RhHB1 mediates the antagonism of gibberellins to ABA and ethylene during rose (Rosa hybrida) petal senescence

Abstract

SUMMARYRose (Rosa hybrida) is one of the most important ornamental plants worldwide; however, senescence of its petals terminates the ornamental value of the flower, resulting in major economic loss. It is known that the hormones abscisic acid (ABA) and ethylene promote petal senescence, while gibberellins (GAs) delay the process. However, the molecular mechanisms underlying the antagonistic effects amongst plant hormones during petal senescence are still unclear. Here we isolated RhHB1, a homeodomain-leucine zipper I transcription factor gene, from rose flowers. Quantitative RT-PCR and GUS reporter analyses showed that RhHB1 was strongly expressed in senescing petals, and its expression was induced by ABA or ethylene in petals. ABA or ethylene treatment clearly accelerated rose petal senescence, while application of the gibberellin GA 3 delayed the process. However, silencing of RhHB1 delayed the ABA-or ethylene-mediated senescence, and resulted in higher petal anthocyanin levels and lower expression of RhSAG12. Moreover, treatment with paclobutrazol, an inhibitor of GA biosynthesis, repressed these delays. In addition, silencing of RhHB1 blocked the ABA-or ethylene-induced reduction in expression of the GA20 oxidase encoded by RhGA20ox1, a gene in the GA biosynthetic pathway. Furthermore, RhHB1 directly binds to the RhGA20ox1 promoter, and silencing of RhGA20ox1 promoted petal senescence. Eight senescence-related genes showed substantial differences in expression in petals after treatment with GA 3 or paclobutrazol. These results suggest that RhHB1 mediates the antagonistic effect of GAs on ABA and ethylene during rose petal senescence, and that the promotion of petal senescence by ABA or ethylene operates through an RhHB1-RhGA20ox1 regulatory checkpoint.

An Ethylene-Induced Regulatory Module Delays Flower Senescence by Regulating Cytokinin Content

“…Previous studies have reported that HD-Zip I transcription factors are involved in abiotic stress responses (Ariel et al, 2010), fruit ripening (Lin et al, 2008), and leaf and flower senescence (Manavella et al, 2006;Lü et al, 2014). Here we showed that RhHB6 acts as a regulator of RhPR10.1, and that its expression was also up-regulated in senescent rose petals and involved in ethylene-induced flower senescence (Figs.…”

“…3A). In addition, the ion leakage rate and expression of the senescence marker gene RhSAG12 (Lü et al, 2014) were significantly higher in RhPR10.1-silenced petal discs than in the TRV control (Fig. 3, B and C).…”

“…A search of an in-house-generated ethylene-treated rose petal transcriptome database (http://bioinfo.bti.cornell.edu/cgi-bin/rose_454/index. cgi) revealed three members of the HD-Zip I subfamily, RhHB1 (RU00522; Lü et al, 2014), RhHB6 (RU12591), and RhHB7 (RU00731), as well as a member of the HD-Zip II subfamily, RhHB2 (RU01217). CTK, 100 mM 6-BA; ABA, 100 mM ABA; GA, 80 mM GA 3 ; BR, 5 mM brassinosteroid.…”

“…For ethylene treatment, the cut rose flowers at opening stage 2 were exposed to 10 mL L 21 ethylene in an airtight chamber for 24 h. NaOH solution (1 M) was also applied in the chamber to prevent the CO 2 accumulation (Lü et al, 2014).…”

“…Among these hormones, ethylene is considered a major regulator of flower senescence (van Doorn, 2001), and in ethylene-sensitive species, flower senescence is associated with a burst of ethylene production, and the coordinated expression of ethyleneresponsive genes, including those with regulatory functions Lerslerwong et al, 2009). For example, in Arabidopsis (Arabidopsis thaliana), the expression of microRNA164 was shown to be repressed by ethylene, thereby promoting senescence , while in rose (Rosa hybrida), a reduction in transcript abundance of a homeodomain-Leu zipper (HD-Zip) I transcription factor gene, RhHB1, was observed to delay senescence (Lü et al, 2014). In contrast, the expression of another member of the HD-Zip I transcription factor family, HaHB-4, was reported to be up-regulated in response to ethylene in sunflower (Helianthus annuus), consequently delaying the senescence process (Manavella et al, 2006), suggesting that antagonistic mechanisms influence ethylene-induced senescence.…”