Cyanide Metabolism, Postharvest Physiological Deterioration and Abiotic Stress Tolerance in Cassava (Manihot esculenta Crantz)
Year: 2011
Type: article
Abstract: Cassava (Manihot esculenta Crantz) is one of the six most important crops in the world, and an important staple for more than 800 million people.Its tolerance to drought and poor soils make an important food security crop especially in sub-Saharan Africa, while its high starch content has made it attractive as a biofuel feedstock.However, cassava produces potentially toxic levels of cyanogenic glycosides and undergoes rapid postharvest physiological deterioration (PPD) within 72 hours of harvest, shortening its shelf-life.The major cyanogenic glycoside in cassava is linamarin (95%).Linamarin is made from valine in the leaves and translocated to the roots where it is believed to contribute to the reduced nitrogen pool of the plant.Linamarin in the roots is hydrolyzed during mechanical damage by linamarase to acetone cyanohydrin, which breaks down spontaneously into cyanide and acetone at pH > 5.0 or temperatures > 35°C.The first objective of this project was to analyze the activities of enzymes involved in nitrogen and cyanide metabolism in cassava in order to understand the effect of cyanogenic potential on nitrogen metabolism.We compared the activity of nitrate reductase, a key enzyme in nitrogen metabolism, in wild-type lines and transgenic low cyanogen lines.The results show that nitrate reductase activity is 3X higher in low cyanogen plants than in wild-type lines, suggesting that cyanogenesis provides a significant source of reduced nitrogen which has to be compensated for by nitrate reduction in low cyanogen plants.In addition, β-cyanoalanine synthase (CAS), nitrilase and rhodanese assays confirmed that CAS is the iii key cyanide metabolizing enzyme in cassava roots, with 3X more activity in the roots compared to shoots, while rhodanese had no detectable activity in cassava roots.We concluded from these assays that the β-cyanoalanine synthase pathway is active in cassava roots, and is likely to be the route for assimilation of cyanide into free amino acids.The second objective was to investigate the potential of redirecting cyanide from cyanogenesis in cassava roots to the production of free amino acids by overexpressing two genes involved in cyanide metabolism; CAS and nitrilase 4 (NIT4).Transgenic plants generated using each of these strategies had limitations because of effects on other pathways, which will be discussed.The third objective was to investigate the role of cyanogenesis in oxidative stress and PPD in cassava.PPD in cassava is marked by an initial oxidative burst, followed by vascular discoloration, ultimately rendering the crop unpalatable.Effective transgenic strategies for extending the shelf-life of cassava require an understanding of the causes of the early events, especially the oxidative burst.We show a causal link between cyanogenesis, which occurs upon mechanical damage in cassava, and accumulation of reactive oxygen species which trigger PPD.By measuring ROS accumulation in transgenic low cyanogen plants and as well as biochemically complementing the low cyanogens by adding 5 mM potassium cyanide, we show that the oxidative burst in cassava roots is cyanogen-induced.In light of these data, we have generated transgenic plants expressing codon-optimized Arabidopsis thaliana Alternate Oxidase (AOX), a cyanide resistant terminal oxidase in the mitochondrial respiratory chain in plants, as a strategy to control PPD by reducing the cyanide-induced ROS accumulation.The transgenic AOX plants show a 10-14X reduction in ROS iv accumulation compared to wild type plants, with at least two week extension in shelflife.The final objective was to test these AOX transgenic lines for tolerance to abiotic stress.The production of ROS is one of the earliest detectable responses of plants to abiotic stresses.Since AOX blocks ROS accumulation, we tested the hypothesis that AOX overexpression protects cassava plants from abiotic stress.Our results show marked tolerance to waterlogging and salt stress, suggesting AOX as a strategy to extend shelf-life of cassava while providing stress tolerance.v For my mother.vi
Author: Tawanda Zidenga
Institution: The Ohio State University
Language: English
Topic: Cassava research and cyanide
Subfield: Plant Science
Domain: Life Sciences
SDG: Zero hunger
Open Access status: green
OpenAlex ID: W2261855148