Receptor-like kinase ACR4 restricts formative cell divisions in the Arabidopsis root

SUPPRESSOR OF FRIGIDA 4 cooperates with the histone methylation reader EBS to positively regulate root development

Maintenance and homeostasis of the quiescent center (QC) in Arabidopsis (Arabidopsis thaliana) root apical meristems are critical for stem cell organization and root development. Despite great progress in relevant research, the molecular mechanisms that determine the root stem cell fate and QC still need further exploration. In Arabidopsis, SUPPRESSOR OF FRIGIDA 4 (SUF4) encodes a C2H2-type zinc finger protein that represses flowering by transcriptional activation of FLOWERING LOCUS C (FLC) through the FRIGIDA (FRI) pathway, and EARLY BOLTING IN SHORT DAYS (EBS) is a bivalent histone reader that prevents premature flowering. Here, we found that SUF4 directly interacts with EBS in vivo and in vitro. Loss of function of SUF4 and/or EBS resulted in disorganization of the QC, aberrant cell division, and stunted root growth. RNA-seq and reverse transcription quantitative real-time polymerase chain reaction analysis revealed that SUF4 and EBS coregulate many root development-related genes. A series of biochemical analyses demonstrated that SUF4 directly binds to the promoter of SCARECROW (SCR), which encodes a key regulator of root development. Chromatin immunoprecipitation assay indicated that both SUF4 and EBS are recruited to the SCR locus in an interdependent manner to promote H3K4me3 levels and suppress H3K27me3 levels, thereby activating the expression of SCR. These findings improve our understanding of the function of SUF4 and EBS and provide insights into the molecular mechanism that couples a transcription factor and a histone methylation reader to modulate QC specification and root development in Arabidopsis.

Plant regeneration in the new era: from molecular mechanisms to biotechnology applications

Plants or tissues can be regenerated through various pathways. Like animal regeneration, cell totipotency and pluripotency are the molecular basis of plant regeneration. Detailed systematic studies on Arabidopsis thaliana gradually unravel the fundamental mechanisms and principles underlying plant regeneration. Specifically, plant hormones, cell division, epigenetic remodeling, and transcription factors play crucial roles in reprogramming somatic cells and reestablishing meristematic cells. Recent research on basal non-vascular plants and monocot crops has revealed that plant regeneration differs among species, with various plant species using distinct mechanisms and displaying significant differences in regenerative capacity. Conducting multi-omics studies at the single-cell level, tracking plant regeneration processes in real-time, and deciphering the natural variation in regenerative capacity will ultimately help understand the essence of plant regeneration, improve crop regeneration efficiency, and contribute to future crop design.

Understanding role of roots in plant response to drought: Way forward to climate-resilient crops

Drought stress leads to a significant amount of agricultural crop loss. Thus, with changing climatic conditions, it is important to develop resilience measures in agricultural systems against drought stress. Roots play a crucial role in regulating plant development under drought stress. In this review, we have summarized the studies on the role of roots and root-mediated plant responses. We have also discussed the importance of root system architecture (RSA) and the various structural and anatomical changes that it undergoes to increase survival and productivity under drought. Various genes, transcription factors, and quantitative trait loci involved in regulating root growth and development are also discussed. A summarization of various instruments and software that can be used for high-throughput phenotyping in the field is also provided in this review. More comprehensive studies are required to help build a detailed understanding of RSA and associated traits for breeding drought-resilient cultivars.

An update on evolutionary, structural, and functional studies of receptor-like kinases in plants

All living organisms must develop mechanisms to cope with and adapt to new environments. The transition of plants from aquatic to terrestrial environment provided new opportunities for them to exploit additional resources but made them vulnerable to harsh and ever-changing conditions. As such, the transmembrane receptor-like kinases (RLKs) have been extensively duplicated and expanded in land plants, increasing the number of RLKs in the advanced angiosperms, thus becoming one of the largest protein families in eukaryotes. The basic structure of the RLKs consists of a variable extracellular domain (ECD), a transmembrane domain (TM), and a conserved kinase domain (KD). Their variable ECDs can perceive various kinds of ligands that activate the conserved KD through a series of auto- and trans-phosphorylation events, allowing the KDs to keep the conserved kinase activities as a molecular switch that stabilizes their intracellular signaling cascades, possibly maintaining cellular homeostasis as their advantages in different environmental conditions. The RLK signaling mechanisms may require a coreceptor and other interactors, which ultimately leads to the control of various functions of growth and development, fertilization, and immunity. Therefore, the identification of new signaling mechanisms might offer a unique insight into the regulatory mechanism of RLKs in plant development and adaptations. Here, we give an overview update of recent advances in RLKs and their signaling mechanisms.

Genetic regulation of lateral root development

Lateral roots (LRs) are an important part of plant root systems. In dicots, for example, after plants adapted from aquatic to terrestrial environments, filamentous pseudorhizae evolved to allow nutrient absorption. A typical plant root system comprises a primary root, LRs, root hairs, and a root cap. Classical plant roots exhibit geotropism (the tendency to grow downward into the ground) and can synthesize plant hormones and other essential substances. Root vascular bundles and complex spatial structures enable plants to absorb water and nutrients to meet their nutrient quotas and grow. The primary root carries out most functions during early growth stages but is later overtaken by LRs, underscoring the importance of LR development water and mineral uptake and the soil fixation capacity of the root. LR development is modulated by endogenous plant hormones and external environmental factors, and its underlying mechanisms have been dissected in great detail in Arabidopsis, thanks to its simple root anatomy and the ease of obtaining mutants. This review comprehensively and systematically summarizes past research (largely in Arabidopsis) on LR basic structure, development stages, and molecular mechanisms regulated by different factors, as well as future prospects in LR research, to provide broad background knowledge for root researchers.

The Genetic Dissection of Nitrogen Use-Related Traits in Flax (Linum usitatissimum L.) at the Seedling Stage through the Integration of Multi-Locus GWAS, RNA-seq and Genomic Selection

Nitrogen (N), the most important macro-nutrient for plant growth and development, is a key factor that determines crop yield. Yet its excessive applications pollute the environment and are expensive. Hence, studying nitrogen use efficiency (NUE) in crops is fundamental for sustainable agriculture. Here, an association panel consisting of 123 flax accessions was evaluated for 21 NUE-related traits at the seedling stage under optimum N (N+) and N deficiency (N-) treatments to dissect the genetic architecture of NUE-related traits using a multi-omics approach integrating genome-wide association studies (GWAS), transcriptome analysis and genomic selection (GS). Root traits exhibited significant and positive correlations with NUE under N- conditions (r = 0.33 to 0.43, p < 0.05). A total of 359 QTLs were identified, accounting for 0.11% to 23.1% of the phenotypic variation in NUE-related traits. Transcriptomic analysis identified 1034 differentially expressed genes (DEGs) under contrasting N conditions. DEGs involved in N metabolism, root development, amino acid transport and catabolism and others, were found near the QTLs. GS models to predict NUE stress tolerance index (NUE_STI) trait were tested using a random genome-wide SNP dataset and a GWAS-derived QTLs dataset. The latter produced superior prediction accuracy (r = 0.62 to 0.79) compared to the genome-wide SNP marker dataset (r = 0.11) for NUE_STI. Our results provide insights into the QTL architecture of NUE-related traits, identify candidate genes for further studies, and propose genomic breeding tools to achieve superior NUE in flax under low N input.

