The role of miR156 in rejuvenation in Arabidopsis thaliana

Transcriptome and Metabolome Analyses of Leaves from Cutting Rejuvenation of Ancient Cinnamomum camphora

Rejuvenation refers to the transition from the state of mature to juvenile. Many ancient Cinnamomum camphora have aged and died due to climatic and anthropic factors. Vegetative propagation can protect valuable germplasm resources. In this study, a 2000-year-old ancient C. camphora and its 2-year-old cutting plantlets were selected as experimental materials. The results indicated that the number of leaves with palisade tissue (Pal) cell layers was different between samples, with two layers in the rejuvenated leaves (RLs) and one layer in the mature leaves (MLs) and young leaves (YLs). Indole-3-acetic acid (IAA), isopentenyladenine (iP) and isopentenyladenosine (iPR) concentrations were significantly higher in RLs than in MLs and YLs, but the abscisic acid (ABA) concentration was lower. Targeted metabolome analysis identified 293 differentially accumulated metabolites (DAMs). Meanwhile, a total of 5241 differentially expressed genes (DEGs) were identified by transcriptome sequencing. According to the KEGG analysis, there were seven important enriched pathways in the MLs, RLs and YLs, including plant hormone signal transduction (57 DEGs), plant-pathogen interaction (56 DEGs) and MAPK signaling pathway-plant (36 DEGs). KEGG enrichment conjoint analyses of DEGs and DAMs identified 16 common pathways. Integrated analyses of cytological, hormone, metabolome and transcriptome elements can provide a research basis in regard to the rejuvenation regulatory mechanism of ancient C. camphora.

Epigenetic regulation and epigenetic memory resetting during plant rejuvenation

Reversal of plant developmental status from the mature to the juvenile phase, thus leading to the restoration of the developmental potential, is referred to as plant rejuvenation. It involves multilayer regulation, including resetting gene expression patterns, chromatin remodeling, and histone modifications, eventually resulting in the restoration of juvenile characteristics. Although plants can be successfully rejuvenated using some forestry practices to restore juvenile morphology, physiology, and reproductive capabilities, studies on the epigenetic mechanisms underlying this process are in the nascent stage. This review provides an overview of the plant rejuvenation process and discusses the key epigenetic mechanisms involved in DNA methylation, histone modification, and chromatin remodeling in the process of rejuvenation, as well as the roles of small RNAs in this process. Additionally, we present new inquiries regarding the epigenetic regulation of plant rejuvenation, aiming to advance our understanding of rejuvenation in sexually and asexually propagated plants. Overall, we highlight the importance of epigenetic mechanisms in the regulation of plant rejuvenation, providing valuable insights into the complexity of this process.

Non-coding RNAs and leaf senescence: Small molecules with important roles

Non-coding RNAs (ncRNAs) are a special class of functional RNA molecules that are not translated into proteins. ncRNAs have emerged as pivotal regulators of diverse developmental processes in plants. Recent investigations have revealed the association of ncRNAs with the regulation of leaf senescence, a complex and tightly regulated developmental process. However, a comprehensive review of the involvement of ncRNAs in the regulation of leaf senescence is still lacking. This manuscript aims to summarize the molecular mechanisms underlying ncRNAs-mediated leaf senescence and the potential applications of ncRNAs to manipulate the onset and progression of leaf senescence. Various classes of ncRNAs, including microRNAs (miRNAs), small interfering RNAs (siRNAs), long noncoding RNAs (lncRNAs), and circular RNAs (circRNAs), are discussed in terms of their regulatory mechanisms in leaf senescence. Furthermore, we explore the interactions between ncRNA and the key regulators of senescence, including transcription factors as well as core components in phytohormone signaling pathways. We also discuss the possible challenges and approaches related to ncRNA-mediated leaf senescence. This review contributes to a further understanding of the intricate regulatory network involving ncRNAs in leaf senescence.

