Is there hormonal control of dehiscence?
Abscission processes have been linked by considerable evidence to the opposing action of the plant hormones ethylene and auxin (reviewed in González-Carranza et al., 1998). Ethylene works by promoting abscission, while auxins act to delay it. Together, the balance between these two hormones appears to be an important factor regulating the timing of abscission. The hormonal induction of pod dehiscence is less well characterized, although some studies have been reported suggesting similar responses. In studies on fruit development of Brassica parthenocarpic pods, a slightly delayed shattering was correlated with a reduced ethylene production, and normal timing of cell separation could be restored by the application of exogenous ethylene (Child et al., 1998). However, Arabidopsis mutants with non-functional ethylene receptors exhibit a normal time-course of silique dehiscence, suggesting that ethylene might have a minor role in shatter induction.
By contrast, auxins are likely to play a dominant role in dehiscence regulation. In Brassica, an increase in cellulase activity in the DZ was shown to correlate with a specific decrease in auxin content in those cells. In addition, the exogenous application of auxin analogues to Brassica developing pods significantly delayed, although it did not prevent, DZ cell separation (Chauvaux et al., 1997). It was observed that these auxin analogues inhibited the increase in cellulase activity and, interestingly, also affected RDPG1 pectinase activity by blocking its secretion into the cell wall (see above, Dal Degan et al., 2001).
Related to the possible role of auxin in dehiscence regulation the pattern of expression reported for the bHLH transcription factor SPATULA (SPT) is suggestive. In spt mutants the formation of the gynoecium apical tissues and the septum is reduced and the fertility is low. SPT is required for the development of carpel tissues specialized in pollen tube growth: stigma, style and transmitting tract (Alvarez and Smyth, 1999). However, SPT is not only expressed in the pollen tract tissues, but it shows a complex pattern of expression in many different tissues and developmental stages, indicating that SPT may act redundantly with other factors to control tissue growth. Especially intriguing is SPT expression related to cell separation: SPT mRNA is found in the anther stomiun, the DZ of grown siliques, and in the seed abscission zone (Heisler et al., 2001). Diverse evidence indicates that SPT is responsive to auxin: it has been shown that ETTIN, an Auxin Response Factor (ARF), represses SPT in some carpel tissues (Sessions et al., 1997; Heisler et al., 2001); several Auxin Response Elements, the binding motifs for ARFs, are found in the SPT promoter, and it appears that, together with repression by ETT, other ARFs could activate SPT expression (Heisler et al., 2001); finally, inhibitors of auxin polar transport suppress spt carpel phenotypes (Nemhauser et al., 2000). It would be interesting to explore whether SPT could be a link between cell differentiation and auxin action related to DZ formation, and other separation processes such as anther dehiscence or seed abscission.
Pod dehiscence and seed abscission are likely to be co-ordinated, and they could share common regulatory signals. Meakin and Roberts (1991) confirmed this correlation in infection-induced pod dehiscence of Brassica fruits. In addition, some of the genes described above, including RDPG1, YJ80-reported gene, and SPT, are expressed in both dehiscence and seed abscission zones and a functional relationship has been established between DZ genes (SHP, Pinyopich et al., 2001). What could be those common signals? So far, little evidence exists to propose such a mechanism, but abscisic acid may be suggested as a possible co-ordinating signal: its role in seed development is well established, and it has been related to the regulation of programmed cell death in processes like cereal seed germination (Young and Gallie, 2000). A parallel can be proposed between pod shatter and anther dehiscence. These are analogous processes of cell separation that share a number of specifically expressed genes (RDPG1, SPT, see above). Jasmonic acid (JA) has been related to anther dehiscence, and mutants defective in JA synthesis show an altered timing of anther opening (Ishiguro et al., 2001; Sanders et al., 2000). So far, there are no reports of a possible role for JA in pod shattering, but the search for a common signalling factor between anther and pod dehiscence is still far from extensive.
It appears that hormone action could have a role in both DZ differentiation and the co-ordination of physiological events leading to cell separation. Few studies have directly addressed these questions so far, but a large collection of Arabidopsis mutants affected in different hormone synthetic pathways or hormone responses is available that could be used for further characterization of the role of these likely regulators.
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