A blast from the past: Understanding stem cell specification in plant roots using laser ablation

In the Arabidopsis root, growth is sustained by the meristem. Signalling from organiser cells, also termed the quiescent centre (QC), is essential for the maintenance and replenishment of the stem cells. Here, we highlight three publications from the founder of the concept of the stem cell niche in Arabidopsis and a pioneer in unravelling regulatory modules governing stem cell specification and maintenance, as well as tissue patterning in the root meristem: Ben Scheres. His research has tremendously impacted the plant field. We have selected three publications from the Scheres legacy, which can be considered a breakthrough in the field of plant developmental biology. van den Berg et al. (1995) and van den Berg et al. (1997) uncovered that positional information-directed patterning. Sabatini et al. (1999), discovered that auxin maxima determine tissue patterning and polarity. We describe how simple but elegant experimental designs have provided the foundation of our current understanding of the functioning of the root meristem.

Interspecies co-expression analysis of lateral root development using inducible systems in rice, Medicago, and Arabidopsis

Lateral roots are crucial for plant growth and development, making them an important target for research aiming to improve crop yields and food security. However, their endogenous ontogeny and, as it were, stochastic appearance challenge their study. Lateral Root Inducible Systems (LRIS) can be used to overcome these challenges by inducing lateral roots massively and synchronously. The combination of LRISs with transcriptomic approaches significantly advanced our insights in the molecular control of lateral root formation, in particular for Arabidopsis. Despite this success, LRISs have been underutilized for other plant species or for lateral root developmental stages later than the initiation. In this study, we developed and/or adapted LRISs in rice, Medicago, and Arabidopsis to perform RNA-sequencing during time courses that cover different developmental stages of lateral root formation and primordium development. As such, our study provides three extensive datasets of gene expression profiles during lateral root development in three different plant species. The three LRISs are highly effective but timing and spatial distribution of lateral root induction vary among the species. Detailed characterization of the stages in time and space in the respective species enabled an interspecies co-expression analysis to identify conserved players involved in lateral root development, as illustrated for the AUX/IAA and LBD gene families. Overall, our results provide a valuable resource to identify potentially conserved regulatory mechanisms in lateral root development, and as such will contribute to a better understanding of the complex regulatory network underlying lateral root development.

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.

Emerging Roles of Receptor-like Protein Kinases in Plant Response to Abiotic Stresses

The productivity of plants is hindered by unfavorable conditions. To perceive stress signals and to transduce these signals to intracellular responses, plants rely on membrane-bound receptor-like kinases (RLKs). These play a pivotal role in signaling events governing growth, reproduction, hormone perception, and defense responses against biotic stresses; however, their involvement in abiotic stress responses is poorly documented. Plant RLKs harbor an N-terminal extracellular domain, a transmembrane domain, and a C-terminal intracellular kinase domain. The ectodomains of these RLKs are quite diverse, aiding their responses to various stimuli. We summarize here the sub-classes of RLKs based on their domain structure and discuss the available information on their specific role in abiotic stress adaptation. Furthermore, the current state of knowledge on RLKs and their significance in abiotic stress responses is highlighted in this review, shedding light on their role in influencing plant-environment interactions and opening up possibilities for novel approaches to engineer stress-tolerant crop varieties.

Allelic variations of ClACO gene improve nitrogen uptake via ethylene-mediated root architecture in watermelon

KEY MESSAGE

The ClACO gene encoding 1-aminocyclopropane-1-carboxylate oxidase enabled highly efficient 15N uptake in watermelon. Nitrogen is one of the most essential nutrient elements that play a pivotal role in regulating plant growth and development for crop productivity. Elucidating the genetic basis of high nitrogen uptake is the key to improve nitrogen use efficiency for sustainable agricultural productivity. Whereas previous researches on nitrogen absorption process are mainly focused on a few model plants or crops. To date, the causal genes that determine the efficient nitrogen uptake of watermelon have not been mapped and remains largely unknown. Here, we fine-mapped the 1-aminocyclopropane-1-carboxylate oxidase (ClACO) gene associated with nitrogen uptake efficiency in watermelon via bulked segregant analysis (BSA). The variations in the ClACO gene led to the changes of gene expression levels between two watermelon accessions with different nitrogen uptake efficiencies. Intriguingly, in terms of the transcript abundance of ClACO, it was concomitant with significant differences in ethylene evolutions in roots and root architectures between the two accessions and among the different genotypic offsprings of the recombinant BC2F1(ZJU132)-18. These findings suggest that ethylene as a negative regulator altered nitrogen uptake efficiency in watermelon by controlling root development. In conclusion, our current study will provide valuable target gene for precise breeding of 'green' watermelon varieties with high-nitrogen uptake efficiencies.

Divided by fate: The interplay between division orientation and cell shape underlying lateral root initiation in Arabidopsis

The development of lateral roots starts with a round of anticlinal, asymmetric cell divisions in lateral root founder cells in the pericycle, deep within the root. The reorientation of the cell division plane occurs in parallel with changes in cell shape and needs to be coordinated with its direct neighbor, the endodermis. This accommodation response requires the integration of biochemical and mechanical signals in both cell types. Recently, it was reported that dynamic changes in the cytoskeleton and possibly the cell wall are part of the molecular mechanism required to correctly orient and position the cell division plane. Here we discuss the latest progress made towards our understanding of the regulation of cell shape and division plane orientation underlying lateral root initiation in Arabidopsis.

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.

GOLVEN peptides regulate lateral root spacing as part of a negative feedback loop on the establishment of auxin maxima

Lateral root initiation requires the accumulation of auxin in lateral root founder cells, yielding a local auxin maximum. The positioning of auxin maxima along the primary root determines the density and spacing of lateral roots. The GOLVEN6 (GLV6) and GLV10 signaling peptides and their receptors have been established as regulators of lateral root spacing via their inhibitory effect on lateral root initiation in Arabidopsis. However, it remained unclear how these GLV peptides interfere with auxin signaling or homeostasis. Here, we show that GLV6/10 signaling regulates the expression of a subset of auxin response genes, downstream of the canonical auxin signaling pathway, while simultaneously inhibiting the establishment of auxin maxima within xylem-pole pericycle cells that neighbor lateral root initiation sites. We present genetic evidence that this inhibitory effect relies on the activity of the PIN3 and PIN7 auxin export proteins. Furthermore, GLV6/10 peptide signaling was found to enhance PIN7 abundance in the plasma membranes of xylem-pole pericycle cells, which likely stimulates auxin efflux from these cells. Based on these findings, we propose a model in which the GLV6/10 signaling pathway serves as a negative feedback mechanism that contributes to the robust patterning of auxin maxima along the primary root.

