Abstract
In this work, the involvement of programmed cell death (PCD) in the wound-induced postharvest browning disorder and senescence in butterhead lettuce (Lactuca sativa L.) fresh-cuts was studied. At the wounded (cut, bruised) sites, rapid browning, loss of chlorophyll and massive cell death, accompanied with accumulation of reactive oxygen species and increased electrolyte leakage occurred in a narrow strip of tissue adjacent the injury. The dead cell morphology (protoplast and nuclei shrinkage) together with the biochemical and physiological changes resembled necrotic PCD type. With a slight delay post-wounding, senescence associated with similar cell death features was initiated in distant non-wounded sites. In addition to necrotic PCD, both in wounded and senescing tissue, the appearance of empty cell corpses was observed, indicating that part of the cells might undergo vacuolar PCD (self-digestion of cellular content after vacuole collapse). The wounding-induced local cell death at the primary site of damage suggested that PCD may serve as a mechanism to seal-off the wound by building a physical barrier of dead cells. However, the cell death at sites remote from the wound suggests the distribution of long-distance senescence-inducing wound messengers. Trichomes in unwounded tissue often were the first to show H2O2 accumulation and dead cells; thereafter, the elevated H2O2 and cell death appeared in connecting cells and senescence progressed over larger areas. This suggests that trichomes may contribute to mediating the wound signalling leading to subsequent senescence. Our findings demonstrate that PCD is an integral part of the wound syndrome in fresh-cut lettuce.
Highlights
The shelf life of fresh-cut lettuce is largely dependent on factors such as genetic background, developmental stage at harvest of the starting material and postharvest handling conditions
Yellowing and slight browning near the cut edge and around the other injured sites initially appeared on day 2 and the severity was increasing until the end of shelf life (Figs. 1 a1, b–d and 2b)
(e.g. protoplast shrinkage) of the dead cells together with activation of caspase 3-like protease and other elements of necrotic programmed cell death (PCD) are documented in pathogen challenged detached lettuce leaves and non-headed Chinese cabbage (Kiba et al 2006, 2009; Li et al 2006) whereas symptoms of vacuolar cell death are described in senescing lettuce leaves and in freshcuts subjected to high pressure stress (Wagstaff et al 2007; Zhang et al 2015)
Summary
The shelf life of fresh-cut lettuce (a demanded ‘ready to use’ vegetable product) is largely dependent on factors such as genetic background, developmental stage at harvest of the starting material and postharvest handling conditions Major deterioration in the leafy fresh-cuts is pinking and browning at the wounded sites (Couture et al 1993; Castañer et al 1996; Cantwell and Suslow 2002; Hodges and Toivonen 2008; Pedreschi and Lurie 2015). Treatments with gaseous compounds (e.g. nitric oxide (NO), ozone, hydrogen sulphide), soluble substances with antioxidant properties, chlorine and calcium-based solutions, hot water, UV radiation, high pressure, modulations of light quality and photoperiod and, genetic manipulations are shown to suppress the wound-induced browning, delay senescence, stimulate the expression of defence genes or downregulate stress- and senescenceassociated genes (Coupe et al 2003; Rico et al 2006; Eason et al 2014; Li et al 2014; Mahajan et al 2014; Iakimova and Woltering 2015; Woltering and Seifu 2015).
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