A sessile lifestyle forces plants to respond promptly to factors that affect their genomic integrity. Therefore, plants have developed checkpoint mechanisms to arrest cell cycle progression upon the occurrence of DNA stress, allowing the DNA to be repaired before onset of division. Previously, the WEE1 kinase had been demonstrated to be essential for delaying progression through the cell cycle in the presence of replication-inhibitory drugs, such as hydroxyurea. To understand the severe growth arrest of WEE1-deficient plants treated with hydroxyurea, a transcriptomics analysis was performed, indicating prolonged S-phase duration. A role for WEE1 during S phase was substantiated by its specific accumulation in replicating nuclei that suffered from DNA stress. Besides an extended replication phase, WEE1 knockout plants accumulated dead cells that were associated with premature vascular differentiation. Correspondingly, plants without functional WEE1 ectopically expressed the vascular differentiation marker VND7, and their vascular development was aberrant. We conclude that the growth arrest of WEE1-deficient plants is due to an extended cell cycle duration in combination with a premature onset of vascular cell differentiation. The latter implies that the plant WEE1 kinase acquired an indirect developmental function that is important for meristem maintenance upon replication stress.

SUMMARYSynchronized cell cultures are an indispensable tool for the identification and understanding of key regulators of the cell cycle. Nevertheless, the use of cell cultures has its disadvantages, because it represents an artificial system that does not completely mimic the endogenous conditions that occur in organized meristems. Here, we present a new and easy method for Arabidopsis thaliana root tip synchronization by hydroxyurea treatment. A major advantage of the method is the possibility of investigating available Arabidopsis cell-cycle mutants without the need to generate cell cultures. As a proof of concept, the effects of over-expression of a dominant negative allele of the B-type cyclin-dependent kinase CDKB1;1 gene on cell-cycle progression were tested. The previously observed prolonged G 2 phase was confirmed, but was found to be compensated for by a reduced G 1 phase. Furthermore, altered S-phase kinetics indicated a functional role for CDKB1;1 during the replication process.

One-sentence summary: 40The loss-of-function of MtNOOT1 and MtNOOT2 leads to the complete loss of nodule identity, prevents 41 the symbiotic process, and results in the absence of nitrogen fixation in Medicago truncatula. 56VZ, SC, GEDO, and PR analyzed the data. KM, KS and PR wrote the article. 58This work was supported by the CNRS, by the grants ANR SVSE 6.2010.1 (LEGUMICS) and ANR-14- 62Agriculture (Dufrenoy Grant, 2011). This work has benefited from the facilities and expertise of the IMAGIF 63Cell Biology Unit of the Gif campus (www.imagif.cnrs.fr) which is supported by the Conseil Général de 64 l'Essonne. 66The author responsible for distribution of materials integral to the findings presented in this article in 67 accordance with the policy described in the instructions for authors (www.plantphysiol.org) is: Pascal 68 Ratet (pascal.ratet@u-psud.fr). 70 71 Acknowledgments 72The Institute of Plant Sciences Paris-Saclay (IPS2, France) benefits from the support of the LabEx Saclay 97fixing root-like structures that were no longer able to host symbiotic rhizobia. This study provides original 98 insights into the molecular basis underlying nodule identity in legumes forming indeterminate nodules. 100 INTRODUCTION 102The symbiotic interaction between legumes and rhizobia results in the formation of root nodules 103 dedicated to host nitrogen-fixing rhizobia. This unique ability to form root nodules is restricted to the 104 Rosids I clade. The predisposition of plants to enter symbiosis with nitrogen-fixing rhizobia seems to have 105 evolved once, between 70 and 100 million years ago and to have derived from an ancestral and 106 widespread symbiosis, the arbuscular mycorrhizal symbiosis (AMS, Soltis et al., 1995; Smith and Read, 107 2008;Bonfante and Genre, 2010;Humphreys et al., 2010;Werner et al., 2014). 109Genetic approaches using nodule-deficient (nod -) and non-functional nodule (fix -) mutant plants 110 allowed the identification of many genes essential for the early steps of root nodule symbiosis. 111Recognition between symbiotic partners, rhizobial infection and nodule organogenesis are initiated by the 112 host plant perception of rhizobial lipo-chitooligosacharidic compounds Jones et al., 113 2007;Kouchi et al., 2010; Horvath et al., 2011;Ovchinnikova et al., 2011; 114 . These compounds are called Nod factors 115and they are structurally similar to the mycorrhization factors (Myc factors) required for AMS initiation 116 (Maillet et al., 2011). 118In the Papilionaceae family, determinate nodules formed in the Phaseoleae, Loteae and 119Dalbergieae tribes have no persistent apical nodule meristem (NM). However, indeterminate nodules 120 formed in the Trifolieae and Fabeae tribes have a persistent apical NM. Indeterminate nodules are highly-121 structured and present different zones; the NM, the infection zone, the nitrogen fixation zone and the older 122 senescent zone (from top to bottom; . The ability of indeterminate nodules to grow 123 continuously results from the presence of the NM. ...

In eukaryotes, transcriptional regulation is determined by dynamic and reversible chromatin modifications, such as acetylation, methylation, phosphorylation, ubiquitination, glycosylation, that are essential for the processes of DNA replication, DNA-repair, recombination and gene transcription. The reversible and rapid changes in histone acetylation induce genome-wide and specific alterations in gene expression and play a key role in chromatin modification. Because of their sessile lifestyle, plants cannot escape environmental stress, and hence have evolved a number of adaptations to survive in stress surroundings. Chromatin modifications play a major role in regulating plant gene expression following abiotic and biotic stress. Plants are also able to respond to signals that affect the maintaince of genome integrity. All these factors are associated with changes in gene expression levels through modification of histone acetylation. This review focuses on the major types of genes encoding for histone acetyltransferases, their structure, function, interaction with other genes, and participation in plant responses to environmental stimuli, as well as their role in cell cycle progression. We also bring together the most recent findings on the study of the histone acetyltransferase HAC1 in the model legumes Medicago truncatula and Lotus japonicus.