Abstract
Cassava, Manihot esculenta Crantz, has been positioned as one of the most promising crops world-wide representing the staple security for more than one billion people mainly in poor countries. Cassava production is constantly threatened by several diseases, including cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. manihotis (Xam), it is the most destructive disease causing heavy yield losses. Here, we report the detection and localization on the genetic map of cassava QTL (Quantitative Trait Loci) conferring resistance to CBB. An F1 mapping population of 117 full sibs was tested for resistance to two Xam strains (Xam318 and Xam681) at two locations in Colombia: La Vega, Cundinamarca and Arauca. The evaluation was conducted in rainy and dry seasons and additional tests were carried out under controlled greenhouse conditions. The phenotypic evaluation of the response to Xam revealed continuous variation. Based on composite interval mapping analysis, 5 strain-specific QTL for resistance to Xam explaining between 15.8 and 22.1% of phenotypic variance, were detected and localized on a high resolution SNP-based genetic map of cassava. Four of them show stability among the two evaluated seasons. Genotype by environment analysis detected three QTL by environment interactions and the broad sense heritability for Xam318 and Xam681 were 20 and 53%, respectively. DNA sequence analysis of the QTL intervals revealed 29 candidate defense-related genes (CDRGs), and two of them contain domains related to plant immunity proteins, such as NB-ARC-LRR and WRKY.
Highlights
Cassava, Manihot esculenta Crantz, is a starchy root crop and one of the main staple food crops over the world due to its essential role for food security in tropical regions
The defense response of 117 genotypes of a F1 mapping population infected with two Xanthomonas axonopodis pv. manihotis (Xam) strains was evaluated in different environments: two locations under rainy and dry seasons and under greenhouse controlled conditions, which allowed for the identification of five strain-specific QTL for resistance to cassava bacterial blight (CBB)
In silico analyses of the QTL intervals revealed 29 candidate defense-related genes (CDRGs) that might operate in the defense response
Summary
Manihot esculenta Crantz, is a starchy root crop and one of the main staple food crops over the world due to its essential role for food security in tropical regions. This crop represents an important source of calories for about one billion people (Ceballos et al, 2010). CBB has been reported in all regions where cassava is grown (López and Bernal, 2012; Taylor et al, 2012), including 56 countries distributed over Asia, Africa, Oceania and North, Central, and South America (http://www.cabi.org/) and the number of countries affected by the disease is increasing. The analysis of 65 Xam genomes revealed that this pathogen harbors 14–22 effector genes, from which nine are conserved in all the strains (Bart et al, 2012)
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