The early transcriptome response of cassava (Manihot esculenta Crantz) to mealybug (Phenacoccus manihoti) feeding.

Molemi E Rauwane,Chrissie Rey,Ian Millar,Damaris A Odeny,Jasper Rees,Annette Reineke

doi:10.1371/journal.pone.0202541

Abstract

The mealybug, Phenacoccus manihoti, is a leading pest of cassava (Manihot esculenta Crantz), damaging this crop globally. Although the biological control of this mealybug using natural predators has been established, resistance breeding remains an important means of control. Understanding plant responses to insect herbivory, by determining and identifying differentially expressed genes (DEGs), is a vital step towards the understanding of molecular mechanisms of defence responses in plants and the development of resistant cultivars by gene editing. Morphological and molecular analysis confirmed the mealybug identity as Phenacoccus manihoti (Matile-Ferrero). The transcriptome response of the green mite resistant cassava genotype AR23.1 was compared to P40/1 with no known resistance at 24 and 72 hours of mealybug infestation compared to non-infested mock. A total of 301 and 206 genes were differentially expressed at 24 and 72 of mealybug infestation for AR23.1 and P40/1 genotypes respectively, using a log2 fold change and P-value ≤ 0.05. Gene ontology functional classification revealed an enrichment of genes in the secondary metabolic process category in AR23.1 in comparison with P40/1, while genes in the regulation of molecular function, cellular component biogenesis and electron carrier categories were more significantly enriched in P40/1 than in AR23.1. Biological pathway analysis, based on KEGG, revealed a significant enrichment of plant-pathogen interaction and plant hormonal signal transduction pathways for a cohort of up-regulated and down-regulated DEGs in both genotypes. Defence-related genes such as 2-oxogluterate, gibberellin oxidase and terpene synthase proteins were only induced in genotype AR23.1 and not in P40/1, and subsequently validated by RT-qPCR. The study revealed a difference in response to mealybug infestation in the two genotypes studied, with AR23.1 showing a higher number of differentially expressed transcripts post mealybug infestation at 24 and 72 hours. Candidate defence-related genes that were overexpressed in the AR23.1 genotype post mealybug infestation will be useful in future functional studies towards the control of mealybugs.

Highlights

Cassava, tapioca or manioc (Manihot esculenta Crantz) is a woody perennial plant that feeds over 500 million people living throughout the tropics [1,2,3,4]
Both the 18S and 28S ribosomal RNA (rRNA) generated sequences of the nine individual mealybug samples clustered with known Phenacoccus manihoti sequences (JQ651008.1, JQ651255.1 and JQ651257.1) that had been retrieved from the Genbank with 100% (Fig 1) and 99% (Fig 2) similarity respectively
UDP- glucosyl transferases have been reported to be responsible for the last step in the biosynthesis of linamarin and lotaustralin, which are the major cyanogenic glucosides in cassava [55]

Summary

Introduction

Tapioca or manioc (Manihot esculenta Crantz) is a woody perennial plant that feeds over 500 million people living throughout the tropics [1,2,3,4]. It is mainly cultivated for its large starchy roots but the leaves are consumed as vegetable, especially in Africa. Native to South America, cassava is a widely cultivated tuber crop in sub-Saharan Africa and an important food staple in terms of per capita food energy consumed [5, 6]. Cassava is grown by the resource-poor farmers in the seasonally humid regions of South Africa as a supplementary crop to maize (Zea mays)

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Results

Conclusion