In banana, infective endogenous banana streak virus (eBSV) sequences are present in all M. balbisiana progenitors (B genome). There are at least three infective eBSV species that can express in either naturally occurring interspecific hybrid of AB, AAB or AAAB genotype or resulting from crosses involving parents carrying the A genome from M. acuminata and the B genome from M. balbisiana. The research conducted by the team aims at understanding the molecular structure and regulation of eBSVs in order to reintroduce M. balbisiana genetic resources in new interspecific hybrid of dessert or cooking banana.

Characterization of eBSVs diversity

Access to the complete molecular structure of an eBSV allows us to evaluate in silico the potential infective risk of the studied integration. In collaboration with the UMR PHIM, we have developed molecular markers that allow the genotyping of the different eBSV alleles and species previously identified. These markers were used to genotype several banana collections (CIRAD, Embrapa in Brazil, International Transit Center in Belgium).

More recently, the team has conducted collection missions in mainland Southeast Asia (North Vietnam, North Laos and South China) with local partners to characterize Musa and eBSV diversity in wild or lightly anthropized banana plants and to search for M. balbisiana resources free of infective eBSV. No sample containing M. balbisiana genome among the 154 collected is totally free of eBSV but for some of them, the structures of the eBSVs revealed are different from those previously characterized. Their infective nature remains to be defined.

The markers used so far are insufficient to have a complete view of eBSV since they only cover certain regions. The team is currently developing, in collaboration with the CNRGV in Toulouse, a locus capture approach based on CRISPR/Cas9 technology associated with long-read sequencing to provide access to the complete sequence of eBSV. This will not only allow the development of new genotyping markers specific to atypical eBSVs that can be used in breeding, but also to define their infective risk in silico.

Regulation of eBSVs

Although carriers of infective eBSVs, some banana plants, such as seedy diploids M. balbisiana (BB), never exhibit systemic plant infection even when plants are subjected to stresses known to activate viral integrants.

Illumina sequencing revealed that eBSVs produce low-abundance transcripts covering most of the viral sequence and generate mainly small interfering RNAs (siRNAs) of 24 nt. Remarkably, siRNA production is limited to the duplicated and inverted viral sequences present in eBSVs. Using bisulfite sequencing, we found that both siRNA-producing and non-producing sequences of eBSVs are highly methylated. These data suggest that eBSVs are controlled at the epigenetic level in the seedy diploids M. balbisiana.

Genomic characterization of dwarfism in banana

Key character for bananas cultivation in tropical or sub-tropical areas subject to storms and hurricanes, dwarfism confers a more robust stature and less sensitivity to wind. In addition, the manual tasks of maintenance and harvesting of the bunches are easier in the dwarf or semi-dwarf varieties. The latter are also better adapted to high density plantations. The understanding of the dwarfism mechanisms impacting the size and the robustness of the pseudo-trunk is thus essential and constitutes a key step for banana plant improvement.

While rare dwarf cultivars occur naturally within the major triploid banana groups, dwarfism has never been observed in diploid ancestral cultivars and wild relatives. Dwarf banana plants are commonly obtained by somaclonal variation after in vitro culture in the Cavendish group and to a lesser extent in other triploid banana groups. This variation may be reversible to normal size. Physiological and biochemical studies of isogenic pairs of dwarf/normal banana plants show that dwarfs are minimally responsive to external gibberellic acid supply and have lower gibberellic acid content than their normal-sized counterparts. The involvement of gibberellins was confirmed by proteomic analyses, but dense RadSeq genotyping failed to identify any dwarf-specific region(s) of the genome. The determinism of dwarfism could therefore be associated with epigenetic regulation mechanisms on which the team plans to work.

Key publications

Chabannes M., Gabriel M., Aksa A., Galzi S., Dufayard J.F., Iskra Caruana M.L., Muller E. 2021. Badnaviruses and banana genomes: A long association sheds light on Musa phylogeny and origin. Molecular Plant Pathology,
22 (2) : p. 216-230. https://doi.org/10.1111/mpp.13019

Iskra Caruana M.L., Thomas J.E., Chabannes M. 2019. Banana streak. In : Jones David R. (ed.). Handbook of diseases of banana, Abacá and Enset. Wallingford : CABI, p. 393-408.

Lava Kumar P., Selvarajan R., Iskra Caruana M.L., Chabannes M., Hanna R.. 2015. Biology, etiology, and control of virus diseases of banana and plantain. In : by Gad Loebenstein and Nikolaos I. Katis. Control of Plant Virus Diseases Vegetatively-Propagated Crops. New-York : Academic Press, p. 229-269. (Advances in Virus Research.
https://doi.org/10.1016/bs.aivir.2014.10.00

Chabannes M., Iskra Caruana M.L.. 2013. Endogenous pararetroviruses - a reservoir of virus infection in plants. Current Opinion in Virology, 3 (6) : p. 615-620.https://doi.org/10.1016/j.coviro.2013.08.012

Chabannes M., Baurens F.C., Duroy P.O., Bocs S., Vernerey M.S., Rodier-Goud M., Barbe V., Gayral P., Iskra Caruana M.L.. 2013. Three infectious viral species lying in wait in the banana genome. Journal of Virology, 87 (15) : p. 8624-8637.https://doi.org/10.1128/JVI.00899-13