Cassava begomovirus species diversity changes during plant vegetative cycles.

Cassava is a root crop important for global food security and the third biggest source of calories on the African continent. Cassava production is threatened by Cassava mosaic disease (CMD), which is caused by a complex of single-stranded DNA viruses (family: Geminiviridae, genus: Begomovirus) that are transmitted by the sweet potato whitefly (Bemisia tabaci). Understanding the dynamics of different cassava mosaic begomovirus (CMB) species through time is important for contextualizing disease trends. Cassava plants with CMD symptoms were sampled in Lake Victoria and coastal regions of Kenya before transfer to a greenhouse setting and regular propagation. The field-collected and greenhouse samples were sequenced using Illumina short-read sequencing and analyzed on the Galaxy platform. In the field-collected samples, African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), East African cassava mosaic Kenya virus (EACMKV), and East African cassava mosaic virus-Uganda variant (EACMV-Ug) were detected in samples from the Lake Victoria region, while EACMV and East African mosaic Zanzibar virus (EACMZV) were found in the coastal region. Many of the field-collected samples had mixed infections of EACMV and another begomovirus. After 3 years of regrowth in the greenhouse, only EACMV-like viruses were detected in all samples. The results suggest that in these samples, EACMV becomes the dominant virus through vegetative propagation in a greenhouse. This differed from whitefly transmission results. Cassava plants were inoculated with ACMV and another EACMV-like virus, East African cassava mosaic Cameroon virus (EACMCV). Only ACMV was transmitted by whiteflies from these plants to recipient plants, as indicated by sequencing reads and copy number data. These results suggest that whitefly transmission and vegetative transmission lead to different outcomes for ACMV and EACMV-like viruses.

Geminivirus Resistance: A Minireview.

A continuing challenge to crop production worldwide is the spectrum of diseases caused by geminiviruses, a large family of small circular single-stranded DNA viruses. These viruses are quite diverse, some containing mono- or bi-partite genomes, and infecting a multitude of monocot and dicot plants. There are currently many efforts directed at controlling these diseases. While some of the methods include controlling the insect vector using pesticides or genetic insect resistance (Rodríguez-López et al., 2011), this review will focus on the generation of plants that are resistant to geminiviruses themselves. Genetic resistance was traditionally found by surveying the wild relatives of modern crops for resistance loci; this method is still widely used and successful. However, the quick rate of virus evolution demands a rapid turnover of resistance genes. With better information about virus-host interactions, scientists are now able to target early stages of geminivirus infection in the host, preventing symptom development and viral DNA accumulation.

Infectivity of Deinbollia mosaic virus, a novel weed-infecting begomovirus in East Africa.

Weed-infecting begomoviruses play an important role in the epidemiology of crop diseases because they can potentially infect crops and contribute to the genetic diversity of crop-infecting begomoviruses. Despite the important epidemiological role that weed-infecting begomoviruses play, they remain insufficiently studied in Africa. Recently, we identified Deinbollia mosaic virus (DMV), a distinct begomovirus found naturally infecting the weed host Deinbollia borbonica (Sapindaceae) in Kenya and Tanzania. In this study, we investigated the capacity of DMV to infect a restricted host range of Solanaceae and Euphorbiaceae species. Biolistic inoculation of Nicotiana benthamiana with concatemeric DNAs resulted in systemic infection associated with yellow mosaic symptoms, while DNA partial dimers caused asymptomatic systemic infection. DMV was not infectious to cassava (Manihot esculenta Crantz), suggesting host resistance to the virus. Here, we demonstrate the first experimental infectivity analysis of DMV in N. benthamiana and cassava.

The E2FD/DEL2 factor is a component of a regulatory network controlling cell proliferation and development in Arabidopsis.

