Occurrence and Localization of 9.5 Cellulase in Abscising and Nonabscising Tissues

Nitrocellulose tissue prints immunoblotted with 9.5 cellulase antibody were used to demonstrate areas of cellulase localization within Phaseolus vulgaris explants on exposure to ethylene. The 9.5 cellulase was induced in the distal and proximal abscission zone and in the stem. In both abscission zones, the 9.5 cellulase was found in the cortical cells of the separation layer, which develops as a narrow band of cells at the place where fracture occurs. The enzyme was also found associated with the vascular traces of the tissues adjacent to the separation layer extending through the first few millimeters at each side of the separation layer. The two abscission zones differed in the way that cellulase distributed through the separation layer as abscission proceeded. In the distal zone, cellulase appeared first in the cells of the separation layer adjacent to vascular traces and extended toward the periphery. In the proximal zone, 9.5 cellulase accumulated first in the cortical cells that lie in the adaxial side and then extended to the abaxial side. In response to ethylene, 9.5 cellulase was also induced in the vascular traces of the stem and the pulvinus without developing a separation layer. The role of 9.5 cellulase in the vascular traces is unknown. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by immunoblotting with 9.5 cellulase antibody identified the same 51-kilodalton protein in both abscising and nonabscising tissues. Therefore, the determinant characteristic of the abscission process is the induction of 9.5 cellulase by cortical cells in the separation layer, and this implies that these cells have a unique mechanism for initiating 9.5 cellulase synthesis.     

Further examination of abscission zone cells as ethylene target cells in higher plants

BACKGROUND AND AIMS: Two aspects of the competence of abscission zone cells as a specific class of hormone target cell are examined. The first is the competence of these target cells to respond to a remote stele-generated signal, and whether ethylene acts in concert with this signal to initiate abscission of the primary leaf in Phaseolus vulgaris. The second is to extend the concept of dual control of abscission cell competence. Can the concept of developmental memory that is retained by abscission cell of Phaseolus vulgaris post-separation in terms of the inductive/repressive control of beta-1,4-glucan endohydrolase (cellulase) activity exerted by ethylene/auxin be extended to the rachis abscission zone cells of Sambucus nigra? METHODS: Abscission assays were performed using the leaf petiole-pulvinus explants of P. vulgaris with the distal pulvinus stele removed. These (-stele) explants do not separate when treated with ethylene and require a stele-generated signal from the distal pulvinus for separation at the leaf petiole-pulvinis abscission zone. Using these explants, the role of ethylene was examined, using the ethylene action blocker, 1-methyl cyclopropene, as well as the significance of the tissue from which the stele signal originates. Further, leaf rachis abscission explants were excised from the compound leaves of S. nigra, and changes in the activity of cellulase in response to added ethylene and auxin post-separation was examined. KEY RESULTS: The use of (-stele) explants has confirmed that ethylene, with the stele-generated signal, is essential for abscission. Neither ethylene alone nor the stelar signal alone is sufficient. Further, in addition to the leaf pulvinus distal to the abscission zone, mid-rib tissue that is excised from senescent or green mid-rib tissue can also generate a competent stelar signal. Experiments with rachis abscission explants of S. nigra have shown that auxin, when added to cells post-separation can retard cellulase activity, with activity re-established with subsequent ethylene treatment. CONCLUSIONS: The triggers that initiate and regulate the separation process are complex with, in bean leaves at least, the generation of a signal (or signals) from remote tissues, in concert with ethylene, a requisite part of the process. Once evoked, abscission cells maintain a developmental memory such that the induction/repression mediated by ethylene/auxin that is observed prior to separation is also retained by the cells post-separation.     

A Role for the Stele in Intertissue Signaling in the Initiation of Abscission in Bean Leaves (Phaseolus vulgaris L.)

