of the EDS1-SAG101-NRG1 module. Collectively, these findings provide new insights into the evolution of NLR genes in the context of ecological adaptation and genome content variation.

Remodeling of Root Growth Under Combined Arsenic and Hypoxia Stress Is Linked to Nutrient Deprivation

Root architecture responds to environmental stress. Stress-induced metabolic and nutritional changes affect the endogenous root development program. Transcriptional and translational changes realize the switch between stem cell proliferation and cell differentiation, lateral root or root hair formation and root functionality for stress acclimation. The current work explores the effects of stress combination of arsenic toxicity (As) and hypoxia (Hpx) on root development in Arabidopsis thaliana. As revealed previously, combined As and Hpx treatment leads to severe nutritional disorder evident from deregulation of root transcriptome and plant mineral contents. Both As and Hpx were identified to pose stress-specific constraints on root development that lead to unique root growth phenotype under their combination. Besides inhibition of root apical meristem (RAM) activity under all stresses, As induced lateral root growth while root hair density and lengths were strongly increased by Hpx and HpxAs-treatments. A dual stimulation of phosphate (Pi)-starvation response was observed for HpxAs-treated plant roots; however, the response under HpxAs aligned more with Hpx than As. Transcriptional evidence along with biochemical data suggests involvement of PHOSPHATE STARVATION RESPONSE 1; PHR1-dependent systemic signaling. Pi metabolism-related transcripts in close association with cellular iron homeostasis modulate root development under HpxAs. Early redox potential changes in meristematic cells, differential ROS accumulation in root hair zone cell layers and strong deregulation of NADPH oxidases, NADPH-dependent oxidoreductases and peroxidases signify a role of redox and ROS signaling in root architecture remodeling under HpxAs. Differential aquaporin expression suggests transmembrane ROS transport to regulate root hair induction and growth. Reorganization of energy metabolism through NO-dependent alternate oxidase, lactate fermentation, and phosphofructokinase seems crucial under HpxAs. TOR and SnRK-signaling network components were potentially involved in control of sustainable utilization of available energy reserves for root hair growth under combined stress as well as recovery on reaeration. Findings are discussed in context of combined stress-induced signaling in regulation of root development in contrast to As and Hpx alone.

Genome-wide sRNA and mRNA transcriptomic profiling insights into dynamic regulation of taproot thickening in radish (Raphanus sativus L.)

BACKGROUND

Taproot is the main edible organ and ultimately determines radish yield and quality. However, the precise molecular mechanism underlying taproot thickening awaits further investigation in radish. Here, RNA-seq was performed to identify critical genes involved in radish taproot thickening from three advanced inbred lines with different root size.

RESULTS

A total of 2606 differentially expressed genes (DEGs) were shared between 'NAU-DY' (large acicular) and 'NAU-YB' (medium obovate), which were significantly enriched in 'phenylpropanoid biosynthesis', 'glucosinolate biosynthesis', and 'starch and sucrose metabolism' pathway. Meanwhile, a total of 16 differentially expressed miRNAs (DEMs) were shared between 'NAU-DY' and 'NAU-YH' (small circular), whereas 12 miRNAs exhibited specific differential expression in 'NAU-DY'. Association analysis indicated that miR393a-bHLH77, miR167c-ARF8, and miR5658-APL might be key factors to biological phenomenon of taproot type variation, and a putative regulatory model of taproot thickening and development was proposed. Furthermore, several critical genes including SUS1, EXPB3, and CDC5 were characterized and profiled by RT-qPCR analysis.

CONCLUSION

This integrated study on the transcriptional and post-transcriptional profiles could provide new insights into comprehensive understanding of the molecular regulatory mechanism underlying taproot thickening in root vegetable crops.

Brassinosteroids regulate root meristem development by mediating BIN2-UPB1 module in Arabidopsis

Plant steroid hormones brassinosteroids (BRs) regulate plant growth and development at many levels. While negative regulatory factors that inhibit development and are counteracted by BRs exist in the root meristem, these factors have not been characterized. The functions of UPB1 transcription factor in BR-regulated root growth have not been established, although its role in regulating root are well documented. Here, we found that BIN2 interacts with and phosphorylates the UPB1 transcription factor consequently promoting UPB1 stability and transcriptional activity. Genetic analysis revealed that UPB1 deficiency could partially recover the short-root phenotype of BR-deficient mutants. Expression of a mutated UPB1^S37AS41A protein lacking a conserved BIN2 phosphorylation sites can rescue shorter root phenotype of bin2-1 mutant. In addition, UPB1 was repressed by BES1 at the transcriptional level. The paclobutrazol-resistant protein family (PRE2/3) interacts with UPB1 and inhibits its transcriptional activity to promote root meristem development, and BIN2-mediated phosphorylation of UPB1 suppresses its interaction with PRE2/3, and subsequently impairing root meristem development. Taken together, our data elucidate a molecular mechanism by which BR promotes root growth via inhibiting BIN2-UPB1 module.

Various physiological and genetic researches have provided ample evidence in support of the role of plant hormones in root development. Brasinosteroids (BR) play important roles in controlling root growth and development, but the mechanism of negatively regulating factors in this process is less well studied. Here, we found BIN2 kinase, a negative component in BR signaling, interacted with and phosphorylated UPB1 to stabilize and promote its binding ability. The upb1-1 mutant was hypersensitive phenotype, while UPB1-overexpression lines showed opposite effects on BR regulated root meristem development, and defect of UPB1 partially suppressed the short-root phenotype in BR-deficient mutants. Moreover, the paclobutrazol-resistant protein family (PRE2/3) interacted with UPB1 and inhibited its transcriptional activity, and this interaction was also inhibited by BIN2 phosphorylation, thus impairing root meristem development. Our findings provide significant insights into BR signaling through BIN2-UPB1 in regulating root meristem.