Blueberry necrotic ring blotch virus in Southern Highbush Blueberry: Insights into In Planta and In-Field Movement.

Blueberry necrotic ring blotch virus (BNRBV) causes an emerging disease of southern highbush blueberry (SHB) in the southeastern United States. Disease incidence and severity vary considerably from year to year within the same planting. Experiments were conducted to determine how the virus spreads in the field. Leaf tissue from symptomatic field plants tested positive for BNRBV in 2011, whereas the same plants were asymptomatic in 2012 and tested negative for the virus. Symptomatic and asymptomatic leaves from individual shoots were tested for the presence of the virus, and symptomatic leaves tested positive (100%), whereas 65.4% of the asymptomatic leaves from the same shoots tested negative. Leaves were selected in which half the leaf blade was symptomatic and the other half was not; symptomatic leaf halves tested positive (100%), whereas 76.0% of the asymptomatic halves from the same leaf tested negative for the virus. When virus-free, potted trap plants were interspersed in the field among established plants that had shown disease symptoms the previous year, disease onset in trap plants was observed 2 to 3 weeks after disease onset in field plants. In a separate experiment, asymptomatic softwood cuttings were collected from mother plants symptomatic for BNRBV, rooted, and monitored for symptom development for a period of 12 to 27 months. No BNRBV symptoms were observed in the progeny, whereas disease incidence was high for cuttings taken at the same time from plants infected with Blueberry red ringspot virus used as a control. Collectively, these studies suggest that BNRBV does not infect SHB plants systemically and is not transmitted through vegetative propagation, and that the virus likely does not persist in plants after natural defoliation in the fall.

Toward understanding the molecular mechanism of a geminivirus C4 protein.

Geminiviruses are ssDNA plant viruses that cause significant agricultural losses worldwide. The viruses do not encode a polymerase protein and must reprogram differentiated host cells to re-enter the S-phase of the cell cycle for the virus to gain access to the host-replication machinery for propagation. To date, 3 Beet curly top virus (BCTV) encoded proteins have been shown to restore DNA replication competency: the replication-initiator protein (Rep), the C2 protein, and the C4 protein. Ectopic expression of the BCTV C4 protein leads to a severe developmental phenotype characterized by extensive hyperplasia. We recently demonstrated that C4 interacts with 7 of the 10 members of the Arabidopsis thaliana SHAGGY-like protein kinase gene family and characterized the interactions of C4 and C4 mutants with AtSKs. Herein, we propose a model of how C4 functions.

The roles of phosphorylation and SHAGGY-like protein kinases in geminivirus C4 protein induced hyperplasia.

Even though plant cells are highly plastic, plants only develop hyperplasia under very specific abiotic and biotic stresses, such as when exposed to pathogens like Beet curly top virus (BCTV). The C4 protein of BCTV is sufficient to induce hyperplasia and alter Arabidopsis development. It was previously shown that C4 interacts with two Arabidopsis Shaggy-like protein kinases, AtSK21 and 23, which are negative regulators of brassinosteroid (BR) hormone signaling. Here we show that the C4 protein interacts with five additional AtSK family members. Bikinin, a competitive inhibitor of the seven AtSK family members that interact with C4, induced hyperplasia similar to that induced by the C4 protein. The Ser49 residue of C4 was found to be critical for C4 function, since: 1) mutagenesis of Ser49 to Ala abolished the C4-induced phenotype, abolished C4/AtSK interactions, and resulted in a mutant protein that failed to induce changes in the BR signaling pathway; 2) Ser49 is phosphorylated in planta; and 3) plant-encoded AtSKs must be catalytically active to interact with C4. A C4 N-myristoylation site mutant that does not localize to the plasma membrane and does not induce a phenotype, retained the ability to bind AtSKs. Taken together, these results suggest that plasma membrane associated C4 interacts with and co-opts multiple AtSKs to promote its own phosphorylation and activation to subsequently compromise cell cycle control.