The importance of earth reference controls in spaceflight -omics research: characterization of nucleolin mutants from the seedling growth experiments

48 p.-4 fig.-1 graph. abst.-4 fig. supl.-1 data supl.-14 tab. reportUnderstanding plant adaptive responses to the space environment is a requisite for enabling space farming. Spaceflight produces deleterious effects on plant cells, particularly affecting ribosome biogenesis, a complex stress-sensitive process coordinated with cell division and differentiation, known to be activated by red light. Here, in a series of ground studies, we have used mutants from the two Arabidopsis nucleolin genes (NUC1 and NUC2, nucleolar regulators of ribosome biogenesis) to better understand their role in adaptive response mechanisms to stress on Earth. Thus, we show that nucleolin stress-related gene NUC2 can compensate for the environmental stress provided by darkness in nuc1 plants, whereas nuc2 plants are not able to provide a complete response to red light. These ground control findings, as part of the ESA/NASA Seedling Growth spaceflight experiments, will determine the basis for the identification of genetic backgrounds enabling an adaptive advantage for plants in future space experiments.This work was supported by the Agencia Estatal de Investigación of the Spanish Ministry of Science and Innovation , Grants # ESP2015-64323-R and # RTI2018-099309-B-I00 (co-funded by EU- ERDF ) to F.J.M., by pre-doctoral fellowships to A.M. and A.V. from the Spanish National Program for Young Researchers Training ( MINECO , Ref. BES-2013-063933 , BES-2016-077976 ), and the Seedling Growth Project to the ISS LSRA2009-0932 /1177, a shared project of ESA-ELIPS Program and NASA . J.Z.K. is funded by Grants NNX12A065G and 80NSSC17K0546 . These results are related to the Space Omics TT funded by the European Space Agency contract ESA 4000131202/20/NL/PG to R.H.Peer reviewe

Light signals counteract alterations caused by simulated microgravity in proliferating plant cells

18 p.-9 fig.Premise: Light and gravity are fundamental cues for plant development. Our understanding of the effects of light stimuli on plants in space, without gravity, is key to providing conditions for plants to acclimate to the environment. Here we tested the hypothesis that the alterations caused by the absence of gravity in root meristematic cells can be counteracted by light.Methods: Seedlings of wild‐type Arabidopsis thaliana and two mutants of the essential nucleolar protein nucleolin (nuc1, nuc2) were grown in simulated microgravity,either under a white light photoperiod or under continuous darkness. Key variables of cell proliferation (cell cycle regulation), cell growth (ribosome biogenesis),and auxin transport were measured in the root meristem using in situ cellular markers and transcriptomic methods and compared with those of a 1 g control.Results: The incorporation of a photoperiod regime was sufficient to attenuate or suppress the effects caused by gravitational stress at the cellular level in the root meristem. In all cases, values for variables recorded from samples receiving light stimuli in simulated microgravity were closer to values from the controls than values from samples grown in darkness. Differential sensitivities were obtained for the two nucleolin mutants.Conclusions: Light signals may totally or partially replace gravity signals, significantly improving plant growth and development in microgravity. Despite that, molecular alterations are still compatible with the expected acclimation mechanisms, which need to be better understood. The differential sensitivity of nuc1 and nuc2 mutants to gravitational stress points to new strategies to produce more resilient plants to travel with humans in new extraterrestrial endeavors.This work was funded by the Agencia Estatal de Investigación of the Spanish Ministry of Science an Innovation, Grants#ESP2015‐64323‐R and #RTI2018‐099309‐B‐I00 (co‐funded by EU‐ERDF) to F.J.M., and Bonus Recherche from the UPVD to J.S.V. The use of the facilities of microgravity simulation was provided by the ESA‐CORA‐Ground Based Facilities Program, contract Ref. #4000105761 to F.J.M. and R.H. A.M. was recipient of a contract of the Program for Young Researchers Training of the Agencia Estatal de Investigación of the Spanish Ministry of Science an Innovation Ref. #BES‐2013‐063933.Peer reviewe

in the 3’ETS in Arabidopsis

protein required for cleavage of the pre-rRN

Upon heat stress processing of ribosomal RNA precursors into mature rRNAs is compromised after cleavage at primary P site in Arabidopsis thaliana

International audienceTranscription and processing of 45S rRNAs in the nucleolus are keystones of ribosome biogenesis. While these processes are severely impacted by stress conditions in multiple species, primarily upon heat exposure, we lack information about the molecular mechanisms allowing sessile organisms without a temperature-control system, like plants, to cope with such circumstances. We show that heat stress disturbs nucleolar structure, inhibits pre-rRNA processing and provokes imbalanced ribosome profiles in Arabidopsis thaliana plants. Notably, the accuracy of transcription initiation and cleavage at the primary P site in the 5'ETS (5' External Transcribed Spacer) are not affected but the levels of primary 45S and 35S transcripts are, respectively, increased and reduced. In contrast, precursors of 18S, 5.8S and 25S RNAs are rapidly undetectable upon heat stress. Remarkably, nucleolar structure, pre-rRNAs from major ITS1 processing pathway and ribosome profiles are restored after returning to optimal conditions, shedding light on the extreme plasticity of nucleolar functions in plant cells. Further genetic and molecular analysis to identify molecular clues implicated in these nucleolar responses indicate that cleavage rate at P site and nucleolin protein expression can act as a checkpoint control towards a productive pre-rRNA processing pathway

Nucleolin is required for DNA methylation state and the expression of rRNA gene variants in Arabidopsis thaliana.

International audienceIn eukaryotes, 45S rRNA genes are arranged in tandem arrays in copy numbers ranging from several hundred to several thousand in plants. Although it is clear that not all copies are transcribed under normal growth conditions, the molecular basis controlling the expression of specific sets of rRNA genes remains unclear. Here, we report four major rRNA gene variants in Arabidopsis thaliana. Interestingly, while transcription of one of these rRNA variants is induced, the others are either repressed or remain unaltered in A. thaliana plants with a disrupted nucleolin-like protein gene (Atnuc-L1). Remarkably, the most highly represented rRNA gene variant, which is inactive in WT plants, is reactivated in Atnuc-L1 mutants. We show that accumulated pre-rRNAs originate from RNA Pol I transcription and are processed accurately. Moreover, we show that disruption of the AtNUC-L1 gene induces loss of symmetrical DNA methylation without affecting histone epigenetic marks at rRNA genes. Collectively, these data reveal a novel mechanism for rRNA gene transcriptional regulation in which the nucleolin protein plays a major role in controlling active and repressed rRNA gene variants in Arabidopsis