Genome-Wide Analysis of SPL/miR156 Module and Its Expression Analysis in Vegetative and Reproductive Organs of Oil Palm (Elaeis guineensis)

The SPL (SQUAMOSA-promoter binding protein-like) gene family is one of the largest plant transcription factors and is known to be involved in the regulation of plant growth, development, and stress responses. The genome-wide analysis of SPL gene members in a diverse range of crops has been elucidated. However, none of the genome-wide studies on the SPL gene family have been carried out for oil palm, an important oil-yielding plant. In this research, a total of 24 EgSPL genes were identified via a genome-wide approach. Phylogenetic analysis revealed that most of the EgSPLs are closely related to the Arabidopsis and rice SPL gene members. EgSPL genes were mapped onto the only nine chromosomes of the oil palm genome. Motif analysis revealed conservation of the SBP domain and the occurrence of 1-10 motifs in EgSPL gene members. Gene duplication analysis demonstrated the tandem duplication of SPL members in the oil palm genome. Heatmap analysis indicated the significant expression of SPL genes in shoot and flower organs of oil palm plants. Among the identified EgSPL genes, a total 14 EgSPLs were shown to be targets of miR156. Real-time PCR analysis of 14 SPL genes showed that most of the EgSPL genes were more highly expressed in female and male inflorescences of oil palm plants than in vegetative tissues. Altogether, the present study revealed the significant role of EgSPL genes in inflorescence development.

DNA methylation variation is crucial to restore adventitious rooting ability during in vitro shoot culture-induced rejuvenation in apple rootstock

In vitro shoot culture has been widely used for restoring adventitious rooting ability in rooting recalcitrant woody perennial species for the past few decades, but its molecular mechanism is largely uncovered. DNA methylation is an essential epigenetic mark that participates in many biological processes. Recent reports suggested a role of DNA methylation in vitro culture in plants. In this study, we characterized the single-base resolution DNA methylome and transcriptome of adult and in vitro shoot culture-induced rejuvenation cuttings of apple rootstock M9T337. We found a global decrease in DNA methylation during rejuvenation, which may be correlated with increased expression of DNA demethylase genes and decreased expression of DNA methyltransferase genes. We additionally documented DNA hypomethylation in 'T337'_R in gene protomer associated with higher transcript levels of several adventitious rooting-related genes. The application of a DNA methylation inhibitor (5-azacytidine) enhanced the adventitious rooting ability and the expression level of adventitious rooting-related genes, such as, MdANT, MdMPK3, MdABCB21, MdCDC48, MdKIN8B, pri-MdMIR156a5 and pri-MdMIR156a12. Together, the DNA hypomethylation is critical for the rejuvenation-dependent adventitious rooting ability in apple rootstock. In addition, increased DNA methylation was also found in thousands of genes in 'T337'_R. We additionally documented that DNA hypermethylation is required for inhibition of adventitious rooting-repressed genes, such as MdGAD5a, encoding glutamate decarboxylase, which can catalyze glutamate decarboxylated to form γ-aminobutyric acid (GABA). Our results revealed that in vitro shoot culture-dependent DNA methylation variation plays important roles in adventitious rooting in apple rootstock.

Effect of Explant Source on Phenotypic Changes of In Vitro Grown Cannabis Plantlets over Multiple Subcultures

Drug-type cannabis is often multiplied using micropropagation methods to produce genetically uniform and disease/insect-free crops. However, micropropagated plantlets often exhibit phenotypic variation, leading to culture decline over time. In cannabis, the source of these changes remains unknown, though several factors (e.g., explant’s sources and prolonged in vitro culture) can result in such phenotypical variations. The study presented herein evaluates the effects of explant sources (i.e., nodal segments derived from the basal, near-basal, middle, and apical parts of the greenhouse-grown mother plant) over multiple subcultures (4 subcultures during 235 days) on multiplication parameters and leaf morphological traits of in vitro cannabis plantlets. While initial in vitro responses were similar among explants sourced from different regions of the plant, there were significant differences in performance over the course of multiple subcultures. Specifically, explant source and/or the number of subcultures significantly impacted plantlet height, number of nodes, and canopy surface area. The explants derived from the basal and near-basal parts of the plant resulted in the tallest shoots with the greatest number of nodes, while the explants derived from the middle and apical regions led to shorter shoots with fewer nodes. Moreover, the basal-derived explants produced cannabis plantlets with shorter but wider leaves which demonstrated the potential of such explants for in vitro rejuvenation practices with minimal culture decline. This study provides new evidence into the long-term impacts of explant source in cannabis micropropagation.

