The MADS-Domain Factors AGAMOUS-LIKE15 and AGAMOUS-LIKE18, along with SHORT VEGETATIVE PHASE and AGAMOUS-LIKE24, Are Necessary to Block Floral Gene Expression during the Vegetative Phase    

Abstract

Multiple factors, including the MADS-domain proteins AGAMOUS-LIKE15 (AGL15) and AGL18, contribute to the regulation of the transition from vegetative to reproductive growth. AGL15 and AGL18 were previously shown to act redundantly as floral repressors and upstream of FLOWERING LOCUS T (FT) in Arabidopsis (Arabidopsis thaliana). A series of genetic and molecular experiments, primarily focused on AGL15, was performed to more clearly define their role. agl15 agl18 mutations fail to suppress ft mutations but show additive interactions with short vegetative phase (svp) mutations in ft and suppressor of constans1 (soc1) backgrounds. Chromatin immunoprecipitation analyses with AGL15-specific antibodies indicate that AGL15 binds directly to the FT locus at sites that partially overlap those bound by SVP and FLOWERING LOCUS C. In addition, expression of AGL15 in the phloem effectively restores wild-type flowering times in agl15 agl18 mutants. When agl15 agl18 mutations are combined with agl24 svp mutations, the plants show upward curling of rosette and cauline leaves, in addition to early flowering. The change in leaf morphology is associated with elevated levels of FT and ectopic expression of SEPALLATA3 (SEP3), leading to ectopic expression of floral genes. Leaf curling is suppressed by sep3 and ft mutations and enhanced by soc1 mutations. Thus, AGL15 and AGL18, along with SVP and AGL24, are necessary to block initiation of floral programs in vegetative organs.Appropriate timing of the shift from vegetative to reproductive growth is an important determinant of plant fitness. The time at which a plant flowers is determined through integration of signals reflecting extrinsic and intrinsic conditions, such as photoperiod, the duration of cold, plant health, and age (for review, see Amasino, 2010). One of the most important pathways regulating the timing of the floral transition is the photoperiod pathway (for review, see Imaizumi and Kay, 2006). Under long-day (LD) inductive conditions in Arabidopsis (Arabidopsis thaliana), photoperiod pathway components act to promote flowering by inducing CONSTANS (CO) and downstream genes. The floral integrator FLOWER-ING LOCUS T (FT) is a major target of multiple flowering pathways and the photoperiod pathway in particular. It is directly activated by CO (Samach et al., 2000). Under LD conditions, the peak of CO expression is coincident with the presence of light, and CO activates FT expression in the leaf vascular system (Yanovsky and Kay, 2003). FT travels through the phloem to the shoot apex (Corbesier et al., 2007), where, together with FLOWER-ING LOCUS D (Abe et al., 2005;Wigge et al., 2005), it activates APETALA1 (AP1) and other floral meristem identity genes, starting the flowering process. Other flowering time pathways converge on FT and/or directly impact gene expression in the meristem. The changes in gene expression that accompany the floral transition must be rapid, robust, largely irreversible, and strictly controlled spatially. This is achieved through positive feedforwa...

The class III peroxidase PRX17 is a direct target of the MADS‐box transcription factor AGAMOUS‐LIKE15 (AGL15) and participates in lignified tissue formation

“…The analysis of the PRX17 promoter sequence in the PLACE database (Table ) revealed the occurrence of three variants of a CArG motif (C[A/T]8G) to which AGL15 preferentially binds in vitro (Tang & Perry, ). AGL15 is involved in the regulation of age‐dependent developmental programs, including the transition to flowering by maintaining an immature or juvenile state (Fernandez et al ., , ; Adamczyk et al ., ). For example, 35S : AGL15 lines show a delay in bolting and silique maturation (Fernandez et al ., ), similar to the phenotype of prx17 mutants (Fig.…”

Floral organ abscission is regulated by a positive feedback loop

“…AGL15 binds a CArG motif with a longer A/T-rich core than other MADS-domain proteins (18). AGL15 has been characterized in great detail (17,18,(20)(21)(22), especially in embryos and the regulation of flowering time, making hypothesis generation easier than for less-studied transcription factors. Moreover, AGL15 mRNA is detectable in stamen abscission zones and is differentially expressed through the process of abscission (Fig.…”

“…FYF does not have any putative MAPK phosphorylation sites, so it is unlikely to be regulated by MAPK phosphorylation. It is well established that AGL15 and AGL18 regulate the transition from vegetative growth to flowering and that AGL15 binds the key flowering time regulator, FLOWERING LOCUS T in vivo (21,22). It will be interesting to see whether MPK3/6 also regulates AGL15 by phosphorylation in the flowering time pathway.…”

Transcription Factors in the Regulation of Somatic Embryogenesis

“…AGL15 has also been suggested to contribute to chromatin repression by increasing the efficiency of the formation of a repressive complex or the recruitment of a corepressor (Fernandez et al 2015). Further analysis is needed to identify the targets of the epigenetic repression that is mediated by AGL15 during embryogenic induction.…”