Regulation of Synthesis

ACS is the major factor regulating the rate of ethylene synthesis. This regulation is in part dependent on the level of ACS. One of the mechanisms used by plants to control the concentration of ACS is the transcriptional regulation of ACS genes.

Using promoter-GUS fusions, Rodrigues-Pousada et al. (1993, 1999) and Tsuchisaka and Theologis (2004b) showed that the different functional ACS genes in Arabidopsis each have a unique pattern of expression. Although the patterns are specific to each gene, they show overlapping regions of expression. The GUS expression also differs according to changes in environmental conditions. For instance, the expression of ACS1/2, 2/4 (see remark concerning ACS gene numbering above), 6, 7, 8 and 11 in five-day-old etiolated seedlings is confined to the elongation zone of the hypocotyl, the embryonic root region, the cotyledons and the root vascular tissue. In the light, however, these genes are expressed in the cotyledons, the embryonic root, the roots, and in primary leaves, while ACS 1/2, 5, 8 and 11 are active in the shoot apex.

In addition, different stress-promoting factors (cold, wounding, heat) have been shown to alter the transcriptions of ACS genes, with each factor altering the transcription of each individual ACS gene in a specific manner (Tsuchisaka and Theologis 2004b). Wounding (by cutting) the hypocotyls of five-day-old light-grown seedlings inhibits the expression of the genes that are constitutively expressed in the intact tissue, like ACS1 and ACS5, and induces the expression of ACS1/2, 2/4, 6, 7 and 8, which were not expressed before wounding. Cold treatment inhibits the expression of ACS5

and ACS 11 and alters the pattern of ACS8 expression, whereas heat enhances the expression of ACS4 and alters the pattern of ACS8 and ACS 11. Moreover, it was shown by RT-PCR that ACS1/2 and ACS7 are salt-inducible (Achard et al. 2006), and that ACS 6, 8, 9, and 11 are induced in low-light conditions (Vandenbussche et al. 2003b). Finally, ACS8 appears to be subject to strong circadian control (Thain et al. 2004).

Hormonal induction of ACS genes has also been documented extensively. First, it was demonstrated that auxins are inducers of ethylene production (Yang and Hoffman 1984; Abeles et al. 1992). Abel et al. (1995) and Tsuchisaka and Theologis (2004b) provided proof of auxin-enhanced transcription and differential expression patterns of ACS 1/2, 2/4, 5, 6, 7, 8 and 11 in the root. Furthermore, ACS1/2 also was proven to be up-regulated by cytokinins in the root (Rodrigues-Pousada et al. 1999). In light-grown seedlings, ACS4, 5, and 7 are responsive to ABA (Wang et al. 2005), while ACS7 is also responsive to GA (gibberellins). Finally, brassinosteroids can stimulate ACS2/4 expression in dark-grown Arabidopsis seedlings (Joo et al. 2006).

As mentioned before, it was shown that ACS functions as a homod-imer. This implies the possibility that functional heterodimers can also form. Tsuchisaka and Theologis (2004a) have been able to prove that 17 functional heterodimers can form in E. coli. As yet this finding has not been confirmed in plants, but this may indicate why ACS is encoded by a multigene family. The unique expression pattern of these genes (which often overlaps) could therefore offer a combination of different isozymes, leading to a high number of heterodimers, which in turn can have different biochemical properties. It was hypothesized that ACC can be synthesized in different tissues and under different conditions due to these different biochemical properties (Tsuchisaka and Theologis 2004b).

Plants also control ACS levels post-transcriptionally. In Arabidopsis this mechanism was revealed by the analysis of eto (ethylene overproduction) mutants. There are three ETO genes in Arabidopsis, and mutants have been characterized for each. eto2 is an allele of ACS5. A dominant mutation (insertion in the C-terminus) in this gene causes an increase of ACS5 protein accumulation (Vogel et al. 1998). Likewise, a recessive mutation in ETO1 causes an increase of ACS5. ETO1 is a BTB (broad complex - tramtrack -brick-a-brack)-domain-containing protein that also possesses a TPR (tetra-tricopeptide repeat) motif (known to interact with other proteins) and could function as an adaptor connecting ACS to a CUL3 ubiquitin E3 ligase, thus promoting ACS degradation (Wang et al. 2004). ETO1 exercises its function through interaction with the C-terminus of ACS5, which also explains the higher stability of ACS5 in the eto2 mutant (Wang et al. 2004). Much like eto2, eto3 is modified in the C-terminal region of ACS9, an ACS closely related to ACS5, rendering it more stable (Chae et al. 2003).

The levels of ethylene are also regulated by ACO. In Arabidopsis it was proven that ACO is regulated by ethylene at the transcriptional level. This regulation is different for different isozymes (van Zhong and Burns 2003; De Paepe et al. 2004b). ACO genes are also differentially regulated by light intensity (Vandenbussche et al. 2003b).

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