Elsevier

Scientia Horticulturae

Volume 272, 15 October 2020, 109584
Scientia Horticulturae

SPLs-mediated flowering regulation and hormone biosynthesis and signaling accompany juvenile-adult phase transition in Pyrus

https://doi.org/10.1016/j.scienta.2020.109584Get rights and content

Highlights

  • Genes related with juvenile-adult phase transition were isolated in Pyrus by RNA-seq.

  • Changes of plant hormone biosynthesis and signal transduction happened during the juvenile-adult phase transition in Pyrus.

  • SPLs-mediated flowering regulation played a role during the juvenile-adult phase transition of Pyrus.

Abstract

A long juvenile period limits the breeding process of fruit trees, such as pears. To understand the molecular mechanisms involved in the juvenile-adult phase change, two cDNA libraries of juvenile and adult phases from seedlings of hybrid offspring (Pyrus. pyrifolia Nakai cv.Whangkeumbae × P. bretschneideri Rehd. cv. Zaosu) were subjected to high-throughput sequencing. A total of 23,995 genes were obtained, of which 608 and 290 genes were down- and up-regulated, respectively, in the adult phase compared with the juvenile phase. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that some differentially expressed genes were involved in hormone biosynthesis and signaling pathways. Determination of hormone contents revealed that the levels of gibberellin A4 (GA4), ethylene, and jasmonic acid (JA) were lower in the adult than in the juvenile phase, while the contents of indole-3-acetic acid (IAA), gibberellin A1 (GA1), and abscisic acid (ABA) were significantly higher in the adult than in the juvenile phase. Combining the quantitative PCR (qPCR) analysis, we suggested that JA and ethylene might play negative roles, while ABA and IAA might play positive roles in the juvenile-adult phase transition in Pyrus. Meanwhile, a series of flowering-related genes were differentially expressed between juvenile and adult phases, including SPL (Squamosa promoter binding protein-like) genes, the flowering-time integrator gene SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1), and some floral meristem identity genes. Through qPCR analysis of SPL family members, 27 SPLs were found to participate in the phase transition in Pyrus. However, the regulatory mechanisms of hormone and SPLs require further study.

Introduction

As woody perennials, fruit trees undergo a gradual and continuous process of ontogenetic phase change. The transition from the juvenile phase to the adult vegetative phase is referred to as vegetative phase change, and that from the adult vegetative phase to the adult reproductive phase is referred to as the flowering process, or the floral transition. The juvenile phase is defined as the inability to initiate flowering (Zimmerman et al., 1985), which can last 3–8 years or longer in fruit trees. When the plant has passed from the juvenile to the adult vegetative phase, the plant can be induced to flower. Understanding the regulatory mechanisms involved in the juvenile-adult phase change is important for shortening the length of the juvenile phase, accelerating flowering and fruiting, and speeding up the breeding process of fruit trees.

In plants, ontogenetic phase change is regulated by external environmental cues and endogenous developmental signals, such as light, day length, temperature, carbohydrates, age, and hormones. The signals that regulate the vegetative phase change overlap considerably with those regulating the flowering process (Bäurle and Dean, 2006). Therefore, early researches on juvenile-adult phase transition in perennial woody plants mainly focused on the homologous genes of flowering regulation, such as flowering-time integrator genes and MADS-box transcription factor genes, which had been well studied in Arabidopsis. For example, overexpression of the apple AFL1 (APPLE FLORICAULA/LEAFY), spruce DAL1 (MADS-box gene), and grapevine VvFT and VvMADS8 genes in Arabidopsis leads to early flowering (Wada et al., 2002; Carlsbecker et al., 2004; Sreekantan and Thomas, 2006). The citrus SOC1-like gene, CsSL1, can functionally complement the late flowering phenotype of the Arabidopsis soc1 mutant (Tan and Swain, 2007). Overexpression of MdFT1 and PtFT1 induces flowering in juvenile apple and poplar trees (Bohlenius et al., 2006; Kotoda et al., 2010), and ectopic expression of an FT homolog from citrus also induces an early flowering phenotype in trifoliate orange (Endo et al., 2005). Constitutive expression of the Betula pendula BpMADS4 gene in apples results in flowering during the in vitro culture of shoots (Flachowsky et al., 2007). Additionally, Kotoda et al. (2006) reported that antisense expression of MdTFL1(TERMINAL FLOWER 1) resulted in flower induction and reduced the juvenile phase in apples.