A combination of microdissection and viscometric endo-[beta]-1,4-glucanhydrolase assays was used to investigate if the early appearance of the abscission-related isoelectric point-9.5 endo-[beta]-1,4-glucanhydrolase in the stele of the pulvinus and abscission zone of the foliar abscission zone of Phaseolus vulgaris L. prior to cell separation (reported by E. del Campillo, P.D. Reid, R. Sexton, L.N.Lewis [1990] Plant Cell 2: 245-254) indicates that the vascular tissue of this region has a specific role in abscission. We find that no endo-[beta]-1,4-glucanhydrolase activity or cell separation is detectable in the abscission zone cortex if the abscission zone cortex is separated from the stele tissue. If the stele is separated from the abscission zone cortex after a lag period but again before any endo-[beta]-1,4-glucanhydrolase activity is present in the abscission zone cortex, then the enzyme is produced in the cortex and abscission ensues. We conclude that the cortex of the abscission zone is able to abscind independently of the vascular tissue only after the vascular tissue has begun to respond to abscission-promoting signals. We suggest that ethylene promotes formation of an abscission-permitting signal in the stele of the abscission zone and pulvinus, and that this signal is an essential elicitor for the synthesis of cell separation enzymes in the target cells of the abscission zone cortex.     

Changes in Two Forms of Membrane-Associated Cellulase during Ethylene-Induced Abscission

Only one form of membrane-associated cellulase was found previously in the lower petiolar pulvinus of Phaseolus vulgaris (cv Red Kidney). The cellulase has an isoelectric point (pI) of 4.5 (DE Koehler, LN Lewis 1979 Plant Physiol 63: 677-679). This enzyme was detected in abscission zones collected before the onset of abscission (control tissue), and was thought to represent a pre-secretory form of another cellulase, the abscission cellulase, which has a basic pI and is secreted during abscission. We now show that this acidic, membrane-associated cellulase is a glycoprotein, tightly bound to the membrane, with maximum activity at pH 5.1, and that it is not immunologically related to the abscission cellulase. Furthermore, when bean explants are induced to abscise with ethylene, the activity of the acidic cellulase declines rapidly to 50% of control levels in the first day. When abscission is fully developed, the membranes contain a basic form of cellulase with a pI of 8.0 to 9.0 and only trace levels of the acidic cellulase. The basic form is not a high mannose glycoprotein; it has maximum activity in a broad pH range (4.0-8.0) and is antigenically related to the abscission cellulase, which is induced during abscission and transported to the cell wall. Antibody raised against the abscission cellulase recognized two proteins in a crude membrane fraction from abscising tissue. One of those proteins comigrated with the abscission cellulase, and the other was 1 to 2 kilodaltons larger. Thus, during abscission, the acidic membrane-associated cellulase rapidly declines before the appearance of the abscission cellulase. We conclude that there is no conversion from the acidic cellulase to the basic cellulase and suggest that the acidic and basic cellulase isoenzymes are proteins derived from two different genes.     

Leaf Abscission in Soybean: Cytochemical and Ultrastructural Changes following Benzylaminopurine Treatment

6-benzylaminopurine (BAP) delays leaf abscission of soybeanGlycine max (L.) Merr. Abscission of the distal pulvinus ofprimary leaves was induced in 12-d-old seedlings or explantsby removal of the leaf blade. BAP applied to the cut end ofthe pulvinus following leaf blade removal delayed abscission.Discoloration of the pulvinus occurred before abscission commencedand the number of grana in chloroplasts within cortical parenchymacells of the pulvinus decreased over time following leaf bladeremoval. BAP prevented discoloration of pulvinus tissues anda decrease in grana number. Starch grains within amyloplastsof cells of the starch sheath in the pulvinus disappeared followingleaf blade removal, whereas starch accumulated within the abscissionzone prior to abscission. BAP prevented this apparent redistributionof starch and instead promoted an increase in starch withinplastids of cortical parenchyma cells of the pulvinus. Duringthe abscission process, cells within the separation layer enlargedand their nuclei and nucleoli became more evident prior to theirseparation from one another. Cell separation resulted from breakdownof middle lamellae and partial degradation of primary cell walls.Cycloheximide applied directly to the external surface of theabscission zone inhibited abscission in a similar way to theBAP treatment. These results suggest that BAP prevents abscissionby altering patterns of starch distribution in the pulvinusand abscission zone and by inhibiting the synthesis of proteinsthat typically appear de novo in induced abscission zone tissues. Key words: Benzylaminopurine, BAP, Soybean, Pulvinus, Abscission, amyloplast.     

Purification of a cellulase from kidney bean abscission zones

The purification of a cellulase isoenzyme with a pI of 9.5 from kidney bean abscission zones is described. An important step in the purification involved the adsorption of the cellulase isoenzyme onto an affinity column of CF-11 cellulose and the subsequent elution with cellobiose. Native and SDS polyacrylamide gel electrophoresis established that there was only one component in the purified cellulase samples. Antibodies raised against the purified pI 9.5 cellulase precipitated this isoenzyme from crude or purified solutions but did not cross react with pI 4.5 cellulase from 2,4-D-treated abscission zones. The antibody was shown to be monospecific by immunoelectrophoresis and by the fact that it precipitated only a single ¹⁴C-labeled protein from an abscission zone extract heavily labeled with ¹⁴C amino acids.     

