Ins and outs of viral infection in plants

Poleroviruses are characterized by their obligate transmission by aphids and their phloem tropism. Recently we discovered a new protein P3a encoded by a cryptic gene conserved among poleroviruses. This small protein expressed from a non-conventional initiation codon is essential for viral long distance movement in the plant. Our objective is to decipher the mode of action of this small protein, in particular its interactions with the other viral proteins involved in viral movement.

The RNA silencing suppressor P0 functions by inducing the degradation of the protein ARGONAUTE 1, the key effector of this antiviral defence pathway. This leads to a number of gene deregulations potentially involved in the establishment of infection. Such deregulations could also play a role in virus transmission by the aphid vector. This hypothesis is investigated in collaboration with the team of Dr. Véronique Brault at INRA of Colmar (ANR VIRAPHIPLANT). In parallel we further investigate the role of P0 by seeking new cellular partners and by site-directed mutagenesis.

BNYVV (Beet necrotic yellow vein virus) is the etiological agent of Rhizomania, a viral disease that induces massive root proliferation in sugar beet. Depending on the isolates, the viral genome consists of four to five single stranded RNAs of positive polarity. RNA-3 undergoes cleavage leading to the accumulation of a non-coding RNA (ncRNA) stabilized by the “coremin” sequence present on RNA-3. Both ncRNA and BNYVV silencing suppressor p14 protein are essential to viral systemic spread in the plant. Current studies aim at understanding the complex interplay between long-distance movement and RNAi.

The four to five genomic BNYVV RNA are packaged in separate rod shape particles. We are exploring the mechanism whereby all viral RNA can reach the same cell to initiate a new replication cycle after long distance movement in the vasculature. Preliminary data obtained in collaboration with Dr. Claudio Ratti (Bologna, Italy) suggest the existence of molecular interactions between viral RNAs. We are currently characterizing these interactions.

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Co-infections of sugar beet by both BNYVV and Beet mild yellowing virus (BMYV, Polerovirus) are occurring in the field in natural conditions and the plants develop more severe symptoms compared to single infections. In collaboration with the breeding company SESVanderHave we are investigating the potential synergism between the two viruses and the consequences for sugar beet production.

The nuclear step of CaMV cycle, including nuclear export of virus RNAs, is largely unknown. However, it is pivotal for the virus replication. CaMV double stranded DNA genome is transcribed in monocistronic 19S RNA and polycistronic pregenomic 35S RNA part of which undergoes complex alternative splicing. We are studying the mechanisms involved in the nuclear export of these intron-less and intron-retaining mRNAs. We are aiming at identification of 35S pregenomic RNA cis-elements and viral and cellular trans-acting factors involved in nuclear export to unravel the pathway(s) of spliced and unspliced mRNA export in plants.