Extending cassava root shelf life via reduction of reactive oxygen species production.

Tawanda Zidenga,Dimuth Siritunga,Hangsik Moon,Richard Sayre,Elisa Leyva-Guerrero

doi:10.1104/pp.112.200345

Abstract

One of the major constraints facing the large-scale production of cassava (Manihot esculenta) roots is the rapid postharvest physiological deterioration (PPD) that occurs within 72 h following harvest. One of the earliest recognized biochemical events during the initiation of PPD is a rapid burst of reactive oxygen species (ROS) accumulation. We have investigated the source of this oxidative burst to identify possible strategies to limit its extent and to extend cassava root shelf life. We provide evidence for a causal link between cyanogenesis and the onset of the oxidative burst that triggers PPD. By measuring ROS accumulation in transgenic low-cyanogen plants with and without cyanide complementation, we show that PPD is cyanide dependent, presumably resulting from a cyanide-dependent inhibition of respiration. To reduce cyanide-dependent ROS production in cassava root mitochondria, we generated transgenic plants expressing a codon-optimized Arabidopsis (Arabidopsis thaliana) mitochondrial alternative oxidase gene (AOX1A). Unlike cytochrome c oxidase, AOX is cyanide insensitive. Transgenic plants overexpressing AOX exhibited over a 10-fold reduction in ROS accumulation compared with wild-type plants. The reduction in ROS accumulation was associated with a delayed onset of PPD by 14 to 21 d after harvest of greenhouse-grown plants. The delay in PPD in transgenic plants was also observed under field conditions, but with a root biomass yield loss in the highest AOX-expressing lines. These data reveal a mechanism for PPD in cassava based on cyanide-induced oxidative stress as well as PPD control strategies involving inhibition of ROS production or its sequestration.

Highlights

One of the major constraints facing the large-scale production of cassava (Manihot esculenta) roots is the rapid postharvest physiological deterioration (PPD) that occurs within 72 h following harvest
We show that overexpression of alternative oxidase (AOX; a cyanide-resistant terminal oxidase in plants) in cassava storage roots reduces the accumulation of reactive oxygen species (ROS) and delays PPD by 10 to 21 d under greenhouse and field conditions
Previous studies have shown that mechanical injury of cassava storage roots triggers cyanogenesis (McMahon et al, 1995) and an associated burst of acetone from cyanohydrins detected throughout the storage root (Iyer et al, 2010), followed by the production of ROS (Reilly et al, 2004)

Summary

Introduction

One of the major constraints facing the large-scale production of cassava (Manihot esculenta) roots is the rapid postharvest physiological deterioration (PPD) that occurs within 72 h following harvest. The delay in PPD in transgenic plants was observed under field conditions, but with a root biomass yield loss in the highest AOX-expressing lines These data reveal a mechanism for PPD in cassava based on cyanideinduced oxidative stress as well as PPD control strategies involving inhibition of ROS production or its sequestration. PPD is initiated by mechanical damage, which typically occurs during harvesting and progresses from the proximal site of damage to the distal end, making the roots unpalatable within 72 h (Wenham, 1995; Buschmann et al, 2000; Iyer et al, 2010) This deterioration is an active process distinct from the secondary deterioration caused by microbial infection (Booth, 1976).

Methods

Results

Conclusion