Rice Protein Tagging Project:A Call for International Collaborations on Genome-wide In-Locus Tagging of Rice Proteins

In September 1997,the International Rice Genome Sequencing Project was launched.This initiative pooled the resources of ten nations to obtain the first complete rice genome sequence,and promoted rice research and breeding into the post-genomics era(Li et al.,2018).In 2008,an internationally coordinated project named "RICE2020" was proposed to systematically and fully characterize all rice genes,transcripts,and proteins(Zhang et al.,2008).While genes and their transcripts can be readily characterized by sequencing-and PCR-based methods,the characterization of protein dynamics including protein levels,subcellular localizations,post-transla-tional modifications,and interactions with macromolecules(e.g.,proteins,DNA,RNA,carbohydrates,and lipids)and small molecules(e.g.,metabolites and ligands)is much more challenging and usually requires antibodies that specifically recognize the protein of interest.Because it is very difficult to systematically produce reliable antibodies for the specific recognition of individual plant proteins,a common practice is to transform a tag-fused open reading frame(ORF)of a gene to the corresponding loss-of-function mutant plants.However,such an ectopically expressed tagged protein may not fully reca-pitulate the properties of the endogenous protein due to the random insertion of the transgene,even when the transgene is expressed under the endogenous gene promoter.A preferred so-lution is to genetically label the coding sequence of the gene of interest,at its in vivo locus,with a sequence encoding a fluores-cent protein tag or an affinity tag such as FLAG or HA.Such "in-locus" protein tagging,as we are naming it here,has been carried out genome-wide in yeast,Caenorhabditis elegans,fly,and mammalian cultured cells,greatly facilitating the characterization of proteins in these organisms(Jiang et al.,2018).In 2017,a Genome Tagging Project was launched in mice,aiming to label every protein using a CRISPR/Cas9-based "artificial spermatids"method(Jiang et al.,2018).Significant funding and efforts have been put into this project,which is expected to provide valuable mouse resources to accelerate biomedical research.In higher plants,in-locus tagging of proteins has been extremely challenging technically.

OsSERK1 regulates rice development but not immunity to Xanthomonas oryzae pv. oryzae or Magnaporthe oryzae

Somatic embryogenesis receptor kinase（SERK）proteins play pivotal roles in regulation of plant development and immunity. The rice genome contains two SERK genes,OsSerk1 and OsSerk2. We previously demonstrated that OsSerk2 is required for rice Xa21-mediated resistance to Xanthomonas oryzae pv. oryzae（Xoo） and for normal development. Here we report the molecular characterization of OsSerk1. Overexpression of OsSerk1 results in a semi-dwarf phenotype whereas silencing of OsSerk1 results in a reduced angle of the lamina joint. OsSerk1 is not required for rice resistance to Xoo or Magnaporthe oryzae. Overexpression of Os Serk1 in OsSerk2-silenced lines complements phenotypes associated with brassinosteroid（BR） signaling defects, but not the disease resistance phenotype mediated by Xa21. In yeast, OsSERK1 interacts with itself forming homodimers, and also interacts with the kinase domains of OsSERK2 and BRI1, respectively.OsSERK1 is a functional protein kinase capable of auto-phosphorylation in vitro. We conclude that, whereas OsSERK2 regulates both rice development and immunity, OsSERK1 functions in rice development but not immunity to Xoo and M. oryzae.