An emerging view of plant cell cycle regulators, including the E2F transcription factors, implicates them in the integration of cell proliferation and development. Arabidopsis encodes six E2F proteins that can act as activators or repressors of E2F-responsive genes. E2FA, E2FB and E2FC interact with the retinoblastoma-like RBR protein and bind to DNA together with their DP partners. In contrast, E2FD, E2FE and E2FF (also known as DEL2, DEL1 and DEL3) are atypical E2Fs that possess duplicated DNA binding regions, lack trans-activating and RBR-binding domains and are believed to act as transcriptional inhibitors/repressors. E2FE/DEL1 has been shown to inhibit the endocycle and E2FF/DEL3 appears to control cell expansion but the role of E2FD/DEL2 has not been reported so far. In this study, we investigated the expression of E2FD/DEL2 and analysed the accumulation of its product. These studies revealed that E2FD/DEL2 accumulation is subject to negative post-translational regulation mediated by the plant hormone auxin. Moreover, the analysis of mutant and transgenic plants characterized by altered expression of E2FD/DEL2 has revealed that this atypical E2F can affect plant growth by promoting cell proliferation and repressing cell elongation. Overexpression of E2FD/DEL2 increased the expression of E2FA, E2FB and E2FE/DEL1 whereas its inactivation led to the up-regulation of genes encoding repressors of cell division. These results suggest that E2FD/DEL2 is part of a regulatory network that controls the balance between cell proliferation and development in Arabidopsis.

Global analysis of Arabidopsis gene expression uncovers a complex array of changes impacting pathogen response and cell cycle during geminivirus infection.

Geminiviruses are small DNA viruses that use plant replication machinery to amplify their genomes. Microarray analysis of the Arabidopsis (Arabidopsis thaliana) transcriptome in response to cabbage leaf curl virus (CaLCuV) infection uncovered 5,365 genes (false discovery rate <0.005) differentially expressed in infected rosette leaves at 12 d postinoculation. Data mining revealed that CaLCuV triggers a pathogen response via the salicylic acid pathway and induces expression of genes involved in programmed cell death, genotoxic stress, and DNA repair. CaLCuV also altered expression of cell cycle-associated genes, preferentially activating genes expressed during S and G2 and inhibiting genes active in G1 and M. A limited set of core cell cycle genes associated with cell cycle reentry, late G1, S, and early G2 had increased RNA levels, while core cell cycle genes linked to early G1 and late G2 had reduced transcripts. Fluorescence-activated cell sorting of nuclei from infected leaves revealed a depletion of the 4C population and an increase in 8C, 16C, and 32C nuclei. Infectivity studies of transgenic Arabidopsis showed that overexpression of CYCD3;1 or E2FB, both of which promote the mitotic cell cycle, strongly impaired CaLCuV infection. In contrast, overexpression of E2FA or E2FC, which can facilitate the endocycle, had no apparent effect. These results showed that geminiviruses and RNA viruses interface with the host pathogen response via a common mechanism, and that geminiviruses modulate plant cell cycle status by differentially impacting the CYCD/retinoblastoma-related protein/E2F regulatory network and facilitating progression into the endocycle.

DNA abrasion onto plants is an effective method for geminivirus infection and virus-induced gene silencing.

Geminiviruses belong to a rapidly growing group of plant pathogens that contribute to crop losses in tropical and subtropical areas of the world. Geminivirus infection is a model for plant DNA replication and virus/host interactions. Geminiviruses are also used as vectors to induce silencing of endogenous genes in several plant species. A method was analyzed for inoculating geminiviruses using plasmid DNA rubbed onto leaves in the presence of an abrasive (DNA abrasion). Although the use of DNA abrasion to inoculate geminiviruses has been described previously, the technique has fallen out of favor and has not been systematically optimized. However, consistent efficiencies of 100% infection rates can be achieved by DNA abrasion. The symptoms of Tomato Golden Mosaic Virus or Cabbage Leaf Curl Virus infection on Nicotiana benthamiana were similar in timing and appearance to the symptoms observed in plants inoculated using Agrobacterium as the delivery method. More importantly, silencing of an endogenous gene was highly efficient when a geminivirus silencing vector was inoculated by the DNA abrasion method. Other plant species successfully inoculated with geminiviruses by DNA abrasion were Nicotiana tabacum, Capsicum annuum and Nicandra physalodes. Unfortunately, Arabidopsis thaliana could not be infected with Cabbage Leaf Curl Virus using leaf abrasion, demonstrating limitation of the method. However, leaf abrasion to inoculate geminiviruses is an easy and inexpensive method that should be considered as an accessible technique to the growing number of researchers using geminiviruses.