Carpel morphogenesis

Abstract: Floral organ specification is controlled by various MADS-box genes in both dicots and monocots, whose expression is often subjected to both genetic and epigenetic regulation in Arabidopsis thaliana. However, little information is known about the role of epigenetic modification of MADS-box genes during rice flower development. Here, we report the characterization of a rice gene, curved chimeric palea 1 (CCP1) that functions in palea development. Mutation in CCP1 resulted in abnormal palea with ectopic stigmatic tissues and other pleiotropic phenotypes. We found that OsMADS58, a C-class gene responsible for carpel morphogenesis, was ectopically expressed in the ccp1 palea, indicating that the ccp1 palea was misspecified and partially acquired carpel-like identity. Constitutive expression of OsMADS58 in the wild-type rice plants caused morphological abnormality of palea similar to that of ccp1, whereas OsMADS58 knockdown by RNAi in ccp1 could rescue the abnormal phenotype of mutant palea, suggesting that the repression of OsMADS58 expression by CCP1 is critical for palea development. Map-based cloning revealed that CCP1 encodes a putative plant-specific emBRYONIC flower1 (EMF1)-like protein. Chromatin immunoprecipitation assay showed that the level of the H3K27me3 at the OsMADS58 locus was greatly reduced in ccp1 compared with that in the wild-type. Taken together, our results show that CCP1 plays an important role in palea development through maintaining H3K27me3-mediated epigenetic silence of the carpel identity-specifying gene OsMADS58, shedding light on the epigenetic mechanism in floral organ development.

Abstract: †BackgroundandAimsCRABSCLAW(CRC)isamemberoftheYABBYfamilyoftranscriptionfactorsinvolvedin carpel morphogenesis, floral determinacy and nectary specification in arabidopsis. CRC orthologues have beenfunctionally characterized across angiosperms, revealing additional roles in leaf vascular development and carpelidentityspecificationinPoaceae.ThesestudiessupportanancestralroleofCRCorthologuesincarpeldevelopment,whilerolesinvasculardevelopmentandnectaryspecificationappeartobederived.Thisstudyaimedtoexpandre-searchonCRCfunctionalconservationtothelegumefamilyinordertobetterunderstandtheevolutionaryhistoryofCRCorthologuesin angiosperms.†Methods CRCorthologuesfromPisumsativumandMedicagotruncatulawereidentified.RNAinsituhybridiza-tionexperimentsdeterminedthecorrespondingexpressionpatternsthroughoutflowerdevelopment.Thephenotypiceffects of reduced CRC activity were investigated in P. sativum using virus-induced gene silencing.†Key Results CRC orthologues from P. sativum and M. truncatula showed similar expression patterns, mainlyrestricted to carpels and nectaries. However, these expression patterns differed from those of other core eudicots,most importantly in a lack of abaxial expression in the carpel and in atypical expression associated with themedialveinoftheovary.CRCdownregulationinpeacauseddefectsincarpelfusionandstyle/stigmadevelopment,both typicallyassociated with CRC function in eudicots, but also affected vascular development in the carpel.†Conclusions ThedatasupporttheconservedrolesofCRCorthologuesincarpelfusion,style/stigmadevelopmentandnectarydevelopment.Inaddition,anintriguingnewaspectofCRCfunctioninlegumeswastheunexpectedrolein vascular development, which could be shared by other species from widely diverged clades within the angios-perms, suggesting that this role could be ancestral rather than derived, as so far generallyaccepted.Key words: Flower development, CRABS CLAW, CRCorthologues, carpel evolution, gynoecium, nectary, carpelvasculature, floral determinacy, Pisum sativum, Medicago truncatula, YABBY transcription factors.

Abstract: The gynoecium is among the most intricately patterned organs of the plant, comprisingdifferent tissue sub-structures, all with the purpose of facilitating propagation to the next generation It is therefore representative of the complexity involved in the initiation and establishment of organ patterning and presents a unique model to study the mechanisms coordinating development As with all other organs, the interplay between genetic and hormonal factors specifies carpel development However, although much is known about the genetic components involved in carpel development, our understanding of hormonal regulation and the cross-talk between these two pathways is limited Thus, the aim of this thesis has been to address this issue by obtaining an integrated view of the genetic and hormonal regulatory mechanisms which act to coordinate gynoecium development It has done so using broadly two approaches: first, by characterising the transcription factor interactions which pattern the carpel, and second, by elucidating the cross-talk between these interactions and the plant hormone auxin Further, it has also studied the role of auxin in carpel morphogenesis Observations from this research have uncovered a novel auxin co-receptor complex formed by the transcription factors IND and ETT The co-receptor binds the IAA molecule directly and exhibits specificity for IAA over the synthetic analogues NAA and 2,4-D This coreceptor functions to coordinate the development of the style and stigmatic tissues of the carpel, possibly via the regulation of PID kinase Further, this work has also identified novel roles in protein-protein dimerisation for the domains involved in this interaction Analyses also indicate that this novel auxin signalling pathway may also be conserved in the Brassicaceae through the ETT orthologues in this family Finally, this project has analysed how ETT’s role as an auxin receptor could be translated into precise spatiotemporal regulation of its target genes to specify the boundaries necessary for gynoecium patterning Together, the results from this work have posed new questions as to the signalling mechanisms through which auxin coordinates its varied and numerous functions in plants