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Edible banana arose from combinations of species Musa acuminata, whose genome is denoted A, and M. balbisiana, whose genome is denoted B. The nuclear genome of all known diploid BB genitors harbors infectious alleles of endogenous Banana streak viruses (eBSV). Their activation by abiotic stresses leads to spontaneous infections by cognate viruses in diploid AB, triploid AAB and tetraploid AAAB interspecific hybrids. Banana streak viruses are naturally transmitted by several species of mealybug. A comprehensive risk assessment study of the activation of infectious eBSV alleles was undertaken in Guadeloupe. Firstly, a large scale prevalence survey was carried out for viral species BSOLV, BSGFV and BSIMV among the main cultivated banana types. Overall prevalence levels were low for BSOLV (0.85%) and BSGFV (0.99%). No sample was found infected by BSIMV. No impact of BSV in infected plants could be observed, with only one infected sample displaying typical BSV leaf streak symptoms. The comparison of prevalence levels monitored in AAB genotypes and in AAA genotypes suggests that BSOLV and BSGFV infections in AAB genotypes result preferentially from the activation of eBSV infectious alleles rather than from vector-borne transmission. Statistical analyses showed a positive correlation between prevalence levels and altitude. Secondly, the activation of eBSOLV and eBSGFV infectious alleles was monitored under field conditions over two production cycles in triploid plantains French Clair (AAB genotype) and Pelipita (ABB genotype), using a random block assay and plants originating from either cell culture or horticultural multiplication. Activation rates were significantly higher for infectious allele GF7 than for infectious allele OL1 and also higher in French Clair than in Pelipita. Results pointed to an influence of the mode of production of planting material on activation, and to an absence of impact of BSGFV and BSOLV infection on production.

Several species of Banana streak virus (BSV; family Caulimoviridae, genus Badnavirus) occur in banana (Musa spp). They cause a wide range of symptoms on the leaves, pseudostem and fruits of infected plants, although the impact of infections on yield and fruit quality has never been properly assessed. BSVs are naturally transmitted by mealybugs. However, infections can also occur spontaneously in interspecific varieties following stress-induced activation of infectious endogenous BSV sequences (eBSVs) integrated in B (Musa balbisiana) genomes. A 484 plants experimental plot using a random block design was set up in Guadeloupe in order to study the kinetics and impact of BSV infections resulting from the activation of infectious eBSVs in two triploid interspecific plantain varieties : French Clair (AAB genotype), and Pelipita (ABB genotype). Both varieties harbor similar infectious endogenous sequences of Banana streak OL virus (BSOLV) and Banana streak GF virus (BSGFV). Planting material was multiplied either by cell culture (vitroplants) or by a corm splitting technique (PIF). Activation of infectious eBSOLV and eBSGFV sequences was monitored every three months by immunocapture-PCR indexing [1, 2] of all the plants of the plot. Data gathered over a 15 months period showed that activation of infectious eBSGFVs occured in both varieties, although activation rates were significantly higher in French Clair. Activation of infectious eBSOLVs occurred only in French Clair, at a much lower rate. These results suggest that infectious eBSGFVs and eBSOLVs are more prone to activation by abiotic stresses under field conditions in French Clair than in Pelipita, pointing to a role of plant genetic background in the activation process. They also suggest that in French Clair, infectious eBSGFVs are more prone to activation than infectious eBSOLVs, confirming previous observations made in cell culture and pointing to differential activation potentials among infectious eBSVs [3]. Our data also showed that activation of eBSGFVs in French Clair was higher in plants originating from PIF than in plants originating from vitroplants. No significant impact of BSOLV and BSGFV infection on plant growth and fruit production was observed, and none of the infected plant did express BSV symptoms.

