Temporal regulation of vegetative phase change in plants

During their vegetative growth, plants reiteratively produce leaves, buds, and internodes at the apical end of the shoot. The identity of these organs changes as the shoot develops. Some traits change gradually, but others change in a coordinated fashion, allowing shoot development to be divided into discrete juvenile and adult phases. The transition between these phases is called vegetative phase change. Historically, vegetative phase change has been studied because it is thought to be associated with an increase in reproductive competence. However, this is not true for all species; indeed, heterochronic variation in the timing of vegetative phase change and flowering has made important contributions to plant evolution. In this review, we describe the molecular mechanism of vegetative phase change, how the timing of this process is controlled by endogenous and environmental factors, and its ecological and evolutionary significance.

Genome-Wide Identification and Expression Analysis of the SPL Gene Family in Three Orchids

SPL transcription factors regulate important processes such as plant growth and development, metabolic regulation, and abiotic stress. They play crucial roles in the development of flower organs. However, little is known about the characteristics and functions of the SPLs in the Orchidaceae. In this study, Cymbidium goeringii Rchb. f., Dendrobium chrysotoxum Lindl., and Gastrodia elata BI. were used as research objects. The SPL gene family of these orchids was analyzed on a genome-wide scale, and their physicochemical properties, phylogenetic relationships, gene structures, and expression patterns were studied. Transcriptome and qRT-PCR methods were combined to investigate the regulatory effect of SPLs on the development of flower organs during the flowering process (bud, initial bloom, and full bloom). This study identifies a total of 43 SPLs from C. goeringii (16), D. chrysotoxum (17), and G. elata (10) and divides them into eight subfamilies according to the phylogenetic tree. Most SPL proteins contained conserved SBP domains and complex gene structures; half of the genes had introns longer than 10 kb. The largest number and variety of cis-acting elements associated with light reactions were enriched, accounting for about 45% of the total (444/985); 13/43 SPLs contain response elements of miRNA156. GO enrichment analysis showed that the functions of most SPLs were mainly enriched in the development of plant flower organs and stems. In addition, expression patterns and qRT-PCR analysis suggested the involvement of SPL genes in the regulation of flower organ development in orchids. There was little change in the expression of the CgoSPL in C. goeringii, but DchSPL9 and GelSPL2 showed significant expression during the flowering process of D. chrysotoxum and G. elata, respectively. In summary, this paper provides a reference for exploring the regulation of the SPL gene family in orchids.

Meristem dormancy in Marchantia polymorpha is regulated by a liverwort-specific miRNA and a clade III SPL gene

The shape of modular organisms depends on the branching architecture, which in plants is determined by the fates of generative centers called meristems. The branches of the liverwort Marchantia polymorpha are derived from two adjacent meristems that develop at thallus apices. These meristems may be active and develop branches or may be dormant and do not form branches. The relative number and position of active and dormant meristems define the overall shape and form of the thallus. We show that the clade III SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factor MpSPL1 is required for meristem dormancy. The activity of MpSPL1 is regulated by the liverwort-specific Mpo-MR13 miRNA, which, in turn, is regulated by PIF-mediated signaling. An unrelated PIF-regulated miRNA, MIR156, represses a different SPL gene (belonging to clade IV) that inhibits branching during the shade avoidance response in Arabidopsis thaliana. This suggests that a conserved light signaling mechanism modulates branching architecture in liverworts and angiosperms and therefore is likely operated in the last common ancestor. However, PIF-mediated signaling represses the expression of different miRNA genes with different SPL targets during dichotomous, apical branching in liverworts and during lateral, subapical branching in angiosperms. We speculate that the mechanism that acts downstream of light and regulates meristem dormancy evolved independently in liverworts and angiosperms.

