GRAS transcription factors regulate cell division planes in moss overriding the default rule

CLE peptides act via the receptor-like kinase CRINKLY 4 in Physcomitrium patens gametophore development

The CLAVATA pathway plays a key role in the regulation of multicellular shoot and root meristems in flowering plants. In Arabidopsis, CLAVATA 3-like signaling peptides (CLEs) act via receptor-like kinases CLAVATA 1 and CRINKLY 4 (CR4). In the moss Physcomitrium patens, PpCLAVATA and PpCR4 were previously studied independently and shown to play conserved roles in the regulation of cell proliferation and differentiation. The plant calpain DEFECTIVE KERNEL 1 (DEK1) has been identified as another key regulator of cell division and cell fate in vascular plants and bryophytes. The functional interaction between CLAVATA, CR4, and DEK1 remains unknown. Here, we show that P. patens crinkly4 and dek1 mutants respond differently to CLE peptide treatments suggesting their distinct roles in the CLAVATA pathway. Reduced CLAVATA-mediated suppression of leafy shoot growth in Δcr4 mutants indicates that PpCR4 is involved in CLV3p perception, most likely as a receptor. The CLV3p strongly suppressed leaf vein development in Δcr4 mutants, suggesting that other receptors are involved in these processes and indicating a potential role of PpCR4 in organ sensitization to CLEs.

MpANT regulates meristem development in Marchantia polymorpha

Meristems are crucial for organ formation, but our knowledge of their molecular evolution is limited. Here, we show that AINTEGUMENTA (MpANT) in the euANT branch of the APETALA2-like transcription factor family is essential for meristem development in the nonvascular plant Marchantia polymorpha. MpANT is expressed in the thallus meristem. Mpant mutants show defects to maintain meristem identity and undergo meristem duplication, while MpANT overexpressers show ectopic thallus growth. MpANT directly upregulates MpGRAS9 in the SHORT-ROOT (SHR) branch of the GRAS family. In the vascular plant Arabidopsis thaliana, the euANT-branch genes PLETHORAs (AtPLTs) and AtANT are involved in the formation and maintenance of root/shoot apical meristems and lateral organ primordia, and AtPLTs directly target SHR-branch genes. In addition, euANTs bind through a similar DNA-binding motif to many conserved homologous genes in M. polymorpha and A. thaliana. Overall, the euANT pathway has an evolutionarily conserved role in meristem development.

CsPAT1, a GRAS transcription factor, promotes lignin accumulation by antagonistic interacting with CsWRKY13 in tea plants

Lignin is an important component of plant cell walls and plays crucial roles in the essential agronomic traits of tea quality and tenderness. However, the molecular mechanisms underlying the regulation of lignin biosynthesis in tea plants remain unclear. CsWRKY13 acts as a negative regulator of lignin biosynthesis in tea plants. In this study, we identified a GRAS transcription factor, phytochrome A signal transduction 1 (CsPAT1), that interacts with CsWRKY13. Silencing CsPAT1 expression in tea plants and heterologous overexpression in Arabidopsis demonstrated that CsPAT1 positively regulates lignin accumulation. Further investigation revealed that CsWRKY13 directly binds to the promoters of CsPAL and CsC4H and suppresses transcription of CsPAL and CsC4H. CsPAT1 indirectly affects the promoter activities of CsPAL and CsC4H by interacting with CsWRKY13, thereby facilitating lignin biosynthesis in tea plants. Compared with the expression of CsWRKY13 alone, the co-expression of CsPAT1 and CsWRKY13 in Oryza sativa significantly increased lignin biosynthesis. Conversely, compared with the expression of CsPAT1 alone, the co-expression of CsPAT1 and CsWRKY13 in O. sativa significantly reduced lignin accumulation. These results demonstrated the antagonistic regulation of the lignin biosynthesis pathway by CsPAT1 and CsWRKY13. These findings improve our understanding of lignin biosynthesis mechanisms in tea plants and provide insights into the role of the GRAS transcription factor family in lignin accumulation.

The landscape of transcription factor promoter activity during vegetative development in Marchantia

Transcription factors (TFs) are essential for the regulation of gene expression and cell fate determination. Characterizing the transcriptional activity of TF genes in space and time is a critical step toward understanding complex biological systems. The vegetative gametophyte meristems of bryophytes share some characteristics with the shoot apical meristems of flowering plants. However, the identity and expression profiles of TFs associated with gametophyte organization are largely unknown. With only ∼450 putative TF genes, Marchantia (Marchantia polymorpha) is an outstanding model system for plant systems biology. We have generated a near-complete collection of promoter elements derived from Marchantia TF genes. We experimentally tested reporter fusions for all the TF promoters in the collection and systematically analyzed expression patterns in Marchantia gemmae. This allowed us to build a map of expression domains in early vegetative development and identify a set of TF-derived promoters that are active in the stem-cell zone. The cell markers provide additional tools and insight into the dynamic regulation of the gametophytic meristem and its evolution. In addition, we provide an online database of expression patterns for all promoters in the collection. We expect that these promoter elements will be useful for cell-type-specific expression, synthetic biology applications, and functional genomics.

