SERKs Regulate Embryo Development Of Arabidopsis Thaliana

Receptor-like protein kinases(RLKs)are single-pass transmembrane protein kinases,which consist of extracellular ligand-binding domain,transmembrane domain and intracellular kinase domain.They can transduce extracellular signals into a cell by the way of substrate phosphorylation and are involved in cell-to-cell and cell-to-environment communications.There are more than 400 typical RLKs in Arabidopsis and were classified into 13 families based on their extracellular domain structures.Among them,leucine-rich repeat(LRR)RLKs belong to the largest family with at least 223 members,including 5 SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASEs(SERKs)in Arabidopsis.The first SERK gene,DcSERK,was identified as a marker gene in carrot suspension cultures to monitor cell identity change from somatic to embryonic cells.Recent studies indicate that Arabidopsis SERKs possess functional diversity and play important roles in regulating distinct signaling pathways such as brassinosteroid(BR)signaling,anther development,cell death control,and various plant innate immunity responses.Analyses of gene expression patterns indicated that at least one of Arabidopsis SERKs,SERK1,expresses during all the stages of Arabidopsis embryo development,suggesting that it may play important roles in mediating embryo development.However,studies on this have not made a breakthrough yet.In order to reveal the functions of SERKs in Arabidopsis embryo development,a series of serk mutants were created by crossing.The serkl serk2 serk3 serk4 quadruple mutant is embryo lethal,whereas a triple mutant serk1 serk2 serk3 displays delayed embryo development and aberrant embryos.These observations provided strong evidence that SERKs are essential to Arabidopsis embryo development.This study investigated the embryo development process of serkl serk1 serk3 and serk1 serk1 serk3 serk4,and analyzed the expression patterns of SERKs.Results showed that embryo defects of serk mutants occur initially at the 32-cell proembryo stage,and the size of hypophysis cells increases abnormally at this time and the cells divide in advance.After that,primordia develop slowly,and cotyledons and hypocotyls become much shorter.In apical cell line,SERKs originally express at the 32-cell proembryo stage,and then express in all primordia after the heart-shaped stage,especially strong in the epidermis,cotyledons and provascular cells.Complementation of SERK1 and SERK3 in serkl serk2 serk3 mutant plants could rescue the embryo defects,indicating that the mutant phenotypes were caused by loss-of-function of SERKs.In order to further study the mutant embryos at the molecular level,the expression patterns of some embryo marker genes were analyzed.Results showed that the expression of WUSCHEL-RELATED HOMEOBOX 5(WOX5),a quiescent center marker gene,is dramatically altered and enhanced in the provascular cells of serkl serk2 serk3 embryos.Meanwhile,the expression of PLETHORA 1(PLT1)is down regulated dramatically in the protoderm of the mutant embryos,and the expression of PIN-FORMED 1(PIN1)was up-regulated.Previous studies showed that WOX5 and PLT1 are extremely important genes in maintaining the activity of stem cells,suggesting that the provascular cells have a lower degree of differentitation and the protoderm is more specialized in the mutant embryos.PIN1 is an auxin efflux,and the change of its expression level indicates that the transportation of auxin may be changed in mutant protoderm.Finally,to reveal the molecular mechanisms of SERKs in regulating embryo development,this study also investigated the components of SERKs-mediated signaling pathway.Previous studies already proved that SERKs function as co-receptors of BRI1 to mediate BR signaling.However,no similar embryo defects and altered marker gene expression were observed in the BR receptor mutant bril-701 brl1 brl3,suggesting that SERKs-mediated embryo development is independent of BR signaling pathway.Our data also indicated that SERKs might form complex with GASSHO1(GSOl)and GS02,other two LRR-RLKs,to regulate the protoderm development during Arabidopsis embryogenesis.Meanwhile,a Mitogen-activated protein kinases(MAPK)pathway may transduce the SERKs-mediated signals to regulate the development of cotyledons and hypocotyl in Arabidopsis embryo.