Several species of Banana streak virus (BSV) occur in banana. They cause a wide range of symptoms on leaves, pseudostem and fruits, although the impact of infections on yield and fruit quality has never been properly assessed. BSVs are naturally transmitted by mealybugs. However, spontaneous infections occur in interspecific varieties such as plantains, following stress-induced activation of infectious endogenous BSV sequences (eBSVs) integrated in B (Musa balbisiana) genomes. The kinetics of activation of infectious eBSVs was monitored in Guadeloupe in an experimental plot, using a random block design. It showed that infectious eBSVs display differential activation potentials in plantain varieties French Clair and Pelipita, pointing to a role of plant genetic background in the activation process. It also showed that the multiplication mode of planting material influences activation levels monitored under field conditions and that infection had no significant impact on plant growth and fruit production of both varieties. A wide range prevalence study of BSVs undertaken throughout Guadeloupe's plantations, Creole gardens, abandoned fields and wild areas among varieites representative of the main dessert banana and plantain types grown in Guadeloupe showed that overall BSV prevalence were low in dessert banana and cooking banana. Compared with a similar survey carried out in 2006, prevalence was very similar for dessert banana but significantly lower for plantains, which carry eBSVs that interfere with molecular diagnostic and cause frequent false positives. It is likely that the recent optimization of BSV molecular diagnostic increased the accuracy of detection. Overall, these results suggest that BSVs have a low prevalence and unmeasurable impact on dessert banana and plantain in Guadeloupe, owing to low vector-borne transmission and low activation of infectious eBSVs. These results also lead to recommendation regarding the management of BSVs through safe multiplication modes of plantain planting material.

A prevalence study of Banana streak virus species BSOLV, BSGFV and BSIMV was carried out in Guadeloupe in 2014-2015. A total of 1500 leaf samples were collected throughout Guadeloupe, in plantations, Creole gardens, abandoned fields and wild areas. Samples were representative of the main banana types grown in Guadeloupe, including dessert banana (AA, AAA and AAB genotypes) and plantain (AAB and ABB genotypes). Each leaf sample was indexed separately for BSOLV, BSGFV and BSIMV by immunocapture PCR [1, 2]. A total of 15 samples were indexed positive for BSOLV and 13 for BSGFV, with no mixed infection registered. No sample was indexed positive for BSIMV, confirming previous results suggesting that this BSV species may not be present in Guadeloupe [3]. Overall BSV prevalence levels were low, with only 1.05% (6/570) of the analyzed AAA dessert banana, 2.57% (19/737) of the AAB types (plantain and Fig Pomme) and 1.87% (3/160) of the ABB plantain types infected. Compared with a similar survey carried out in 2006 on a more reduced number of samples [3], prevalence levels were very similar for dessert banana but significantly lower for AAB and ABB genotypes, which carry infectious endogenous BSV (eBSV) sequences that interfere with PCR-based diagnostic and cause frequent false positives. It is likely that the recent optimization of IC-PCR based diagnostic of BSVs [2] resulted in a more accurate detection of episomal BSVs. Plantain and Fig pomme types with an AAB genome accounted for 66.7% (10/15) and 69.3% (9/13) of the samples infected by BSOLV and BSGFV, respectively, whereas dessert bananas with an AAA genome accounted for 20% (3/15) and 23% (3/13) of the samples infected by BSOLV and BSGFV, respectively and plantain types with an ABB genome accounted for 13.3% (2/15) and 7.7% (1/13) of the samples infected by BSOLV and BSGFV, respectively. Overall, these results show that BSVs have a low prevalence in Guadeloupe in both dessert banana and plantain. Furthermore, none of the infected plants showed symptom, suggesting that BSV infections have a very limited (if any) impact on production in Guadeloupe. These results also provide insights into the epidemiology of BSVs in Guadeloupe, which will be discussed.