Molecular Mechanisms of Regulation of Root Development by Plant Peptides

Peptides perform many functions, participating in the regulation of cell differentiation, regulating plant growth and development, and also involved in the response to stress factors and in antimicrobial defense. Peptides are an important class biomolecules for intercellular communication and in the transmission of various signals. The intercellular communication system based on the ligand-receptor bond is one of the most important molecular bases for creating complex multicellular organisms. Peptide-mediated intercellular communication plays a critical role in the coordination and determination of cellular functions in plants. The intercellular communication system based on the receptor-ligand is one of the most important molecular foundations for creating complex multicellular organisms. Peptide-mediated intercellular communication plays a critical role in the coordination and determination of cellular functions in plants. The identification of peptide hormones, their interaction with receptors, and the molecular mechanisms of peptide functioning are important for understanding the mechanisms of both intercellular communications and for regulating plant development. In this review, we drew attention to some peptides involved in the regulation of root development, which implement this regulation by the mechanism of a negative feedback loop.

CLE3 and its homologs share overlapping functions in the modulation of lateral root formation through CLV1 and BAM1 in Arabidopsis thaliana

Lateral roots are important for a wide range of processes, including uptake of water and nutrients. The CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION-RELATED (CLE) 1 ~ 7 peptide family and their cognate receptor CLV1 have been shown to negatively regulate lateral root formation under low-nitrate conditions. However, little is known about how CLE signaling regulates lateral root formation. A persistent obstacle in CLE peptide research is their functional redundancies, which makes functional analyses difficult. To address this problem, we generate the cle1 ~ 7 septuple mutant (cle1 ~ 7-cr1, cr stands for mutant allele generated with CRISPR/Cas9). cle1 ~ 7-cr1 exhibits longer lateral roots under normal conditions. Specifically, in cle1 ~ 7-cr1, the lateral root density is increased, and lateral root primordia initiation is found to be accelerated. Further analysis shows that cle3 single mutant exhibits slightly longer lateral roots. On the other hand, plants that overexpress CLE2 and CLE3 exhibit decreased lateral root lengths. To explore cognate receptor(s) of CLE2 and CLE3, we analyze lateral root lengths in clv1 barely any meristem 1(bam1) double mutant. Mutating both the CLV1 and BAM1 causes longer lateral roots, but not in each single mutant. In addition, genetic analysis reveals that CLV1 and BAM1 are epistatic to CLE2 and CLE3. Furthermore, gene expression analysis shows that the LATERAL ORGAN BOUNDARIES DOMAIN/ASYMMETRIC LEAVES2-LIKE (LBD/ASL) genes, which promote lateral root formation, are upregulated in cle1 ~ 7-cr1 and clv1 bam1. We therefore propose that CLE2 and CLE3 peptides are perceived by CLV1 and BAM1 to mediate lateral root formation through LBDs regulation.

Advances in structure and function of auxin response factor in plants

Auxin is a crucial phytohormone that has various effects on the regulators of plant growth and development. Auxin signal transduction is mainly controlled by two gene families: auxin response factor (ARF) and auxin/indole-3-acetic acid (Aux/IAA). ARFs are plant-specific transcription factors that bind directly to auxin response elements in the promoters of auxin-responsive genes. ARF proteins contain three conserved regions: a conserved N-terminal B3 DNA-binding domain, a variable intermediate middle region domain that functions in activation or repression, and a C-terminal domain including the Phox and Bem1p region for dimerization, similar to the III and IV elements of Aux/IAA, which facilitate protein-protein interaction through homodimerization of ARF proteins or heterodimerization of ARF and Aux/IAA proteins. In the two decades following the identification of the first ARF, 23 ARF members have been identified and characterized in Arabidopsis. Using whole-genome sequencing, 22, 25, 23, 25, and 36 ARF genes have been identified in tomato, rice, wheat, sorghum, and maize, respectively, in addition to which the related biofunctions of some ARFs have been reported. ARFs play crucial roles in regulating the growth and development of roots, leaves, flowers, fruits, seeds, responses to biotic and abiotic stresses, and phytohormone signal crosstalk. In this review, we summarize the research progress on the structures and functions of ARFs in Arabidopsis, tomato, and cereal crops, to provide clues for future basic research on phytohormone signaling and the molecular design breeding of crops.

The plant specific SHORT INTERNODES/STYLISH (SHI/STY) proteins: Structure and functions

Plant specific SHORT INTERNODES/STYLISH (SHI/STY) protein is a transcription factor involved in the formation and development of early lateral organs in plants. However, research on the SHI/STY protein family is not focused enough. In this article, we review recent studies on SHI/STY genes and explore the evolution and structure of SHI/STY. The biological functions of SHI/STYs are discussed in detail in this review, and the application of each biological function to modern agriculture is discussed. All SHI/STY proteins contain typical conserved RING-like zinc finger domain and IGGH domain. SHI/STYs are involved in the formation and development of lateral root, stem extension, leaf morphogenesis, and root nodule development. They are also involved in the regulation of pistil and stamen development and flowering time. At the same time, the regulation of some GA, JA, and auxin signals also involves these family proteins. For each aspect, unanswered or poorly understood questions were identified to help define future research areas. This review will provide a basis for further functional study of this gene family.

Cellular and gene expression patterns associated with root bifurcation in Selaginella

The roots of lycophytes branch through dichotomy or bifurcation, during which the root apex splits into two daughter roots. This is morphologically distinct from lateral root (LR) branching in the extant euphyllophytes, with LRs developing along the root axis at different distances from the apex. Although the process of root bifurcation is poorly understood, such knowledge can be important, because it may represent an evolutionarily ancient strategy that roots recruited to form new stem cells or meristems. In this study, we examined root bifurcation in the lycophyte Selaginella moellendorffii. We characterized an in vitro developmental time frame based on repetitive apex bifurcations, allowing us to sample different stages of dichotomous root branching and analyze the root meristem and root branching in S. moellendorffii at the microscopic and transcriptomic level. Our results showed that, in contrast to previous assumptions, initial cells (ICs) in the root meristem are mostly not tetrahedral but rather show an irregular shape. Tracking down the early stages of root branching argues for the occurrence of a symmetric division of the single IC, resulting in two apical stem cells that initiate root meristem bifurcation. Moreover, we generated a S. moellendorffii root branching transcriptome that resulted in the delineation of a subset of core meristem genes. The occurrence of multiple putative orthologs of meristem genes in this dataset suggests the presence of conserved pathways in the control of meristem and root stem cell establishment or maintenance.

Complementary roles for auxin and auxin signalling revealed by reverse engineering lateral root stable prebranch site formation

Priming is the process through which periodic elevations in auxin signalling prepattern future sites for lateral root formation, called prebranch sites. Thus far, the extent to which elevations in auxin concentration and/or auxin signalling are required for priming and prebranch site formation has remained a matter of debate. Recently, we discovered a reflux-and-growth mechanism for priming generating periodic elevations in auxin concentration that subsequently dissipate. Here, we reverse engineer a mechanism for prebranch site formation that translates these transient elevations into a persistent increase in auxin signalling, resolving the prior debate into a two-step process of auxin concentration-mediated initial signal and auxin signalling capacity-mediated memorization. A crucial aspect of the prebranch site formation mechanism is its activation in response to time-integrated rather than instantaneous auxin signalling. The proposed mechanism is demonstrated to be consistent with prebranch site auxin signalling dynamics, lateral inhibition, and symmetry-breaking mechanisms and perturbations in auxin homeostasis.