The use of immunological methods to study the activity of cellulase isozymes (B 1:4 glucan 4-glucan hydrolase) in bean leaf abscission

Abstract The changes in the levels of two different isozymes of cellulase (EC 3.2.1.4) have been followed during the abscission of the primary leaves of bean (Phaseolus vulgaris c.v. Red Kidney), using antibodies raised against the 9.5 form of the enzyme. Data from both radioimmune and direct assay show that the 9.5 form of cellulase is undetectable prior to the induction of abscission. After a 12 h lag this isozyme increases in activity, the increase preceding a decrease in integrity of the abscission zone cell walls. The results are consistent not only with the view that this specific isozyme is involved in wall hydrolysis but also with previous data which showed that cellulase is synthesized ‘de novo’. The 4.5 isozyme of cellulase is more widely spread throughout the plant, being most active in young tissues. During abscission the activity of this isozyme in the abscission layer falls and consequently it is not thought to be involved directly in the abscission process.     

LOCALIZATION OF DEHYDROGENASE AND ACID PHOSPHATASE IN THE ABSCISSION ZONE OF BEAN LEAVES

Leaf abscission in Phaseolus vulgaris L. cv. ‘Contender’ is associated with enzymatic changes during and prior to separation. Deblading resulted in a localized increase in dehydrogenase and acid phosphatase in the abscission zone. Increased enzyme activities were observed 24–48 hr after deblading. In debladed plants separation was complete in 6–8 days. At separation, dehydrogenase activity appeared to decrease and localization was specific to the protective layer, while the petiole side had no activity. In contrast, acid phosphatase activity was observed in some layers of cells on the petiole side after separation. Ethylene treatment promoted abscission and separation occurred in 24–48 hr in both debladed and intact plants. No protective layer was formed during ethylene-induced abscission. Enzymatic changes similar to those observed in debladed control plants were observed with ethylene treatment. Ethylene induced an additional abscission layer between the pulvinus and petiole, where an abscission layer normally does not form. In this ethylene-induced abscission layer, similar enzyme activities were detected.     

Occurrence of 9.5 cellulase and other hydrolases in flower reproductive organs undergoing major cell wall disruption

The occurrence of enzymes associated with bean leaf abscission was investigated in bean (Phaseolus vulgaris) flower reproductive organs in which catabolic cell wall events are essential during anther and pistil development. Cellulase activity was detected in high levels in both pistil and anthers of bean flowers before anthesis. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by immunoblotting with 9.5 cellulase antibody identified a protein in anthers and pistil with the same size (51 kilodaltons) and serologically closely related to the abscission cellulase. The accumulation of 9.5 cellulase protein in the anther is developmentally regulated and increases from undetectable levels at very young stages of anther development to high levels as the anther matures. In the pistil, the 9.5 cellulase was localized in the upper part of the pistil where the stigma and the stylar neck reside and was detected in the youngest developmental stage analyzed. Antibodies against basic chitinase, which accumulates to high levels in abscission zones after exposure to ethylene, identified a protein with the same size (33 kilodaltons) and serologically closely related, in both anthers and upper portion of the pistil. In contrast, a 45-kilodalton protein and the basic β-1,3-glucanase associated with abscission were undetected in bean reproductive organs. Interestingly, β-1,3-glucanase activity was detected in young bean anthers and decreased at anthesis, but the anther β-1,3-glucanase is serologically unrelated to the basic β-1,3-glucanase. Thus, it appears that the basic cellulase and chitinase occur in combination in many plant processes that require major cell wall disruption, whereas hemicellulases such as β-1,3-glucanase are specific to each process.     

