Early induction of apple fruitlet abscission is characterized by an increase of both isoprene emission and abscisic acid content.

Eccher Giulia,Boschetti Andrea,Ramina Angelo,Dimauro Mariano,Botton Alessandro,Ruperti Benedetto

doi:10.1104/pp.112.208470

Abstract

Apple (Malus domestica) fruitlet abscission represents an interesting model system to study the early phases of the shedding process, during which major transcriptomic changes and metabolic rearrangements occur within the fruit. In apple, the drop of fruits at different positions within the cluster can be selectively magnified through chemical thinners, such as benzyladenine and metamitron, acting as abscission enhancers. In this study, different abscission potentials were obtained within the apple fruitlet population by means of the above-cited thinners. A metabolomic study was conducted on the volatile organic compounds emitted by abscising fruitlets, allowing for identification of isoprene as an early marker of abscission induction. A strong correlation was also observed between isoprene production and abscisic acid (ABA) levels in the fruit cortex, which were shown to increase in abscising fruitlets with respect to nonabscising ones. Transcriptomic evidence indicated that abscission-related ABA is biologically active, and its increased biosynthesis is associated with the induction of a specific ABA-responsive 9-cis-epoxycarotenoid dioxygenase gene. According to a hypothetical model, ABA may transiently cooperate with other hormones and secondary messengers in the generation of an intrafruit signal leading to the downstream activation of the abscission zone. The shedding process therefore appears to be triggered by multiple interdependent pathways, whose fine regulation, exerted within a very short temporal window by both endogenous and exogenous factors, determines the final destiny of the fruitlets.

Highlights

Apple (Malus domestica) fruitlet abscission represents an interesting model system to study the early phases of the shedding process, during which major transcriptomic changes and metabolic rearrangements occur within the fruit
Natural and chemically induced fruit drop dynamics were followed until their conclusion in two commercial apple genotypes, cv Golden Delicious and Red Chief, which are characterized by significant differences in their natural abscission potential (AP) and sensitivity to thinning chemicals
A hypothetical model originally developed by Botton et al (2011) was extended on the basis of the main findings described to explain the regulatory action exerted by abscisic acid (ABA) in apple cortex during the early phases of abscission induction and the putative role of isoprene in the process (Fig. 9)

Summary

Introduction

Apple (Malus domestica) fruitlet abscission represents an interesting model system to study the early phases of the shedding process, during which major transcriptomic changes and metabolic rearrangements occur within the fruit. Apple (Malus domestica) represents an interesting model system to study the molecular mechanisms regulating early fruit development and fruit developmental plasticity in response to endogenous and environmental cues, because it develops fruit clusters with a clear gradient of correlative dominance related to the position and size of the fruit This dominance can be regulated by means of chemical treatments that induce fruit drop (Greene et al, 1992; Bangerth, 2000), a practice known as fruit thinning adopted by horticulturists to reduce fruit load and improve the final fruit size and quality. A hypothetical model for this process was proposed based upon both transcriptomic and metabolic data, indicating a strong link between abscission induction and the nutritional stress occurring within the tree, which was magnified by the BA treatment (Botton et al, 2011) This physiological condition is primarily perceived at the fruitlet cortex, in which the molecular mechanisms linking the sugar starvation to abscisic acid (ABA) signaling are activated. Despite this critical situation, which is irreversible in the experimental conditions adopted by Botton et al (2011), the tree activates specific homeostatic mechanisms, among which the detoxification systems against the ROS may represent an interesting starting point for the identification of new physiological markers of fruitlet abscission

Results

Discussion

Conclusion