HAE is a transmembrane serine/threonine protein kinase located in the plasma membrane [36]. HAE is expressed in the abscision areas of the floral organs, as well as at the base of the petioles and stems. Expression in flowers depends on the stage and is first observed in maturing flowers, which coincides with the competence for self-pollination. Transgenic plants that express an antisense constitutive construction of HAE have a defective cleavage of the floral organs, suggesting that HAE plays a role in abscision [36]. ABSCISSION-DEFICIENT INFLORESCENCE (IDA) encodes a small protein thought to be secreted necessary for the cleavage of floral organs, increasing the possibility that it may be the HAESA ligand [37]. The physiological control of abscision is discussed in more detail in Chapter 3 by Kozlowski and Pallardy (1997). Agustí, J., Merelo, P., Cercos, M., Tadeo, F. R., and Talon, M. (2008). Differential gene expression induced by ethylene during abscision of citrus leaves. J. Exp. Bot 59, 2717–2733.

doi: 10.1093/jxb/ern138 FIGURE 6-7. Schematic formation of a layer of abscesses around a diseased site of a prunus leaf. [After Samuel (1927).] (A–C) Leaf spots and ball holes by Xanthomonas arboricola pv. prunibacteria on (A) ornamental cherry leaves; Characteristic broad light green halos form around the infected area before all affected tissue falls, (B) on the peach and (C) on the plum. The shooting hole effect is especially evident on plum leaves. The cleavage of the floral organs in Arabidopsis is divided into four different stages of development.21 The first stage, which usually occurs early in the development of the flower, is the differentiation of specialized AZ cells, in which the organ later separates from the plant. When AZ cells are not differentiated, abscision does not occur, as in the blade-on-petiole 1 and 2 double mutant (BOP1 BOP2), which does not represent anatomically different AZ cells.17 The second phase begins when the organs prepare to separate from the plant, often with an environmental or hormonal stimulus, and the AZ cells become competent to respond to abscision signals. Subsequently, pectin begins to break down in the cell walls between the AZs, followed by an expansion in the size of the AZ cells.24 Ultimately, the actual separation of the organ overlaps and a woody protective layer develops on the distal side of the AZ to protect the plant from pathogenic attacks. Peng, G., Wu, J., Lu, W. and Li, J. (2013).

A polygalacturonase gene grouped in clade E is involved in the cleavage of lychee fruits. Sci. Hortic. 150, 244–250. doi: 10.1016/j.scienta.2012.10.029 Figure 26. Fresh cross-sections, which show the general characteristics of the xylem of various components of the cut rose. The sections are colored with toluidine blue O. (a) stem; (b) abscision zone; (c) the median region of the stem; (d) distal end of the stem.

Bar = 0.5 mm. Abscision can also occur in premature leaves as a defense of plants. Premature leaf cleavage has been shown to occur in response to an infestation of biliary aphids. By abbcing the leaves housing leafy poplars, plants have been shown to massively reduce the pest population, as 98% of aphids died in stripped galls. Abscision is selective and the risk of dropping leaves increases with the increase in the number of galls. A leaf with three or more galls was four times more likely to abstain than a leaf with a gall, and 20 times more likely to drop it than a leaf without galls. [1] The reduction in chlorophyll production in the leaves due to reduced sun exposure in autumn explains why some leaves turn yellow. However, the yellow color can attract aphids, so some trees instead color the leaves red by injecting a light pigment. [7] The loss of chlorophyll may also contribute to the abscision process.

Petersen, M., Sander, L., Child, R., Van Onckelen, H., Ulvskov, P., and Borkhardt, B. (1996). Isolation and characterization of a polygalacturonase specific to the dehiscence zone of Brassica napus pods. Plant Mol. Biol. 31, 517–527. doi: 10.1007/BF00042225 Berüter and Droz (1991) showed that during the period of apple fall before June, various treatments (shading, proximal border of the abscision zone, removal of seeds from attached fruits) induced fruit abscision. If the same treatments were applied after June, none of the fruits fall off. The data showed that during the lowering period before June, abscision was induced by blocking nutrient intake to the fruit abscision zone.