Root growth is modulated by environmental factors and depends on cell production in the root meristem (RM). New cells in the meristem are generated by stem cells and transit-amplifying cells, which together determine RM cell number. Transcription factors and chromatin-remodeling factors have been implicated in regulating the switch from stem cells to transit-amplifying cells. Here, we show that two Arabidopsis thaliana paralogs encoding plant-specific histone deacetylases, HDT1 and HDT2, regulate a second switch from transit-amplifying cells to expanding cells. Knockdown of HDT1/2 (hdt1,2i) results in an earlier switch and causes a reduced RM cell number. Our data show that HDT1/2 negatively regulate the acetylation level of the C 19 -GIBBERELLIN 2-OXIDASE2 (GA2ox2) locus and repress the expression of GA2ox2 in the RM and elongation zone. Overexpression of GA2ox2 in the RM phenocopies the hdt1,2i phenotype. Conversely, knockout of GA2ox2 partially rescues the root growth defect of hdt1,2i. These results suggest that by repressing the expression of GA2ox2, HDT1/ 2 likely fine-tune gibberellin metabolism and they are crucial for regulating the switch from cell division to expansion to determine RM cell number. We propose that HDT1/2 function as part of a mechanism that modulates root growth in response to environmental factors.

BackgroundMicrobial-associated molecular patterns activate several MAP kinases, which are major regulators of the innate immune response in Arabidopsis thaliana that induce large-scale changes in gene expression. Here, we determine whether microbial-associated molecular pattern-triggered gene expression involves modifications at the chromatin level.ResultsHistone acetylation and deacetylation are major regulators of microbial-associated molecular pattern-triggered gene expression and implicate the histone deacetylase HD2B in the reprogramming of defence gene expression and innate immunity. The MAP kinase MPK3 directly interacts with and phosphorylates HD2B, thereby regulating the intra-nuclear compartmentalization and function of the histone deacetylase.ConclusionsBy studying a number of gene loci that undergo microbial-associated molecular pattern-dependent activation or repression, our data reveal a mechanistic model for how protein kinase signaling directly impacts chromatin reprogramming in plant defense.Electronic supplementary materialThe online version of this article (doi:10.1186/s13059-017-1261-8) contains supplementary material, which is available to authorized users.

BackgroundThe Chinese chestnut (Castanea mollissima) is widely cultivated in China for nut production. This plant also plays an important ecological role in afforestation and ecosystem services. To facilitate and expand the use of C. mollissima for breeding and its genetic improvement, we report here the whole-genome sequence of C. mollissima.FindingsWe produced a high-quality assembly of the C. mollissima genome using Pacific Biosciences single-molecule sequencing. The final draft genome is ∼785.53 Mb long, with a contig N50 size of 944 kb, and we further annotated 36,479 protein-coding genes in the genome. Phylogenetic analysis showed that C. mollissima diverged from Quercus robur, a member of the Fagaceae family, ∼13.62 million years ago.ConclusionsThe high-quality whole-genome assembly of C. mollissima will be a valuable resource for further genetic improvement and breeding for disease resistance and nut quality.

Parasponia andersonii is a fast-growing tropical tree that belongs to the Cannabis family (Cannabaceae). Together with 4 additional species, it forms the only known non-legume lineage able to establish a nitrogen-fixing nodule symbiosis with rhizobium. Comparative studies between legumes and P. andersonii could provide valuable insight into the genetic networks underlying root nodule formation. To facilitate comparative studies, we recently sequenced the P. andersonii genome and established Agrobacterium tumefaciens-mediated stable transformation and CRISPR/Cas9-based genome editing. Here, we provide a detailed description of the transformation and genome editing procedures developed for P. andersonii. In addition, we describe procedures for the seed germination and characterization of symbiotic phenotypes. Using this protocol, stable transgenic mutant lines can be generated in a period of 2-3 months. Vegetative in vitro propagation of T 0 transgenic lines allows phenotyping experiments to be initiated at 4 months after A. tumefaciens co-cultivation. Therefore, this protocol takes only marginally longer than the transient Agrobacterium rhizogenes-based root transformation method available for P. andersonii, though offers several clear advantages. Together, the procedures described here permit P. andersonii to be used as a research model for studies aimed at understanding symbiotic associations as well as potentially other aspects of the biology of this tropical tree. Video LinkThe video component of this article can be found at https://www.jove.com/video/59971/ 4 . Additionally, it was demonstrated that the root nodule symbiosis in legumes is founded on the much older, and widespread arbuscular mycorrhizal symbiosis 5 . Phylogenomic comparisons suggest that the nitrogen-fixing nodule symbioses of legumes, Parasponia, as well as, the so-called actinorhizal plant species that host diazotrophic Frankia bacteria, have a shared evolutionary origin 6,7,8 . To determine whether the genes identified to be involved in the legume nodule formation are the part of a conserved genetic basis, studies on non-legume species are essential. To this end, we propose to use P. andersonii as a comparative research model, alongside legumes, to identify the core genetic networks underlying root nodule formation and functioning.P. andersonii is a pioneer that can be found on the slopes of volcanic hills. It can meet growth speeds of 45 cm per month and reach lengths of up to 10 meters 9 . P. andersonii trees are wind-pollinated, which is facilitated by the formation of separate male and female flowers 3,10. We recently sequenced and annotated the diploid genome (2n = 20; 560 Mb/1C) of P. andersonii, and assembled draft genome sequences of 2 additional Parasponia species; P. rigida and P. rugosa 6 . This revealed ~35,000 P. andersonii gene models that can be clustered in >20,000 orthogroups together with genes from M. truncatula, soybean (Glycine max), Arabidopsis thaliana, woodland strawberry (Fragaria vesca), Trema orientalis, black cot...