Identification and expression of genes in response to cassava bacterial blight infection

Abstract

Cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. manihotis (or XAM) is a serious disease of cassava (Manihot esculenta Crantz). In this study, quantitative trait loci (QTL) associated with CBB infection were identified in the F1 progenies of a cross between the "Huay Bong 60" and "Hanatee" cassava cultivars. The phenotype of disease severity was observed at 7, 10, and 12 days after inoculation (DAI). A total of 12 QTL were identified, of which 5, 6, and 1 were detected in 7, 10, and 12 DAI samples, respectively. Among all identified QTL, CBB14_10dai_1, CBB14_10dai_2, and CBB14_12dai showed the most significant (P < 0.0001) associations with CBB infection, and explained 21.3, 13.8, and 26.5% of phenotypic variation, respectively. Genes underlying the QTL were identified and their expression was investigated in resistant and susceptible cassava plants by real-time quantitative RT-PCR. The results identified candidate genes that showed significant differences in expression between resistant and susceptible lines, including brassinosteroid insensitive 1-associated receptor kinase 1-related (Manes.04G059100), cyclic nucleotide-gated ion channel 2 (Manes.02G051100), and autophagy-related protein 8a-related (Manes.17G026600) at 7 DAI, and regulator of nonsense transcripts 1 homolog (Manes.17G021900) at both 7 and 12 DAI. The expression pattern of all genes showed higher levels in resistant (B82, B32, B20, and B70) as compared to susceptible (HB60, B100, B95, and B47) plants. Overall, this study has identified QTL and markers linked to CBB infection trait, and identified candidate genes involved in CBB resistance. This information will be of use for better understanding defense mechanisms in cassava to bacterial blight disease.

Cassava diseases caused byXanthomonas phaseolipv.manihotisandXanthomonas cassavae

“…QTLs were associated to CBB resistance, explaining up to 26.5% of the genotypic variance, and five colocalized genes were shown to be differentially upregulated in resistant genotypes (Tappiban et al, 2018). Table S6 summarizes all the cassava QTLs (more than 100) reported to be involved in CBB resistance.…”

“…Due to the quantitative nature of CBB resistance, and the complex interaction between the host genetic background and the environment, the search for resistance sources against Xpm in cassava is mainly restricted to the identification of quantitative trait loci (QTLs) (Jorge et al, 2000 , 2001 ; López Carrascal et al, 2007 ; Soto‐Sedano et al, 2017 ; Tappiban et al, 2018 ; Wydra et al, 2004 ). Identified QTLs were strain‐specific in all cases, explaining up to 61% of the resistance variance (reviewed by López & Bernal, 2012 ).…”

“…Furthermore, three of these QTLs were shown to be significantly influenced by environmental conditions, especially humidity. More recently, using an SSR‐based genetic map derived from the F 1 offspring of an Asian cassava cultivar cross, 10 QTLs were associated to CBB resistance, explaining up to 26.5% of the genotypic variance, and five colocalized genes were shown to be differentially upregulated in resistant genotypes (Tappiban et al, 2018 ). Table S6 summarizes all the cassava QTLs (more than 100) reported to be involved in CBB resistance.…”

“…Autophagy regulation is altered during Xpm infection, which significantly impacts immunity. Four members of the cassava autophagy‐related protein 8 ( MeATG8 ) family, including MeATG8f which colocalizes with a resistance QTL (Tappiban et al, 2018 ) and whose product was reported to interact with MeWRKY20 to induce defence against Xpm (Yan et al, 2017 ). Furthermore, MeATG8b and Me8ATG8e have antagonistic roles against the two cassava glyceraldehyde‐3‐phosphate dehydrogenases MeGAPC4 and MeGAPC6, whose downregulation results in higher autophagic activity, higher H 2 O 2 levels, and increased callose deposition (Zeng et al, 2018 ).…”

“…QTL name, name of the QTL associated with CBB resistance; ExpVar, percentage of phenotypic variance explained by a given QTL; linkage group, linkage group where a given QTL can be found, linkage groups depend on the map used for associations; strain, strain used to inoculate the assessed plants (in most cases, QTLs are strain‐specific); natural inoculum, the study was performed in the field and the quantified disease resulted from infections with naturally occurring strains; undefined strain, the study states that only one strain was used to inoculate the assessed plants, but authors do not provide the strain identifier; technique, disease quantitation technique used to associate phenotypes with markers; AUDPC, area under the disease progression curve; 100‐DI, percentage of diseased plants (100‐disease incidence); MDR, mean disease rating; marker, peak marker associated with a given QTL; map reference, cassava map used to associate QTLs. The maps developed by Fregene et al ( 1997 ) and Soto et al ( 2015 ) derived from the female parental TMS 30572 and the male parental CM 2177–2; the map developed by Wydra et al ( 2004 ) derived from five F 1 male individuals (CM7857‐4, CM7857‐10, CM7857‐51, CM7857‐77, and CM7857‐115) and the female recurrent parent TMS30572; the map developed by Tappiban et al ( 2018 ) derived from the parentals Huay Bong 60 (HB60) and Hanatee (HN). Study reference, the reference to the study where QTLs were found…”