Integrated transgene and transcriptome reveal the molecular basis of MdWRKY87 positively regulate adventitious rooting in apple rootstock

For most fruit and forest species vegetative propagated from elite genotypes, adventitious rooting is essential. The ability to form adventitious roots significantly decreased during the juvenile to adult phase change. Apart from the miR156-SPL pathway, whether there is another regulation mechanism controlling age-dependent adventitious rooting ability remained largely unknown. In the present study, we showed that MdWRKY87 expression level was positively correlation with adventitious rooting ability. In addition, over-expressing of MdWRKY87 in tobacco leads to enhanced adventitious rooting ability, more adventitious root number and accelerated adventitious rooting process. Comparative transcriptome profiling indicated that MdWRKY87 overexpression can activate the expression of adventitious rooting-induced genes, such as WOX11 and AIL. In addition, MdWRKY87 overexpression can inhibit the transcription of adventitious rooting-repressed genes, such as AUX/IAAs and type-B cytokinin RRs. Collectively, here we demonstrated that higher expression level of MdWRKY87 contributes to age-dependent adventitious rooting-competent in juvenile apple rootstock.

A study on the effect of host plants on Chinese gallnut morphogenesis

Galls are products of the hyperplasia of host plant structures induced by gall-inducing organisms and have been considered as an extended phenotype of the inducers. There is little evidence regarding the effect of host plants on gall morphology. We hypothesised that the morphology and developmental pattern of galls are different because of the different location of their stimulation, even though two kinds of inducers are close relatives. We observed that horned galls and their leaflets of their host plant, Rhus chinensis required a longer rapid growth stage than fusiform galls and Rhus potaninii leaflets. The distribution of trichomes showed positional dependence. Molecular analysis showed that in the fusiform gall, the target genes that regulate the plastochron of leaflets and serration development were hardly expressed, and CUP-SHAPED COTYLEDON-2 may be a key gene that regulates the formation of the horns. In summary, horned and fusiform galls showed a developmental pattern similar to those of their host plant leaflets. We suggest that the inducing site is important in the morphology and development of galls.

SQUINT Positively Regulates Resistance to the Pathogen Botrytis cinerea via miR156-SPL9 Module in Arabidopsis

SQUINT (SQN) regulates plant maturation by promoting the activity of miR156, which functions primarily in the miR156-SQUAMOSA PROMOTER BINDING PROTEIN-LIKE9 (SPL9) module regulating plant growth and development. Here, we show that SQN acts in the jasmonate (JA) pathway, a major signaling pathway regulating plant responses to insect herbivory and pathogen infection. Arabidopsis thaliana sqn mutants showed elevated sensitivity to the necrotrophic fungus Botrytis cinerea compared with wild type. However, SQN is not involved in the early pattern-triggered immunity response often triggered by fungal attack. Rather, SQN positively regulates the JA pathway, as sqn loss-of-function mutants treated with B. cinerea showed reduced JA accumulation, JA response and sensitivity to JA. Furthermore, the miR156-SPL9 module regulates plant resistance to B. cinerea: mir156 mutant, and SPL9 overexpression plants displayed elevated sensitivity to B. cinerea. Moreover, constitutively expressing miR156a or reducing SPL9 expression in the sqn-1 mutant restored the sensitivity of Arabidopsis to B. cinerea and JA responses. These results suggest that SQN positively modulates plant resistance to B. cinerea through the JA pathway, and the miR156-SPL9 module functions as a bridge between SQN and JA to mediate plant resistance to this pathogen.

