MALE AND FEMALE FLOWERS OF THE DIOECIOUS PLANT SORREL SHOW DIFFERENT PATTERNS OF MADS BOX GENE-EXPRESSION

Male and female flowers of the dioecious plant sorrel (Rumex acetosa) each produce three whorls of developed floral organs: two similar whor ls of three perianth segments and either six stamens (in the male) or a gynoecium consisting of a fertile carpel and two sterile carpels (in the female), In the developing male flower, there is no significant p roliferation of cells in the center of the flower, in the position nor mally occupied by the carpels of a hermaphrodite plant, In the female flower, small stamen primordia are formed. To determine whether the or gan differences are associated with differences in the expression of o rgan identity genes, cDNA clones representing the putative homologs of B and C function MADS box genes were isolated and used in an in situ hybridization analysis, The expression of RAD1 and RAD2 (two different DEFICIENS homologs) in males and females was confined to the stamen w horl; the lack of expression in the second, inner perianth whorl corre lated with the sepaloid nature of the inner whorl of perianth segments . Expression of RAP1 (a PLENA homolog) occurred in the carpel and stam en whorls in very young flower primordia from both males and females. However, as soon as the inappropriate set of organs ceased to develop, RAP1 expression became undetectable in those organs. The absence of e xpression of RAP1 may be the cause of the arrest in organ development or may be a consequence.