SUMMARYIn indeterminate inflorescences, floral meristems develop on the flanks of the shoot apical meristem, at positions determined by auxin maxima. The floral identity of these meristems is conferred by a handful of genes called floral meristem identity genes, among which the LEAFY (LFY) transcription factor plays a prominent role. However, the molecular mechanism controlling the early emergence of floral meristems remains unknown. A body of evidence indicates that LFY may contribute to this developmental shift, but a direct effect of LFY on meristem emergence has not been demonstrated. We have generated a LFY allele with reduced floral function and revealed its ability to stimulate axillary meristem growth. This role is barely detectable in the lfy single mutant but becomes obvious in several double mutant backgrounds and plants ectopically expressing LFY. We show that this role requires the ability of LFY to bind DNA, and is mediated by direct induction of REGULATOR OF AXILLARY MERISTEMS1 (RAX1) by LFY. We propose that this function unifies the diverse roles described for LFY in multiple angiosperm species, ranging from monocot inflorescence identity to legume leaf development, and that it probably pre-dates the origin of angiosperms.

Aphid transmission of poleroviruses is highly specific, but the viral determinants governing this specificity are unknown. We used a gene exchange strategy between two poleroviruses with different vectors, Beet western yellows virus (BWYV) and Cucurbit aphid-borne yellows virus (CABYV), to analyze the role of the major and minor capsid proteins in vector specificity. Virus recombinants obtained by exchanging the sequence of the readthrough domain (RTD) between the two viruses replicated in plant protoplasts and in whole plants. The hybrid readthrough protein of chimeric viruses was incorporated into virions. Aphid transmission experiments using infected plants or purified virions revealed that vector specificity is driven by the nature of the RTD. BWYV and CABYV have specific intestinal sites in the vectors for endocytosis: the midgut for BWYV and both midgut and hindgut for CABYV. Localization of hybrid virions in aphids by transmission electron microscopy revealed that gut tropism is also determined by the viral origin of the RTD.The genera Polerovirus, Luteovirus, and Enamovirus constitute the Luteoviridae family (luteovirids). Luteovirids are restricted to the phloem tissue of host plants and are strictly transmitted by aphids in a persistent, circulative, and nonpropagative manner (17,20). Virions, acquired by aphids while feeding from sieve tubes of an infected plant, are transported through the gut epithelium and released in the hemolymph. In this compartment, virions interact with symbionin (of endosymbiont origin), which may protect them from the immune system and/or modify structural properties of virions (42). Virus particles in the hemolymph can be taken up by accessory salivary gland (ASG) epithelial cells from which they are released via the salivary duct during feeding.Transmission electron microscope (TEM) observations indicate that transport of virions through the gut and ASG epithelia occurs by receptor-mediated endocytosis/exocytosis. Passage across the gut wall can take place at two sites, the posterior midgut and the hindgut. Barley yellow dwarf virus-MAV (Luteovirus) (16), Cereal yellow dwarf virus (Polerovirus) (17), and Soybean dwarf virus (unassigned member of the Luteoviridae) (19) are internalized at the hindgut, while the posterior midgut is used by two poleroviruses, Beet western yellows virus (BWYV) (37) and Potato leafroll virus (14). Cucurbit aphid-borne yellows virus (CABYV) (Polerovirus) is unique among studied luteovirids in that its virions are taken up at both sites (38).Luteovirus transmission is highly specific (21), but the molecular mechanisms controlling specificity are unknown. One hypothesis is that specificity is mediated by interaction between motifs on the virion and receptors at the epithelial cell plasmalemma during endocytosis. Experiments with different combinations of luteovirids and vector or nonvector aphid species indicate that the basal plasmalemma of ASG epithelial cells is an important site for such differential interactions, although the ASG basal lamina...