Multispecies transcriptomes reveal core fruit development genes

During angiosperm evolution there have been repeated transitions from an ancestral dry fruit to a derived fleshy fruit, often with dramatic ecological and economic consequences. Following the transition to fleshy fruits, domestication may also dramatically alter the fruit phenotype via artificial selection. Although the morphologies of these fruits are well documented, relatively less is known about the molecular basis of these developmental and evolutionary shifts. We generated RNA-seq libraries from pericarp tissue of desert tobacco and both cultivated and wild tomato species at common developmental time points and combined this with corresponding, publicly available data from Arabidopsis and melon. With this broadly sampled dataset consisting of dry/fleshy fruits and wild/domesticated species, we applied novel bioinformatic methods to investigate conserved and divergent patterns of gene expression during fruit development and evolution. A small set of 121 orthologous "core" fruit development genes show a common pattern of expression across all five species. These include key players in developmental patterning such as orthologs of KNOLLE, PERIANTHIA, and ARGONAUTE7. GO term enrichment suggests that these genes function in basic cell division processes, cell wall biosynthesis, and developmental patterning. We furthermore uncovered a number of "accessory" genes with conserved expression patterns within but not among fruit types, and whose functional enrichment highlights the conspicuous differences between these phenotypic classes. We observe striking conservation of gene expression patterns despite large evolutionary distances, and dramatic phenotypic shifts, suggesting a conserved function for a small subset of core fruit development genes.

Overexpression of a SHORT-ROOT transcriptional factor enhances the auxin mediated formation of adventitious roots and lateral roots in poplar trees

SHORT-ROOT (SHR) defines root stem cells and maintains radial patterning, but its involvement in adventitious root (AR) formation has not been reported. In this study, we showed that PtSHR2 was transcriptionally upregulated by excision before the formation of AR and responded dynamically to auxin. PtSHR2 overexpression (SHR2B^OE) in hybrid poplars resulted in an increased number of ARs with an initial delay. Despite a lower endogenous content in the stems than in wild-type plants, indole-3-acetic acid (IAA) content at the SHR2B^OE basal stem increased rapidly after cutting and reached a higher maximum than in wild-type plants, which was accompanied by a more sustained and stronger induction of AR formation marker genes. In addition, the higher auxin content in SHR2B^OE ARs resulted in more and longer lateral roots (LRs). Application of auxin abolished the early delay in the formation of AR and largely other AR phenotypes of SHR2B^OE plants, whereas the polar auxin transport inhibitor N-1-naphthylphthalamic acid completely inhibited both AR and LR abnormalities. Since the enhanced rooting ability of SHR2B^OE stem cuttings in hydroponics was clearly confirmed, our results suggest a novel role of poplar SHR2 as a positive regulator during the organogenesis of AR and LR by affecting local auxin homeostasis.

Histidine kinase inhibitors impair shoot regeneration in Arabidopsis thaliana via cytokinin signaling and SAM patterning determinants

Reversible protein phosphorylation is a post-translational modification involved in virtually all plant processes, as it mediates protein activity and signal transduction. Here, we probe dynamic protein phosphorylation during de novo shoot organogenesis in Arabidopsis thaliana. We find that application of three kinase inhibitors in various time intervals has different effects on root explants. Short exposures to the putative histidine (His) kinase inhibitor TCSA during the initial days on shoot induction medium (SIM) are detrimental for regeneration in seven natural accessions. Investigation of cytokinin signaling mutants, as well as reporter lines for hormone responses and shoot markers, suggests that TCSA impedes cytokinin signal transduction via AHK3, AHK4, AHP3, and AHP5. A mass spectrometry-based phosphoproteome analysis further reveals profound deregulation of Ser/Thr/Tyr phosphoproteins regulating protein modification, transcription, vesicle trafficking, organ morphogenesis, and cation transport. Among TCSA-responsive factors are prior candidates with a role in shoot apical meristem patterning, such as AGO1, BAM1, PLL5, FIP37, TOP1ALPHA, and RBR1, as well as proteins involved in polar auxin transport (e.g., PIN1) and brassinosteroid signaling (e.g., BIN2). Putative novel regeneration determinants regulated by TCSA include RD2, AT1G52780, PVA11, and AVT1C, while NAIP2, OPS, ARR1, QKY, and aquaporins exhibit differential phospholevels on control SIM. LC-MS/MS data are available via ProteomeXchange with identifier PXD030754.

Text-based experiment retrieval in genomic databases

With the growing number of genomic data in public repositories, efficient search methodologies have become a basic need to reach the relevant genomic data. However, this need cannot be fulfilled with the current repositories because they offer a limited search option which is a lexical matching of textual descriptions or metadata of the experiments. This technique is insufficient to get the required information needed to detect similarities between experiments within a large data collection. Due to the limitation of the existing repositories, in this study, we develop a text-based experiment retrieval framework by using both lexical and semantic similarity approaches to find similarities between experiments, and their retrieval performance was compared. This study is the first attempt to use text-driven semantic analysis approaches for developing a retrieval framework for experiments. An empirical study was conducted on a large textual description of Arabidopsis microarray experiments from the Gene Expression Omnibus database. In the proposed model, Jaccard similarity was used as a lexical similarity approach; Latent Semantic Analysis, Probabilistic Latent Semantic Analysis and Latent Dirichlet allocation were used as semantic similarity approaches to detect similarities between the textual descriptions of the experiments. According to the experimental results, relevant experiments can be retrieved successfully by text-driven semantic similarity approaches compared with the lexical similarity approach.

Genome-wide identification, structural analysis and expression profiles of short internodes related sequence gene family in quinoa

Based on the whole genome data information of Chenopodium quinoa Willd, the CqSRS gene family members were systematically identified and analyzed by bioinformatics methods, and the responses of CqSRS genes to NaCl (100 mmol/L), salicylic acid (200 umol/L) and low temperature (4°C) were detected by qRT-PCR. The results showed that a total of 10 SHI related sequence genes were identified in quinoa, and they were distributed on 9 chromosomes, and there were four pairs of duplicated genes. The number of amino acids encoded ranged from 143 aa to 370 aa, and the isoelectric point ranged from 4.81 to 8.90. The secondary structure was mainly composed of random coil (Cc). Most of the SRS gene encoding proteins were located in the cytoplasm (5 CqSRS). Phylogenetic analysis showed that the CqSRS genes were divided into three groups, and the gene structure showed that the number of exons of CqSRS was between two-five. Promoter analysis revealed that there are a total of 44 elements related to plant hormone response elements, light response elements, stress response elements and tissue-specific expression in the upstream regin of the gene. Protein interaction showed that all 10 CqSRS proteins appeared in the known protein interaction network diagram in Arabidopsis. Expression profile analysis showed that CqSRS genes had different expression patterns, and some genes had tissue-specific expression. qRT-PCR showed that all SRS family genes responded to ABA、NaCl、drought and low-temperature treatments, but the expression levels of different CqSRS genes were significantly different under various stresses. This study lays a foundation for further analyzed the function of CqSRS genes.