Reproductive competence is regulated independently of vegetative phase change in Arabidopsis thaliana

A long-standing question in plant biology is how the acquisition of reproductive competence is related to the juvenile-to-adult vegetative transition. We addressed this question by examining the expression pattern and mutant phenotypes of two families of miRNAs-miR156/miR157 and miR172-that operate in the same pathway and play important roles in these processes. The phenotype of mutants deficient for miR156/miR157, miR172, and all three miRNAs demonstrated that miR156/miR157 regulate the timing of vegetative phase change but have only a minor effect on reproductive competence, whereas miR172 has a minor role in vegetative phase change but has a major effect on reproductive competence. MIR172B is directly downstream of the miR156/SPL module, but temporal variation in the level of miR156 in the shoot apex and leaf-to-leaf variation in miR156 expression in young primordia was not associated with a change in the level of miR172 in these tissues. Additionally, although miR172 levels increase from leaf to leaf later in leaf development, this variation is largely insensitive to changes in the abundance of miR156. Our results indicate that the acquisition of reproductive competence in Arabidopsis is regulated by miR172 through a mechanism that is independent of the vegetative phase change pathway.

miR156-independent repression of the ageing pathway by longevity-promoting AHL proteins in Arabidopsis

Plants age by developmental phase changes. In Arabidopsis, the juvenile to adult vegetative phase change (VPC) is marked by clear heteroblastic changes in leaves. VPC and the subsequent vegetative to reproductive phase change are promoted by SQUAMOSA PROMOTOR BINDING PROTEIN-LIKE (SPL) transcription factors and repressed by miR156/157 targeting SPL transcripts. By genetic, phenotypic, and gene expression analyses, we studied the role of the longevity-promoting AT-HOOK MOTIF NUCLEAR LOCALIZED 15 (AHL15) and family members in SPL-driven plant ageing. Arabidopsis ahl loss-of-function mutants showed accelerated VPC and flowering, whereas AHL15 overexpression delayed these phase changes. Expression analysis and tissue-specific AHL15 overexpression revealed that AHL15 affects VPC and flowering time directly through its expression in the shoot apical meristem and young leaves, and that AHL15 represses SPL2/9/13/15 gene expression in a miR156/157-independent manner. The juvenile traits of spl loss-of-function mutants appeared to depend on enhanced expression of the AHL15 gene, whereas SPL activity prevented vegetative growth from axillary meristem by repressing AHL15 expression. Our results place AHL15 and close family members together with SPLs in a reciprocal regulatory feedback loop that modulates VPC, flowering time, and axillary meristem development in response to both internal and external signals.

Identification of genes differentially expressed between prostrate shoots and erect shoots in the lycophyte Selaginella nipponica using an RNA-seq approach

Lycophytes are the earliest vascular plants and Selaginella is the most studied genus among them. Prostrate shoots are produced during early growth and erect shoots emerge later in S. nipponica, thus providing an opportunity for exploring the evolution of the mechanism underlying the transition between growth phases. Six libraries were sequenced for the prostrate and the erect shoots, and a total of 206 768 genes were identified. Some genes were differentially expressed in prostate and erect shoot, with relatively high expression in the prostate shoots being related to hormone responses and defence reactions, while higher expression in the erect shoots was related to spore formation and shoot development. Some SPL genes possessed a miR156 binding site and were highly expressed in the erect shoots, while AP2-like genes were more highly expressed in the prostrate shoots but simultaneously lacked any miR172 binding site. MiR156 was detected at a higher concentration in the prostrate shoots. Thus, the mechanism for the vegetative to reproductive transition of sporophytes probably originated in the common ancestor of vascular plants and must have experienced stepwise development during evolution.

Genome-wide characterization of SPL family in Medicago truncatula reveals the novel roles of miR156/SPL module in spiky pod development

BACKGROUND

SQUAMOSA Promoter Binding Protein-Likes (SPLs) proteins are plant-specific transcription factors that play many crucial roles in plant growth and development. However, there is little information about SPL family in the model legume Medicago truncatula.

RESULTS

In this study, a total of 23 MtSPL genes were identified in M. truncatula genome, in which 17 of the MtSPLs contained the putative MtmiR156 binding site at the coding or 3' UTR regions. Tissue-specific expression pattern analysis showed that most MtmiR156-targeted MtSPLs were highly expressed in seed and pod. The observation of MtmiR156B-overexpressing plants reveals that MtmiR156/MtSPL modules are not only involved in the development of leaves and branches, but also in the seed pod development, especially the formation of spine on pod.

CONCLUSION

The spines on pods are developed in many plant species, which allow pods to adhere to the animals, and then be transported on the outside. This study sheds light on the new function of SPL family in seed dispersal by controlling the formation of spiky pod, and provides insights on understanding evolutionary divergence of the members of SPL gene family among plant species.

