In the tropics, cassava ranks third as a source of calories just behind rice and maize, and is typically grown by resource-poor smallholder farmers on marginal lands. A naturally drought-tolerant crop, it provides a critical staple food to many populations vulnerable to food insecurity. But cassava is often seen as a poor cousin in the world’s family of staple crops.
Improvement and expanded adoption of crops suited to growth with limited water resources on marginal lands is critical to ensuring food security, given the limited arable land and population growth, further compounded by the effects of climate change. In sub-Saharan Africa and throughout much of the tropics and sub-tropics, the development and use of crop varieties with high water-use efficiency is particularly important for marginal areas with poor soils, unreliable rainfall and where irrigation is unavailable or unaffordable for resource-poor farmers. In this respect, cassava deserves particular attention because of its status and further potential as both a food security and a cash crop for most households living in marginal areas of the tropics and sub-tropics.
It has been estimated that moisture or drought stress is the most adverse crop environmental stress, accounting for over 70% of potential agriculture yield losses worldwide. In Africa, the cassava growth cycle is typically interrupted by 3–6 months of drought, influencing various plant physiological processes resulting in depressed growth, development and economic yield. In general, cassava can withstand significant periods of drought stress. However, there is a range of drought-tolerance levels in available germplasm, and its growth and productivity in marginal areas are constrained by severe drought stress, especially during the earlier stages of growth. Development of cassava varieties with farmer-preferred traits and increased drought tolerance will allow its expanded cultivation and elevated yields in marginal areas.
Given the inherent challenges with cassava breeding, an understanding of the molecular basis of cassava drought responses and tolerance can help greatly in the development of appropriate varieties. Conventional breeding has been hindered by cassava’s high heterozygosity, genotype by environment interaction, long life cycle and limited seed production, while molecular breeding is hindered by limited information on genomic regions and genes associated with drought tolerance in cassava. Efforts to improve cassava’s water-use efficiency through conventional breeding have been limited in many parts of the world. Breeding programmes in Latin America have successfully identified germplasm with increased levels of drought tolerance, with 2–3 times the yield of typical cassava genotypes in semi-arid conditions.
Plant tolerance to drought stress is a complex trait with several interacting layers of molecular and physiological responses. Drought stress responses and tolerance genes have been well characterized in a number of plant species, lending insight into the general pathways involved and potential tolerance mechanisms and genes in other species. Plant resistance to drought stress can be achieved through escape (e.g. early flowering time in drier environments), avoidance (e.g. transpiration control by stomata and development of extensive root systems), phenotypic flexibility, water conservation in tissues, antioxidant defences, plant growth regulation by hormones and osmotic adjustment. Drought stress induces accumulation of metabolites and drought-related proteins.
Ecophysiologically, mechanisms of drought tolerance in cassava have been identified such as avoidance, through partial stomatal closure to reduce transpiration, development of extensive root systems and proportionally strategic reductions in leaf canopy; however, in some studies greater leaf retention has been correlated with drought tolerance, so the relationship between leaf retention and drought tolerance depends on the genotype and probably on environmental factors (e.g. severity of drought). While a limited number of molecular studies have sequenced normalized expressed sequence tag libraries from cassava under drought stress, no molecular studies have been conducted that quantify gene expression in single or contrasting cassava genotypes under conditions resembling those in the field, which would enable the identification of both drought-responsive and candidate drought-tolerance genes most relevant to cassava drought improvement efforts.
A new study on the open access journal AoB PLANTS has confirmed the drought-tolerant and drought-susceptible status of improved and farmer-preferred cassava varieties which are now part of the germplasm being integrated into the breeding programme at the National Crops Resources Research Institute in Uganda to develop drought-tolerant cassava with other farmer-preferred traits. The morphological and physiological responses of the two genotypes to drought stress were assessed. The relative expression levels of genes previously demonstrated to be functionally involved in, or associated with, drought stress responses in other species were also analysed. This study provides a general characterization of drought responses in cassava, yielding expression-based markers and candidate drought-tolerance genes for ongoing cassava improvement efforts. A molecular understanding of the drought responses of this drought-tolerant species can also provide insights for increasing the drought tolerance of more drought-sensitive species.
Physiological and molecular characterization of drought responses and identification of candidate tolerance genes in cassava. (2013) AoB PLANTS 5: plt007 doi: 10.1093/aobpla/plt007
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.
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