Peach fruit acquired tolerance to low temperature stress by accumulation of linolenic acid and N-acylphosphatidylethanolamine in plasma membrane

Abstract

Peach fruit (Prunus persica L. cv. Beijing 33) did not show symptoms of chilling injury in 0 °C-Air or 0 °C-CA, but did in 5 °C-Air after 21 d. The mechanisms by which 0 °C storage could activate chilling tolerance of peach fruit were investigated by analysing characteristics of plasma membrane. We found that peach fruit stored in 0 °C-Air and 0 °C-CA had much higher linolenic acid content and unsaturation degree of plasma membrane than did that in 5 °C-Air. In addition, the fruits stored in 0 °C-CA showed a higher membrane fluidity and membrane integrity than did that in 0 °C-Air, which was related to the accumulation of N-acylphosphatidylethanolamine (NAPE) of peach fruits stored in 0 °C-CA. Based on these results, it appears that a higher unsaturation degree of membrane lipid and NAPE accumulation are beneficial for maintaining membrane fluidity, leading to an enhanced tolerance of peach fruit to chilling stress.

Introduction

Many plant species, especially those of tropical or temperate origin, are severely injured by exposure to low but not freezing, temperatures (Saltveit & Morris, 1990). Exposure of these chilling-sensitive plants to unfavourable low temperatures often results in numerous cellular and metabolic dysfunctions, such as altered respiration rates, impaired photosynthetic activity, and changes in membrane permeability (Allen & Ort, 2001). Peach fruit is also sensitive to low temperature stress and chilling injury (CI) occurs easily when it is exposed to the low temperature for long periods (Saltveit & Morris, 1990). In a previous experiment, we found that peach fruits stored at 5 °C for 21 d usually showed CI, but no CI symptom occurred at 0 °C (Zhang & Tian, 2009). In addition, peach fruits kept at controlled atmosphere (CA), with 5% O₂ plus 5% CO₂ showed a stronger resistance to low temperature stress (Wang, Tian, & Xu, 2005) and application of methyl salicylate (MeSA) could effectively enhance tolerance of peach fruits to CI (Meng, Han, Wang, & Tian, 2009). A better understanding of the processes of chilling tolerance in fruit is now required as this may lead to important agricultural and economic benefits.

A biochemical basis to explain the CI mechanism of plant has not yet been established. Lyons (1973) proposes the existence of a primary event that results in a series of secondary events which, in turn, results in the symptoms of CI. The cell membranes are likely sites of primary effects of chilling. Mikami and Murata (2003) reported that unsaturated fatty acid (UFA) level in membrane lipids was positively correlated with plant chilling tolerance. Genetic manipulation of the level of UFAs led to the eventual modification of the cold sensitivity of tobacco plants (Murata et al., 1992). In cyanobacteria, sensitivity to cold is also closely correlated with the level of unsaturation of membrane lipids (Tasaka et al., 1996). Aside from the interest given to the responses of the UFA level to low temperature stress, the role of an unusual phospholipid class, N-acylphosphatidylethanolamines (NAPEs), in membrane protection and stabilisation has received considerable attention (Hansen, Moesgaard, Hansen, & Petersen, 2000). This compound is characterised by the presence of a third fatty acyl residue linked to the N-atom of the phosphatidylethanolamine headgroup by an amide bond and shows a propensity to accumulate under various stress conditions involving degenerative membrane changes (Schmid, Schmid, & Natarajan, 1990). For instance, NAPEs synthesis was observed in cultivated potato cells submitted to anoxia stress and the capacity to increase its NAPE level may confer some additional protection to the cell (Rawyler et al., 2002, Rawyler and Braendle, 2001). However, studies on NAPEs biosynthesis and its involvement in the response of plant tissues to low temperature stress have, so far, been rare.

As compared with plant responses to chilling stress, there are a number of papers describing that the development of CI of fruit is caused by an imbalance between oxidative and reductive processes due to metabolic gas gradients inside the fruit (Franck et al., 2007, Wang et al., 2005). Accumulation of reactive oxygen species may induce loss of membrane integrity which becomes macroscopically visible through the enzymatic oxidation of phenolic compounds to brown-coloured polymers. However, plasma membrane involvement in the process of CI development has received little attention to date. In order to fully understand whether the biochemical and biophysical characteristics of plasma membrane lipids are correlated with chilling-resistance of peach fruit, we investigated the function of linolenic acid (C18:3) and unsaturation degree of the plasma membrane in enhancing tolerance of peach fruit to low temperature stress. Here, we present the first evidence that peach fruit acquired chilling-tolerance by accumulation of C18:3 and NAPEs in plasma membrane, and explain why peach fruit stored at 0 °C showed a higher chilling tolerance than did that at 5 °C.