Genome-wide identification and characterization of the SBP-box gene family in Petunia

Background

SQUAMOSA PROMOTER BINDING PROTEIN (SBP)-box genes encode a family of plant-specific transcription factors (TFs) that play important roles in many growth and development processes including phase transition, leaf initiation, shoot and inflorescence branching, fruit development and ripening etc. The SBP-box gene family has been identified and characterized in many species, but has not been well studied in Petunia, an important ornamental genus.

Results

We identified 21 putative SPL genes of Petunia axillaris and P. inflata from the reference genome of P. axillaris N and P. inflata S6, respectively, which were supported by the transcriptome data. For further confirmation, all the 21 genes were also cloned from P. hybrida line W115 (Mitchel diploid). Phylogenetic analysis based on the highly conserved SBP domains arranged PhSPLs in eight groups, analogous to those from Arabidopsis and tomato. Furthermore, the Petunia SPL genes had similar exon-intron structure and the deduced proteins contained very similar conserved motifs within the same subgroup. Out of 21 PhSPL genes, fourteen were predicted to be potential targets of PhmiR156/157, and the putative miR156/157 response elements (MREs) were located in the coding region of group IV, V, VII and VIII genes, but in the 3’-UTR regions of group VI genes. SPL genes were also identified from another two wild Petunia species, P. integrifolia and P. exserta, based on their transcriptome databases to investigate the origin of PhSPLs. Phylogenetic analysis and multiple alignments of the coding sequences of PhSPLs and their orthologs from wild species indicated that PhSPLs were originated mainly from P. axillaris. qRT-PCR analysis demonstrated differential spatiotemperal expression patterns of PhSPL genes in petunia and many were expressed predominantly in the axillary buds and/or inflorescences. In addition, overexpression of PhSPL9a and PhSPL9b in Arabidopsis suggested that these genes play a conserved role in promoting the vegetative-to-reproductive phase transition.

Conclusion

Petunia genome contains at least 21 SPL genes, and most of the genes are expressed in different tissues. The PhSPL genes may play conserved and diverse roles in plant growth and development, including flowering regulation, leaf initiation, axillary bud and inflorescence development. This work provides a comprehensive understanding of the SBP-box gene family in Petunia and lays a significant foundation for future studies on the function and evolution of SPL genes in petunia.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4537-9) contains supplementary material, which is available to authorized users.

Genome-wide identification and expression analysis of SBP-like transcription factor genes in Moso Bamboo (Phyllostachys edulis)

BACKGROUND

The SQUAMOSA promoter binding protein-like (SPL) proteins are plant-specific transcription factors (TFs) that function in a variety of developmental processes including growth, flower development, and signal transduction. SPL proteins are encoded by a gene family, and these genes have been characterized in two model grass species, Zea mays and Oryza sativa. The SPL gene family has not been well studied in moso bamboo (Phyllostachys edulis), a woody grass species.

RESULTS

We identified 32 putative PeSPL genes in the P. edulis genome. Phylogenetic analysis arranged the PeSPL protein sequences in eight groups. Similarly, phylogenetic analysis of the SBP-like and SBP proteins from rice and maize clustered them into eight groups analogous to those from P. edulis. Furthermore, the deduced PeSPL proteins in each group contained very similar conserved sequence motifs. Our analyses indicate that the PeSPL genes experienced a large-scale duplication event ~15 million years ago (MYA), and that divergence between the PeSPL and OsSPL genes occurred 34 MYA. The stress-response expression profiles and tissue-specificity of the putative PeSPL gene promoter regions showed that SPL genes in moso bamboo have potential biological functions in stress resistance as well as in growth and development. We therefore examined PeSPL gene expression in response to different plant hormone and drought (polyethylene glycol-6000; PEG) treatments to mimic biotic and abiotic stresses. Expression of three (PeSPL10, -12, -17), six (PeSPL1, -10, -12, -17, -20, -31), and nine (PeSPL5, -8, -9, -14, -15, -19, -20, -31, -32) genes remained relatively stable after treating with salicylic acid (SA), gibberellic acid (GA), and PEG, respectively, while the expression patterns of other genes changed. In addition, analysis of tissue-specific expression of the moso bamboo SPL genes during development showed differences in their spatiotemporal expression patterns, and many were expressed at high levels in flowers and leaves.

