The most common cucumber plants (Cucumis sativus L.) show monoecious sex expression and produce both male and female flowers on the same plant, whereas plants of the gynoecious type produce only female flowers under natural conditions. Ethylene is a plant hormone that affects sex differentiation in cucumber plants. In the gynoecious cucumber, application of the ethylene biosynthesis inhibitor aminoethoxyvinyl glycine (AVG) induced only male flowers, whereas application of the ethylene action inhibitor silver nitrate (AgNO 3 ) induced male and morphologically bisexual flowers. To investigate whether the morphologically bisexual flowers that were induced by AgNO 3 in gynoecious cucumber plants have functional sexual organs, we analyzed the function of pistils and stamens. Seeds were produced by cross-fertilization and the pollen tubes were elongated on sucrose medium in eight of the twenty-five morphologically bisexual flowers. These results show that the application of AgNO 3 to gynoecious cucumber plants produces functional bisexual flowers. In functional bisexual flowers, the length of pistils was similar to that of the female flowers of gynoecious cucumber plants and the length of stamens was similar to that of male and bisexual flowers of andromonoecious cucumber plants. In contrast, stamen length was inversely proportional to pistil length in the group of morphologically bisexual flowers that were induced by AgNO 3 . These results suggest that the morphology of the sexual organs is different in morphologically bisexual flowers and in functional bisexual flowers. It is possible that the functional bisexual flowers that were induced by AgNO 3 in gynoecious cucumber plants are the result of exclusive blockade of ethylene signals that inhibit stamen development.
Monoecious cucumber (Cucumis sativus L.) produces male and female flowers on the same plant. In the early bisexual stage, all flower buds contain primordia for both stamens and pistils and sexual differences are established by the selective arrest of sexual organ primordia. Recently, the Cs1-MMP gene was isolated from a cucumber cotyledon. Cs1-MMP encodes a putative matrix metalloproteinase (MMP) and is expressed at the boundary of senescence and programmed cell death (PCD) of the cotyledon. In animals, MMPs make up a major group of enzymes that degrade the extracellular cell matrix (ECM). In cucumber flower organs, Cs1-MMP expression was analyzed in order to clarify the relationship between the arrest of sexual organs and PCD at the molecular level. In male flowers, strong Cs1-MMP expression was detected in both sepals and the area where pistil primordia became arrested. In female flowers, the highest levels of Cs1-MMP expression were observed in the sepals and the area where stamen primordia were arrested. In both male and female flowers, Cs1-MMP expression was detected from the early stage of development to anthesis. Our results suggest that Cs1-MMP plays an important role in PCD-mediated arrest of sexual organ primordia in cucumber flowers. Thus, it is possible that sex determination in cucumber requires continuous PCD to occur at the pistil or stamen primordia.
These physiological phenomena were confirmed at the molecular level. Two genes (CS-ACS1G and CS-ACS2) which encode a key enzyme in the ethylene biosynthesis pathway, 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, were identified as being related to sex expression in cucumber plants. The CS-ACS1G gene, which exists in gynoecious cucumber plants but not in monoecious cucumber plants, was closely linked to the F gene (Trebitsh et al., 1997 Monoecious cucumber (Cucumis sativus L.) produces male and female flowers on the same plant, but application of ethylene promotes expression of female flowers. In the early bisexual stage, all flower buds contain primordia for both stamens and pistils and sexual differences are established by the selective arrest of sexual organ primordia. The expression of six cell cycle-related genes [Cs Cyclin A (CsCycA), CsCycB, CsCycD3; 1, CsCycD3; 2, Cs Cyclin-dependent kinase A CsCDKA), and CsCDKB] was analyzed in male and female flower buds to investigate cell cycle activity in sexual organs. CsCycA was expressed specifically in the area containing arrested stamen primordia (ACASP) in female flower buds, but was barely detected in male flower buds. Expression of the other five genes was detected in the ACASP but not in pistils of female flower buds. Expression of all six genes was elevated transiently following the application of 2-chloroethylphosphonic acid (ethephon), an ethylene-releasing agent, to the shoot apices. The correlation of expression of these genes with the formation of female flowers suggests they play a role in the arrest of stamens of female flowers in monoecious cucumber plants.
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