| dc.description.abstracteng | In higher plants, xenobiotic chemicals induce transcriptional activation of genes
involved in their detoxification. A generally accepted concept explaining gene
regulatory networks is that, activation of pre-existing primary transcription factors
regulates transcription of secondary transcription factors that in turn induce genes
that execute the appropriate response. In the detoxification program, TGA
transcription factors and their transcriptional co-activator SCL14 bind to promoters
containing activation sequence-1 (as-1)–like cis-elements. Microarray analysis of
plants containing either lower or higher amounts of SCL14 had identified two NAC
transcription factors – ANAC032 and ATAF1 – whose expression was affected by
SCL14 (Fode et al., 2008). Thus, these are candidate secondary transcription factors that lead to the expression of downstream genes. In the current study, a microarray of transgenic 35S:ANAC032 plants was compared to a previous array that had identified genes that were activated by the xenobiotic TIBA. Seventy-eight genes were up-regulated upon TIBA treatment or ectopic expression of ANAC032 and were thus potential direct or indirect target genes of the NAC TFs in the detoxification response. Three such potential target genes – AKR4C9, bHLH585 and At3g04000 – were found to be induced by TIBA in a TGA/SCL14-dependent manner with induction being compromised in varying degrees in the ataf1 and anac032 single knockout and ataf1anac032 double knockout plants. Additionally, transient expression assays indicated that the promoters of the three genes could be induced by ANAC032 and ATAF1, although only when fused to the strong activating domain VP16. High SCL14/TGA-dependent induction of NAC TFs and the three target genes, AKR4C9, bHLH585 and At3g04000 was also observed upon wounding. This wound response was not dependent on the plant hormone jasmonic acid. Ectopic expression of the two NAC TFs was also found to suppress SA-, JA/ET-, JA and ABA- responsive genes. Since their expression is also triggered by these hormones, they are candidates to mediate the antagonism between the
corresponding pathways. This could not be confirmed using the ataf1anac032 double knock out, which might be due to the redundancy with the two related transcription factors ANAC102 and ATAF2 or NAC-independent mechanisms.
Lastly, ectopically expressed ANAC032 and ATAF1 led to developmental defects
including dwarfism, curled leaves showing early yellowing and delayed or absent
flower initiation. The latter may be due to increased expression of FLC observed in
the 35S:ANAC032 and 35S:ATAF1 plants. Consistently, these plants show increased juvenility which is characteristic of plants over-expressing the FLC gene. As revealed by increased dormancy of ataf1anac032 seeds, ANAC032 and ATAF1 seem to negatively regulate seed dormancy. The molecular basis of this regulation needs to be carefully studied in the future. However, clues may be provided by the microarray which indicated that 35S:ANAC032 plants show an up-regulation of genes related to negative regulation of seed dormancy. To sum up, the ATAF-type NAC TFs seem to have two major roles, one in modulating responses under stress conditions and the other in regulating seed dormancy. | de |