Transdifferentiation of Mature Cortical Cells to Functional Abscission Cells in Bean

Abscission explants of bean (Phaseolus vulgaris L.) were treated with ethylene to induce cell separation at the primary abscission zone. After several days of further incubation of the remaining petiole in endogenously produced ethylene, the distal two-thirds of the petiole became senescent, and the remaining (proximal) portion stayed green. Cell-to-cell separation (secondary abscission) takes place precisely at the interface between the senescing yellow and the enlarging green cells. The expression of the abscission-associated isoform of β-1,4-glucanhydrolase, the activation of the Golgi apparatus, and enhanced vesicle formation occurred only in the enlarging cortical cells on the green side. These changes were indistinguishable from those that occur in normal abscission cells and confirm the conversion of the cortical cells to abscission-type cells. Secondary abscission cells were also induced by applying auxin to the exposed primary abscission surface after the pulvinus was shed, provided ethylene was added. Then, the orientation of development of green and yellow tissue was reversed; the distal tissue remained green and the proximal tissue yellowed. Nevertheless, separation still occurred at the junction between green and yellow cells and, again, it was one to two cell layers of the green side that enlarged and separated from their senescing neighbors. Evaluation of Feulgen-stained tissue establishes that, although nuclear changes occur, the conversion of the cortical cell to an abscission zone cell is a true transdifferentiation event, occurring in the absence of cell division.     

Cellulase and Abscission in the Red Kidney Bean (Phaseolus vulgaris)

Cellulase (β-1, 4-glucan-glucanohydrolase EC 3.2.1.4) activity in the abscission zone of red kidney bean (Phaseolus vulgaris) was previously shown to exist in at least two different molecular forms. The form of the enzyme which has an isoelectric point of 4.5 is present in both abscising and nonabscising tissue and requires grinding for extraction. Another form of the enzyme which has an isoelectric point of 9.5 is present only in tissue in which the abscission process has been induced. Further, much of this form of cellulase can be removed from the tissue by vacuum infiltration with buffer. Time course studies indicate that while the increase in measurable cellulase activity in tissue which is actively undergoing abscission was due primarily to the appearance of cellulase 9.5, this form of the enzyme cannot be removed by vacuum infiltration until after the breakstrength of the abscission zone has decreased nearly to zero. The intracellular localization of these two forms of cellulase is discussed.     

Characterization of a non-abscission mutant in Lupinus angustifolius. I. Genetic and structural aspects

A spontaneous mutant, Abs, that does not abscise any organs despite an apparently normal pattern of growth and senescence was isolated from among plants of Lupinus angustifolius cv. 'Danja'. Abs was found to be a recessive single gene mutation, and it was proposed that the gene for the original mutant phenotype, referred to as Abs, be designated abs1. An artificially induced mutant allelic to abs1 was also obtained and a non-allelic mutant phenotype, Delabs (delayed abscission), which was designated abs2. Morphological and cytological features of the abscission process under conditions of natural and ethylene-induced senescence were compared in the wild-type parent and Abs mutant. In the parent genotype abscission under natural conditions is similar to many other species, consisting of a stage of cell division forming an abscission zone, activation of the cytoplasm of zone cells, dissolution of the middle lamella, disorganization of fibrillar wall structure, and cell separation. A slightly different pattern of abscission zone development was observed for ethylene-treated explants of the parent, mainly with respect to features of cell division and cell enlargement. In Abs no abscission occurred for any abscission sites under conditions of natural senescence or with ethylene treatment of small shoot explants. However, relatively normal abscission zones were differentiated at all sites in the mutant except that extensive cell wall disorganization did not occur. Ethylene production by leaves or other organs of the mutant was no different from that of Danja. Application of copper salts or hydrogen peroxide, droughting, waterlogging, or application of abscisic acid (ABA) increased ethylene production equally in both genotypes but did not result in abscission in the mutant. Release of root cap border cells, the only other cell separation process examined, was similar in each genotype. The study concludes that the mutation is quite specific to the abscission process and may be due to a lack of or delay in the expression of hydrolytic enzyme(s) associated specifically with abscission zone differentiation and separation.     

Jasmonates promote abscission in bean petiole expiants: Its relationship to the metabolism of cell wall polysaccharides and cellulase activity