PpSCARECROW1 (PpSCR1) regulates leaf blade and mid-vein development in Physcomitrium patens

In plants, asymmetric cell divisions result in distinct cell fates forming large and small daughter cells, adding to the cellular diversity in an organ. SCARECROW (SCR), a GRAS domain-containing transcription factor controls asymmetric periclinal cell divisions in flowering plants by governing radial patterning of ground tissue in roots and cell proliferation in leaves. Though SCR homologs are present across land plant lineages, the current understanding of their role in cellular patterning and leaf development is mostly limited to flowering plants. Our phylogenetic analysis identified three SCR homologs in moss Physcomitrium patens, amongst which PpSCR1 showed highest expression in gametophores and its promoter activity was prominent at the mid-vein and the flanking leaf blade cells pointing towards its role in leaf development. Notably, out of the three SCR homologs, only the ppscr1 knock-out lines developed slender leaves with four times narrower leaf blade and three times thicker mid-vein. Detailed histology studies revealed that slender leaf phenotype is either due to the loss of anticlinal cell divisions or failure of periclinal division suppression in the leaf blade. RNA-Seq analyses revealed that genes responsible for cell division and differentiation are expressed differentially in the mutant. PpSCR1 overexpression lines exhibited significantly wider leaf lamina, further reconfirming the role in leaf development. Together, our data suggests that PpSCR1 is involved in the leaf blade and mid-vein development of moss and that its role in the regulation of cell division and proliferation is ancient and conserved among flowering plants and mosses.

Genome-wide identification and expression pattern analysis of the kiwifruit GRAS transcription factor family in response to salt stress

BACKGROUND

GRAS is a family of plant-specific transcription factors (TFs) that play a vital role in plant growth and development and response to adversity stress. However, systematic studies of the GRAS TF family in kiwifruit have not been reported.

RESULTS

In this study, we used a bioinformatics approach to identify eighty-six AcGRAS TFs located on twenty-six chromosomes and phylogenetic analysis classified them into ten subfamilies. It was found that the gene structure is relatively conserved for these genes and that fragmental duplication is the prime force for the evolution of AcGRAS genes. However, the promoter region of the AcGRAS genes mainly contains cis-acting elements related to hormones and environmental stresses, similar to the results of GO and KEGG enrichment analysis, suggesting that hormone signaling pathways of the AcGRAS family play a vital role in regulating plant growth and development and adversity stress. Protein interaction network analysis showed that the AcGRAS51 protein is a relational protein linking DELLA, SCR, and SHR subfamily proteins. The results demonstrated that 81 genes were expressed in kiwifruit AcGRAS under salt stress, including 17 differentially expressed genes, 13 upregulated, and four downregulated. This indicates that the upregulated AcGRAS55, AcGRAS69, AcGRAS86 and other GRAS genes can reduce the salt damage caused by kiwifruit plants by positively regulating salt stress, thus improving the salt tolerance of the plants.

CONCLUSIONS

These results provide a theoretical basis for future exploration of the characteristics and functions of more AcGRAS genes. This study provides a basis for further research on kiwifruit breeding for resistance to salt stress. RT-qPCR analysis showed that the expression of 3 AcGRAS genes was elevated under salt stress, indicating that AcGRAS exhibited a specific expression pattern under salt stress conditions.

Control of stem cell behavior by CLE-JINGASA signaling in the shoot apical meristem in Marchantia polymorpha

Land plants undergo indeterminate growth by the activity of meristems in both gametophyte (haploid) and sporophyte (diploid) generations. In the sporophyte of the flowering plant Arabidopsis thaliana, the apical meristems are located at the shoot and root tips in which a number of regulatory gene homologs are shared for their development, implying deep evolutionary origins. However, little is known about their functional conservation with gametophytic meristems in distantly related land plants such as bryophytes, even though genomic studies have revealed that the subfamily-level diversity of regulatory genes is mostly conserved throughout land plants. Here, we show that a NAM/ATAF/CUC (NAC) domain transcription factor, JINGASA (MpJIN), acts downstream of CLAVATA3 (CLV3)/ESR-related (CLE) peptide signaling and controls stem cell behavior in the gametophytic shoot apical meristem of the liverwort Marchantia polymorpha. In the meristem, strong MpJIN expression was associated with the periclinal cell division at the periphery of the stem cell zone (SCZ), whereas faint MpJIN expression was found at the center of the SCZ. Time course observation indicates that the MpJIN-negative cells are lost from the SCZ and respecified de novo at two separate positions during the dichotomous branching event. Consistently, the induction of MpJIN results in ectopic periclinal cell division in the SCZ and meristem termination. Based on the comparative expression data, we speculate that the function of JIN/FEZ subfamily genes was shared among the shoot apical meristems in the gametophyte and sporophyte generations in early land plants but was lost in certain lineages, including the flowering plant A. thaliana.

