title: Acquiring competence for shoot development in Arabidopsis: ARR2 directly targets A-type ARR genes that are differentially activated by CIM preincubation.
abstract: Shoots can be regenerated from roots in Arabidopsis by treating root explants with cytokinin, however, shoot regeneration requires preincubation on callus induction medium (CIM) prior to induction on cytokinin-rich shoot induction medium (SIM). A cytokinin-inducible marker gene, RESPONSE REGULATOR 15 (ARR15), was identified through a "CIM dropout experiment" with similar requirements for CIM preincubation. The requirements for ARR15 contrasted to ARR5, another cytokinin-inducible ARR gene that does not require CIM preincubation. We show here that despite their differences, both ARR5 and ARR15 are direct targets of the transcriptional B-type response regulator, ARR2. This was demonstrated by identifying genes upregulated following beta estradiol induced nuclear relocation of an ARR2-estradiol receptor fusion protein. The differences in CIM preincubation requirements for ARR5 and ARR15 expression indicate an additional layer of control for these A-type ARR genes during SIM incubation. For ARR15, the CIM requirement is a transcriptional effect, because the expression of ARR15 promoter:GUS reporter gene constructs is also affected by CIM preincubation. A testable model is that transcription of ARR15, but not ARR5, is blocked by a repressor and that the effects of the repressor are relieved by CIM preincubation.
authors.affiliation: Plant Sciences Institute; Roy J. Carver Co-Laboratory; Iowa State University; Ames Iowa USA.
authors.forename: Ping
authors.initials: P
authors.lastname: Che
authors.forename: Sonia
authors.initials: S
authors.lastname: Lall
authors.forename: Stephen H
authors.initials: SH
authors.lastname: Howell
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