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. 2021 Sep 13;13(9):1820. doi: 10.3390/v13091820

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© 2021 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

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Proposed models of WRKY functions, hormones and downstream signaling of responses to SACMV in T200 and TME3 at 32 dpi. (A) TME3 exhibits milder symptoms compared to T200 in response to SACMV. This is associated with a number of upregulated hormone genes, JA, ABA, SA and ET, as a general response to infection. While a number of SA genes are upregulated, they are not significantly represented in the enriched GO (biological process) categories. In contrast to mock-inoculated plants, upregulation of MeWRKY3, 11 and 18 expression is observed. These transcription factors positively regulate hormone biosynthesis and signaling and biotic and oxidative stress functions. Differential expression of MeWRKY11 is specifically associated with an ABA response, a hallmark of abiotic and biotic stress. These responses to WRKY expression contribute at a later stage to a lower viral load and symptom recovery observed at 67 dpi and tolerance to SACMV. (B) T200 exhibits severe symptoms in response to SACMV. This is associated with a number of upregulated hormones JA, ABA, SA and ET genes as a general response to infection. However, while a number of ABA genes are upregulated, they are not significantly represented in the enriched GO (biological process) categories. In contrast to mock-inoculated plants, upregulated expression of MeWRKY3, 5/55 and 70 is observed. AtWRKY12 is a known positive regulator of plant defense, and downregulation of expression of its MeWRKY70 homolog contributes to severe leaf curl/chlorosis disease symptoms in T200. AtWRKY41 has been shown to regulate hormone signaling and biotic stress responses, and in cassava MeWRKY3, a homolog of AtWRKY41 plays a role in regulation of hormone signaling and biotic stress responses to SACMV. Arabidopsis NPR1 gene controls the onset of systemic acquired resistance (SAR)/immunity to a broad spectrum of pathogens, and AtWRKY53 is a known target of nonexpressor of pathogenesis-related gene (NPR1) during SAR in Arabidopsis. However, in the absence of SAR or the hypersensitive response (HR) in cassava, it is likely that SA-mediated responses of MeWRKY55, uniquely upregulated in T200, are associated with oxidative and other biotic stresses through cross-talk with JA and ET. The AtWRKY53 homolog of MeWRKY55 is a positive regulator of senescence suggesting that MeWRKY55 may play a role in other disease-associated metabolic perturbations associated with severe symptoms. (C) Mitogen-activated phosphate kinase (MPK3) is uniquely upregulated in T200 upon SACMV infection at 32 dpi (and at 67 dpi). Phosphorylation of MPK3 leads to upregulation of MeWRKY27/68 (AtWRKY33 homolog), a negative regulator of ABA and oxidative stress, which may contribute to high stress levels in T200 reflected by severe symptoms and high SACMV replication. While a number of ABA genes are upregulated in T200, there are no significantly enriched ABA biological processes observed in T200. This could also be a result of antagonism between SA, JA and ABA. MeWRKY27/68 positively regulates an increase in ACC which is involved in ethylene biosynthesis. Ethylene while having many biological roles is widely hallmarked as the stress hormone. Ethylene responsive factors (ERF5 and 6) are upregulated; however, ERF5 does not appear to play a direct role in defense to SACMV in T200. ERF5 and 6 may play a general role in regulation of biotic stress.

Proposed models of WRKY functions, hormones and downstream signaling of responses to SACMV in T200 and TME3 at 32 dpi. (A) TME3 exhibits milder symptoms compared to T200 in response to SACMV. This is associated with a number of upregulated hormone genes, JA, ABA, SA and ET, as a general response to infection. While a number of SA genes are upregulated, they are not significantly represented in the enriched GO (biological process) categories. In contrast to mock-inoculated plants, upregulation of MeWRKY3, 11 and 18 expression is observed. These transcription factors positively regulate hormone biosynthesis and signaling and biotic and oxidative stress functions. Differential expression of MeWRKY11 is specifically associated with an ABA response, a hallmark of abiotic and biotic stress. These responses to WRKY expression contribute at a later stage to a lower viral load and symptom recovery observed at 67 dpi and tolerance to SACMV. (B) T200 exhibits severe symptoms in response to SACMV. This is associated with a number of upregulated hormones JA, ABA, SA and ET genes as a general response to infection. However, while a number of ABA genes are upregulated, they are not significantly represented in the enriched GO (biological process) categories. In contrast to mock-inoculated plants, upregulated expression of MeWRKY3, 5/55 and 70 is observed. AtWRKY12 is a known positive regulator of plant defense, and downregulation of expression of its MeWRKY70 homolog contributes to severe leaf curl/chlorosis disease symptoms in T200. AtWRKY41 has been shown to regulate hormone signaling and biotic stress responses, and in cassava MeWRKY3, a homolog of AtWRKY41 plays a role in regulation of hormone signaling and biotic stress responses to SACMV. Arabidopsis NPR1 gene controls the onset of systemic acquired resistance (SAR)/immunity to a broad spectrum of pathogens, and AtWRKY53 is a known target of nonexpressor of pathogenesis-related gene (NPR1) during SAR in Arabidopsis. However, in the absence of SAR or the hypersensitive response (HR) in cassava, it is likely that SA-mediated responses of MeWRKY55, uniquely upregulated in T200, are associated with oxidative and other biotic stresses through cross-talk with JA and ET. The AtWRKY53 homolog of MeWRKY55 is a positive regulator of senescence suggesting that MeWRKY55 may play a role in other disease-associated metabolic perturbations associated with severe symptoms. (C) Mitogen-activated phosphate kinase (MPK3) is uniquely upregulated in T200 upon SACMV infection at 32 dpi (and at 67 dpi). Phosphorylation of MPK3 leads to upregulation of MeWRKY27/68 (AtWRKY33 homolog), a negative regulator of ABA and oxidative stress, which may contribute to high stress levels in T200 reflected by severe symptoms and high SACMV replication. While a number of ABA genes are upregulated in T200, there are no significantly enriched ABA biological processes observed in T200. This could also be a result of antagonism between SA, JA and ABA. MeWRKY27/68 positively regulates an increase in ACC which is involved in ethylene biosynthesis. Ethylene while having many biological roles is widely hallmarked as the stress hormone. Ethylene responsive factors (ERF5 and 6) are upregulated; however, ERF5 does not appear to play a direct role in defense to SACMV in T200. ERF5 and 6 may play a general role in regulation of biotic stress.