Abstract
BackgroundNaphthaleneacetic acid (NAA), a synthetic auxin analogue, is widely used as an effective thinner in apple orchards. When applied shortly after fruit set, some fruit abscise leading to improved fruit size and quality. However, the thinning results of NAA are inconsistent and difficult to predict, sometimes leading to excess fruit drop or insufficient thinning which are costly to growers. This unpredictability reflects our incomplete understanding of the mode of action of NAA in promoting fruit abscission.ResultsHere we compared NAA-induced fruit drop with that caused by shading via gene expression profiling performed on the fruit abscission zone (FAZ), sampled 1, 3, and 5 d after treatment. More than 700 genes with significant changes in transcript abundance were identified from NAA-treated FAZ. Combining results from both treatments, we found that genes associated with photosynthesis, cell cycle and membrane/cellular trafficking were downregulated. On the other hand, there was up-regulation of genes related to ABA, ethylene biosynthesis and signaling, cell wall degradation and programmed cell death. While the differentially expressed gene sets for NAA and shading treatments shared only 25% identity, NAA and shading showed substantial similarity with respect to the classes of genes identified. Specifically, photosynthesis, carbon utilization, ABA and ethylene pathways were affected in both NAA- and shading-induced young fruit abscission. Moreover, we found that NAA, similar to shading, directly interfered with leaf photosynthesis by repressing photosystem II (PSII) efficiency within 10 minutes of treatment, suggesting that NAA and shading induced some of the same early responses due to reduced photosynthesis, which concurred with changes in hormone signaling pathways and triggered fruit abscission.ConclusionsThis study provides an extensive transcriptome study and a good platform for further investigation of possible regulatory genes involved in the induction of young fruit abscission in apple, which will enable us to better understand the mechanism of fruit thinning and facilitate the selection of potential chemicals for the thinning programs in apple.
Highlights
Naphthaleneacetic acid (NAA), a synthetic auxin analogue, is widely used as an effective thinner in apple orchards
By comparing the gene expression profiles of young fruit abscission caused by NAA and shading, we found that the number of genes affected by these two treatments was positively correlated with the severity of the thinning responses
We found that two auxin transport-related genes (PIN and auxin efflux carrier gene (AEC)) were repressed by NAA in the fruit abscission zone (FAZ) and fruit cortex (FC), i.e., starting from 3 d, and their expression levels remained lower than the control, indicating that auxin efflux from the fruitlets was blocked (Figure 6)
Summary
Naphthaleneacetic acid (NAA), a synthetic auxin analogue, is widely used as an effective thinner in apple orchards. The physiological mechanisms by which NAA promotes the abscission of young apple fruitlets have been discussed [1,2,3] Principal among these mechanisms is a reduction in carbohydrate availability to the developing fruit either by interference with photosynthesis [4,5] or by reduced translocation of metabolites, including photosynthates, from leaves to the fruit [6]. Normal fruitlet abscission, which can occur both shortly after anthesis and during the “June drop”, has been at least partly attributed to the competition for carbohydrates among young fruit and between fruit and vegetative shoots [7,9] Together these findings indicate that photosynthesis is critical for fruit development and treatments that alter the levels of carbohydrates available for translocation to the developing fruit can influence the fruit set of apple trees
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