Molecular evolutionary analysis of the SHI/STY gene family in land plants: A focus on the Brassica species

The plant-specific SHORT INTERNODES/STYLISH (SHI/STY) proteins belong to a family of transcription factors that are involved in the formation and development of early lateral roots. However, the molecular evolution of this family is rarely reported. Here, a total of 195 SHI/STY genes were identified in 21 terrestrial plants, and the Brassica species is the focus of our research. Their physicochemical properties, chromosome location and duplication, motif distribution, exon-intron structures, genetic evolution, and expression patterns were systematically analyzed. These genes are divided into four clades (Clade 1/2/3/4) based on phylogenetic analysis. Motif distribution and gene structure are similar in each clade. SHI/STY proteins are localized in the nucleus by the prediction of subcellular localization. Collinearity analysis indicates that the SHI/STYs are relatively conserved in evolution. Whole-genome duplication is the main factor for their expansion. SHI/STYs have undergone intense purifying selection, but several positive selection sites are also identified. Most promoters of SHI/STY genes contain different types of cis-elements, such as light, stress, and hormone-responsive elements, suggesting that they may be involved in many biological processes. Protein-protein interaction predicted some important SHI/STY interacting proteins, such as LPAT4, MBOATs, PPR, and UBQ3. In addition, the RNA-seq and qRT-PCR analysis were studied in detail in rape. As a result, SHI/STYs are highly expressed in root and bud, and can be affected by Sclerotinia sclerotiorum, drought, cold, and heat stresses. Moreover, quantitative real-time PCR (qRT-PCR) analyses indicates that expression levels of BnSHI/STYs are significantly altered in different treatments (cold, salt, drought, IAA, auxin; ABA, abscisic acid; 6-BA, cytokinin). It provides a new understanding of the evolution and expansion of the SHI/STY family in land plants and lays a foundation for further research on their functions.

A Dof-CLE circuit controls phloem organization

The phloem consists of sieve elements (SEs) and companion cells (CCs). Here we show that Dof-class transcription factors preferentially expressed in the phloem (phloem-Dofs) are not only necessary and sufficient for SE and CC differentiation, but also induce negative regulators of phloem development, CLAVATA3/EMBRYO SURROUNDING REGION-RELATED25 (CLE25), CLE26 and CLE45 secretory peptides. CLEs were perceived by BARELY ANY MERISTEM (BAM)-class receptors and CLAVATA3 INSENSITIVE RECEPTOR KINASE (CIK) co-receptors, and post-transcriptionally decreased phloem-Dof proteins and repressed SE and CC formation. Multiple mutations in CLE-, BAM- or CIK-class genes caused ectopic formation of SEs and CCs, producing an SE/CC cluster at each phloem region. We propose that while phloem-Dofs induce phloem cell formation, they inhibit excess phloem cell formation by inducing CLEs. Normal-positioned SE and CC precursor cells appear to overcome the effect of CLEs by reinforcing the production of phloem-Dofs through a positive feedback transcriptional regulation.

Prohibitin 3 gives birth to a new lateral root primordium

This article comments on: Li S, Li Q, Tian X, Mu L, Ji M, Wang X, Li N, Liu F, Shu J, Crawford NM, Wang Y. 2022. PHB3 regulates lateral root primordia formation via NO-mediated degradation of AUXIN/INDOLE-3-ACETIC ACID proteins. Journal of Experimental Botany 73,4034–4045.

Molecular Determinants of in vitro Plant Regeneration: Prospects for Enhanced Manipulation of Lettuce (Lactuca sativa L.)

In vitro plant regeneration involves dedifferentiation and molecular reprogramming of cells in order to regenerate whole organs. Plant regeneration can occur via two pathways, de novo organogenesis and somatic embryogenesis. Both pathways involve intricate molecular mechanisms and crosstalk between auxin and cytokinin signaling. Molecular determinants of both pathways have been studied in detail in model species, but little is known about the molecular mechanisms controlling de novo shoot organogenesis in lettuce. This review provides a synopsis of our current knowledge on molecular determinants of de novo organogenesis and somatic embryogenesis with an emphasis on the former as well as provides insights into applying this information for enhanced in vitro regeneration in non-model species such as lettuce (Lactuca sativa L.).

Essential roles of SERKs in the ROOT MERISTEM GROWTH FACTOR-mediated signaling pathway

ROOT MERISTEM GROWTH FACTORs (RGFs), a group of peptide hormones, play key roles in root apical meristem development. In Arabidopsis (Arabidopsis thaliana), there are 11 members of RGFs, in which at least RGF1, RGF2, and RGF3 are expressed at the root tip and are involved in root stem cell niche maintenance. RGFs are perceived by five functionally redundant receptor-like protein kinases, RGF1 INSENSITIVE 1 (RGI1) to RGI5, to maintain the expression of two downstream APETALA 2 (AP2) transcription factor genes, PLETHORA 1 (PLT1) and PLT2, and to stabilize PLT2. RGI1 to RGI3 were also named RGF RECEPTOR 1 (RGFR1) to RGFR3, respectively. Although previous studies have suggested that BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) and its paralogs, SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASEs (SERKs), may act as coreceptors of RGIs, comprehensive genetic and biochemical analyses have not been well documented. Here, we report that single, double, and triple mutants of SERKs show various degrees of short root phenotypes and insensitivity to exogenously applied RGF1. The interaction between RGIs and BAK1 and their mutual phosphorylation are RGF1 dependent. We also found that RGF1-induced MAPK activation relies on both RGIs and SERKs. We demonstrate that RGIs play redundant roles in regulating root apical meristem development. Therefore, we genetically and biochemically substantiated that SERKs, as coreceptors, play essential roles in the RGF1-mediated signaling pathway.

Control of Root Stem Cell Differentiation and Lateral Root Emergence by CLE16/17 Peptides in Arabidopsis

Secreted peptide-mediated cell-to-cell communication plays a crucial role in the development of multicellular organisms. A large number of secreted peptides have been predicated by bioinformatic approaches in plants. However, only a few of them have been functionally characterized. In this study, we show that two CLAVATA3/EMBRYO SURROUNDING REGION-RELATED (CLE) peptides CLE16/17 are required for both stem cell differentiation and lateral root (LR) emergence in Arabidopsis. We further demonstrate that the CLE16/17 peptides act through the CLAVATA1-ARABIDOPSIS CRINKLY4 (CLV1-ACR4) protein kinase complex in columella stem cell (CSC) differentiation, but not in LR emergence. Furthermore, we show that CLE16/17 promote LR emergence probably via activating the expression of HAESA/HAESA-LIKE2 (HAE/HSL2) required for cell wall remodeling. Collectively, our results reveal a CLV1-ACR4-dependent and -independent dual-function of the CLE16/17 peptides in root development.

