Abstract
Among the numerous biological constraints that hinder cassava (Manihot esculenta Crantz) production, foremost is cassava mosaic disease (CMD) caused by virus members of the family Geminiviridae, genus Begomovirus. The mechanisms of CMD tolerance and susceptibility are not fully understood; however, CMD susceptible T200 and tolerant TME3 cassava landraces have been shown to exhibit different large-scale transcriptional reprogramming in response to South African cassava mosaic virus (SACMV). Recent identification of 85 MeWRKY transcription factors in cassava demonstrated high orthology with those in Arabidopsis, however, little is known about their roles in virus responses in this non-model crop. Significant differences in MeWRKY expression and regulatory networks between the T200 and TME3 landraces were demonstrated. Overall, WRKY expression and associated hormone and enriched biological processes in both landraces reflect oxidative and other biotic stress responses to SACMV. Notably, MeWRKY11 and MeWRKY81 were uniquely up and downregulated at 12 and 67 days post infection (dpi) respectively in TME3, implicating a role in tolerance and symptom recovery. AtWRKY28 and AtWRKY40 homologs of MeWRKY81 and MeWRKY11, respectively, have been shown to be involved in regulation of jasmonic and salicylic acid signaling in Arabidopsis. AtWRKY28 is an interactor in the RPW8-NBS resistance (R) protein network and downregulation of its homolog MeWRKY81 at 67 dpi in TME3 suggests a negative role for this WRKY in SACMV tolerance. In contrast, in T200, nine MeWRKYs were differentially expressed from early (12 dpi), middle (32 dpi) to late (67 dpi) infection. MeWRKY27 (homolog AtWRKY33) and MeWRKY55 (homolog AtWRKY53) were uniquely up-regulated at 12, 32 and 67 dpi in T200. AtWRKY33 and AtWRKY53 are positive regulators of leaf senescence and oxidative stress in Arabidopsis, suggesting MeWRKY55 and 27 contribute to susceptibility in T200.
Highlights
Cassava (Manihot esculenta Crantz) is a perennial food security crop largely grown in developing countries, with worldwide cassava productions measuring over 300 million tonnes in 2019
Since ET and jasmonic acid (JA) biological processes (BPs) are highly represented in both landraces at 32 dpi but only in T200 at 67 dpi, we suggest that these hormones are principally associated with biotic stress at 32 dpi as there was no evidence for cassava mosaic disease (CMD) resistance responses
The differentially expressed (DE) WRKYs in TME3 at 32 dpi share only two upregulated transcription factors (TFs) (AtWRKY41 and 70 homologs) with T200, which implies that a different set of WRKYs respond to South African cassava mosaic virus (SACMV) in T200 and TME3 even though they share a similar stress response. We conclude that these WRKYs are positive activators of JA, but in TME3 we suggest JA concomitantly plays a greater role in temperance of disease symptoms (Figure 2)
Summary
Cassava (Manihot esculenta Crantz) is a perennial food security crop largely grown in developing countries, with worldwide cassava productions measuring over 300 million tonnes in 2019 (http://www.fao.org/faostat/en/#data). CMD presents as a spectrum of symptoms such as leaf curl, stunted growth, mosaic and chlorosis [4]. This subsequently leads to a reduction in cassava crop production, which can be as high as a 100% loss in yield [2]. Cassava mosaic begomoviruses are bipartite, composed of two circular ssDNA components (DNA A and B) that are packaged separately into twinned geminate virion particles [6,7]. Each DNA component ranges between 2.5 and
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