Stress In Cassava Research Articles

Genetic Variation of Postharvest Physiological Deterioration Susceptibility in a Cassava Germplasm

ABSTRACTPostharvest physiological deterioration (PPD) is an unresolved major stress in cassava (Manihot esculenta Crantz), which constrains the crop suitability for commercial and industrial purposes. The present study sourced for PPD tolerance from different genetic resources available within a cassava germplasm. Six hundred and twelve cassava accessions within International Institute of Tropical Agriculture (IITA)‐Ibadan germplasm were evaluated 2, 4, 6, and 8 days after harvest (DAH) for PPD and rate of PPD per day (PPD d–1) over the evaluation dates. Sixteen cassava genotypes that had mean PPD d–1 of 0.0% d–1 were classified as non‐deteriorators (NoDs) with possible delayed‐PPD trait and nine genotypes with mean PPD d–1 ≥ 70% d–1 as extra‐super fast deteriorators (xSPDs) with possible early‐PPD trait. The PPD was more pronounced at the proximal end of a root than at the middle and distal regions. Polyploid genotypes showed higher PPD tolerance than diploids but local varieties were more susceptible to PPD than the improved varieties. The study suggests that polyploidy could be an additional genetic resource for PPD tolerance in cassava. In addition, the knowledge of PPD status of a cassava germplasm allows its proper utilization. The NoD genotypes could sustain storage and transport within 8 d for commercial and industrial purposes and xSPD genotypes could be immediately processed into fermented foods and dried chips. The results presented in this study are explanatory in nature and could be explored further for integration into improvement programs in cassava.

English

Cassava’s ( Manihot esculenta Crantz) importance as a food security crop in Sub Saharan Africa is enhanced by its special traits such as tolerance to drought and high yields under drought stress. However, full understanding of tolerance mechanisms under hydrothermal stress in cassava is a key in developing highly tolerant varieties with increased yield. In our study, the effects of low soil moisture and increased temperature on cassava physiology were investigated. Twenty (20) cassava varieties were evaluated in a Randomized Complete Block Design in western Uganda. Hydrothermal stress was described as a period of no rainfall for a period of eight weeks leading to low soil moisture (contents between 28 to 35%) and average daily temperatures of ≥35°C. The average daily relative humidity during this period was considerably low (≤40%) further complementing already enhanced stress conditions. As such, the contents of important biochemicals and secondary metabolites in the plants were altered in a bid to counteract the effects of stress. Significant differences occurred in accumulation of main biochemicals such as soluble proteins (P<0.05), free reducing sugars (P<0.05) and bound reducing sugars (P<0.05) while reductions in the total starch yield by 70 to 100% of the original composition before stress were observed in all the test varieties. Changes in pigment properties were also observed with a decrease in the total carotenoid content (~65%) and chlorophyll a ( Chla ) (~40%) but no significant changes were observed for chlorophyll b ( Chlb ). Secondary metabolites such as phenolics and tannins too depicted varied but non-significant changes and they existed in low quantities. There were also significant changes in the phenotype (foliar portion of the plant) with at least two mechanisms of tolerance identified. The study showed the importance of carbohydrate and nitrogen cycle related metabolites in mediating tolerance in cassava by affecting their phenotypic expression in the plant. Keywords: Hydrothermal stress, bio-chemicals, pigments, secondary metabolites, cassava. African Journal of Biotechnology , Vol 13(31) 3173-3186

Physiological and molecular characterization of drought responses and identification of candidate tolerance genes in cassava

Cassava is an important root crop to resource-poor farmers in marginal areas, where its production faces drought stress constraints. Given the difficulties associated with cassava breeding, a molecular understanding of drought tolerance in cassava will help in the identification of markers for use in marker-assisted selection and genes for transgenic improvement of drought tolerance. This study was carried out to identify candidate drought-tolerance genes and expression-based markers of drought stress in cassava. One drought-tolerant (improved variety) and one drought-susceptible (farmer-preferred) cassava landrace were grown in the glasshouse under well-watered and water-stressed conditions. Their morphological, physiological and molecular responses to drought were characterized. Morphological and physiological measurements indicate that the tolerance of the improved variety is based on drought avoidance, through reduction of water loss via partial stomatal closure. Ten genes that have previously been biologically validated as conferring or being associated with drought tolerance in other plant species were confirmed as being drought responsive in cassava. Four genes (MeALDH, MeZFP, MeMSD and MeRD28) were identified as candidate cassava drought-tolerance genes, as they were exclusively up-regulated in the drought-tolerant genotype to comparable levels known to confer drought tolerance in other species. Based on these genes, we hypothesize that the basis of the tolerance at the cellular level is probably through mitigation of the oxidative burst and osmotic adjustment. This study provides an initial characterization of the molecular response of cassava to drought stress resembling field conditions. The drought-responsive genes can now be used as expression-based markers of drought stress tolerance in cassava, and the candidate tolerance genes tested in the context of breeding (as possible quantitative trait loci) and engineering drought tolerance in transgenics.

