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PROLIFERATING INFLORESCENCE MERISTEM, a MADS-Box Gene That Regulates Floral Meristem Identity in Pea1
SQUAMOSA and APETALA1 are floral meristem identity genes from snapdragon (Antirrhinum majus) and Arabidopsis, respectively. Here, we characterize the floral meristem identity mutation proliferating inflorescence meristem (pim) from pea (Pisum sativum) and show that it corresponds to a defect in the PEAM4 gene, a homolog of SQUAMOSA and APETALA1. The PEAM4 coding region was deleted in the pim-1 allele, and this deletion cosegregated with the pim-1 mutant phenotype. The pim-2 allele carried a nucleotide substitution at a predicted 5' splice site that resulted in mis-splicing of pim-2 mRNA. PCR products corresponding to unspliced and exon-skipped mRNA species were observed. The pim-1 and pim-2 mutations delayed floral meristem specification and altered floral morphology significantly but had no observable effect on vegetative development. These floral-specific mutant phenotypes and the restriction of PIM gene expression to flowers contrast with other known floral meristem genes in pea that additionally affect vegetative development. The identification of PIM provides an opportunity to compare pathways to flowering in species with different inflorescence architectures.
The transition from the vegetative to the reproductive phase in plants commences when a signal from the leaves evokes a response in the shoot apical meristem that results in the development of flowers. The genes regulating the cascade of processes that occur in the shoot apex after this switch to reproductive growth have been well studied in the herbaceous species snapdragon (Antirrhinum majus) and Arabidopsis (Simpson et al., 1999; Theissen, 2001). For example, in snapdragon, the floral meristem identity gene SQUAMOSA (SQUA) is required for the transition to flowering and floral organ specification. This was determined by the phenotypes of squa null mutants, which typically produce reiterated inflorescences in place of flowers (Huijser et al., 1992). Flower formation, when it occurs, includes a wide range of floral abnormalities, especially in the two outer whorls (Huijser et al., 1992), suggesting that SQUA also functions in first- and second-whorl organ specification in snapdragon.
In Arabidopsis, a corresponding role in floral development is carried out by APETALA1 (API). Flowers on Arabidopsis plants carrying strong apl mutant alleles retain many inflorescence-like characteristics; first-whorl organs are converted into bract or leaflike organs bearing axillary flowers, which then repeat...