Physiology and molecular biology of fruit abscission
During development, specific organs may undergo programmed separation from the main plant body, a process called abscission. Abscission plays crucial roles in the health and reproductive success of plants. For example, shedding of senescent leaves facilitates the recycling of mineral nutrients, abscission of floral organs after pollination allows for a focus of energy on reproduction, and dropping of diseased or infected organs reduces the spread of disease. Abscission of ripening fruits and mature seeds is an important process contributing to seed dispersal.
Organ separation typically occurs in a pre-determined position, the abscission zone. The abscission zone is a few layers of small, densely cytoplasmic cells, generally arranged transversely to the organ axis. During initiation of abscission, these separation layer cells expand and may divide. Subsequently, secretion of hydrolytic enzymes, increased peroxidase activity, and loss of calcium and pectin from the wall of separation layer cells presumably lead to the dissolution of the pectin-rich middle lamella, weakening the cell wall and allowing disintegration of abscission zone tissues. Cells basal to the separation layers may undergo a process of transdifferentiation to form a protective layer continuous with the periderm of the stem (Addicott 1982). The vasculature, which passes through the separation layers, may not always participate in abscission, thus providing a final connection to the main plant body that can be broken by physical force.
Various environmental and developmental signals have been shown to induce abscission by influencing the ratio between auxins and ethylene within the organ and adjacent abscission zone cells. According to a widely accepted model developed with leaf explants, loss of basipetal flow of auxin through the abscission zone, for example during leaf senescence, activates abscission by derepressing sensitivity of separation layer cells to ethylene. Thus a balance between auxin and ethylene signaling, rather than absolute levels of the hormones, seems to be the predominant effector of abscission.
Developing fruit constitute a strong source of auxin, which is transported basipetally across the separation layers of the fruit pedicel, and loss of auxin transport is associated with fruit abscission. In many fruits, ripening is accompanied by the production of significant amounts of ethylene, but whether this ethylene acts directly or indirectly to promote abscission remains unknown. Maturity in many fruits that naturally abscise is not associated with high levels of ethylene production, suggesting either that low levels are sufficient to promote abscission or that natural abscission can occur independently of ethylene. Exogenous ethylene accelerates abscission of ripening fruit in a variety of fruit species, even those that do not produce high levels of endogenous ethylene. As shown for a leaf explant abscission model, increased levels of ethylene in the fruit may reduce basipetal transport of auxin to the abscission zone, at least in part by decreasing auxin transport capacity. This mechanism may be superimposed on the endogenous decrease in auxin synthesis in the fruit associated with maturity, a phenomenon that may itself derepress ethylene generation.
It is now known that abscisic acid (ABA), originally identified by Frederick Adddicott's group in the mid-60's as a promoter of cotton fruit abscission, acts indirectly by promoting ethylene production.
The genetics of fruit abscission has been studied in tomato, where the JOINTLESS (J) gene is required for formation of the abscission zone (Mao et al. 2000). In Arabidopsis, which does not abscise fruit, abscission has been studied in the context of floral organs. Here, the NPR1-like genes BLADE-ON-PETIOLE1 (BOP1) and BOP2 are required for abscission zone formation (McKim et al. 2008). Besides genes involved in auxin and ethylene signaling, several other genes have been identified that are required for the normal abscission of sepals, petals and stamens following pollination:
INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) encodes on of several related proteins containing a C-terminal motif that is processed to a small peptide (Butenko et al. 2003). Constitutive overexpression of IDA, or simply adding the IDA peptide exogenously, can ectopically induce floral organ abscission (Stenvik et al. 2008).
HAESA is one of multiple related receptor-like kinases (RLKs) that is required for absicssion in response to IDA peptides, suggesting it might act as a receptor for the peptide. Consistent with this function, it is expressed in floral organ abscission layers (Jinn et al. 2000).