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Title: Transcriptome profiling reveals regulatory mechanisms underlying corolla senescence in petunia

item WANG, HONG - University Of California
item CHANG, XIAOXIAO - Guangdong Academy Of Agricultural Sciences
item LIN, JING - Jiangsu Academy Agricultural Sciences
item CHANG, YOUHONG - Jiangsu Academy Agricultural Sciences
item CHEN, JEN-CHIH - University Of California
item REID, MICHAEL - University Of California
item Jiang, Cai-Zhong

Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2018
Publication Date: 4/1/2018
Citation: Wang, H., Chang, X., Lin, J., Chang, Y., Chen, J., Reid, M.S., Jiang, C. 2018. Transcriptome profiling reveals regulatory mechanisms underlying corolla senescence in petunia. Horticulture Research. 5:16.

Interpretive Summary: Petal senescence is the irreversible and final stage of floral differentiation and development, beginning once a flower has been successfully pollinated. The maintenance of petals is costly in terms of respiratory energy, nutrients and water loss; therefore, petal senescence is associated with complex and dynamic changes. These changes include morphological characteristics, the collapsing of cell structure, the degradation of macromolecules, structural and compositional changes in the plasma membrane and essential nutrient remobilization. However, it is not clear how the process is regulated genetically. The onset of floral senescence is believed to involve alterations in the levels of plant hormones. In ethylene (ET)-sensitive flowers, the first sign of visible senescence is accompanied by a transient and sudden rise of ethylene production4. Other hormones such as cytokinin (CK), abscisic acid (ABA), auxin, gibberellic acid (GA) and jasmonic acid (JA) are also involved in ET-sensitive petal senescence. Treatment with ABA promotes the large increase in ET production and hastens petal wilting in carnation flowers. A negative relationship was observed between the level of cytokinins and petal senescence in petunia and carnation. The increase of CK content in rose antagonized petal senescence prompted by ET. Similarly, applying CKs delayed petunia petal senescence. Auxin also plays a role in ET-sensitive petal senescence. Application of auxin prompted ET production and petal wilting in cut carnation flowers. In addition, 2,4-dichlorophenoxyacetic acid (2,4-D), a synthetic auxin, induced the expression of ACS genes in petals. In most research, these hormones are used as exogenous regulators to observe ET sensitivity and floral longevity in ET-sensitive species. Although the enrichment of response to auxin stimulus and ACC was observed 12 hours after pollination in the petals of petunia, the differential expression patterns of genes related to these hormones in petal senescence is unclear. Petal senescence is regulated by transcription factors (TFs). On one hand, ETHYLENE INSENSITVE-LIKE (EIL) and ETHYLENE RESPONSE FACTORS (ERFs) are correlated with the ET response signaling pathway. EIL3, a homologous of EIN3 in carnation, is a pivotal switch of ET-induced gene expression. DAFSAG9, which is homologous to ERF2, was significantly up-regulated in senescing daffodil petals. On the other hand, a large group of other TFs, such as B-box zinc finger, bHLH DNA-binding, homeodomain-like (HD), MADS-box, MYB, and NAC, display differential expression when ET-insensitivity is induced in the etr1-1 transgenic petunia. More than 20 members from the ERF, NAC, bZIP, HD-Zip and WRKY transcription factor families showed differential expression in petals at the early stage of pollination-induced senescence in petunia. In addition, NAC, Aux/IAA, MYB, bZIP, and MADS are differentially expressed during carnation petal senescence. These studies indicate that these TFs play regulating roles in ET-dependent petal senescence. However, the biological functions of these TFs are largely unknown. High-throughput gene expression analysis using mRNA sequencing (RNA-Seq) represents the most powerful tool to elucidate the underlying regulatory mechanism. Recently, pollination- and ET-induced petal senescence in petunia has been studied through RNA-Seq analysis, however, the regulatory mechanisms that govern the onset of natural petal senescence from opening to wilting in petunia is unclear. Therefore, identifying the dynamic processes and regulatory factors in transcription is a crucial step in determining the master switches in petal senescence. We employed RNA-seq technology to investigate the global and chronological sequence of transcriptional events during the initial petal senescence in petunia. We identified large numbers of differentially expressed genes (

Technical Abstract: Genetic regulatory mechanisms that govern petal natural senescence in petunia is complicated and unclear. To identify key genes and pathways that regulate the process, we initiated a transcriptome analysis in petunia petals at four developmental time points, including petal opening without anthesis (D0), petal expansion, two days after anthesis (D2), petal with initial signs of senescence (D4), and wilting petal (D7). We identified large numbers of differentially expressed genes (DEGs), ranging from 4626 between transition from D0 and D2, 1116 between D2 and D4, a transition to the onset of flower senescence, and to 327 between D4 and D7, a developmental stage representing flower senescence. Pathway analysis showed significant enrichment of hormone biosynthesis and signaling transduction pathways, especially relating to the auxin and ethylene during the flower development. Ethylene emission was detected at the D2 to D4 transition, followed by a large eruption at the D4 to D7 transition. Furthermore, large numbers of transcription factors (TFs) were activated over the course of senescence. Functional analysis by virus-induced gene silencing (VIGS) experiments demonstrated that inhibition of the expression of TFs, such as ethylene-related ERF, auxin-related ARF, bHLH, bZIP, HB, MADS and MYB, significantly either extended or shortened flower longevity. Our data suggest that hormonal interaction between ethylene and auxin may play critical regulatory roles in the onset of petal natural senescence in petunia.