REBELOTE,SQUINT, andULTRAPETALA1Function Redundantly in the Temporal Regulation of Floral Meristem Termination inArabidopsis thaliana 

Abstract

In Arabidopsis thaliana, flowers are determinate, showing a fixed number of whorls. Here, we report on three independent genes, a novel gene REBELOTE (RBL; protein of unknown function), SQUINT (SQN; a cyclophilin), and ULTRAPETALA1 (ULT1; a putative transcription factor) that redundantly influence floral meristem (FM) termination. Their mutations, combined with each other or with crabs claw, the genetic background in which they were isolated, trigger a strong FM indeterminacy with reiterations of extra floral whorls in the center of the flower. The range of phenotypes suggests that, in Arabidopsis, FM termination is initiated from stages 3 to 4 onwards and needs to be maintained through stage 6 and beyond, and that RBL, SQN, and ULT1 are required for this continuous regulation. We show that mutant phenotypes result from a decrease of AGAMOUS (AG) expression in an inner 4th whorl subdomain. However, the defect of AG activity alone does not explain all reported phenotypes, and our genetic data suggest that RBL, SQN, and, to a lesser extent, ULT1 also influence SUPERMAN activity. Finally, from all the molecular and genetic data presented, we argue that these genes contribute to the more stable and uniform development of flowers, termed floral developmental homeostasis.

Master and servant: Regulation of auxin transporters by FKBPs and cyclophilins

“…The squint mutation in the Arabidopsis SQN/Cyp40 cyclophilin gene results in a delay of vegetative shoot maturation [95] and a delay of flower meristem termination when combined with other mutations [96]. Similar to TWD1/FKBP42 [65], Arabidopsis SQN/Cyp40 forms a complex with HSP90, facilitating HPS90-mediated protein complex assembly [97].…”

Time to Stop: Flower Meristem Termination

“…Nonetheless, in both sup and wild-type flowers, cells incorporate BrdU on both sides of the boundary between whorl 3 and FM center, indicating that the latter is still proliferating (Breuil-Broyer et al, 2004). Additionally, the delay in FM termination in sup mutant flowers has recently been associated with transient maintenance of WUS expression beyond stage 6 (Prunet et al, 2008), suggesting that the FM keeps functioning normally while the extra whorls of stamens are produced. Also, the occasional fasciation of ag-1 sup-1 double mutant flowers (Bowman et al, 1992) indicates that SUP represses cell divisions in the FM center, and not only in whorl 3.…”

“…pAP3::AG ag-3 transgenic plants, which lack functional AG in the whole fourth whorl, but still express it in the third, exhibit flowers with stamens but no carpel, and utterly lacking FM termination (Jack et al, 1997), showing the importance of AG expression in the fourth whorl to switch WUS expression. Moreover, the delay or loss of FM termination without alteration of floral organ identity observed in clv, ult1, and pan single mutants as well as in crabs claw squint (crc sqn) and crc ult1 double mutants is associated with a transient or persistent defect in AG expression in an inner subdomain of whorl 4 (Clark et al, 1993;Fletcher, 2001;Prunet et al, 2008;Das et al, 2009;Figure 3. Main genetic pathways promoting FM termination.…”

“…However, a total loss of FM termination was more recently observed in several double mutants (Alvarez and Smyth, 1999;Prunet et al, 2008) and transgenic plants (Chen, 2004;Zhao et al, 2007), while some mutants, including clv, ult1, perianthia (pan), and superman (sup) only display a delay of FM termination, and produce a limited number of supernumerary whorls before stem cell maintenance eventually stops (Fig. 1, E and F;Schultz et al, 1991;Bowman et al, 1992;Clark et al, 1993Clark et al, , 1995Kayes and Clark, 1998;Fletcher, 2001;Carles et al, 2004;Das et al, 2009;Maier et al, 2009).…”

“…1, E and F;Schultz et al, 1991;Bowman et al, 1992;Clark et al, 1993Clark et al, , 1995Kayes and Clark, 1998;Fletcher, 2001;Carles et al, 2004;Das et al, 2009;Maier et al, 2009). However, like in ag mutants, these phenotypes are always associated with a transient or persistent maintenance of WUS expression beyond stage 6, depending on the strength of the loss of FM termination (Schoof et al, 2000;Carles et al, 2004;Zhao et al, 2007;Prunet et al, 2008;Das et al, 2009;Maier et al, 2009). …”