CONCLUSIONS

The PeSPL genes play important roles in plant growth and development, including responses to stresses, and most of the genes are expressed in different tissues. Our study provides a comprehensive understanding of the PeSPL gene family and may enable future studies on the function and evolution of SPL genes in moso bamboo.

Genomic organization, phylogenetic comparison, and expression profiles of the SPL family genes and their regulation in soybean

SQUAMOSA Promoter-Binding Protein-Like (SPL) genes form a major family of plant-specific transcription factors and play an important role in plant growth and development. In this study, we report the identification of 41 SPL genes (GmSPLs) in the soybean genome. Phylogenetic analysis revealed that these genes were divided into five groups (groups 1-5). Further, exon/intron structure and motif composition revealed that the GmSPL genes are conserved within their same group. The N-terminal zinc finger 1 (Zn1) of the SBP domain was a CCCH (Cys3His1) and the C terminus zinc finger 2 (Zn2) was a CCHC (Cys2HisCys) type. The 41 GmSPL genes were distributed unevenly on 17 of the 20 chromosomes, with tandem and segmental duplication events. We found that segmental duplication has made an important contribution to soybean SPL gene family expansion. The Ka/Ks ratios revealed that the duplicated GmSPL genes evolved under the effect of purifying selection. In addition, 17 of the 41 GmSPLs were found as targets of miR156; these might be involved in their posttranscriptional regulation through miR156. Importantly, RLM-RACE analysis confirmed the GmmiR156-mediated cleavage of GmSPL2a transcript in 2-4 mm stage of soybean seed. Alternative splicing events in 9 GmSPLs were detected which produces transcripts and proteins of different lengths that may modulate protein signaling, binding, localization, stability, and other properties. Expression analysis of the soybean SPL genes in various tissues and different developmental stages of seed suggested distinct spatiotemporal patterns. Differences in the expression patterns of miR156-targeted and miR156-non-targeted soybean SPL genes suggest that miR156 plays key functions in soybean development. Our results provide an important foundation for further uncovering the crucial roles of GmSPLs in the development of soybean and other biological processes.

Differential SPL gene expression patterns reveal candidate genes underlying flowering time and architectural differences in Mimulus and Arabidopsis

Evolutionary transitions in growth habit and flowering time responses to variable environmental signals have occurred multiple times independently across angiosperms and have major impacts on plant fitness. Proteins in the SPL family of transcription factors collectively regulate flowering time genes that have been implicated in interspecific shifts in annuality/perenniality. However, their potential importance in the evolution of angiosperm growth habit has not been extensively investigated. Here we identify orthologs representative of the major SPL gene clades in annual Arabidopsis thaliana and Mimulus guttatus IM767, and perennial A. lyrata and M. guttatus PR, and characterize their expression. Spatio-temporal expression patterns are complex across both diverse tissues of the same taxa and comparable tissues of different taxa, consistent with genic sub- or neo-functionalization. However, our data are consistent with a general role for several SPL genes in the promotion of juvenile to adult phase change and/or flowering time in Mimulus and Arabidopsis. Furthermore, several candidate genes were identified for future study whose differential expression correlates with growth habit and architectural variation in annual versus perennial taxa.

Functional Evolution in the Plant SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE (SPL) Gene Family

The SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE (SPL) family of transcription factors is functionally diverse, controlling a number of fundamental aspects of plant growth and development, including vegetative phase change, flowering time, branching, and leaf initiation rate. In natural plant populations, variation in flowering time and shoot architecture have major consequences for fitness. Likewise, in crop species, variation in branching and developmental rate impact biomass and yield. Thus, studies aimed at dissecting how the various functions are partitioned among different SPL genes in diverse plant lineages are key to providing insight into the genetic basis of local adaptation and have already garnered attention by crop breeders. Here we use phylogenetic reconstruction to reveal nine major SPL gene lineages, each of which is described in terms of function and diversification. To assess evidence for ancestral and derived functions within each SPL gene lineage, we use ancestral character state reconstructions. Our analyses suggest an emerging pattern of sub-functionalization, neo-functionalization, and possible convergent evolution following both ancient and recent gene duplication. Based on these analyses we suggest future avenues of research that may prove fruitful for elucidating the importance of SPL gene evolution in plant growth and development.