The RpTOE1-RpFT Module Is Involved in Rejuvenation during Root-Based Vegetative Propagation in Robinia pseudoacacia

Vegetative propagation is an important method of reproduction and rejuvenation in horticulture and forestry plants with a long lifespan. Although substantial juvenile clones have been obtained through the vegetative propagation of ornamental plants, the molecular factors that regulate rejuvenation during vegetative propagation are largely unknown. Here, root sprouting and root cutting of Robinia pseudoacacia were used as two vegetative propagation methods. From two consecutive years of transcriptome data from rejuvenated seedlings and mature trees, one gene module and one miRNA module were found to be specifically associated with rejuvenation during vegetative propagation through weighted gene co-expression network analysis (WGCNA). In the gene module, a transcription factor-encoding gene showed high expression during vegetative propagation, and it was subsequently named RpTOE1 through homology analysis. Heterologous overexpression of RpTOE1 in wild-type Arabidopsis and toe1 toe2 double mutants prolonged the juvenile phase. The qRT-PCR results predicted RpFT to be a downstream gene that was regulated by RpTOE1. Further investigation of the protein-DNA interactions using yeast one-hybrid, electrophoretic mobility shift, and dual luciferase reporter assays confirmed that RpTOE1 negatively regulated RpFT by binding directly to the TOE binding site (TBS)-like motif on its promoter. On the basis of these results, we showed that the high expression of RpTOE1 during vegetative propagation and its inhibition of RpFT played a key role in the phase reversal of R. pseudoacacia.

Rejuvenation increases leaf biomass and flavonoid accumulation in Ginkgo biloba

Rejuvenation refers to the transition from an adult state to a juvenile state. Trunk truncation at the base of the tree can result in tree rejuvenation. However, little is known about the association of rejuvenation with leaf biomass and flavonoid accumulation. The results of this study showed that, compared with control leaves, leaves of renewed Ginkgo biloba shoots were larger, thicker, and more lobed and had higher fresh/dry weights and chlorophyll contents. The leaf biomass per hectare of rejuvenated trees was twofold higher than that of the untruncated controls. Moreover, we observed a marked increase in the accumulation of flavonol glycosides via metabolomic analysis and detected upregulated expression of genes involved in flavonoid biosynthesis, including CHS, FLS, F3'H, DFR, and LAR. Overexpression of GbCHS in ginkgo calli confirmed that GbCHS plays an important role in flavonoid biosynthesis. Interestingly, the contents of gibberellins significantly increased in the rejuvenated leaves. Moreover, exogenous gibberellin treatment significantly increased GbCHS expression and flavonoid contents. Our findings show that truncation can stimulate tree rejuvenation by altering hormone levels, representing an effective and feasible approach for enhancing the biomass and flavonoid content of G. biloba leaves.

The Regulation of Plant Vegetative Phase Transition and Rejuvenation: miRNAs, a Key Regulator

In contrast to animals, adult organs in plants are not formed during embryogenesis but generated from meristematic cells as plants advance through development. Plant development involves a succession of different phenotypic stages and the transition between these stages is termed phase transition. Phase transitions need to be tightly regulated and coordinated to ensure they occur under optimal seasonal, environmental conditions. Polycarpic perennials transition through vegetative stages and the mature, reproductive stage many times during their lifecycles and, in both perennial and annual species, environmental factors and culturing methods can reverse the otherwise unidirectional vector of plant development. Epigenetic factors regulating gene expression in response to internal cues and external (environmental) stimuli influencing the plant’s phenotype and development have been shown to control phase transitions. How developmental and environmental cues interact to epigenetically alter gene expression and influence these transitions is not well understood, and understanding this interaction is important considering the current climate change scenarios, since epigenetic maladaptation could have catastrophic consequences for perennial plants in natural and agricultural ecosystems. Here, we review studies focusing on the epigenetic regulators of the vegetative phase change and highlight how these mechanisms might act in exogenously induced plant rejuvenation and regrowth following stress.