Combining Fine Mapping, Whole-Genome Re-Sequencing, and RNA-Seq Unravels Candidate Genes for a Soybean Mutant with Short Petioles and Weakened Pulvini

A short petiole is an important agronomic trait for the development of plant ideotypes with high yields. However, the genetic basis underlying this trait remains unclear. Here, we identified and characterized a novel soybean mutant with short petioles and weakened pulvini, designated as short petioles and weakened pulvini (spwp). Compared with the wild type (WT), the spwp mutant displayed shortened petioles, owing to the longitudinally decreased cell length, and exhibited a smaller pulvinus structure due to a reduction in motor cell proliferation and expansion. Genetic analysis showed that the phenotype of the spwp mutant was controlled by two recessive nuclear genes, named as spwp1 and spwp2. Using a map-based cloning strategy, the spwp1 locus was mapped in a 183 kb genomic region on chromosome 14 between markers S1413 and S1418, containing 15 annotated genes, whereas the spwp2 locus was mapped in a 195 kb genomic region on chromosome 11 between markers S1373 and S1385, containing 18 annotated genes. Based on the whole-genome re-sequencing and RNA-seq data, we identified two homologous genes, Glyma.11g230300 and Glyma.11g230600, as the most promising candidate genes for the spwp2 locus. In addition, the RNA-seq analysis revealed that the expression levels of genes involved in the cytokinin and auxin signaling transduction networks were altered in the spwp mutant compared with the WT. Our findings provide new gene resources for insights into the genetic mechanisms of petiole development and pulvinus establishment, as well as soybean ideotype breeding.

The hidden half comes into the spotlight: Peeking inside the black box of root developmental phases

Efficient use of natural resources (e.g., light, water, and nutrients) can be improved with a tailored developmental program that maximizes the lifetime and fitness of plants. In plant shoots, a developmental phase represents a time window in which the meristem triggers the development of unique morphological and physiological traits, leading to the emergence of leaves, flowers, and fruits. Whereas developmental phases in plant shoots have been shown to enhance food production in crops, this phenomenon has remained poorly investigated in roots. In light of recent advances, we suggest that root development occurs in three main phases: root apical meristem appearance, foraging, and senescence. We provide compelling evidence suggesting that these phases are regulated by at least four developmental pathways: autonomous, non-autonomous, hormonal, and periodic. Root developmental pathways differentially coordinate organ plasticity, promoting morphological alterations, tissue regeneration, and cell death regulation. Furthermore, we suggest how nutritional checkpoints may allow progression through the developmental phases, thus completing the root life cycle. These insights highlight novel and exciting advances in root biology that may help maximize the productivity of crops through more sustainable agriculture and the reduced use of chemical fertilizers.

Transcriptional Regulation of Different Rhizome Parts Reveal the Candidate Genes That Regulate Rhizome Development in Poa pratensis

A strong rhizome can enhance the ability of a plant to resist drought, low temperature, and other stresses, as it can help plants rapidly obtain water and nutrients. Poa pratensis var. anceps Gaud. cv. Qinghai (QH) is a variant of P. pratensis that is widely distributed in natural grasslands above 3000 m above sea level on the Qinghai-Tibet Plateau. It forms turf easily and has strong soil-fixing ability due to its well-developed rhizomes. Understanding the molecular mechanism of rhizome development in this species is essential for cultivating new varieties of rhizome-type pasture for ecological protection. To clarify the transcriptional regulatory changes in different parts of the rhizome, we analyzed three different rhizome parts (rhizome buds, rhizome nodes, and rhizome internodes) of QH and weak-rhizome wild P. pratensis material (SN) using RNA sequencing. A total of 3806 genes were specifically expressed in Q_B, 1104 genes were specifically expressed in Q_N, and 1181 genes were specifically expressed in Q_I. Analysis showed that MYB, B3, NAC, BBR-BPC, AP2 MIKC_MADS, BSE1, and C2H2 may be key transcription factors regulating rhizome development. These genes interacted with multiple functional genes related to carbohydrate, secondary metabolism, and signal transduction, thus ensuring the normal development of the rhizomes. In particular, SUS (sucrose synthase) [EC:2.4.1.13] is specifically expressed in Q_I, which may be an inducing factor for the production of new plants from Q_B and Q_N. Additionally, PYL, PP2C, and SNRK2, which are involved in the abscisic acid signaling pathway, were differentially expressed in Q_N. In addition, genes related to protein modification and degradation, such as CIPKs, MAPKs, E2, and E3 ubiquitin ligases, were also involved in rhizome development. This study laid a foundation for further functional genomics studies on rhizome development in P. pratensis.

Integration of Cell Growth and Asymmetric Division during Lateral Root Initiation in Arabidopsis thaliana

Lateral root formation determines to a large extent the ability of plants to forage their environment and thus their growth. In Arabidopsis thaliana and other angiosperms, lateral root initiation requires radial cell expansion and several rounds of anticlinal cell divisions that give rise to a central core of small cells, which express different markers than the larger surrounding cells. These small central cells then switch their plane of divisions to periclinal and give rise to seemingly morphologically similar daughter cells that have different identities and establish the different cell types of the new root. Although the execution of these anticlinal and periclinal divisions is tightly regulated and essential for the correct development of the lateral root, we know little about their geometrical features. Here, we generate a four-dimensional reconstruction of the first stages of lateral root formation and analyze the geometric features of the anticlinal and periclinal divisions. We identify that the periclinal divisions of the small central cells are morphologically dissimilar and asymmetric. We show that mother cell volume is different when looking at anticlinal vs. periclinal divisions and the repeated anticlinal divisions do not lead to reduction in cell volume, although cells are shorter. Finally, we show that cells undergoing a periclinal division are characterized by a strong cell expansion. Our results indicate that cells integrate growth and division to precisely partition their volume upon division during the first two stages of lateral root formation.

A single-cell view of the transcriptome during lateral root initiation in Arabidopsis thaliana

Root architecture is a major determinant of plant fitness and is under constant modification in response to favorable and unfavorable environmental stimuli. Beyond impacts on the primary root, the environment can alter the position, spacing, density, and length of secondary or lateral roots. Lateral root development is among the best-studied examples of plant organogenesis, yet there are still many unanswered questions about its earliest steps. Among the challenges faced in capturing these first molecular events is the fact that this process occurs in a small number of cells with unpredictable timing. Single-cell sequencing methods afford the opportunity to isolate the specific transcriptional changes occurring in cells undergoing this fate transition. Using this approach, we successfully captured the transcriptomes of initiating lateral root primordia in Arabidopsis thaliana and discovered many upregulated genes associated with this process. We developed a method to selectively repress target gene transcription in the xylem pole pericycle cells where lateral roots originate and demonstrated that the expression of several of these targets is required for normal root development. We also discovered subpopulations of cells in the pericycle and endodermal cell files that respond to lateral root initiation, highlighting the coordination across cell files required for this fate transition.

Reconstruction of lateral root formation through single-cell RNA sequencing reveals order of tissue initiation

Postembryonic organogenesis is critical for plant development. Underground, lateral roots (LRs) form the bulk of mature root systems, yet the ontogeny of the LR primordium (LRP) is not clear. In this study, we performed the single-cell RNA sequencing through the first four stages of LR formation in Arabidopsis. Our analysis led to a model in which a single group of precursor cells, with a cell identity different from their pericycle origins, rapidly reprograms and splits into a mixed ground tissue/stem cell niche fate and a vascular precursor fate. The ground tissue and stem cell niche fates soon separate and a subset of more specialized vascular cells form sucrose transporting phloem cells that appear to connect to the primary root. We did not detect cells resembling epidermis or root cap, suggesting that outer tissues may form later, preceding LR emergence. At this stage, some remaining initial precursor cells form the primordium flanks, while the rest create a reservoir of pluripotent cells that are able to replace the LR if damaged. Laser ablation of the central and lateral LRP regions showed that remaining cells restart the sequence of tissue initiation to form a LR. Collectively, our study reveals an ontological hierarchy for LR formation with an early and sequential split of main root tissues and stem cells.

