NAA and Ethylene Regulate Expression of Genes Related to Ethylene Biosynthesis, Perception, and Cell Wall Degradation During Fruit Abscission and Ripening in ‘Delicious’ Apples

Expression of genes for ethylene biosynthesis, ethylene perception, and cell wall degradation in the fruit cortex and abscission zone was examined during fruit abscission and ripening in ‘Delicious’ apples (Malus × domestica). An autocatalytic burst of fruit ethylene production and accelerated fruit softening were associated with increased expression of genes related to ethylene biosynthesis (MdACS and MdACO), whereas reduced expression of ethylene receptor genes (MdETR and MdERS), increased expression of an ethylene signal transduction gene (MdCTR1), and increased expression of genes related to cell wall degradation (MdPG and MdEG) in the fruit cortex occurred during fruit ripening. Aminoethoxyvinylglycine (AVG) or 1−methylcyclopropene (1-MCP) inhibited fruit ethylene production, suppressed expression of MdACS1, MdACO1, MdERS1, and MdPG1 in the fruit cortex, and delayed fruit softening, whereas naphthaleneacetic acid (NAA) increased fruit ethylene production, increased expression of MdACS1, MdACO1, MdERS1 and MdPG1 in the fruit cortex, and accelerated fruit softening. Fruit abscission and expression of MdACS5A, MdACS5B, MdACO1, MdPG2, and MdEG1 in the fruit abscission zone were reduced by AVG and 1-MCP. NAA also reduced fruit abscission while reducing expression of MdPG2 and MdEG1 only in the fruit abscission zone. The levels of MdETR1, MdETR2, MdERS1, and MdERS2 transcripts in the fruit abscission zone decreased during fruit abscission and fruit ripening regardless of treatment. The combination of NAA and AVG was more effective in inhibiting expression of MdPG2 and MdEG1 in the fruit abscission zone and reducing fruit abscission than was either NAA or AVG used alone.