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Title:   Author  item
                                            CHANG, XIAOXIAO - Northwest Agricultural & Forestry Universityitem
                                            DONNELLY, LINDA - US Department Of Agriculture (USDA)item
                                            RAO, JINGPING - Northwest Agricultural & Forestry Universityitem
                                            REID, MICHAEL - University Of Californiaitem
                                            Jiang, Cai-Zhong
  Submitted to: PLOS ONE
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 1/7/2014
                                    Publication  Date: 2/14/2014
                                    Publication URL: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0088320Citation: Chang, X., Donnelly, L., Rao, J., Reid, M.S., Jiang, C. 2014. A petunia homeodomain-leucine zipper protein, PhHD-Zip, plays an important role in flower senescence. PLoS One. 9(2):1-10.Interpretive Summary: Flower senescence is a process stimulated by developmental and environmental signals, and requires gene transcriptional regulation. We identified a homeodomain-leucine zipper transcription factor, named as PhHD-Zip, which was up-regulated during petunia flower senescence. Silencing PhHD-Zip using virus-induced gene silencing (VIGS) method resulted in extended flower longevity by 20% in both unpollinated and pollinated flowers compared with wild-type and vector control plants. Transcript abundance of ethylene biosynthesis-related genes (ACS, ACO), ABA biosynthesis-related gene (NCED) and senescence-related genes (SAG12, SAG29) and ethylene production were dramatically reduced in the PhHD-Zip silenced flowers. On the other hand, over-expression of this gene in petunia caused earlier flower senescence. Furthermore, PhHD-Zip transcript level in petunia flower was induced by hormones (ethylene, ABA) and abiotic stresses (dehydration, NaCl and cold). The results suggest that PhHD-Zip plays an important role in regulating petunia flower senescence.
                                Technical Abstract: Flower senescence is mediated in part by changes of plant hormones, such as ethylene, cytokinin and abscisic acid (ABA). Ethylene is known to control flower senescence in many species, especially ethylene sensitive flowers, like petunia, carnation and rose. During flower senescence in petunia and other flowers, ethylene production has a climacteric rise at the late stage of the petal senescence. Petunia flowers treated with ethylene accelerates corolla senescence and the expression of senescence-related genes; while senescence can be significantly delayed by treating flowers with ethylene action inhibitors, such as silver thiosulfate (STS) and 1-methylcyclopropen (1-MCP). However, even in ethylene sensitive flowers, other plant hormones also play important roles. The exogenous application of cytokinin to detached petunia flowers delays corolla senescence. Exogenous ABA application to petunia flowers does not accelerate flower senescence, suggesting that the increase in the ABA content of the corollas is a by-product of senescence rather than a regulator. ABA appears to increase the sensitivity of flowers to ethylene, as transcript abundance of some ethylene receptors increased after exogenous ABA treatment. Therefore, ABA may accelerate senescence by increasing the sensitivity of the flower, acting at the level of ethylene receptors. However, it is not well understood what regulates ethylene or ABA pathway during flower senescence at transcriptional levels.
Transcription factors (TFs) are involved in plant development and senescence process. Detailed analysis from Arabidopsis petal senescence data revealed that the most represented families amongst TFs specifically up-regulated in petals were AP2-EREBP, homeobox (HB) and AUX-IAA. The up-regulation of the AP2-EREB TFs establishes the role of ethylene in Arabidopsis.  In petunia flowers, the ethylene-responsive element binding factor (ERF) family genes were studied in detail. Group VII ERFs, previously associated with fruit ripening and senescence were also associated with corolla senescence in petunia. Their expression was down-regulated by STS, and up-regulated by ethylene and ABA supporting the link between ethylene and ABA responses.
The homeodomain-leucine zipper (HD-Zip) family transcription factors are unique to plants and contain a leucine zipper motif (LZ) immediately downstream of the homeodomain (HD).  The HD-Zip I transcription factors play an important role in the regulation of development in response to changes to environmental conditions and hormone stimuli, especially under water deficit stress and different light conditions. Increased expression levels of ATHB5 in transgenic Arabidopsis plants cause an enhanced sensitivity to the inhibitory effect of ABA on seed germination and seedling growth. The expression of HD-Zip I TFs such as ATHB6, ATHB7 and ATHB12 in Arabidopsis are induced by water deficit, osmotic stress and exogenous ABA. T-DNA insertion lines of ATHB12 in Arabidopsis show reduced sensitivity to ABA, whereas over-expression lines of ATHB12 and/or ATHB7 show hypersensitivity to ABA in root elongation assays. 
The expression of HD-Zip I subfamily genes from other species, such as HaHB4 from sunflower and NaHD20 from tobacco, is strongly induced by water deficit and ABA. Furthermore, over-expression of sunflower HaHB4 in Arabidopsis delays leaf senescence and reduces ethylene sensitivity, suggesting that its encoding protein could serve as a new component of ethylene signaling pathways. Expression of a HD-Zip gene in Mirabilis jalapa is up-regulated during flower senescence. But the function of HD-Zip on flower senescence hasn’t been demonstrated. Petunia is an ideal model system for studies of flower senescence because of the short life cycle and the large flower amenity for biochemical and molecular analysis. Virus induced gene silencing (VIGS) based on


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