Root evolution: Evidence for convergent evolution of root meristem

The root system is thought to have evolved independently during the process of plant terrestrialization. It is unclear how the molecular regulation of the root meristem has been modified during these independent root evolutions. Two new reports provide important evidence in support of the putative convergent evolution of lycophyte roots.

Genome and transcriptome of Selaginella kraussiana reveal evolution of root apical meristems in vascular plants

The evolution of roots allowed vascular plants to adapt to land environments. Fossil evidence indicates that roots evolved independently in euphyllophytes (ferns and seed plants) and lycophytes, the two lineages of extant vascular plants. Based on a high-quality genome assembly, mRNA sequencing (mRNA-seq) data, and single-cell RNA-seq data for the lycophyte Selaginella kraussiana, we show that the two root origin events in lycophytes and euphyllophytes adopted partially similar molecular modules in the regulation of root apical meristem (RAM) development. In S. kraussiana, the RAM initiates from the rhizophore primordium guided by auxin and duplicates itself by dichotomous branching. The auxin signaling pathway directly upregulates euAINTEGUMENTAb (SkeuANTb), and then SkeuANTb directly promotes the expression of SkeuANTa and the WUSCHEL-RELATED HOMEOBOX13b (SkWOX13b) for RAM maintenance, partially similar to the molecular pathway involving the euANT-branch PLETHORA (AtPLT) genes and AtWOX5 in root initiation in the seed plant Arabidopsis thaliana. Other molecular modules, e.g., SHORT-ROOT and SCARECROW, also have partially similar expression patterns in the RAMs of S. kraussiana and A. thaliana. Overall, our study not only provides genome and transcriptome tools of S. kraussiana but also indicates the employment of some common molecular modules in RAMs during root origins in lycophytes and euphyllophytes.

Properties of INDETERMINATE DOMAIN Proteins from Physcomitrium patens: DNA-Binding, Interaction with GRAS Proteins, and Transcriptional Activity

INDETERMINATE DOMAIN (IDD) proteins are plant-specific transcription factors that interact with GRAS proteins, such as DELLA and SHORT ROOT (SHR), to regulate target genes. The combination of IDD and DELLA proteins regulates genes involved in gibberellic acid (GA) synthesis and GA signaling, whereas the combination of IDD with the complex of SHR and SCARECROW, another GRAS protein, regulates genes involved in root tissue formation. Previous bioinformatic research identified seven IDDs, two DELLA, and two SHR genes in Physcomitrium patens, a model organism for non-vascular plants (bryophytes), which lack a GA signaling pathway and roots. In this study, DNA-binding properties and protein-protein interaction of IDDs from P. patens (PpIDD) were analyzed. Our results showed that the DNA-binding properties of PpIDDs were largely conserved between moss and seed plants. Four PpIDDs showed interaction with Arabidopsis DELLA (AtDELLA) proteins but not with PpDELLAs, and one PpIDD showed interaction with PpSHR but not with AtSHR. Moreover, AtIDD10 (JACKDAW) interacted with PpSHR but not with PpDELLAs. Our results indicate that DELLA proteins have modified their structure to interact with IDD proteins during evolution from moss lineage to seed plants, whereas the interaction of IDD and SHR was already present in moss lineage.

Genome-Wide Identification, Expression and Stress Analysis of the GRAS Gene Family in Phoebe bournei

GRAS genes are important transcriptional regulators in plants that govern plant growth and development through enhancing plant hormones, biosynthesis, and signaling pathways. Drought and other abiotic factors may influence the defenses and growth of Phoebe bournei, which is a superb timber source for the construction industry and building exquisite furniture. Although genome-wide identification of the GRAS gene family has been completed in many species, that of most woody plants, particularly P. bournei, has not yet begun. We performed a genome-wide investigation of 56 PbGRAS genes, which are unequally distributed across 12 chromosomes. They are divided into nine subclades. Furthermore, these 56 PbGRAS genes have a substantial number of components related to abiotic stress responses or phytohormone transmission. Analysis using qRT-PCR showed that the expression of four PbGRAS genes, namely PbGRAS7, PbGRAS10, PbGRAS14 and PbGRAS16, was differentially increased in response to drought, salt and temperature stresses, respectively. We hypothesize that they may help P. bournei to successfully resist harsh environmental disturbances. In this work, we conducted a comprehensive survey of the GRAS gene family in P. bournei plants, and the results provide an extensive and preliminary resource for further clarification of the molecular mechanisms of the GRAS gene family in P. bournei in response to abiotic stresses and forestry improvement.

Redundant mechanisms in division plane positioning

Redundancies in plant cell division contribute to the maintenance of proper division plane orientation. Here we highlight three types of redundancy: 1) Temporal redundancy, or correction of earlier defects that results in proper final positioning, 2) Genetic redundancy, or functional compensation by homologous genes, and 3) Synthetic redundancy, or redundancy within or between pathways that contribute to proper division plane orientation. Understanding the types of redundant mechanisms involved provides insight into current models of division plane orientation and opens up new avenues for exploration.