Plant-specific histone deacetylases are essential for early and late stages of Medicago nodule development

Legume and rhizobium species can establish a nitrogen-fixing nodule symbiosis. Previous studies have shown that several transcription factors that play a role in (lateral) root development are also involved in nodule development. Chromatin remodeling factors, like transcription factors, are key players in regulating gene expression. However, studies have not investigated whether chromatin remodeling genes that are essential for root development are also involved in nodule development. Here, we studied the role of Medicago (Medicago truncatula) histone deacetylases (MtHDTs) in nodule development. Arabidopsis (Arabidopsis thaliana) orthologs of HDTs have been shown to play a role in root development. MtHDT expression is induced in nodule primordia and is maintained in the nodule meristem and infection zone. Conditional, nodule-specific knockdown of MtHDT expression by RNAi blocks nodule primordium development. A few nodules may still form, but their nodule meristems are smaller, and rhizobial colonization of the cells derived from the meristem is markedly reduced. Although the HDTs are expressed during nodule and root development, transcriptome analyses indicate that HDTs control the development of each organ in a different manner. During nodule development, the MtHDTs positively regulate 3-hydroxy-3-methylglutaryl coenzyme a reductase 1 (MtHMGR1). Decreased expression of MtHMGR1 is sufficient to explain the inhibition of primordium formation.

Conserved and differentiated functions of CIK receptor kinases in modulating stem cell signaling in Arabidopsis

The shoot apical meristem (SAM) and root apical meristem (RAM) act as pools of stem cells that give rise to aboveground and underground tissues and organs in higher plants, respectively. The CLAVATA3 (CLV3)-WUSCHEL (WUS) negative-feedback loop acts as a core pathway controlling SAM homeostasis, while CLV3/EMBRYO SURROUNDING REGION (ESR) 40 (CLE40) and WUSCHEL-RELATED HOMEOBOX5 (WOX5), homologs of CLV3 and WUS, direct columella stem cell fate. Moreover, CLV3 INSENSITIVE KINASES (CIKs) have been shown to be essential for maintaining SAM homeostasis, whereas whether they regulate the distal root meristem remains to be elucidated. Here, we report that CIKs are indispensable for transducing the CLE40 signal to maintain homeostasis of the distal root meristem. We found that the cik mutant roots displayed disrupted quiescent center and delayed columella stem cell (CSC) differentiation. Biochemical assays demonstrated that CIKs interact with ARABIDOPSIS CRINKLY4 (ACR4) in a ligand-independent manner and can be phosphorylated by ACR4 in vitro. In addition, the phosphorylation of CIKs can be rapidly induced by CLE40, which partially depends on ACR4. Although CIKs act as conserved and redundant regulators in the SAM and RAM, our results demonstrated that they exhibit differentiated functions in these meristems.

Lateral Root Initiation and the Analysis of Gene Function Using Genome Editing with CRISPR in Arabidopsis

Lateral root initiation is a post-embryonic process that requires the specification of a subset of pericycle cells adjacent to the xylem pole in the primary root into lateral root founder cells. The first visible event of lateral root initiation in Arabidopsis is the simultaneous migration of nuclei in neighbouring founder cells. Coinciding cell cycle activation is essential for founder cells in the pericycle to undergo formative divisions, resulting in the development of a lateral root primordium (LRP). The plant signalling molecule, auxin, is a major regulator of lateral root development; the understanding of the molecular mechanisms controlling lateral root initiation has progressed tremendously by the use of the Arabidopsis model and a continual improvement of molecular methodologies. Here, we provide an overview of the visible events, cell cycle regulators, and auxin signalling cascades related to the initiation of a new LRP. Furthermore, we highlight the potential of genome editing technology to analyse gene function in lateral root initiation, which provides an excellent model to answer fundamental developmental questions such as coordinated cell division, growth axis establishment as well as the specification of cell fate and cell polarity.

Advances and perspectives in discovery and functional analysis of small secreted proteins in plants

Small secreted proteins (SSPs) are less than 250 amino acids in length and are actively transported out of cells through conventional protein secretion pathways or unconventional protein secretion pathways. In plants, SSPs have been found to play important roles in various processes, including plant growth and development, plant response to abiotic and biotic stresses, and beneficial plant-microbe interactions. Over the past 10 years, substantial progress has been made in the identification and functional characterization of SSPs in several plant species relevant to agriculture, bioenergy, and horticulture. Yet, there are potentially a lot of SSPs that have not been discovered in plant genomes, which is largely due to limitations of existing computational algorithms. Recent advances in genomics, transcriptomics, and proteomics research, as well as the development of new computational algorithms based on machine learning, provide unprecedented capabilities for genome-wide discovery of novel SSPs in plants. In this review, we summarize known SSPs and their functions in various plant species. Then we provide an update on the computational and experimental approaches that can be used to discover new SSPs. Finally, we discuss strategies for elucidating the biological functions of SSPs in plants.

Identification of tomato root growth regulatory genes and transcription factors through comparative transcriptomic profiling of different tissues

Tomato is an economically important vegetable crop and a model for development and stress response studies. Although studied extensively for understanding fruit ripening and pathogen responses, its role as a model for root development remains less explored. In this study, an Illumina-based comparative differential transcriptomic analysis of tomato root with different aerial tissues was carried out to identify genes that are predominantly expressed during root growth. Sequential comparisons revealed ~ 15,000 commonly expressed genes and ~ 3000 genes of several classes that were mainly expressed or regulated in roots. These included 1069 transcription factors (TFs) of which 100 were differentially regulated. Prominent amongst these were members of families encoding Zn finger, MYB, ARM, bHLH, AP2/ERF, WRKY and NAC proteins. A large number of kinases, phosphatases and F-box proteins were also expressed in the root transcriptome. The major hormones regulating root growth were represented by the auxin, ethylene, JA, ABA and GA pathways with root-specific expression of certain components. Genes encoding carbon metabolism and photosynthetic components showed reduced expression while several protease inhibitors were amongst the most highly expressed. Overall, the study sheds light on genes governing root growth in tomato and provides a resource for manipulation of root growth for plant improvement.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01015-0.

