| Flower is the reproductive structure and the most distinguishing organ in angiosperms.Flower opening is essential for pollination and thus successful sexual reproduction.In addition to this,flowers are the most important ornamental organs of many ornamental plants.The process of flower opening directly controls the economic value of flower.Rose is one of the most important ornamental plants in the world.Ethylene promotes rose flower opening,but often causes abnonnal opening.However,little is known about the intrinsic biological mechanism of ethylene-promoted the flower opening in rose.Here,we report that PETAL MOVEMENT-RELATED PROTEIN 1(RhPMPl)gene,encoding a rose HD-Zip I subfamily transcription factor,is upregulated by ethylene treatment.Our findings thus define the RhPMP1 is involved in ethylene-induced flower opening via regulating asymmetric growth of petal base.1)Ethylene accelerates flower opening via stimulating asymmetric growth of petal base.We observed that petal base exhibited dramatically curving and bulging during flower opening.The petal thickness increased during flower opening(stage 2 to stage 5).Moreover,morphological changes of petal base were cause by increase in both cell divivion and cell expansion in the adaxial side of petal base.After ethylene treatment,petal unfolded without upper petal expansion,indicating that petal movement did not depend on upper petal expansion.Transversal sections showed that after 24 h of treatment ethylene promoted the bulging of adaxial side parenchyma tissue at petal base,and outer morphology and anatomy in the petal base is similar to that of petal base of fully opened flower(stage 5),indicating that ethylene stimulated flower opening and asymmetric growth in petal base,mimicking natural development in petal base.2)RhPMP1 promotes flower opening by regulating asymmetric growth of the base of the petal.After RNA-Seq analysis of ethylene-treated and untreated petal basal tissues of rose,the results showed that a HD-Zip I family gene RhPMP1 was significantly induced by ethylene,and its expression increased with flowers opening.After ethylene treatment,the expression level of RhHD-ZIP in the adaxial side of petal base was higher than that in the abaxial side.In the process of natural opening,the expression level of RhHD-ZIP in the adaxial side of petal base at stage 5 was higher than that in the abaxial side.Expression of RhPMP1 was silenced in rose plantlets by virus-induce gene silencing(VIGS).The time of flower opening in RhPMP1-silenced plantlets was significantly longer than that in TRV-control plantlets,suggesting that it plays a vital role in promoting flower opening and petal movement.RhPMP1-silenced plants attenuated asymmetric growth of the base of the petal by reducing cell division and cell expansion in adaxial side of parenchyma at petal base.RhPMP1-silenced plants delay ethylene-induced flower opening by affecting the asymmetric growth of the base of the petal.3)RhPMP1 is a direct downstream gene to RhEIN3,which is the primary transcription factor in ethyene signaling pathway.Yeast one hybrid(Y1H)assay and electrophoretic mobility shift assay(EMSA)showed that the master transcription factor in ethylene signaling pathway,RhEIN3,bound to the promoter of RhPMP1,and activated the expression of RhPMP1.The dual-luciferase reporter assay and chromatin immunoprecipitation(ChIP)-PCR assay indicated that RhEIN3 could bind to the RhPMP1 promoter in vivo.4)RhPMP1 delays the ethylene-induced flower opening process by directly positively regulating the expression of RhAPC3b.RNA-Seq data of RhPMP1-silenced petals showed that a large number of cell expansion-and division-related functional genes were down-regulated in RhPMP1-silenced petals.Screening of 9 candidate genes by Y1H assay,found that RhPMPl bound to the promoter of RhAPC3b.The dual-luciferase reporter assay and ChIP-PCR assay showed that RhPMPl bound to the promoter of RhAPC3b in vivo and activated its expression.RhAPC3b encodes a homolog of the subunit of the anaphase-promoting complex 3(APC3),which is an important component of APC/C complex regulating cell cycle.APC/C can simultaneously regulate cell division and expansion.Similar to the expression pattern of RhPMP1,RhAPC3b expression increased in the adaxial side of the petal base during flower opening.A significant increase in expression of RhAPC3b was measured in the adaxial side of petal base after 24 h of ethylene treatment.Similar to RhPMP1-silenced phenotype,RhAPC3b silencing impaired asymmetric growth of petal base by reducing cell expansion and cell division.Moreover,RhAPC3b silencing also delayed the opening process in response to ethylene.Taken together,we speculate that during the unfolding of petal,the master transcription facter in ethylene signaling pathway,RhEIN3,directly targets RhPMP1 promoter and positively regulates RhPMP1 transcription in the adaxial side at petal base.RhPMPl directly up-regulates RhAPC3b expression,and promotes both cell division and cell expansion in adaxial side at petal base.RhPMP1 promotes petal movement and flower opening via controlling the asymmetric growth in petal base. |
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