Roles of Ethylene Production and Ethylene Receptor Expression in Regulating Apple Fruitlet Abscission.

Giulia Eccher,Benedetto Ruperti,Andrea Boschetti,Alessandro Botton,Maura Begheldo

doi:10.1104/pp.15.00358

Abstract

Apple (Malus × domestica) is increasingly being considered an interesting model species for studying early fruit development, during which an extremely relevant phenomenon, fruitlet abscission, may occur as a response to both endogenous and/or exogenous cues. Several studies were carried out shedding light on the main physiological and molecular events leading to the selective release of lateral fruitlets within a corymb, either occurring naturally or as a result of a thinning treatment. Several studies pointed out a clear association between a rise of ethylene biosynthetic levels in the fruitlet and its tendency to abscise. A direct mechanistic link, however, has not yet been established between this gaseous hormone and the generation of the abscission signal within the fruit. In this work, the role of ethylene during the very early stages of abscission induction was investigated in fruitlet populations with different abscission potentials due either to the natural correlative inhibitions determining the so-called physiological fruit drop or to a well-tested thinning treatment performed with the cytokinin benzyladenine. A crucial role was ascribed to the ratio between the ethylene produced by the cortex and the expression of ethylene receptor genes in the seed. This ratio would determine the final probability to abscise. A working model has been proposed consistent with the differential distribution of four receptor transcripts within the seed, which resembles a spatially progressive cell-specific immune-like mechanism evolved by apple to protect the embryo from harmful ethylene.

Highlights

Apple (Malus 3 domestica) is increasingly being considered an interesting model species for studying early fruit development, during which an extremely relevant phenomenon, fruitlet abscission, may occur as a response to both endogenous and/or exogenous cues
Ethylene biosynthesis was measured in the cortex of two fruitlet classes: (1) lateral fruitlets that abscise only upon a successful thinning treatment; and (2) central fruitlets that persist even upon the treatment (Fig. 1)
Additional information is given and discussed under a new perspective. Concerning the latter experiment, despite the fact that it was carried out according to the usual standards in terms of time of BA application, the fruit drop of treated trees did not differ significantly from the controls (Supplemental Fig. S1). Both lateral and central fruitlets that were treated with BA (LB3 and CB3) displayed, already at 16 days after petal fall (DAPF), an enhanced ethylene biosynthesis, significantly higher than that found in control samples

Summary

Introduction

Apple (Malus 3 domestica) is increasingly being considered an interesting model species for studying early fruit development, during which an extremely relevant phenomenon, fruitlet abscission, may occur as a response to both endogenous and/or exogenous cues. This experimental system allowed light to be shed on the signaling pathways mediating the induction of apple fruitlet abscission, which can be summarized in a hypothetical model describing the cortex as the primary sensor of the nutritional stress occurring within the tree In this tissue, the molecular mechanisms linking nutrient starvation to hormone signaling are activated, mainly involving abscisic acid (Eccher et al, 2013) and ethylene, whose levels peak at 2 and 4 to 5 d after abscission induction, respectively. Since the seed is the primary source of auxin within the fruit, its abortion decreases the supply of this hormone to the AZ, leading to its activation according to the same model proposed for the leaf by Sakamoto et al (2008)

Results

Discussion

Conclusion