Jasmonic acid (JA) and its methyl ester (JA-Me) promoted the abscission of bean petiole expiants in the dark and light, and the activity of these compounds was almost same. JA and JA-Me did not enhance ethylene production in bean petiole expiants in the light, indicating that the abscission-promoting effects of these compounds are not the result of ethylene. Cells in the petiole adjacent to the abscission zone expanded during abscission but not in the pulvinus, and JA-Me promoted cell expansion in the petiole and the pulvinus. JA-Me had no effect on the total amounts of pectic and hemicellulosic polysaccharides in 2-mm segments of the abscission region, which included 1 mm of pulvinus and 1 mm of petiole from the abscission zone. On the other hand, the total amounts of cellulosic polysaccharides in this region were reduced significantly by the addition of JA-Me in the light. JA-Me had no effect on the neutral sugar composition of hemicellulosic polysaccharides during abscission. The decrease in the endogenous levels of UDP-sugars in the petiole adjacent to the abscission zone was accelerated during abscission by the addition of JA-Me in the light. Cellulase activities of pulvinus and petiole in 10-day-old seedlings were enhanced by the addition of JA. These results suggest that the promoting effect of JA or JA-Me on the abscission of bean petiole explants is due to the change of sugar metabolism in the abscission zone, in which the increase in cellulase activity involves the degradation of cell wall polysaccharides. Jasmonic acid (JA) and its methyl ester (JA-Me) are considered to be putative plant hormones for a number of reasons, including their wide occurrence in the plant kingdom, biologic, activities in multiple aspects at low concentrations, and their interaction with other plant hormones (for reviews see Parthier 1991, Hamberg and Gardner 1992, Sembdner and Parthier 1993, Ueda et al. 1994a). We have already reported that JA and JA-Me and C₁₈-unsaturated fatty acids, which are considered to be the substrates of the biosynthesis of jasmonates, are powerful senescence-promoting substances (Ueda et al. 1982b, 1991a). Senescence symptoms induced by these compounds are identical to those of natural senescence. Recently we have also found that JA inhibited indole-3-acetic acid (IAA)-induced elongation of oat (Avena sativa L. cv. Victory) coleoptile segments by inhibiting the synthesis of cell wall polysaccharides (Ueda et al. 1994b, 1995). These facts led us to study the mode of actions of JA and JA-Me on promoting abscission, which is considered the last dramatic phenomenon of senescence. In this paper we report that JA and JA-Me promote abscission in bean (Phaseolus vulgaris L. cv. Masterpiece) petiole expiants and that the changes in the metabolism of cell wall polysaccharides in the petiole and the pulvinus adjacent to the abscission zone are involved in the promotive effects of these compounds.Abbreviations ABA abscisic acid - ACC 1-aminocyclopropane-1-carboxylic acid - DCB 2,6-dichlorobenzonitrile - HPLC high performance liquid chromatography - IAA indole-3-acetic acid - JA jasmonic acid - JA-Me methyl jasmonate - MES 2-(N-morpholino)ethane-sulfonic acid, monohydrate - TCA trichloroacetic acid - Tris 2-amino-2-hydroxymethy-1,3-propanediole     

Effects of Ethylene and 2,4-D on the Activity of Cellulase Isoenzymes in Abscission Zones of the Developing Orange Fruit

The role of ethylene and 2,4-D in the abscission process, and the induction of cellulase isoenzymes in the abscission zones of Citrus fruit at two physiological stages of fruit development, were studied using a new staining technique for the detection of cellulase isoenzymes in polyacrylamide gels following electrophoretic separation. Four to seven isoenzymes were detected in the shoot-peduncle (zone A) and peduncle-fruit (zone C) abscission zones; at least two of them could be detected at excision time, and of these at least one could not be connected with abscission. In the young fruit, ethylene enhanced and 2,4-D delayed both abscission and the formation of several isoenzymes. In the older fruit, ethylene enhanced and 2,4-D delayed the formation of isoenzymes at a time where no abscission occurred any more in zone A. A slower but significant increase in most of the isoenzyme activity detected was also observed in abscission zone A of untreated older fruit explants after excision. These results fully agree with those reported earlier in relation to total cellulase and polygalacturonase activity (Greenberg et al., Physiol. Plant. 34: 1, 1975) tested at the same stages of fruit development. It is suggested, that the generality of the concept that a rise in hydrolytic enzymes in the abscission zone is necessarily followed by separation of the organ should be re-evaluated.     

Abscission: role of cellulase

Cellulase (β-1,4-glucan-glucanohydrolase EC 3.2.1.4) activity increased during abscission and was localized in the cell separation layer of Phaseolus vulgaris L. cv. Red Kidney (bean), Gossypium hirsutum L. cv. Acala 4-42 (Cotton) and Coleus blumei Benth. Princeton strain (Coleus) abscission zone explants. Cellulase activity was optimum at pH 7, was reduced by one-half after heating to 55° for 10 min, and was associated with the soluble components of the cell. Explants treated with aging retardants (indoleacetic acid, ⁶N-benzyladenine, and coumarin), CO₂, actinomycin D or cycloheximide had less cellulase activity than untreated controls. Ethylene increased cellulase activity of aged explants after a 3-hr lag period but had no effect on cellulase activity of freshly excised explants. It was concluded that 1 of the roles of ethylene in abscission is to regulate the production of cellulase which in turn is required for cell separation.     

