Molecular insights into Cassava brown streak virus susceptibility and resistance by profiling of the early host response.

Ravi B Anjanappa,Michal J Okoniewski,Alicja Szabelska‐Berȩsewicz,Devang Mehta,Wilhelm Gruissem,Hervé Vanderschuren

doi:10.1111/mpp.12565

Ravi B Anjanappa, Michal J Okoniewski + Show 4 more

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https://doi.org/10.1111/mpp.12565

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Abstract

Summary Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) are responsible for significant cassava yield losses in eastern sub‐Saharan Africa. To study the possible mechanisms of plant resistance to CBSVs, we inoculated CBSV‐susceptible and CBSV‐resistant cassava varieties with a mixed infection of CBSVs using top‐cleft grafting. Transcriptome profiling of the two cassava varieties was performed at the earliest time point of full infection (28 days after grafting) in the susceptible scions. The expression of genes encoding proteins in RNA silencing, salicylic acid pathways and callose deposition was altered in the susceptible cassava variety, but transcriptional changes were limited in the resistant variety. In total, the expression of 585 genes was altered in the resistant variety and 1292 in the susceptible variety. Transcriptional changes led to the activation of β‐1,3‐glucanase enzymatic activity and a reduction in callose deposition in the susceptible cassava variety. Time course analysis also showed that CBSV replication in susceptible cassava induced a strong up‐regulation of RDR1, a gene previously reported to be a susceptibility factor in other potyvirus–host pathosystems. The differences in the transcriptional responses to CBSV infection indicated that susceptibility involves the restriction of callose deposition at plasmodesmata. Aniline blue staining of callose deposits also indicated that the resistant variety displays a moderate, but significant, increase in callose deposition at the plasmodesmata. Transcriptome data suggested that resistance does not involve typical antiviral defence responses (i.e. RNA silencing and salicylic acid). A meta‐analysis of the current RNA‐sequencing (RNA‐seq) dataset and selected potyvirus–host and virus–cassava RNA‐seq datasets revealed that the conservation of the host response across pathosystems is restricted to genes involved in developmental processes.

Highlights

Cassava brown streak disease (CBSD) is one of the most damaging virus diseases of cassava in Africa, causing harvest losses of up to 70% in susceptible varieties (Hillocks et al, 2001; Legg et al, 2011)
Cassava brown streak virus (CBSV) inoculation induces greater transcriptome modulation in susceptible cassava Following successful grafting of 60444 and KBH 2006/18 virusfree scions onto rootstocks with a mixed infection of CBSV (TAZDES-01) and Ugandan cassava brown streak virus (UCBSV) (TAZ-DES-02) (CBSV infection in short), we performed a time course sampling of the scions at 16, 22 and 28 days after grafting (Fig. S1, see Supporting Information)
Considering the over-representation of chloroplast-related genes (TAIR gene ontology (GO):0009507) in our datasets, we further investigated the relative abundance of nuclear genes encoding chloroplast proteins (NGCPs) transcripts regulated by CBSV infection

Summary

Introduction

Cassava brown streak disease (CBSD) is one of the most damaging virus diseases of cassava in Africa, causing harvest losses of up to 70% in susceptible varieties (Hillocks et al, 2001; Legg et al, 2011). CBSD is caused by two phylogenetically distinct potyvirus species: Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) (Mbanzibwa et al, 2011), collectively referred to as CBSVs. CBSVs are positive single-stranded RNA [(1)ssRNA] viruses belonging to the genus Ipomovirus, family Potyviridae (Mbanzibwa et al, 2009; Monger et al, 2001), and are transmitted by whiteflies (Bemisia tabaci) (Maruthi et al, 2005). Two recent field analyses of different cassava genotypes for the presence of CBSV and UCBSV found that both viruses occurred as mixed infections in the majority of tested genotypes (Kaweesi et al, 2014; Ogwok et al, 2015)

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