| Organ abscission is a developmentally and environmentally regulated cell separation process initiated in specialized tissues, the abscission zones (AZs). Understanding the basis of organ abscission is of fundamental interest to elucidate underlying mechanism of organ abscission and is of applied interest as abscission regulation is critical to achieve optimized market value. A model developed from physiological studies has established that the balance between auxin and ethylene within the AZs regulates leaf abscission. Genetic analyses in model plant species have identified both ethylene-dependent and ethylene-independent pathways to regulate floral organ abscission. In this dissertation, I sought to: investigate natural variation in fruit abscission-related traits in Malus species, identify gene expression changes within the pedicel abscission zone during apple fruit abscission, analyze promoter activity of members of the PECTATE LYASE-LIKE gene family in cell separation and wall loosening in Arabidopsis. In the first study, I evaluated 144 Malus accessions representing wild species, domestic cultivars, and hybrids for abscission-related traits. I found that seasonal timing of fruit abscission in wild species and hybrids showed a broad distribution similar to that seen for domestic cultivars, and that internal ethylene concentration at the time of abscission varied by over three orders of magnitude. Wild species, domestic cultivars, and hybrids all included representatives that showed abscission of fruit prior to substantial production of ethylene, as well as accessions that retained fruit for a significant period of time following ethylene production. For all accessions that retained fruit, fruit removal resulted in abscission of the pedicel, and exogenous ethylene promoted abscission, suggesting that the abscission zone was functional. Our results suggest important roles for mechanisms independent of fruit ethylene production in abscission. In the second study, we identified transcriptional changes accompanying the transition from competent-quiescent to activated AZs in the apple fruit pedicel. The abscission-associated genes identified in this work contribute to our understanding of fruit abscission, while suggesting a common molecular mechanism of fruit abscission induced under various conditions. In the third study, we documented the spatial and temporal promoter activity of 23 of the 26 Arabidopsis PLL genes throughout development. Numerous gene family members showed activity in localized domains programmed for abscission, such as the abscission zones (AZs) of the sepal, petal, and stamen, and seed, as well as the fruit dehiscence zone. Several other members showed activity in cell types expected to facilitate separation, including the endosperm layers during seed germination, and root endodermal and cortical layers during lateral root emergence. Other PLL promoters were active in domains not obviously programmed for separation, including the apparent vestigial AZs of the branch and pedicel. These results suggest potential for unique and overlapping activity of PLL genes, and provide guidance for analysis of individual gene function through reverse genetics. |