ULTRASTRUCTURAL STUDIES OF ABSCISSION IN PHASEOLUS: ETHYLENE EFFECTS ON CELL WALLS

This paper reports light and electron microscope observations of changes in the walls of cortical cells in the laminar abscission region of red kidney bean (Phaseolus vulgaris L.). In intact plants two or three rows of cells comprise the abscission zone. Pectic substances are not present in the walls of these cells when wall breaks occur. The separation cavity involves breaks in both radial and longitudinal cell walls. In ethylene-treated explants pectic substances are present in the cell walls when breaking occurs. The separation cavity involves breaks in longitudinal walls only, and breaking is confined to a single row of cortical cells. Prior to cell wall break the plasma membrane frequently invaginates. In intact plants this may be associated with plasmolysis and with the formation of secondary vacuoles. In ethylene-treated explants it may also be related to plasmolysis. At the time of cell wall break many unidentifiable inclusions of varying sizes and shapes are present in the cell wall region. Chloroplasts and mitochondria are structurally altered but recognizable in the cell at the time of wall break. Plasmodesmata are frequently observed in abscission cells and may be structurally elaborate. The observations of the nature of cell wall changes during abscission in ethylene-treated material fail to confirm physiological studies of other workers suggesting that pectin dissolution is necessary and may be sufficient for formation of a separation layer.     

Cellulase and cell differentiation in Acer pseudoplatanus

Summary Homogenates of differentiating xylem and phloem tissue have higher cellulase activities than cambial samples; the highest activity is always found in phloem. Callus tissue, in which no vascular differentiation occurs, contains only low cellulase activity. The results suggest that cellulase is involved in vascular differentiation. Different pH optima of cellulase activity were found: in cambium, xylem and phloem tissue, cellulase activity with an optimum at about pH 5.9 is predominantly membrane-bound; it is sedimentable at 100,000 g and releasable by Triton X-100. The same may be true of activity with an optimum at pH 5.3. Phloem tissue also contains a soluble, cytoplasmic cellulase of high activity at pH 7.1, and xylem tissue contains cytoplasmic cellulase with an optimum at pH 6.5. Low cellulase activity with a pH optimum similar to that of xylem homogenates was found in xylem sap. Cellulase activity in abscission zones increases greatly just before leaf abscission. Abscission zone cellulase has two pH optima, et 5.3 and 5.9; both activities are increased by Triton treatment of homogenates. The possible existence of several different cellulases forming part of a cellulase complex, and the rôle of the enzymes in hydrolysing wall material during cell differentiation are discussed.     

Abscission of flowers and floral parts

The abscission of inflorescences, flowers, petals, sepals, styles,and stamens is discussed, with emphasis on the anatomy and ultrastructureof the abscission zones, and the role of cell wall degradingenzymes and hormonal control. Shedding of these parts is usuallydue to cell wall dissolution, but abscission of petals, stamens,and styles in some species occurs due to the forces generatedby the growing fruit. Flower abscission is clearly regulatedby ethylene, whilst auxins apparently decrease the sensitivityto ethylene. Petal, style and stamen abscission also seems tobe controlled by endogenous ethylene. Auxin is apparently involvedin abscission of styles and stamens, but in petals its roleis at yet unclear. The ultrastructural data indicate high proteinsynthesis and high secretory activity of material toward cellwalls of abscission zone cells. The physiological evidence indicatesa role of both polygalacturonase and cellulase in cell walldissolution, whilst the role of other cell wall degrading enzymesis still unknown. The physiological processes occurring in thewalls of the separating cells should be distinguished from thoserelating to defence against microbial intrusion, such as depositionof lignin and suberin and tylose formation. Experimentationusing mutants and transgenic plants may aid in separating theseprocesses. Sequencing of the isoenzymes specific for the abscissionzone and a search for abscission zone-specific promoters seemsa requirement for the successful evaluation of the enzymes involvedin cell wall degradation. Key words: Abscission, anatomy, abscission zone, hormonal control, cell wall degrading enzymes, inflorescences