Infectious alleles of endogenous Banana streak viruses (eBSVs) are present in the genome of Musa balbisiana spp. Their activation by biotic and abiotic stresses lead to spontaneous infections by several species of Banana streak virus (BSV) in triploid (AAB) and tetraploid (AAAB) interspecific hybrids, making them the main constraint for breeding new banana interspecific hybrids and for exchanging germplasm. However the risk of spreading BSVs associated to the cultivation of interspecific hybrids has never been assessed, although such hybrids are grown over important areas throughout the Caribbean, Latin America and Africa. In order to tackle this issue, a study of the kinetics of activation of infectious alleles OL 1 and GF7, whose expression lead to spontaneous infections by BSOLV and BSGFV, respectively, was carried out in natural plantains French Clair, Pelipita and Macho x Hembra and hybrids FHIA-18 and FHIA-21 under field conditions in the Dominican Republic, Cuba and Guadeloupe. Experimental plots using similar randomized block designs and planting material originating from suckers, vitroplants or stem fragments were set up. Plants were monitored visually for symptoms and checked every 3 months by multiplex immunocapture PCR indexing for the presence of BSOLV and BSGFV. Results collected over 24 months showed that infectious alleles OL 1 and GF7 display differential expression levels, pointing to the existence of additional (plant) factors involved in their activation. They also showed that the mode of multiplication of the planting material influences expression levels. Preliminary results also suggested that infection by BSOLV and BSGFV does not have a major effect on fruit production, although additional data are needed to reach definite conclusions in this regard. Overall, this work contributes significantly to the development and implementation of appropriate strategies for evaluating and recommendations for mitigating the risks of spreading BSVs associated with the cultivation of banana interspecific hybrids.

International audience; Lateral root (LR) development is a major component of the efficiency of a plant to capture soil resources. A remarkable facet of lateral root behavior is stochasticity affecting the different stages of LR growth, leading to a large range of lateral root lengths along the primary root. It has been argued that such variability among LR could be an adaptive trait enabling a greater plasticity and efficiency of the root system in front of the spatial and temporal heterogeneity of soil resources (Forde, 2009). In order to characterize variability in LR growth and evaluate the role of hormonal and biochemical signals that might influence it, we analyzed the patterns of development of hundreds of lateral roots in a set of maize seedlings grown in rhizotrons. The SmartRoot toolbox was used to provide a comprehensive recording of individual root growth dynamics. Using a clustering method based on growth variables such as elongation rate and growth duration, we could identify 3 main different elongation patterns in maize LRs composed of accelerating, slowly decelerating and rapidly arrested LRs, as found in other species (Pagès, 1995). The molecular and cellular characterization of these root types revealed differences in meristem size, apical diameters as well as apical diameter variation. Moreover, fast growing roots showed increased sugar content along their tips. Finally, by affecting either the sugar or the auxin status (by pruning seminal roots or using auxin transport mutants) we differentially altered the proportion of the different root types, suggesting that both signals contribute to shape the root system in response to variations of environmental conditions.FORDE, B. (2009), J. Exp. Bot. 60 (14): 3989-4002PAGES, L. (1995), New Phytologist, 130: 503–509

This study focuses on the diversity of circular single-stranded DNA (ssDNA) viruses associated with Poaceae growing in the fynbos ecosystem and its neighbouring agroecosystems in the Cape floristic region of Southern Africa. We have developed a sequenceindependent approach that combined two methods, including a first step of rolling circle amplification (RCA) followed by a second step of whole genome amplification (WGA). We have first tested and validated this approach using controls from our laboratory, including 4 Geminiviridae (sugarcane/Sugarcane streak Egypt virus; Datura stramonium/Tomato leaf curl virus, Datura stramonium/Tomato yellow leaf curl virus and Euphorbia caputmedusae/ Euphorbia caput-medusae stunt virus) and 2 double-stranded DNA Caulimoviridae (banana/Banana streak Obino l'Ewai virus and Arabidopsis thaliana/Cauliflower mosaic virus). The RCA/WGA approach was successful for unambiguously detecting the 4 Geminiviridae tested but failed to detect both Caulimoviridae. In addition to the 4 expected Geminiviridae, 2 Mastrevirus (located within the sugarcane plant), two Circovirus (banana and turnip) and three mycoviruses (banana, turnip and Euphorbia caput-medusae) were detected by performing Blast searches. This approach was then used for detecting the presence of circular ssDNA viruses within 23 South African Poaceae. Again, several ssDNA viruses were detected, including Mastrevirus, Begomovirus, Circovirus, mycoviruses and unknown ssDNA viruses. This study confirms the presence of a wealth of ssDNA viral sequences within plants that blur the boundaries between previously well-defined groups and stresses the need to better understand the evolutionary history of circular ssDNA viruses and to revisit current taxonomic classification schemes (Rosario et al., 2012). (Texte intégral)