Understanding Cassava to Feed Africa

Cassava 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.



AJ Cann. ORCID 0000-0002-9014-3720

Alan Cann is a Senior Lecturer in the School of Biological Sciences at the University of Leicester and Internet Consulting Editor for AoB.