Fruit detachment in blueberry
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A better understanding of fruit detachment and the processes mediating it is essential to improve the efficiency of mechanical harvesting in blueberry (Vaccinium sp.). In blueberry, fruit detachment may occur either at the point of attachment of the pedicel to the peduncle (peduncle-pedicel junction, PPJ) or at the point of attachment of the pedicel to the fruit (fruit-pedicel junction, FPJ). Whether fruit detachment at these junctions is mediated by the physiological process of abscission or through physical separation of the organ from the parent plant is not well understood. Abscission is a physiological process that involves the programmed separation of entire organs at an anatomically distinct layer called the abscission zone (AZ). Additionally, the spatial and temporal changes in the cell wall composition and metabolism and the regulation of fruit detachment process is not completely understood. In this study, a series of experiments were performed to understand physiological, biochemical and molecular aspects of fruit detachment in blueberry. Anatomical, physiological and microscopic analysis revealed that PPJ is the true abscission zone of blueberry and detachment at FPJ is as result of physical breakage. Glycome profiling and immuno-localization of PPJ indicated that alteration in pectins and hemicellulose plays a key role in cell separation during abscission. A number of cell-wall carbohydrate metabolism related genes were altered upon induction of abscission. RNA-Seq analysis showed that abscission agent induced abscission was associated with extensive changes in the expression of genes associated with the biosynthesis and signaling of phytohormones such as ethylene, jasmonic acid and auxin. Also, potentially a plant hormone cross-talk and interaction plays an important role in abscission. Scanning electron microscopy analysis and fruit detachment in response to mechanical shaking of southern highbush cultivar 'Suziblue' and accession line TH729 revealed that variability in ease of fruit detachment is due to stronger PPJ. A decrease in expression of cell wall hydrolysis related gene was observed with stronger PPJ. Potentially these phytohormone and cell wall hydrolysis genes play an important role in the genetic ease of fruit detachment/abscission.