With a deepening understanding of phase transition, microRNA156 (miR156), which is abundant in the juvenile phase and decreases with time, is regarded as providing an endogenous age cue for plant development and negatively regulates the vegetative phase change and floral induction through repression of the expression of Squamosa promoter binding protein-like (SPL) (Wu and Poethig, 2006; Wang et al., 2009; Wu et al., 2009; Yamaguchi et al., 2009). In recent years, an increasing number of studies on miR156 and their target genes in the phase change of woody plants have been reported. In Malus, miR156 expression in juvenile tissues was significantly higher than that in adult tissues (Jia et al., 2017; Xu et al., 2017). Furthermore, the ectopic expression of apple miR156 h reduced the expression of AtSPL9 and AtSPL15, and delayed flowering time in transgenic Arabidopsis (Sun et al., 2013). Overexpression of Arabidopsis miR156 in Populus x canadiensis resulted in a prolonged juvenile phase (Wang et al., 2011). The PdSPL9 gene of Paeonia delavayi and VpSBP11 of grapevine shortened juvenility and advanced the flowering time in transgenic Arabidopsis (Hou et al., 2017; Zhu et al., 2018). High-throughput screening of miRNA and the corresponding target genes in the phase-change process indicates that hormones are considered to participate in the phase transition in apple (Xing et al., 2014; Guo et al., 2017).

Although some genes associated with the juvenile-adult phase change have been cloned and analyzed in woody plants, little is known about the molecular mechanisms involved in the juvenile-adult phase change of woody plants, especially in pears. In the present study, we performed transcriptome analysis and isolated the differentially expressed genes (DEGs) during the juvenile-adult phase change process in pear trees. Then the DEGs were enriched into putative functional categories and pathways using the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG), seperately. The main metabolism and signaling pathways were analyzed and further verified by qPCR and hormone content detection. The results may lay a foundation for further studis on regulation mechanisms involved in the juvenile-adult phase change in pear trees.

Section snippets

Plant materials

Leaf samples were collected from three independent 6-year-old hybrid offspring (Pyrus. pyrifolia Nakai cv. Whangkeumbae × P. bretschneideri Rehd. cv. Zaosu) (planted with 1.0 m row spacing at normal water and fertilizer management level) in the pear experimental plot of Qingdao Agricultural University in April, 2017. First, we investigated the first flower node number of 32 hybrid seedlings that had flowered; the average was 96.3. Because the basal part or the basal suckers of a woody perennial

RNA-sequencing of pear leaves

To analyze the expression profile of mRNA during the juvenile-adult phase change, juvenile and adult cDNA libraries were constructed from leaves and sequenced. An overview of the sequencing and mapping results were listed in Table 1. Approximately 26,105,954–32,944,626 raw reads per library were obtained. After removing the low-quality raw reads, 25,383,204–32,044,344 clean reads were obtained. After mapping clean reads to the Chinese white pear genome, approximately 63.58 %–65.85 % reads were

Juvenile-adult phase change in perennial woody plants

During the juvenile-adult phase transition process in perennial woody plants, some morphological and physiological changes can be detected, such as leaf shape, phyllotaxis, thorniness, angles between lateral shoots and central axis, shoot growth vigor, photosynthetic capacity, hormone content, and rooting ability of cuttings (Hillman et al., 1974; Hackett, 1985; Zhuo, 1995; James and Bell, 2001; Dick and Leakey, 2006; Moreno-Alías et al., 2009; Piper and Cavieres, 2010; Xing et al., 2014). This

Conclusions

In conclusion, we elucidated the changes in expression of key genes in flowering regulation as well as hormone biosynthesis and signaling in the juvenile-adult phase change in pear trees, and verified SPLs and phytohormones played roles during the process. JA and ethylene may play negative roles, while ABA and IAA may play positive roles.

CRediT authorship contribution statement

Minyan Song: Validation, Investigation, Writing - original draft. Rihong Wang: Validation, Investigation. Fengli Zhou: Validation, Formal analysis. Ran Wang: Resources, Supervision, Funding acquisition. Shaoling Zhang: Resources, Funding acquisition. Dingli Li: Conceptualization. Jiankun Song: Project administration. Shaolan Yang: Validation. Yingjie Yang: Conceptualization, Writing - review & editing, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the China Agricultural Research System (CARS-29-07), the Doctoral Foundation of Shandong Province (ZR2019BC003), the Taishan Scholar Foundation of Shandong Province, and the Breeding Project of Shandong Province (2019LZGC008).

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