Spermidine application to young developing peach fruits leads to a slowing down of ripening by impairing ripening-related ethylene and auxin metabolism and signaling

Peach (Prunus persica var. laevis Gray) was chosen to unravel the molecular basis underlying the ability of spermidine (Sd) to influence fruit development and ripening. Field applications of 1 mM Sd on peach fruit at an early developmental stage, 41 days after full bloom (dAFB), i.e. at late stage S1, led to a slowing down of fruit ripening. At commercial harvest (125 dAFB, S4II) Sd-treated fruits showed a reduced ethylene production and flesh softening. The endogenous concentration of free and insoluble conjugated polyamines (PAs) increased (0.3-2.6-fold) one day after treatment (short-term response) but soon it declined to control levels; starting from S3/S4, when soluble conjugated forms increased (up to 5-fold relative to controls at ripening), PA levels became more abundant in treated fruits, (long-term response). Real-time RT-PCR analyses revealed that peaks in transcript levels of fruit developmental marker genes were shifted ahead in accord with a developmental slowing down. At ripening (S4I-S4II) the up-regulation of the ethylene biosynthetic genes ACO1 and ACS1 was dramatically counteracted by Sd and this led to a strong down-regulation of genes responsible for fruit softening, such as PG and PMEI. Auxin-related gene expression was also altered both in the short- (TRPB) and in the long-term (GH3, TIR1 and PIN1), indicating that auxin plays different roles during development and ripening processes. Messenger RNA amounts of other hormone-related ripening-regulated genes, such as NCED and GA2-OX, were strongly down-regulated at maturity. Results suggest that Sd interferes with fruit development/ripening by interacting with multiple hormonal pathways.