Exploring floral senescence without a model – the rich rewards of diversity

Floral senescence does not follow the same pattern in all flowering species. Flowers can be broadly divided between those in which petals abscise while still turgid and those in which petals wilt and may or may not abscise. Furthermore, flower senescence can also be divided into a further two groups. In one group of species ethylene is produced by the flowers as the senescence progresses, and acts as a major regulator of this process. In another group ethylene is not produced, or only at very low levels and does not act as a major senescence regulator. Broadly most species that show rapid abscission without wilting are ethylene sensitive whereas those that wilt are generally ethylene insensitive, although the demarcations are not absolute. Many monocotyledonous flowers including lilies, as well as most Asteraceae are ethylene insensitive, while ethylene sensitive species include orchids, carnations petunia and Brassicaceae including Arabidopsis. Given these major differences it has been difficult to find a suitable model to study floral senescence. However new tools such as transcriptomic analyses are offering new approaches to investigate common and divergent regulatory mechanisms. In the obvious model, Arabidopsis, senescence proceeds rapidly and within 48 h petals are abscised with only moderate wilting. Some progress has been made in identifying genes regulating ethylene-regulated senescence in Arabidopsis flowers, and some of these genes may also regulate senescence progression in other ethylene sensitive species. Abscission signals also appear to be shared across different floral senescence groups. A comparison of two lily genotypes that have contrasting senescence programmes identified genes related to abscission previously identified in Arabidopsis. However the regulation of senescence in ethylene-insensitive species remains problematic. Recent transcriptomic studies in lily and other species reveal families of transcription factors that may be involved. A further complexity in floral senescence is offered by composite flowers. A transcriptomic analysis of Dahlias is comparing senescence between florets and the whole inflorescence.