A Physcomitrella PIN protein acts in spermatogenesis and sporophyte retention

Diversification of CLE expression patterns and nonmeristematic roles for CLAVATA receptor-like kinases in a moss

The CLAVATA pathway controls meristematic cell proliferation and multiple nonmeristematic processes in Arabidopsis development. While CLAVATA ancestrally regulates meristematic proliferation in nonseed plant gametophytes, ancestral sporophytic and nonmeristematic functions in land plants are unknown. Here, we analysed the promoter activities of all peptide (PpCLE) and receptor-encoding (PpCLV1a, PpCLV1b and PpRPK2) genes throughout the moss (Physcomitrium patens) life cycle and validated our expression analyses using mutant phenotype data. In gametophore apices, PpCLE3 expression marked apical cells, and PpCLV1b and PpRPK2 overlapped. In nonmeristematic tissues, gametophytes showed highly focal PpCLE but broader receptor-encoding gene expression, and many genes were co-expressed. Mutant phenotype analysis revealed roles for PpCLV1a, PpCLV1b and PpRPK2 in fertility and male and female reproductive development. In sporophytes, no PpCLE expression specifically marked the apical cells, and PpCLV1b and PpRPK2 expression initially marked distinct apical and basal domains, but later overlapped at the intercalary meristem. Overall, fewer genes were co-expressed in sporophytes than in gametophytes, but all genes were co-expressed in guard cells. Our data indicate that nonmeristematic CLAVATA functions in gametangium development and stomatal development may be ancestral within land plants. Peptide encoding (CLE) gene copy numbers amplified in mosses, and promoter evolution was a likely driver of cell type diversification during moss evolution.

Illuminating the role of the calyptra in sporophyte development

The study of moss calyptra form and function began almost 250 years ago, but calyptra research has remained a niche endeavor focusing on only a small number of species. Recent advances have focused on calyptra cuticular waxes, which function in dehydration protection of the immature sporophyte apex. The physical presence of the calyptra also plays a role in sporophyte development, potentially via its influence on auxin transport. Progress developing genomic resources for mosses beyond the model Physcomitrium patens, specifically for species with larger calyptrae and taller sporophytes, in combination with advances in CRISPR-Cas9 genome editing will enable the influence of the calyptra on gene expression and the production of RNAs and proteins that coordinate sporophyte development to be explored.

Control of leaf development in the water fern Ceratopteris richardii by the auxin efflux transporter CrPINMa in the CRISPR/Cas9 analysis

BACKGROUND

PIN-FORMED genes (PINs) are crucial in plant development as they determine the directionality of auxin flow. They are present in almost all land plants and even in green algae. However, their role in fern development has not yet been determined. This study aims to investigate the function of CrPINMa in the quasi-model water fern Ceratopteris richardii.

RESULTS

CrPINMa possessed a long central hydrophilic loop and characteristic motifs within it, which indicated that it belonged to the canonical rather than the non-canonical PINs. CrPINMa was positioned in the lineage leading to Arabidopsis PIN6 but not that to its PIN1, and it had undergone numerous gene duplications. CRISPR/Cas9 genome editing had been performed in ferns for the first time, producing diverse mutations including local frameshifts for CrPINMa. Plants possessing disrupted CrPINMa exhibited retarded leaf emergence and reduced leaf size though they could survive and reproduce at the same time. CrPINMa transcripts were distributed in the shoot apical meristem, leaf primordia and their vasculature. Finally, CrPINMa proteins were localized to the plasma membrane rather than other cell parts.

CONCLUSIONS

CRISPR/Cas9 genome editing is feasible in ferns, and that PINs can play a role in fern leaf development.