Signaling Pathways Mediating the Induction of Apple Fruitlet Abscission      

Abstract

Apple (Malus 3 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.

Ethylene-dependent effects on generative organ abscission of Lupinus luteus

“…The increasing expression of the genes coding for ET biosynthesis enzymes, the increase in the ACC content, as well as studies of ACC immunolocalization in the abscission zone, all suggest that ethylene is directly involved in the functioning of the flower abscission zone in L. luteus. This hypothesis was also confirmed by the results of research performed in other plant species, in which the factors activating abscission of particular organs stimulate the expression of both ACC synthases and oxidases, e.g., in apple (MdACO3, MdACO4, MdACO1) or tomato (SlACO5, LACS2) [41][42][43][44]. Moreover, it was found that immediately before flower abscission in Ecballium elaterum and S. lycopersicum [45,46], the ethylene production rate increases.…”

The photosynthesis inhibitor metamitron is a highly effective thinner for ‘Golden Delicious’ apple in a warm climate

“…This can be achieved by limiting the production of carbohydrates in the tree during the 2-to 3-weeks period after bloom, precisely when the demand for assimilates by the fruitlets is highest [17]. This is supported by Botton et al [8], who showed a carbohydrate deficit in the trees and fruit cortex immediately after treatment with 6-BA, which caused lateral fruit abscission, and by Dash et al [18], who showed a reduction in early fruit growth of apple and thinning after severe shading which limited carbohydrate production and decreased cell production and expansion. Recent reports have accumulated from the USA and Europe on the PSII inhibitor MM, whose reduction of photosynthesis reduces fruit set in apples when applied at the 6-to 10-mm diameter fruit stage [10][11][12].…”

“…These compounds are involved in the formation of abscission layers and cause fruit drop through activation of ethylene, which induces the hydrolytic enzymes polygalacturonase and cellulase [5]; they also increase dark respiration and decrease photosynthesis [6,7]. Another compound used for thinning is the cytokinine 6-benzyladenine (BA), which has been shown to block embryo development by causing severe reductions in carbohydrate levels; this, in turn, reduces polar auxin transport across the fruit pedicel and enhances the abscission zone's sensitivity to ethylene, leading to fruit drop [8,9].…”

MADS Box Transcript Amount is Affected by Ethylene during Abscission

“…The expression pattern in seed of MdERS1, MdETR5 and MdEIN2; higher in NAF than AF followed by a sudden decrease may be seen as a delay in development and a total block by day 7. This hypothesis is supported by the drop in both MdAHS and MdPIN1 transcripts previously reported [10,13] and recently revisited in [66]. MdCTR2 transcript level on the other hand may be mainly related to the transduction of the high ethylene level during the first stages of abscission induction [12,8].…”