A chromosome-scale assembly for 'd'Anjou' pear.

Cultivated pear consists of several Pyrus species with Pyrus communis (European pear) representing a large fraction of worldwide production. As a relatively recently domesticated crop and perennial tree, pear can benefit from genome-assisted breeding. Additionally, comparative genomics within Rosaceae promises greater understanding of evolution within this economically important family. Here, we generate a fully phased chromosome-scale genome assembly of P. communis 'd'Anjou.' Using PacBio HiFi and Dovetail Omni-C reads, the genome is resolved into the expected 17 chromosomes, with each haplotype totaling nearly 540 Megabases and a contig N50 of nearly 14 Mb. Both haplotypes are highly syntenic to each other and to the Malus domestica 'Honeycrisp' apple genome. Nearly 45,000 genes were annotated in each haplotype, over 90% of which have direct RNA-seq expression evidence. We detect signatures of the known whole-genome duplication shared between apple and pear, and we estimate 57% of d'Anjou genes are retained in duplicate derived from this event. This genome highlights the value of generating phased diploid assemblies for recovering the full allelic complement in highly heterozygous crop species.

CYTOKININ RESPONSE FACTOR 2 is involved in modulating the salt stress response.

Cytokinin has strong connections to development and a growing role in the abiotic stress response. Here we show that CYTOKININ RESPONSE FACTOR 2 (CRF2) is additionally involved in the salt (NaCl) stress response. CRF2 promoter-GUS expression indicates CRF2 involvement in the response to salt stress as well as the previously known cytokinin response. Interestingly, CRF2 mutant seedlings are quite similar to the wild type (WT) under non-stressed conditions yet have many distinct changes in response to salt stress. Cytokinin levels measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) that increased in the WT after salt stress are decreased in crf2, potentially from CRF2 regulation of cytokinin biosynthesis genes. Ion content measured by inductively coupled plasma optical emission spectrometry (ICP-OES) was increased in the WT for Na, K, Mn, Ca and Mg after salt stress, whereas the corresponding Ca and Mg increases are lacking in crf2. Many genes examined by RNA-seq analysis were altered transcriptionally by salt stress in both the WT and crf2, yet interestingly approximately one-third of salt-modified crf2 transcripts (2655) showed unique regulation. Different transcript profiles for salt stress in crf2 compared with the WT background was further supported through an examination of co-expressed genes by weighted gene correlation network analysis (WGCMA) and principal component analysis (PCA). Additionally, Gene Ontology (GO) enrichment terms found from salt-treated transcripts revealed most photosynthesis-related terms as only being affected in crf2, leading to an examination of chlorophyll levels and the efficiency of photosystem II (via the ratio of variable fluorescence to maximum fluorescence, Fv /Fm ) as well as physiology after salt treatment. Salt stress-treated crf2 plants had both reduced chlorophyll levels and lower Fv /Fm values compared with the WT, suggesting that CRF2 plays a role in the modulation of salt stress responses linked to photosynthesis.