Expression of Ethylene Biosynthesis and Signaling Genes during Differential Abscission Responses of Sweet Orange Leaves and Mature Fruit

Abstract

When applying abscission agents to tree fruit to facilitate harvest, it is desirable to loosen fruit and not leaves or other organs, but mechanisms controlling leaf and fruit drop are not fully understood. The effect of 450 μL·L−1 ethephon (ethylene-releasing agent) alone or in combination with 1-methylcyclopropene [1-MCP (ethylene perception inhibitor)] on leaf and mature fruit abscission of ‘Valencia’ sweet orange (Citrus sinensis) was studied. Leaf abscission increased and fruit detachment force (FDF) decreased significantly especially 4 days after ethephon treatment. Leaf drop rose to over 80% 7 days after application, whereas FDF was only 30% less than untreated control fruit. When 1-MCP was combined with ethephon and applied to ‘Valencia’ sweet orange canopies, leaf abscission was greatly reduced, but reduction in FDF proceeded unabated. We hypothesized that differential response of ‘Valencia’ sweet orange fruit and leaves to 1-MCP was correlated with expression of ethylene biosynthetic and signaling genes and their downstream action. Partial or full-length nucleotide sequences were obtained for ‘Valencia’ sweet orange homologs of 1-amino-cyclopropane-1-carboxylate synthase-1 (CsACS1), 2 (CsACS2), 1-amino-cyclopropane-1-carboxylate oxidase (CsACO), ethylene response sensor-1 (CsERS1), ethylene response-1 (CsETR1), 2 (CsETR2), 3 (CsETR3), constitutive triple response-1 (CsCTR1), ethylene insensitive-2 (CsEIN2), and ethylene insensitive 3-like-1 (CsEIL1) and 2 (CsEIL2). Ethephon application increased expression of biosynthesis genes CsACS1 and CsACO and receptors CsERS1 and CsETR2 in the abscission zones of leaves and mature fruit. Ethephon-induced increase in gene expression was completely suppressed by 1-MCP application in all but CsACS1 and CsACO in fruit abscission zones. Although gene expression was suppressed initially, CsACS1 and CsACO expression in fruit abscission zones treated with 1-MCP in the presence or absence of ethephon increased over the 7-day measurement period, suggesting that CsACS1 and CsACO expression were negatively regulated by basal ethylene production in this tissue. However, 1-MCP treatment alone did not loosen fruit, indicating that CsACS1 and CsACO played minor roles in fruit abscission. To determine if the difference in ethylene sensitivity was the basis of differential response to ethylene within the same organ, potted ‘Valencia’ sweet orange plants were treated with ethylene, and rates of blade and petiole drop and detachment forces at the laminar and petiolar abscission zones were studied. Although leaf blades abscised earlier than petioles, the force of detachment was similar, indicating no differences in ethylene sensitivity. Overall, the most significant difference between fruit and leaf abscission zones was seen in the expression of CsACS1 and CsACO genes, but the expression pattern was poorly correlated with abscission.