Genome-Wide Characterization of SPL Gene Family in Codonopsis pilosula Reveals the Functions of CpSPL2 and CpSPL10 in Promoting the Accumulation of Secondary Metabolites and Growth of C. pilosula Hairy Root

SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors play critical roles in regulating diverse aspects of plant growth and development, including vegetative phase change, plant architecture, anthocyanin accumulation, lateral root growth, etc. In the present study, 15 SPL genes were identified based on the genome data of Codonopsis pilosula, a well-known medicinal plant. Phylogenetic analysis clustered CpSPLs into eight groups (G1-G8) along with SPLs from Arabidopsis thaliana, Solanum lycopersicum, Oryza sativa and Physcomitrella patens. CpSPLs in the same group share similar gene structure and conserved motif composition. Cis-acting elements responding to light, stress and phytohormone widely exist in their promoter regions. Our qRT-PCR results indicated that 15 CpSPLs were differentially expressed in different tissues (root, stem, leaf, flower and calyx), different developmental periods (1, 2 and 3 months after germination) and various conditions (NaCl, MeJA and ABA treatment). Compared with the control, overexpression of CpSPL2 or CpSPL10 significantly promoted not only the growth of hairy roots, but also the accumulation of total saponins and lobetyolin. Our results established a foundation for further investigation of CpSPLs and provided novel insights into their biological functions. As far as we know, this is the first experimental research on gene function in C. pilosula.

BZR1 Physically Interacts with SPL9 to Regulate the Vegetative Phase Change and Cell Elongation in Arabidopsis

As sessile organisms, the precise development phase transitions are very important for the success of plant adaptability, survival and reproduction. The transition from juvenile to the adult phase—referred to as the vegetative phase change—is significantly influenced by numbers of endogenous and environmental signals. Here, we showed that brassinosteroid (BR), a major growth-promoting steroid hormone, positively regulates the vegetative phase change in Arabidopsis thaliana. The BR-deficient mutant det2-1 and BR-insensitive mutant bri1-301 displayed the increased ratio of leaf width to length and reduced blade base angle. The plant specific transcription factors SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) are key masters for the vegetative phase transition in plants. The expression levels of SPL9, SPL10 and SPL15 were significantly induced by BR treatment, but reduced in bri1-116 mutant compared to wild-type plants. The gain-of-function pSPL9:rSPL9 transgenic plants displayed the BR hypersensitivity on hypocotyl elongation and partially suppressed the delayed vegetative phase change of det2-1 and bri1-301. Furthermore, we showed that BRASSINAZOLE-RESISTANT 1 (BZR1), the master transcription factor of BR signaling pathway, interacted with SPL9 to cooperatively regulate the expression of downstream genes. Our findings reveal an important role for BRs in promoting vegetative phase transition through regulating the activity of SPL9 at transcriptional and post-transcriptional levels.

Genome-wide identification, gene cloning, subcellular location and expression analysis of SPL gene family in P. granatum L

BACKGROUNDS

Pomegranate is an excellent tree species with nutritional, medicinal, ornamental and ecological values. Studies have confirmed that SPL factors play an important role in floral transition and flower development.

RESULTS

Used bioinformatics methods, 15 SPL (SQUAMOSA promoter-binding protein-like) genes were identified and analyzed from the 'Taishanhong' pomegranate (P. granatum L.) genome. Phylogenetic analysis showed that PgSPLs were divided into six subfamilies (G1 ~ G6). PgSPL promoter sequences contained multiple cis-acting elements associated with abiotic stress or hormonal response. Based on the transcriptome data, expression profiles of different tissues and different developmental stages showed that PgSPL genes had distinct temporal and spatial expression characteristics. The expression analysis of miR156 in small RNA sequencing results showed that miR156 negatively regulated the expression of target genes. qRT-PCR analysis showed that the expression levels of PgSPL2, PgSPL3, PgSPL6, PgSPL11 and PgSPL14 in leaves were significantly higher than those in buds and stems (p < 0.05). The expression levels of PgSPL5, PgSPL12 and PgSPL13 in flower buds were significantly higher than that in leaves and stems (p < 0.05). The full-length of coding sequence of PgSPL5 and PgSPL13 were obtained by homologous cloning technology. The full length of PgSPL5 is 1020 bp, and PgSPL13 is 489 bp, which encodes 339 and 162 amino acids, respectively. Further investigation revealed that PgSPL5 and PgSPL13 proteins were located in the nucleus. Exogenous plant growth regulator induction experiments showed that PgSPL5 was up-regulated in leaves and stems. PgSPL13 was up-regulated in leaves and down-regulated in stems. When sprayed with 6-BA, IBA and PP333 respectively, PgSPL5 and PgSPL13 were up-regulated most significantly at P2 (bud vertical diameter was 5.1 ~ 12.0 mm) stage of bisexual and functional male flowers.

