Srdx transcription4/19/2023 D, Siliques of 35S:ARR1-SRDX transgenic plants compared with wild-type plants. C, Flower morphology of 35S:ARR1-SRDX transgenic plants compared with the wild type. The graph shows the time (DAG) of flower bud opening in transgenic plants compared with the wild type. B, Quantitative analysis of the early-flowering phenotype of 35S:ARR1-SRDX transgenic plants. The plants were grown under long-day conditions and photographed at 19 DAG. A, Flower induction occurs earlier in 35S:ARR1-SRDX transgenic plants compared with wildtype plants. Reproductive development of 35S:ARR1-SRDX transgenic plants. This study demonstrates the usefulness of chimeric repressor silencing technology to overcome redundancy in transcription factor families for functional studies. In addition, a role for B-type ARRs in mediating cross talk with other pathways is supported by the resistance of 35S:ARR1-SRDX seeds to phytochrome B-mediated inhibition of germination by far-red light. Collectively, the suppression of pleiotropic cytokinin activities by a dominant repressor version of a B-type ARR indicates that this protein family is involved in mediating most, if not all, of the cytokinin activities in Arabidopsis. The transcript levels of more than 500 genes were more than 2.5-fold reduced in 35S:ARR1-SRDX transgenic seedlings, suggesting a broad function of B-type ARRs. The rapid induction of a large part of the cytokinin response genes was dampened. Several bioassays showed that 35S:ARR1-SRDX plants have an increased resistance toward cytokinin. 35S:ARR1-SRDX transgenic Arabidopsis plants showed phenotypic changes reminiscent of plants with a reduced cytokinin status, such as a strongly reduced leaf size, an enhanced root system, and larger seeds. In a protoplast test system, ARR1-SRDX suppressed ARR6:beta-glucuronidase reporter gene activation by different B-type ARRs. We generated a dominant repressor version of the Arabidopsis (Arabidopsis thaliana) response regulator ARR1 (ARR1-SRDX) using chimeric repressor silencing technology in order to study the extent of the contribution of B-type response regulators to cytokinin activities. In planta functional analysis of this family is hampered by the high level of functional redundancy of its 11 members. One component of this system are B-type response regulators, transcription factors mediating at least part of the response to cytokinin. These data highlight a novel transcription control of the chloroplast aspartate pathway that operates under energy limiting conditions.The signal transduction of the phytohormone cytokinin is mediated by a multistep histidine-to-aspartate phosphorelay system. Hence it is likely that heterodimerization with DPBF4-SRDX inhibits the binding of redundantly functioning bZIP-TFs to the promoters of AK genes and thereby releases the repressing effect. This effect was verified by showing that DPBF4-SRDX fails to recognize the AK/HSD1 enhancer sequence in yeast one-hybrid assays, but increases heterodimmer formation with DPBF4 and ABI5, as estimated by yeast two-hybrid assays. A dominant-negative version of DPBF4 fused to the SRDX repressor domain of SUPERMAN could counteract the repression and stimulate AK expression under low sugar and darkness in planta. However, in abi5 or dpbf4 mutant and abi5, dpbf4 double mutant the repression of AK expression is maintained, indicating a functional redundancy with other bZIP-TFs. Concomitantly, it also increases the expression of asparagines synthetase 1 (ASN1) that shifts aspartate utilization towards asparagine formation. Overexpression of ABI5, but not DPBF4, further increases this AK transcription suppression. Elevated transcript levels of DPBF4 and ABI5 under darkness and low sugar conditions coincide with the repression of AK gene expression. By using yeast one-hybrid assays and complementary chromatin immunoprecipitation analyses in Arabidopsis cells, the bZIP transcription factors ABI5 and DPBF4 were identified, capable of interacting with the G-box-containing enhancer of AK/HSD1 promoter. Here, we show that transcription of all AK genes is negatively regulated under darkness and low sugar conditions. Initial steps of aspartate-derived biosynthesis pathway (Asp pathway) producing Lys, Thr, Met and Ile are catalyzed by bifunctional (AK/HSD) and monofunctional (AK-lys) aspartate kinase (AK) enzymes. Ufaz, Shai Shukla, Vijaya Soloveichik, Yulia Golan, Yelena Breuer, Frank Koncz, Zsuzsa Galili, Gad Koncz, Csaba Zilberstein, Aviah Transcriptional control of aspartate kinase expression during darkness and sugar depletion in Arabidopsis: involvement of bZIP transcription factors Transcriptional control of aspartate kinase expression during darkness and sugar depletion in.
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