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Guanosine pentaphosphate and tetraphosphate (together referred to as ppGpp) are hyperphosphorylated nucleotides found in bacteria and the chloroplasts of plants and algae. In plants and algae artificial ppGpp accumulation can inhibit chloroplast gene expression, and influence photosynthesis, nutrient remobilization, growth, and immunity. However, it is so far unknown whether ppGpp is required for abiotic stress acclimation in plants. Here, we demonstrate that ppGpp biosynthesis is necessary for acclimation to nitrogen starvation in Arabidopsis. We show that ppGpp is required for remodeling the photosynthetic electron transport chain to downregulate photosynthetic activity and for protection against oxidative stress. Furthermore, we demonstrate that ppGpp is required for coupling chloroplastic and nuclear gene expression during nitrogen starvation. Altogether, our work indicates that ppGpp is a pivotal regulator of chloroplast activity for stress acclimation in plants.

Sunflower broomrape (Orobanche cumana Wallr.) was detected in 2010 for the first time in sunflower fields of Tunisia, in the Béja region. No information is available on the race composition and genetic diversity of the broomrape populations parasitizing sunflower in that area. Plant tissue and seeds were collected from nine populations in Béja Sud (ORD, ORE), Amdoun (ORF, ORG), and Béja Nord (ORH to ORL) areas of the Béja region. Virulence studies on populations ORD, ORH and ORK revealed that the ORD population was essentially race E, whereas race G individuals were present in ORH and ORK poulations, with greater frequency in the ORK population. Cluster analysis of inter-population relatedness indicated that Tunisian populations were more related to Eastern European than to Spanish populations, with the Béja Nord populations clustering separately from the rest of Tunisian populations. Analysis at the intra-population level of the ORD, ORG, ORH and ORK populations revealed the existence of two gene pools (GP1 and GP2), that were present in all populations at different frequencies. GP2 was at a lower frequency (1/14) in ORD and ORG but at higher frequencies in ORH (5/15) and ORK (8/14). This indicates that GP1 was probably an initial introduction of a race E population, while GP2 is most likely a later introduction of a race G population, introduced initially in Béja Nord area but now spreading to other areas in the region. The risk of a generalized expansion of race G of broomrape in this sunflower cultivation area is discussed.

Combining landscape ecology and genetics provides an excellent framework to appreciate pest population dynamics and dispersal. The genetic architectures of many species are always shaped by environmental constraints. Because little is known about the ecological and genetic traits of Tunisian whitefly populations, the main objective of this work is to highlight patterns of biodiversity, genetic structure and migration routes of this pest. We used nuclear microsatellite loci to analyze B. tabaci populations collected from various agricultural areas across the country and we determine their biotype status. Molecular data were subsequently interpreted in an ecological context supplied from a species distribution model to infer habitat suitability and hereafter the potential connection paths between sampling localities. An analysis of landscape resistance to B. tabaci genetic flow was thus applied to take into account habitat suitability, genetic relatedness and functional connectivity of habitats within a varied landscape matrix. We shed light on the occurrence of three geographically delineated genetic groups with high levels of genetic differentiation within each of them. Potential migration corridors of this pest were then established providing significant advances toward the understanding of genetic features and the dynamic dispersal of this pest. This study supports the hypothesis of a long-distance dispersal of B. tabaci followed by infrequent long-term isolations. The Inference of population sources and colonization routes is critical for the design and implementation of accurate management strategies against this pest.

We report the nucleotide sequences of three citrus viroids belonging to three different genera: Citrus exocortis viroid (CEVd), Hop stunt viroid (HSVd) and Citrus viroid-III (CVd-III) isolated from a single natural infected Citrus reticulata var. Clementine tree growing in a tree nursery in Manouba (near Tunis Capital). We describe the sequence variability of these viroids from their natural host without using an alternative passage by an indicator host or an artificial inoculation. This work confirms that naturally occurring viroid infections contain a mixture of sequence variants. These are the first sequences of citrus viroids from Africa.

We report the nucleotide sequence of Citrus exocortis viroid (CEVd) from a single natural infected citrus tree of the clementinier variety (Citrus clementina) in Tunisia. The sequence variability of this viroid from its natural host without using an alternative passage by an indicator host or an artificial inoculation was found to occur at 14 positions, giving a variability of 3.8%. This work confirms that naturally - occurring viroids belonging to the Pospiviroid genera contain a mixture of sequence variants. In addition, this study confirms the worldwide distribution of this viroid.

Tunisian pomegranate Hop stunt viroid (HSVd) variants are described. Dot-blot hybridization, S-Page, and reverse transcription polymerase chain reaction (RT-PCR) of RNA extracts from infected tissues were carried out. Results obtained by these techniques were confirmed by cDNA sequencing. The genetic diversity among the Tunisian variants was investigated, which also involved analysis of sequences of previously described HSVd variants from Tunisian citrus var. clementine and fig, and from fruit trees from other Mediterranean countries. Phylogenetic analysis showed that Tunisian pomegranate HSVd variants were clustered into two groups: a cachexia strain within the citrus type group and a recombinant citrus-plum type group. Results also showed a high haplotype diversity which was not related either to the host or to the geographical origin. Selective neutrality and genetic network tests suggest that the HSVd isolates have spread rapidly.