Transcriptome profiling of low temperature-treated cassava apical shoots showed dynamic responses of tropical plant to cold stress.

BackgroundCassava is an important tropical root crop adapted to a wide range of environmental stimuli such as drought and acid soils. Nevertheless, it is an extremely cold-sensitive tropical species. Thus far, there is limited information about gene regulation and signalling pathways related to the cold stress response in cassava. The development of microarray technology has accelerated the study of global transcription profiling under certain conditions.ResultsA 60-mer oligonucleotide microarray representing 20,840 genes was used to perform transcriptome profiling in apical shoots of cassava subjected to cold at 7°C for 0, 4 and 9 h. A total of 508 transcripts were identified as early cold-responsive genes in which 319 sequences had functional descriptions when aligned with Arabidopsis proteins. Gene ontology annotation analysis identified many cold-relevant categories, including 'Response to abiotic and biotic stimulus', 'Response to stress', 'Transcription factor activity', and 'Chloroplast'. Various stress-associated genes with a wide range of biological functions were found, such as signal transduction components (e.g., MAP kinase 4), transcription factors (TFs, e.g., RAP2.11), and reactive oxygen species (ROS) scavenging enzymes (e.g., catalase 2), as well as photosynthesis-related genes (e.g., PsaL). Seventeen major TF families including many well-studied members (e.g., AP2-EREBP) were also involved in the early response to cold stress. Meanwhile, KEGG pathway analysis uncovered many important pathways, such as 'Plant hormone signal transduction' and 'Starch and sucrose metabolism'. Furthermore, the expression changes of 32 genes under cold and other abiotic stress conditions were validated by real-time RT-PCR. Importantly, most of the tested stress-responsive genes were primarily expressed in mature leaves, stem cambia, and fibrous roots rather than apical buds and young leaves. As a response to cold stress in cassava, an increase in transcripts and enzyme activities of ROS scavenging genes and the accumulation of total soluble sugars (including sucrose and glucose) were also detected.ConclusionsThe dynamic expression changes reflect the integrative controlling and transcriptome regulation of the networks in the cold stress response of cassava. The biological processes involved in the signal perception and physiological response might shed light on the molecular mechanisms related to cold tolerance in tropical plants and provide useful candidate genes for genetic improvement.

Molecular cloning and characterization of a novel RING zinc-finger protein gene up-regulated under in vitro salt stress in cassava

Cassava (Manihot esculenta Crantz) is one of the world's most important food crops. It is cultivated mainly in developing countries of tropics, since its root is a major source of calories for low-income people due to its high productivity and resistance to many abiotic and biotic factors. A previous study has identified a partial cDNA sequence coding for a putative RING zinc finger in cassava storage root. The RING zinc finger protein is a specialized type of zinc finger protein found in many organisms. Here, we isolated the full-length cDNA sequence coding for M. esculenta RZF (MeRZF) protein by a combination of 5' and 3' RACE assays. BLAST analysis showed that its deduced amino acid sequence has a high level of similarity to plant proteins of RZF family. MeRZF protein contains a signature sequence motif for a RING zinc finger at its C-terminal region. In addition, this protein showed a histidine residue at the fifth coordination site, likely belonging to the RING-H2 subgroup, as confirmed by our phylogenetic analysis. There is also a transmembrane domain in its N-terminal region. Finally, semi-quantitative RT-PCR assays showed that MeRZF expression is increased in detached leaves treated with sodium chloride. Here, we report the first evidence of a RING zinc finger gene of cassava showing potential role in response to salt stress.