By contrast to the increasing number of transcription factors that have been related to DZ specification, little is known about the signalling mechanisms that must exist to ensure the co-ordination of events leading to pod shatter. The identification and characterization of regulatory elements in the promoters of the genes encoding the dehiscence-related enzymes listed above, and of putative targets of post-translational regulation in the encoded proteins could provide some insights as to how the process is co-ordinated. Such enzymatic activities are probably downstream in the regulatory cascade that originates with the transcription factors involved in DZ formation, but there are no consistent data on the nature and components of this cascade. So far, neither mechanisms of cell-to-cell communication nor signalling molecules have been identified as unequivocally related to this process. However, some candidates could be proposed based on diverse evidence.
Two putative membrane-bound proteins identified by the enhancer trap strategy could be suggested as participants based on sequence similarity and expression pattern analyses, although no functional data are reported yet. YJ80 shows reporter expression at the valve margin and in the seed abscission zone. The YJ80 T-DNA is inserted close to a gene encoding a protein with weak similarity to mammalian ankyrins, and which appears to be genetically downstream of SHP1/SHP2 and IND1 (Østergaard et al., 2001). The YJ115 T-DNA insertion is upstream of a gene of unknown function that contains a putative transmembrane domain. YJ115 shows reporter expression in the abaxial replum and the valve margins, and is regulated by the SHP genes (Roeder et al., 2001).
It has recently been reported that DEFENSE, NO DEATH1 (DND1), a gene encoding a cyclic nucleotide-gated ion channel (CNGC), is expressed in senescing organs and in the DZ of Arabidopsis siliques (Köhler et al., 2001). The dnd1 mutation was first recognized for producing a defective response to pathogen infection. dnd1 mutants are dwarf and, upon infection by avirulent pathogens, are unable to undergo the characteristic programmed cell death associated with the hypersensitive response (Yu et al., 1998). The CNGC encoded by DND1 is a membrane-protein able to conduct K+ and Ca2+ ions, a shared functional characteristic with other animal CNGCs involved in signal transduction (Leng et al., 1999). Although possible defects in dehiscence have not been studied in these mutants, it is tempting to speculate that the process of cell separation in the DZ could be affected by a failure in inducing specific programmed cell death.
As pointed out before, dehiscence and abscission are related processes. A leucine-rich repeat receptor-like protein kinase (LRR-RLK), HAESA, was shown to be involved in the regulation of floral organ abscission in Arabidopsis (Jinn et al., 2000). LRR-RKs are a large family in Arabidopsis, and some of their members have been related to different developmental processes based mainly on mutant phenotypes. Whether there is any LRR-RLK involved in fruit dehiscence regulation is still unknown, although it would be interesting to explore this possibility. SAC29 is an mRNA specifically up-regulated in Brassica napus DZ during late pod development. SAC29 encodes a protein with homology to the receiver domain of response regulator proteins (Whitelaw et al., 1999). In plants, several of these systems, organized in diverse modular arrangements, have been implicated in the response to different stimuli such as ethylene or cytokinin signals (reviewed in (D’Agostino and Kieber, 1999). Although no functional role has been assigned to SAC29, it represents an exciting starting point to study signal transduction mechanisms related to dehiscence.
Clearly, much work needs to be done to begin to clarify how fruit dehiscence is co-ordinated and which are the signals and signalling cascades involved. The study of the functional significance of some of the data presented above should be complemented with new efforts in this direction.
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