Flower senescence

The postproduction quality of flowering horticultural crops is often limited by the longevity of individual blooms, which is influenced by senescence.  Senescence represents the last stage of flower development, ultimately culminating in the programmed death of the petals.  While the senescence of whole plants and plant organs occurs naturally, environmental and biotic stresses during production, in retail environments, and in the home and garden can accelerate senescence. Symptoms of senescence that reduce the quality of ornamentals include flower wilting, abscission of flowers and flower parts, and fading of blossoms.  Creating plants with delayed senescence will produce cut flowers with longer vase lives and potted and garden plants with enhanced bloom displays.  In 2002 the wholesale value of floriculture crops in the U.S. was in excess of $4.5 billion.  Reducing postproduction losses, which can be as high as 30 to 50%, would have a significant economic impact on the industry.  Research in Dr. Jones' laboratory investigates the molecular and biochemical regulation of flower senescence.  The goal of this research is to create genetically engineered ornamentals with delayed flower senescence.    

Identifying senescence pathways in petunia

The senescence program is regulated by highly coordinated changes in gene expression, and the later stages of senescence share many characteristics of programmed cell death.  Flower petals provide an excellent model system for studies of senescence because they have a finite lifespan and their death is under tight developmental control.  While flowers serve an essential role in sexual reproduction, the maintenance of floral structures is costly in terms of respiratory energy and water loss.  The programmed senescence of the corolla, once a flower is pollinated or no longer receptive to pollination, allows the plant to breakdown macromolecules and organelles and remobilize their constituents to developing tissues.  In support of this recycling program, a number of genes encoding hydrolytic enzymes have been identified in screens for genes up regulated during petal senescence (Jones, 2004).   We are currently using DNA microarrays to identify genes that are differentially regulated during senescence.  We are also taking a proteomic approach to identify components of the senescence program in flower petals using 2-dimensional gel electrophoresis.   

Hormonal regulation of flower senescence- Ethylene

The senescence program in flowers is mediated in part by changes in endogenous hormone levels and hormone sensitivity.  Transgenic plants with altered hormone biosynthesis or perception are being used to study the hormonal regulation of senescence and the interactions between these signaling pathways.  Floral senescence in many species is accelerated by the plant hormone ethylene.  In these flowers, petal senescence is associated with an increase in endogenous ethylene production, and inhibitors of ethylene biosynthesis and action delay senescence.  The identification of mutants with impaired ethylene production or perception has allowed for a more precise analysis of ethylene function during senescence than could be obtained from inhibitor studies alone.   Transgenic petunias that are insensitive to ethylene (35S:etr1-1) and exhibit delayed flower senescence provide a means of investigating ethylene’s role in regulating flower senescence.  We are conducting comparative studies of the molecular and biochemical changes accompanying petal senescence in wild type and ethylene insensitive transgenic petunias to determine ethylene’s role in the initiation and execution of the senescence program in petals.    

Postpollination signaling and senescence

Successful sexual reproduction in flowering plants is dependent on highly specific interactions between the male gametophyte (pollen), the female reproductive organ (pistil), and accessory tissues, primarily the corolla.  Following pollen recognition and acceptance the pistil tissues provide physical and chemical support and directional guidance for the growing pollen tube.  During a compatible pollination, the stigma communicates via a transmissible signal to the ovary and petals that a successful pollination has occurred.    This signal results in a series of post-pollination events, which vary between species, but may include ovule maturation, ovary growth, pigmentation changes, wilting (senescence) or abscission of the petals.  Pollination-induced flower senescence is observed mainly among species in which flower longevity (the length of time from anthesis to death) is sensitive to exogenous ethylene, and therefore the effects of pollination on flower senescence are likely mediated by endogenous ethylene.  In support of ethylene’s role in inter-organ signaling, flowers that are insensitive to ethylene, due to treatment with ethylene action inhibitors or transformation with a mutated ethylene receptor (etr1-1), do not exhibit pollination-induced corolla senescence.  Comparative studies of the molecular and biochemical changes occurring following pollination of Wild type and etr1-1 petunia flowers are being conducted to identify components of the pollination signaling pathways and to determine how they are regulated by ethylene.  The specific goals of this research are to gain a broader understanding of the metabolic changes that accompany a compatible pollination and to determine ethylene’s role in inter-organ communication within the flower. 

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