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In Arabidopsis hypocotyls, phototropins are the dominant photoreceptors for the positive phototropism response towards unilateral ultraviolet-B (UV-B) radiation. We report a stark contrast of response mechanism with inflorescence stems with a central role for UV RESISTANCE LOCUS 8 (UVR8). The perception of UV-B occurs mainly in the epidermis and cortex with a lesser contribution of the endodermis. Unilateral UV-B exposure does not lead to a spatial difference in UVR8 protein levels but does cause differential UVR8 signal throughout the stem with at the irradiated side 1) increase of the transcription factor ELONGATED HYPOCOTYL 5 (HY5), 2) an associated strong activation of flavonoid biosynthesis genes and flavonoid accumulation, 3) increased GA2oxidase expression, diminished gibberellin1 levels and accumulation of DELLA protein REPRESSOR OF GA1 (RGA) and, 4) increased expression of the auxin transport regulator, PINOID, contributing to local diminished auxin signalling. Our molecular findings are in support of the Blaauw theory (1919), suggesting that differential growth occurs trough unilateral photomorphogenic growth inhibition. Together the data indicate phototropin independent inflorescence phototropism through multiple locally UVR8-regulated hormone pathways.

Ultraviolet-B radiation (UV-B - 280-315nm) was only recently described as a phototropism-inducing type of radiation. We have identified that phototropic growth towards UV-B of Arabidopsis hypocotyls and inflorescence stems are both regulated by the same photoreceptor pathways but display a shift in dominance during plant development. The phototropin pathway is dominant in etiolated seedlings (Vanhaelewyn et al., 2016a), while the UVR8 pathway is predominant in inflorescence stems. The role of key-players in the UVR8 pathway, such as HY5 and HYH has been evaluated for their importance in this phototropic response. Unilaterally UV-B irradiated inflorescence stem tissue demonstrates a lateral UVR8-mediated signal gradient. In addition, the function of UVR8 in different cell types was validated by use of cell type specific complementation lines of UVR8 in a uvr8-6 mutant background. This reveals that UVR8 signaling is important in various cell types within the inflorescence stem. As UV-B is known to affect plant hormones (Vanhaelewyn et al., 2016b), we investigated their involvement in this phototropic response. By means of reporter line analysis, mutant analysis, gene expression assays and pharmacological assays, we designate a role for both gibberellins and auxins to UV-B induced phototropism of Arabidopsis inflorescence stems. These combined data provide a mechanistic framework for UV-B induced phototropism.

Erratum to: Ethylene Controls Adventitious Root Initiation Sites in Arabidopsis Hypocotyls Independently of Strigolactones; International audience

The Elongator complex (hereafter Elongator) promotes RNA polymerase II-mediated transcript elongation through epigenetic activities such as histone acetylation. Elongator regulates growth, development, immune response and sensitivity to drought and abscisic acid. We demonstrate that elo mutants exhibit defective hypocotyl elongation but have a normal apical hook in darkness and are hyposensitive to light during photomorphogenesis. These elo phenotypes are supported by transcriptome changes, including downregulation of circadian clock components, positive regulators of skoto- or photomorphogenesis, hormonal pathways and cell wall biogenesis-related factors. The downregulated genes LHY, HFR1 and HYH are selectively targeted by Elongator for histone H3K14 acetylation in darkness. The role of Elongator in early seedling development in darkness and light is supported by hypocotyl phenotypes of mutants defective in components of the gene network regulated by Elongator, and by double mutants between elo and mutants in light or darkness signaling components. A model is proposed in which Elongator represses the plant immune response and promotes hypocotyl elongation and photomorphogenesis via transcriptional control of positive photomorphogenesis regulators and a growth-regulatory network that converges on genes involved in cell wall biogenesis and hormone signaling., The research was funded by the EC Marie Curie Intra European fellowship (FP7 People: Marie-Curie Actions; FP7-PEOPLE-2010-IEF-273068) (LightEr) to M.W.; the Initial Research Training network FP7-PEOPLE-2013-ITN-607880 (CHIP-ET) to M.V.L., P.M. and fellows S.L.G. and J.F.; IWT predoctoral fellowship to S.L.G; and Fonds Wetenschappelijk Onderzoek (FWO) (G.0656.13N) to D.V.D.S.

Biofortification of staple crops has been proposed as a strategy to address micronutrient malnutrition, particularly with respect to insufficient intake of vitamin A, iron, zinc, and folate. The World Health Organization, in collaboration with the Food and Agriculture Organization of the United Nations and the Sackler Institute for Nutrition Science at the New York Academy of Sciences, convened a technical consultation entitled "Staple Crops Biofortified with Vitamins and Minerals: Considerations for a Public Health Strategy" in April 2016. Participants of the consultation reviewed the definition of biofortification of staple crops, patterns of crops production, processing, consumption, seed varieties, and micronutrient stability and bioavailability, as well as farmers' adoption and acceptability of the modified crops. Also discussed were economic, environmental, safety, and equity aspects of biofortified crops, as well as legal, policy, regulatory, and ethical issues for the implementation of biofortification strategies in agriculture and nutrition. Consultation working groups identified important and emerging technical issues, lessons learned, and research priorities to better support the evidence of improved nutrition and unintended adverse effects of biofortification. This paper provides the background and rationale of the technical consultation, synopsizes the presentations, and provides a summary of the main considerations proposed by the working groups.

Micronutrient deficiency is a widespread phenomenon, most prevalent in developing countries. Being causally linked to the occurrence of a range of diseases, it affects billions of people worldwide. Enhancing the content of micronutrients in crop products through biotechnology is a promising technique to fight micronutrient malnutrition worldwide. Micronutrient fortification of food products has been implemented in a number of Western countries, but remains inaccessible for poor rural populations in a major part of the developing world. Moreover, evidence of the negative impacts of this practice on human health, at least for some vitamins, is accumulating. Biofortification of crop plants-the enhancement of vitamins and minerals through plant biotechnology-is a promising alternative or complement in the battle against micronutrient deficiencies. Owing to a growing knowledge about vitamin metabolism, as well as mineral uptake and reallocation in plants, it is today possible to enhance micronutrient levels in crop plants, offering a sustainable solution to populations with a suboptimal micronutrient intake.