Interaction of the TCP4 protein with other transcription factors and its effect on Arabidopsis shoot apex

Abstract

The TEOSINTE BRANCHED1, CYCLOIDEA, PROLIFERATING CELL FACTORs (TCP) class of proteins are plant-specific transcription factors with a non-canonical basic helix-loop-helix (bHLH) domain involved in both DNA-binding and protein-protein interaction. The Arabidopsis genome encodes 24 TCP proteins that are grouped into class I (or TCP-P/ PCF class; 13 members) and class II (TCP-C class; 11 members) based on sequence similarity in their bHLH domain. The class II TCPs are further divided into two clades namely the CINCINNATA (CIN) clade (8 members) and the TB1/ CYC clade (3 members). Out of the eight TCPs belonging to the CIN clade, five members, namely TCP2, 3, 4, 10 & 24 (also called as JAW-TCPs), are post-transcriptionally suppressed by the mature microRNA miR319, which is produced by three loci namely MIR319A, B & C. Down-regulation of JAW-TCPs by miR319 over-expression produces leaves which are jagged and wrinkled, hence the name jaw-D. The JAW-TCPs are involved in regulating diverse developmental processes such as leaf morphogenesis, leaf senescence, petal growth, hypocotyl elongation, and photo-periodic flowering transition. Most of the phenotypic effects of JAW-TCP modulation has been observed in the axillary organs such as leaves and floral organs such as petals, stamens, and to a small extent in pistils, whereas the only effect found on the main meristem is in the timing of flowering transition of the vegetative meristem. However, though the TCP4 promoter activity is found in the shoot apical meristem (SAM) in glucuronidase (GUS) reporter assay, its protein product is not detectable in the SAM. Interestingly, when a miR319-resistant form of TCP4 is induced in its endogenous domain, meristem function is adversely affected, suggesting a role for miR319 within the SAM in degrading JAW-TCP transcripts. However, the promoter of MIR319C, the major miR319-producing gene expressed in the vegetative stage, in expressed only in the periphery of SAM and not within the SAM., indicating a movement of the mature miR319 microRNA from the periphery to the main SAM that degrades JAW-TCP transcripts. Whatever the mechanism might be, a reduced SAM activity in the transgenic lines expressing miR319-resistant form of TCP4 suggests a role for the miR319-JAW-TCP module in SAM function, which deserves further investigation. We have pursued this objective by identifying the interacting proteins of TCP4 in the meristem zone and studying their role is meristem function by transgenic and phenotypic analysis.