B and C floral organ identity functions require SEPALLATA MADS-box genes

Abstract

Abnormal flowers have been recognized for thousands of years, but only in the past decade have the mysteries of flower development begun to unfold. Among these mysteries is the differentiation of four distinct organ types (sepals, petals, stamens and carpels), each of which may be a modified leaf. A landmark accomplishment in plant developmental biology is the ABC model of flower organ identity. This simple model provides a conceptual framework for explaining how the individual and combined activities of the ABC genes produce the four organ types of the typical eudicot flower. Here we show that the activities of the B and C organ-identity genes require the activities of three closely related and functionally redundant MADS-box genes, SEPALLATA1/2/3 (SEP1/2/3). Triple mutant Arabidopsis plants lacking the activity of all three SEP genes produce flowers in which all organs develop as sepals. Thus SEP1/2/3 are a class of organ-identity genes that is required for development of petals, stamens and carpels.

APETALA1 and SEPALLATA3 interact to promote flower development

“…Using the yeast two-hybrid assay, we found that AP1 and CAL can each interact with a shared set of proteins, including SEP3, SOC1, SVP and AGL24. Thē ower-development roles of SEP3, SOC1 and SVP have all been recently documented (Hartman et al, 2000;Pelaz et al, 2000;Samach et al, 2000), whereas the function of AGL24 has yet to be determined. Given the fact that only one or a few clones were isolated for some of the genes in the yeast screens, it is likely that not all of the interacting factors have been identi®ed.…”

“…If some of the activities of AP1 require an interaction with SEP3, then mutations in SEP3 might be expected to resemble mutant alleles of AP1. We recently identi®ed two independently derived En-1 transposon insertion alleles of SEP3, and have described the phenotype of sep1 sep2 sep3 triple mutants in which the three inner whorls of organs become sepaloid (Pelaz et al, 2000).…”

“…These data indicate that the ability of either transgene to promotē owering is considerably stronger when the other transgene is also present, providing additional genetic evidence in support of the idea that SEP3 and AP1 interact durinḡ ower development. In addition to the interactions between SEP3 and AP1, recent yeast two-hybrid studies, as well as genetic data, have shown that SEP3 interacts with the AP3, PI and AḠ ower-organ identity gene products (Fan et al, 1997;Homma and Goto, 2001;Pelaz et al, 2000;Pelaz et al, 2001). Thus the speci®c function of SEP3 during¯ower development is determined in part by interactions with other MADS-box proteins.…”

“…In Arabidopsis, the A-function genes are AP1 and APETALA2 (AP2); B-function genes are APETALA3 (AP3), PISTILLATA (PI); and the C-function gene is AGAMOUS (AG). In addition, recent studies have shown that a trio of closely related genes, SEPALLATA1/2/3 (SEP1/ 2/3), are required for petal, stamen and carpel identity, and are thus necessary for the activities of the B-and Cfunction genes (Pelaz et al, 2000). Remarkably, with the exception of the AP2 gene, all the other organ-identity genes belong to the extended family of MADS-box genes, a family that is known to include more than 44 distinct sequences in Arabidopsis (Alvarez- Buylla et al, 2000a;Davies and Schwarz-Sommer, 1994;Purugganan et al, 1995;Rounsley et al, 1995).…”

Relaxed Selection Among Duplicate Floral Regulatory Genes in Lamiales

“…Second, Blanc and Wolfe (2004b) and Maere et al (2005) show duplicate genes that constitute subunits of protein complexes are retained at a higher rate than other functional classes of genes, possibly due to stoichiometric requirements or dominant negative phenotypes following loss of one copy (Veitia 2003). AP3/DEF constitutes one subunit of a multidomain protein (Pelaz et al 2000) and thus might be disproportionately affected by such dosage effects. However, because these hypotheses predict retention of paralogs in an undifferentiated state and we see divergence between functional domains of DEFA/B, it is unlikely that they have contributed significantly to their preservation in Lamiales.…”