Update on Abscission and Dehiscence in Arabidopsis Cutting Loose. Abscission and Dehiscence in Arabidopsis

Delayed cell separation historically was one of the first agricultural traits selected for by man. Successful collection of fruits and seeds of crops such as wheat (Triticum monococcum), rice (Oryza sativa), and a variety of legumes was only possible due to the selection for delayed fruit abscission or pod shatter in which seeds were retained on the stalk rather than rapidly shed. This selection pressure was further imposed as human beings began to harvest by sickle rather than collection in baskets because only the grains that remained longer on the plant were harvested and propagated for future use. A glance at some of the wild relatives of wheat and barley (Hordeum vulgare) shows the presence of brittle, easily shed, seed-bearing stalks rather than the tougher, seed-retaining stalks of some of today’s cultivars. In crops like amaranth (Amaranth caudatus) and rice, there is still a strong emphasis on additional selection for delayed seed shatter because major crop loss occurs. Although plant breeders have selected for nonabscising or early abscission traits for centuries, it is only in the last 50 years that scientists have begun to understand the processes regulating abscission. Scientists initially identified abscissic acid (ABA) as the primary substance responsible for abscission of leaves and fruits. The primary role of ABA in regulating seed dormancy and stomatal opening and closing subsequently was recognized. Although the initial name, ABA, has been retained, the role of ABA in regulating abscission is minor. Furthermore, the identification of ethylene as the gas responsible for leaf abscission and senescence associated with gas lighting nearly 100 years ago by Anton Nicolyovitch Neljubov, led researchers to focus on ethylene as a primary regulator of abscission. In addition, the elucidation of the biosynthetic pathway of ethylene synthesis by Yang and Hoffman (1984) provided additional methods to understand ethylene’s involvement in abscission. Abscission is an active process and has a variety of roles during plant development. Plant parts such as pollen, fruits, seeds, and leaflets may be shed in response to developmental cues to guarantee efficient dispersal or propagation of the plant. Unwanted organs such as flower petals, sepals, and filaments alternatively may be shed when they no longer serve a functional role to the plant. Damaged or infected organs may be rapidly shed as a mechanism of defense. Early studies clearly define the anatomy of the abscission zone using bean (Phaseolus vulgaris), tomato (Lycopersicum esculentum), and Sambucus nigra as model systems (Jensen and Valdovinos, 1967; Addicott, 1982; Osborne, 1989). To speak generally, the abscission zone encompasses several layers of small densely cytoplasmic cells at the juncture of the organ and the main body of the plant. These cells are predetermined at an early stage and proceed through a series of morphological changes associated with the developmental position or stage of the organ being shed. They are characterized by increased rough endoplasmic reticulum associated with the plasma membrane and Golgi. Accumulation of microbodies and invaginations of the plasma membrane are also observed. Associated with this invagination is swelling of cells on either the proximal or distal side of the abscission layer. Irregular cellulose microfibril rearrangement has also been observed in cells within the abscission zone. Delineating the timing of these structural changes is a goal of today’s researchers. In the past, many biochemical changes within the cells of the abscission zone were measured in association with the process of abscission. Modifications of the elemental composition of the cells, changes in hormones, and increased expression of cell wall hydrolytic enzymes are some of the most frequent observations. To be specific, lower levels of calcium have been observed in active abscission zones in correlation with the conversion of insoluble pectins to soluble pectic acids. Changes in the levels of pectin methylesterases and pectate lyases are thought to be involved in demethylation of the pectins, and thus the breakdown of the middle lamella. Other cell wall hydrolytic enzymes that have been demonstrated to be up-regulated in correlation with abscission include glucanases, xyloglucan hydrolyases, and polygalacturonases (PGs; Hadfield and Bennett, 1998; Roberts et al., 2000). In some cases, the genes coding for these enzymes are represented by very large families and the identification of the specific genes involved in the abscission process will be much more difficult. Last, hormones such as ethylene and auxin have long been associated with regulating abscission, and levels of ethylene have been shown to be higher within abscission zone tissues. The addition of ethylene or ethylene analogs to many plants similarly has been shown to accelerate the abscission process, whereas auxin and auxin analogs delay abscission. * E-mail [email protected]; fax 608 –262– 4349.