Distinct genetic basis for root responses to lipo-chitooligosaccharide signal molecules from different microbial origins

Lipo-chitooligosaccharides (LCOs) were originally found as symbiotic signals called Nod Factors (Nod-LCOs) controlling the nodulation of legumes by rhizobia. More recently, LCOs were also found in symbiotic fungi and, more surprisingly, very widely in the kingdom Fungi, including in saprophytic and pathogenic fungi. The LCO-V(C18:1, fucosylated/methyl fucosylated), hereafter called Fung-LCOs, are the LCO structures most commonly found in fungi. This raises the question of how legume plants such as Medicago truncatula can discriminate between Nod-LCOs and Fung-LCOs. To address this question, we performed a genome-wide association study on 173 natural accessions of M. truncatula, using a root branching phenotype and a newly developed local score approach. Both Nod-LCOs and Fung-LCOs stimulated root branching in most accessions, but the root responses to these two types of LCO molecules were not correlated. In addition, the heritability of the root response was higher for Nod-LCOs than for Fung-LCOs. We identified 123 loci for Nod-LCO and 71 for Fung-LCO responses, of which only one was common. This suggests that Nod-LCOs and Fung-LCOs both control root branching but use different molecular mechanisms. The tighter genetic constraint of the root response to Fung-LCOs possibly reflects the ancestral origin of the biological activity of these molecules.

Unraveling Root Development Through Single-Cell Omics and Reconstruction of Gene Regulatory Networks

Over the last decades, research on postembryonic root development has been facilitated by "omics" technologies. Among these technologies, microarrays first, and RNA sequencing (RNA-seq) later, have provided transcriptional information on the underlying molecular processes establishing the basis of System Biology studies in roots. Cell fate specification and development have been widely studied in the primary root, which involved the identification of many cell type transcriptomes and the reconstruction of gene regulatory networks (GRN). The study of lateral root (LR) development has not been an exception. However, the molecular mechanisms regulating cell fate specification during LR formation remain largely unexplored. Recently, single-cell RNA-seq (scRNA-seq) studies have addressed the specification of tissues from stem cells in the primary root. scRNA-seq studies are anticipated to be a useful approach to decipher cell fate specification and patterning during LR formation. In this review, we address the different scRNA-seq strategies used both in plants and animals and how we could take advantage of scRNA-seq to unravel new regulatory mechanisms and reconstruct GRN. In addition, we discuss how to integrate scRNA-seq results with previous RNA-seq datasets and GRN. We also address relevant findings obtained through single-cell based studies and how LR developmental studies could be facilitated by scRNA-seq approaches and subsequent GRN inference. The use of single-cell approaches to investigate LR formation could help to decipher fundamental biological mechanisms such as cell memory, synchronization, polarization, or pluripotency.

Two types of bHLH transcription factor determine the competence of the pericycle for lateral root initiation

The molecular basis of the competence of the pericycle cell to initiate lateral root primordium formation is totally unknown. Here, we report that in Arabidopsis, two types of basic helix-loop-helix (bHLH) transcription factors, named PERICYCLE FACTOR TYPE-A (PFA) proteins and PERICYCLE FACTOR TYPE-B (PFB) proteins, govern the competence of pericycle cells to initiate lateral root primordium formation. Overexpression of PFA genes confers hallmark pericycle characteristics, including specific marker gene expression and auxin-induced cell division, and multiple loss-of-function mutations in PFA genes or the repression of PFB target genes results in the loss of this specific pericycle function. PFA and PFB proteins physically interact and are under mutual- and self-regulation, forming a positive feedback loop. This study unveils the transcriptional regulatory system that determines pericycle participation in lateral root initiation.

Volatile compounds from beneficial rhizobacteria Bacillus spp. promote periodic lateral root development in Arabidopsis

Lateral root formation is coordinated by both endogenous and external factors. As biotic factors, plant growth-promoting rhizobacteria can affect lateral root formation, while the regulation mechanism is unclear. In this study, by applying various marker lines, we found that volatile compounds (VCs) from Bacillus amyloliquefaciens SQR9 induced higher frequency of DR5 oscillation and prebranch site formation, accelerated the development and emergence of the lateral root primordia and thus promoted lateral root development in Arabidopsis. We demonstrated a critical role of auxin on B. amyloliquefaciens VCs-induced lateral root formation via respective mutants and pharmacological experiments. Our results showed that auxin biosynthesis, polar transport and signalling pathway are involved in B. amyloliquefaciens VCs-induced lateral roots formation. We further showed that acetoin, a major component of B. amyloliquefaciens VCs, is less active in promoting root development compared to VC blends from B. amyloliquefaciens, indicating the presence of yet uncharacterized/unknown VCs might contribute to B. amyloliquefaciens effect on lateral root formation. In summary, our study revealed an auxin-dependent mechanism of B. amyloliquefaciens VCs in regulating lateral root branching in a non-contact manner, and further efforts will explore useful VCs to promote plant root development.

Pickle Recruits Retinoblastoma Related 1 to Control Lateral Root Formation in Arabidopsis

Lateral root (LR) formation is an example of a plant post-embryonic organogenesis event. LRs are issued from non-dividing cells entering consecutive steps of formative divisions, proliferation and elongation. The chromatin remodeling protein PICKLE (PKL) negatively regulates auxin-mediated LR formation through a mechanism that is not yet known. Here we show that PKL interacts with RETINOBLASTOMA-RELATED 1 (RBR1) to repress the LATERAL ORGAN BOUNDARIES-DOMAIN 16 (LBD16) promoter activity. Since LBD16 function is required for the formative division of LR founder cells, repression mediated by the PKL–RBR1 complex negatively regulates formative division and LR formation. Inhibition of LR formation by PKL–RBR1 is counteracted by auxin, indicating that, in addition to auxin-mediated transcriptional responses, the fine-tuned process of LR formation is also controlled at the chromatin level in an auxin-signaling dependent manner.

Temperature-dependent fasciation mutants provide a link between mitochondrial RNA processing and lateral root morphogenesis

Although mechanisms that activate organogenesis in plants are well established, much less is known about the subsequent fine-tuning of cell proliferation, which is crucial for creating properly structured and sized organs. Here we show, through analysis of temperature-dependent fasciation (TDF) mutants of Arabidopsis, root redifferentiation defective 1 (rrd1), rrd2, and root initiation defective 4 (rid4), that mitochondrial RNA processing is required for limiting cell division during early lateral root (LR) organogenesis. These mutants formed abnormally broadened (i.e. fasciated) LRs under high-temperature conditions due to extra cell division. All TDF proteins localized to mitochondria, where they were found to participate in RNA processing: RRD1 in mRNA deadenylation, and RRD2 and RID4 in mRNA editing. Further analysis suggested that LR fasciation in the TDF mutants is triggered by reactive oxygen species generation caused by defective mitochondrial respiration. Our findings provide novel clues for the physiological significance of mitochondrial activities in plant organogenesis.

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.

An auxin-regulable oscillatory circuit drives the root clock in Arabidopsis

In Arabidopsis, the root clock regulates the spacing of lateral organs along the primary root through oscillating gene expression. The core molecular mechanism that drives the root clock periodicity and how it is modified by exogenous cues such as auxin and gravity remain unknown. We identified the key elements of the oscillator (AUXIN RESPONSE FACTOR 7, its auxin-sensitive inhibitor IAA18/POTENT, and auxin) that form a negative regulatory loop circuit in the oscillation zone. Through multilevel computer modeling fitted to experimental data, we explain how gene expression oscillations coordinate with cell division and growth to create the periodic pattern of organ spacing. Furthermore, gravistimulation experiments based on the model predictions show that external auxin stimuli can lead to entrainment of the root clock. Our work demonstrates the mechanism underlying a robust biological clock and how it can respond to external stimuli.