Bacillus thuringiensis subsp. israelensis is a bioinsecticide increasingly used worldwide for mosquito control. Despite its apparent low level of persistence in the field due to the rapid loss of its insecticidal activity, an increasing number of studies suggested that the recycling of B. thuringiensis subsp. israelensis can occur under specific, unknown conditions. Decaying leaf litters sampled in mosquito breeding sites in the French Rhône-Alpes region several months after a treatment were shown to exhibit a high level of larval toxicity and contained large amounts of spores. In the present article, we show that the high concentration of toxins found in these litters is consistent with spore recycling in the field, which gave rise to the production of new crystal toxins. Furthermore, in these toxic leaf litter samples, Cry4Aa and Cry4Ba toxins became the major toxins instead of Cyt1Aa in the commercial mixture. In a microcosm experiment performed in the laboratory, we also demonstrated that the toxins, when added in their crystal form to nontoxic leaf litter, exhibited patterns of differential persistence consistent with the proportions of toxins observed in the field-collected toxic leaf litter samples (Cry4 > Cry11 > Cyt). These results give strong evidence that B. thuringiensis subsp. israelensis recycled in specific breeding sites containing leaf litters, and one would be justified in asking whether mosquitoes can become resistant when exposed to field-persistent B. thuringiensis subsp. israelensis for several generations.

As major alterations are occurring in climate and pest ranges, it is imperative to evaluate their combined contribution to tree mortality in order to propose mitigation measures and limit losses in forest productivity. The objective of this study was to explore the association between declines in tree growth resulting from climatic and biotic (spruce budworm) disturbances, and their interactions on tree mortality of two dominant tree species, Abies balsamea and Picea mariana, of the eastern North‐American boreal forest. We disentangle the influences of abiotic and biotic components on growth through a combination of model‐data comparison techniques. First, we characterized the variability in tree growth and mortality in the study area using a network of tree‐ring width measurements collected from living and dead trees. Subsequently, a bioclimatic simulation model was used to estimate the past annual, nonlinear, responses of stand‐level net primary production (NPP) to climate variability (period 1902–2012). From these two data sources, we defined the biotic stress events as the variance in the tree‐ring data unexplained by the bioclimatic forest growth simulation. Throughout the 20th century, two periods of adverse climatic conditions preceded spruce budworm outbreaks episodes and induced tree mortality. Climatic stress events were associated with cold springs, warmer than average summers. We found that past stress history in interaction with tree characteristics and species predisposed trees to mortality. In addition, co‐occurring events (climatic and biotic) increased the severity of mortality episodes. Synthesis. Our study challenges the belief that spruce budworm outbreak is the primary driver of broad‐scale tree mortality in eastern boreal forest. Rather, tree mortality is the result of cumulative events that combine unfavourable conditions for growth, resulting in loss of tree vigour and subsequently, mortality. Co‐occurrence of stresses in the future may lead to more severe episodes of mortality, as extreme climatic events become more frequent.

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Polychlorinated biphenyls (PCBs) represent a large group of recalcitrant environmental pollutants. Up to now, many studies have focused on bioremediation of PCBs by fungal strains; however the mechanisms of adaptation of these strains towards PCBs remain unknown despite their importance in developing effective bioremediation processes. We studied five species, each consisting of two strains isolated either from PCB-polluted or unpolluted substrates (control strains). We investigated their responses to PCB contamination by studying their tolerance to PCBs, their ability to reduce these pollutants and their expression level of Laccase genes. In Thermothelomyces thermophila, Thermothelomyces heterothallica, Thermoascus crustaceus and Fusarium solani, all the studied strains showed a similar tolerance and PCB degradation regardless of their origin. In Schizophyllum commune, while both strains showed similar resistance to PCBs i.e PCBs and their degradation products presented no toxicity for these strains, the rate of PCB degradation of the strain from a PCB-polluted environment was significantly slightly higher. The PCB degradation did not correlate with the expression level of genes encoding Laccases. These results demonstrate that the tolerance and PCB degradation by the fungal strains, which did not involve Laccase activities, required different adaptation systems which seem to be constitutive or rapidly inducible by PCB according to the fungal species.