Impact of 1-methylcyclopropene and controlled atmosphere storage on polyamine and 4-aminobutyrate levels in "Empire" apple fruit

Abstract

1-Methylcyclopropene (1-MCP) delays ethylene-meditated ripening of apple (Malus domestica Borkh.) fruit during controlled atmosphere (CA) storage. Here, we tested the hypothesis that 1-MCP and CA storage enhances the levels of polyamines (PAs) and 4-aminobutyrate (GABA) in apple fruit. A 46-week experiment was conducted with "Empire" apple using a split-plot design with four treatment replicates and 3°C, 2.5 kPa O2, and 0.03 or 2.5 kPa CO2 with or without 1 μL L(-1) 1-MCP. Total PA levels were not elevated by the 1-MCP treatment. Examination of the individual PAs revealed that: (i) total putrescine levels tended to be lower with 1-MCP regardless of the CO2 level, and while this was mostly at the expense of free putrescine, large transient increases in soluble conjugated putrescine were also evident; (ii) total spermidine levels tended to be lower with 1-MCP, particularly at 2.5 kPa CO2, and this was mostly at the expense of soluble conjugated spermidine; (iii) total spermine levels at 2.5 kPa CO2 tended to be lower with 1-MCP, and this was mostly at the expense of both soluble and insoluble conjugated spermine; and (iv) total spermidine and spermine levels at 0.03 kPa were relatively unaffected, compared to 2.5 kPa CO2, but transient increases in free spermidine and spermine were evident. These findings might be due to changes in the conversion of putrescine into higher PAs and the interconversion of free and conjugated forms in apple fruit, rather than altered S-adenosylmethionine availability. Regardless of 1-MCP and CO2 treatments, the availability of glutamate showed a transient peak initially, probably due to protein degradation, and this was followed by a steady decline over the remainder of the storage period which coincided with linear accumulation of GABA. This pattern has been attributed to the stimulation of glutamate decarboxylase activity and inhibition of GABA catabolism, rather than a contribution of PAs to GABA production.

Keywords: 1-methylcyclopropene (1-MCP); 4-aminobutyrate (GABA); abiotic stress; apple fruit; controlled atmosphere storage; high performance liquid chromatography; polyamines.

FIGURE 1

Simplified diagram of metabolic relationships among ethylene, polyamines, and GABA [adapted from Shelp et al. (2012c)]. 1-MCP inhibits ethylene binding to its receptor and the expression of both ACC synthase and ACC oxidase (see text for detailed explanation). Abbreviations: ACC, 1-aminocyclopropane-1-carboxylic acid; APAL, 3-aminopropionaldehyde; BCPAO, back-conversion polyamine oxidase; dcSAM, decarboxylated S-adenosylmethionine; DAO, diamine oxidase; GABA, 4-aminobutyrate; GAD, glutamate decarboxylase; 1-MCP, 1-methylcyclopropene; PAO, polyamine oxidase; SAM, S-adenosylmethionine; SAMDC, SAM decarboxylase; SPDS, spermidine synthase; SPMS, spermine synthase.

FIGURE 2

Effects of 1-MCP and elevated CO₂ on total (A), total free (B), total soluble conjugated (C), and total insoluble conjugated (D) polyamines in “Empire” fruit during controlled atmosphere storage. With the exception of the beginning and end of 1-MCP treatment (-1 day to 0 week, the shaded area), all data are mean estimates of four storage replicates. Different letter groupings indicate significant differences between treatments within weeks (P ≤ 0.05). Underlined letters indicate a shared letter for overlapping data; where letters are absent at a time point, there were no significant differences.

FIGURE 3

Effects of 1-MCP and elevated CO₂ on total (A), free (B), soluble conjugated (C), and insoluble conjugated (D) putrescine in “Empire” fruit during controlled atmosphere storage. With the exception of the beginning and end of 1-MCP treatment (-1 day to 0 week, the shaded area), all data are mean estimates of four storage replicates. Different letter groupings indicate significant differences between treatments within weeks (P ≤ 0.05). Underlined letters indicate a shared letter for overlapping data; where letters are absent at a time point, there were no significant differences.

FIGURE 4

Effects of 1-MCP and elevated CO₂ on total (A), free (B), soluble conjugated (C), and insoluble conjugated (D) spermidine in “Empire” fruit during controlled atmosphere storage. With the exception of the beginning and end of 1-MCP treatment (-1 day to 0 week, the shaded area), all data are mean estimates of four storage replicates. Different letter groupings indicate significant differences between treatments within weeks (P ≤ 0.05). Underlined letters indicate a shared letter for overlapping data; where letters are absent at a time point, there were no significant differences.

FIGURE 5

Effects of 1-MCP and elevated CO₂ on total (A), free (B), soluble conjugated (C), and insoluble conjugated (D) spermine in “Empire” fruit during controlled atmosphere storage. With the exception of the beginning and end of 1-MCP treatment (-1 day to 0 week, the shaded area), all data are mean estimates of four storage replicates. Different letter groupings indicate significant differences between treatments within weeks (P ≤ 0.05). Underlined letters indicate a shared letter for overlapping data; where letters are absent at a time point, there were no significant differences.

FIGURE 6

Effects of 1-MCP and elevated CO₂ on GABA (A) and glutamate (B) levels in “Empire” fruit during controlled atmosphere storage. With the exception of the beginning and end of 1-MCP treatment (-1 day to 0 week, the shaded area), all data are mean estimates of four storage replicates. Different letter groupings indicate significant differences between treatments within weeks (P ≤ 0.05). Underlined letters indicate a shared letter for overlapping data; where letters are absent at a time point, there were no significant differences.