CONCLUSIONS

Our findings suggested that PgSPL2, PgSPL3, PgSPL6, PgSPL11 and PgSPL14 played roles in leaves development of pomegranate. PgSPL5, PgSPL12 and PgSPL13 played roles in pomegranate flower development. PgSPL5 and PgSPL13 were involved in the response process of different plant hormone signal transduction in pomegranate development. This study provided a robust basis for further functional analyses of SPL genes in pomegranate.

Reinvigoration/Rejuvenation Induced through Micrografting of Tree Species: Signaling through Graft Union

Trees have a distinctive and generally long juvenile period during which vegetative growth rate is rapid and floral organs do not differentiate. Among trees, the juvenile period can range from 1 year to 15–20 years, although with some forest tree species, it can be longer. Vegetative propagation of trees is usually much easier during the juvenile phase than with mature phase materials. Therefore, reversal of maturity is often necessary in order to obtain materials in which rooting ability has been restored. Micrografting has been developed for trees to address reinvigoration/rejuvenation of elite selections to facilitate vegetative propagation. Generally, shoots obtained after serial grafting have increased rooting competence and develop juvenile traits; in some cases, graft-derived shoots show enhanced in vitro proliferation. Recent advances in graft signaling have shown that several factors, e.g., plant hormones, proteins, and different types of RNA, could be responsible for changes in the scion. The focus of this review includes (1) a discussion of the differences between the juvenile and mature growth phases in trees, (2) successful restoration of juvenile traits through micrografting, and (3) the nature of the different signals passing through the graft union.

The Molecular Basis of Age-Modulated Plant De Novo Root Regeneration Decline in Arabidopsis thaliana

Plants possess a regeneration capacity that enables them to survive after wounding. For example, detached Arabidopsis thaliana leaves are able to form adventitious roots from their cutting sites even in the absence of exogenous hormone supplements, as process termed de novo root regeneration (DNRR). Wounding rapidly induces auxin biosynthesis at the cutting sites and then elicits a signaling cascade to promote cell fate transitions and finally generate the adventitious roots. However, rooting rates in older plants are much lower than in younger leaf explants. In this review, we highlight the recent breakthroughs in the understanding of DNRR decay in older plants from at least two independent signaling routes: (i) via the accumulation of EIN3 protein in older plants, which directly suppresses expression of WUSCHEL RELATED HOMEOBOX (WOX) genes to inhibit rooting; (ii) the miR156-SPLs-AP2/ERFs pathway, which modulates root regeneration by reducing auxin biosynthesis.

Getting to the root of grafting-induced traits

Grafting is an ancient technique that involves the physical joining of genotypically distinct shoot and root systems, in order to achieve a desirable compound plant. This practice is widely used in modern agriculture to improve biotic and abiotic stress tolerance, modify plant architecture, induce precocious flowering and rejuvenate old perennial varieties, boost yield, and more. Beneficial new rootstock-scion combinations are currently identified through an inefficient trial and error process, which presents a significant bottleneck for the application of grafting to combat new environmental challenges. Identifying the mechanisms that underlie beneficial grafting-induced traits will facilitate rapid breeding and genetic engineering of new rootstock x scion combinations that exhibit superior performance across varying agricultural environments.

