Transformation of moss Physcomitrella patens gametophytes using a biolistic projectile delivery system

CRISPR/Cas9-Mediated Knockout of Physcomitrella patens Phytochromes

Here we describe procedures for gene disruption and excision in Physcomitrella using CRISPR/Cas9 (clustered regularly interspaced short palindromic repeat/CRISPR-associated 9) methods, exemplarily targeting phytochrome (PHY) gene loci. Thereby double-strand breaks (DSBs) are induced using a single guide RNA (sgRNA) with the Cas9 nuclease, leading to insertions or deletions (indels) due to incorrect repair by the nonhomologous-end joining (NHEJ) mechanism. We also include protocols for excision of smaller genomic fragments or whole genes either with or without homologous recombination-assisted repair. The protocol can be adapted to target several loci simultaneously, thereby allowing the physiological analysis of phenotypes that would be masked by functional redundancy. In our particular case, multiple PHY gene knockouts would likely be valuable in understanding phytochrome functions in mosses and, perhaps, higher plants too. Target sites for site-directed induction of DSBs are predicted with the CRISPOR online-tool and are inserted in silico into sequence matrices for the design of sgRNA expression cassettes. The resulting DNAs are cloned into Gateway DONOR vectors and the respective expression plasmids used for moss cotransformation with a Cas9 expression plasmid and a selectable marker (either on a separate plasmid or on one of the other plasmids). After the selection process, genomic DNA is extracted and transformants are analyzed by PCR fingerprinting.

Somatic hybridization provides segregating populations for the identification of causative mutations in sterile mutants of the moss Physcomitrella patens

Forward genetics is now straightforward in the moss Physcomitrella patens, and large mutant populations can be screened relatively easily. However, perturbation of development before the formation of gametes currently leaves no route to gene discovery. Somatic hybridization has previously been used to rescue sterile mutants and to assign P. patens mutations to complementation groups, but the cellular basis of the fusion process could not be monitored, and there was no tractable way to identify causative mutations. Here we use fluorescently tagged lines to generate somatic hybrids between Gransden (Gd) and Villersexel (Vx) strains of P. patens, and show that hybridization produces fertile diploid gametophytes that form phenotypically normal tetraploid sporophytes. Quantification of genetic variation between the two parental strains reveals single nucleotide polymorphisms at a frequency of 1/286 bp. Given that the genetic distinction between Gd and Vx strains exceeds that found between pairs of strains that are commonly used for genetic mapping in other plant species, the spore populations derived from hybrid sporophytes provide suitable material for bulk segregant analysis and gene identification by genome sequencing.