Signaling pathways mediating the induction of apple fruitlet abscission.

Alessandro Botton,Claudio Forcato,Monica Zermiani,Alberto Ferrarini,Angelo Ramina,Stefano Moscatello,Giulia Eccher,Benedetto Ruperti,Alberto Battistelli,Riccardo Velasco,Maura Begheldo

doi:10.1104/pp.110.165779

Abstract

Apple (Malus × domestica) represents an interesting model tree crop for studying fruit abscission. The physiological fruitlet drop occurring in this species can be easily magnified by using thinning chemicals, such as benzyladenine (BA), to obtain fruits with improved quality and marketability. Despite the economic importance of this process, the molecular determinants of apple fruitlet abscission are still unknown. In this research, BA was used to obtain fruitlet populations with different abscission potentials to be analyzed by means of a newly released 30K oligonucleotide microarray. RNAs were extracted from cortex and seed of apple fruitlets sampled over a 4-d time course, during which BA triggers fruit drop, and used for microarray hybridization. Transcriptomic profiles of persisting and abscising fruitlets were tested for statistical association with abscission potential, allowing us to identify molecular signatures strictly related to fruit destiny. A hypothetical model for apple fruitlet abscission was obtained by putting together available transcriptomic and metabolomic data. According to this model, BA treatment would establish a nutritional stress within the tree that is primarily perceived by the fruitlet cortex whose growth is blocked by resembling the ovary growth inhibition found in other species. In weaker fruits, this stress is soon visible also at the seed level, likely transduced via reactive oxygen species/sugar and hormones signaling cross talk, and followed by a block of embryogenesis and the consequent activation of the abscission zone.

Highlights

Apple (Malus 3 domestica) represents an interesting model tree crop for studying fruit abscission
In order to identify the molecular events responsible for apple fruitlet abscission, fruitlet subpopulations characterized by different abscission potentials and fruit drop dynamics were first obtained and sampled
To the best of our knowledge, this study provides the first global monitoring of gene expression changes occurring during the early phases of apple fruitlet abscission induction

Summary

Introduction

Apple (Malus 3 domestica) represents an interesting model tree crop for studying fruit abscission. Data obtained from both tomato and Arabidopsis (Arabidopsis thaliana) have suggested that the transformation of the ovary into fruit is prevented by a negative control exerted by Auxin/Indole Acetic Acid (AUX/IAA) and Auxin Response Factor (ARF) proteins The removal of this negative regulation, following pollination/ fertilization or treatment with auxins, leads to cell proliferation and to fruit set. Transcriptomic profiling studies carried out in tomato (Vriezen et al, 2008) have reinforced the view that fruit development appears to rely on the removal of a negative feedback regulation of ovary growth This inhibition is established by a negative control exerted mainly by abscisic acid (ABA)- and ethylenedependent pathways. Very little or no information is available on how these factors could be modulated by the plant to restrain the development of a fraction of fruits in response to endogenous/environmental perturbations

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Discussion

Conclusion