Molecular Bases for the Regulation of Adventitious Root Generation in Plants

The formation of adventitious roots (ARs) is an ecologically and economically important developmental process in plants. The evolution of AR systems is an important way for plants to cope with various environmental stresses. This review focuses on identified genes that have known to regulate the induction and initiation of ARs and offers an analysis of this process at the molecular level. The critical genes involved in adventitious rooting are the auxin signaling-responsive genes, including the AUXIN RESPONSE FACTOR (ARF) and the LATERAL ORGAN BOUNDARIES-DOMAIN (LOB) gene families, and genes associated with auxin transport and homeostasis, the quiescent center (QC) maintenance, and the root apical meristem (RAM) initiation. Several genes involved in cell wall modulation are also known to be involved in the regulation of adventitious rooting. Furthermore, the molecular processes that play roles in the ethylene, cytokinin, and jasmonic acid signaling pathways and their crosstalk modulate the generation of ARs. The crosstalk and interaction among many molecular processes generates complex networks that regulate AR generation.

Juvenile Leaves or Adult Leaves: Determinants for Vegetative Phase Change in Flowering Plants

Vegetative leaves in Arabidopsis are classified as either juvenile leaves or adult leaves based on their specific traits, such as leaf shape and the presence of abaxial trichomes. The timing of the juvenile-to-adult phase transition during vegetative development, called the vegetative phase change, is a critical decision for plants, as this transition is associated with crop yield, stress responses, and immune responses. Juvenile leaves are characterized by high levels of miR156/157, and adult leaves are characterized by high levels of miR156/157 targets, SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factors. The discovery of this miR156/157-SPL module provided a critical tool for elucidating the complex regulation of the juvenile-to-adult phase transition in plants. In this review, we discuss how the traits of juvenile leaves and adult leaves are determined by the miR156/157-SPL module and how different factors, including embryonic regulators, sugar, meristem regulators, hormones, and epigenetic proteins are involved in controlling the juvenile-to-adult phase transition, focusing on recent insights into vegetative phase change. We also highlight outstanding questions in the field that need further investigation. Understanding how vegetative phase change is regulated would provide a basis for manipulating agricultural traits under various conditions.

Arabidopsis NUCLEAR FACTOR Y A8 inhibits the juvenile-to-adult transition by activating transcription of MIR156s

Vegetative (juvenile-to-adult) and flowering (vegetative-to-reproductive) phase changes are crucial in the life cycle of higher plants. MicroRNA156 (miR156) and its target SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes are master regulators that determine vegetative phase changes. The miR156 level gradually declines as a plant ages and its expression is rapidly repressed by sugar. However, the underlying regulatory mechanism of transcriptional regulation of the MIR156 gene remains largely unknown. In this study, we demonstrated that Arabidopsis NUCLEAR FACTOR Y A8 (NF-YA8) binds directly to CCAAT cis-elements in the promoters of multiple MIR156 genes, thus activating their transcription and inhibiting the juvenile-to-adult transition. NF-YA8 was highly expressed in juvenile-stage leaves, and significantly repressed with developmental age and by sugar signals. Our results suggest that NF-YA8 acts as a signaling hub, integrating internal developmental age and sugar signals to regulate the transcription of MIR156s, thus affecting the juvenile-to-adult and flowering transitions.

The Lifecycle of the Plant Immune System

Throughout their life span, plants confront an endless barrage of pathogens and pests. To successfully defend against biotic threats, plants have evolved a complex immune system responsible for surveillance, perception, and the activation of defense. Plant immunity requires multiple signaling processes, the outcome of which vary according to the lifestyle of the invading pathogen(s). In short, these processes require the activation of host perception, the regulation of numerous signaling cascades, and transcriptome reprograming, all of which are highly dynamic in terms of temporal and spatial scales. At the same time, the development of a single immune event is subjective to the development of plant immune system, which is co-regulated by numerous processes, including plant ontogenesis and the host microbiome. In total, insight into each of these processes provides a fuller understanding of the mechanisms that govern plant-pathogen interactions. In this review, we will discuss the "lifecycle" of plant immunity: the development of individual events of defense, including both local and distal processes, as well as the development and regulation of the overall immune system by ontogenesis regulatory genes and environmental microbiota. In total, we will integrate the output of recent discoveries and theories, together with several hypothetical models, to present a dynamic portrait of plant immunity.