Functional Studies Of Inositol Polyphosphate Kinase Gene In Arabidopsis And Rice

Plant inositol polyphosphate kinase(IPK2)is an enzyme which possesses a broad-specificity that converts Ins(1,4,5)P3 into Ins(1,4,5,6)P4 and subsequently Ins(1,3,4,5,6)Ps.There are two homologous IPK2 genes in Arabidopsis,AtIPK2a and AtIPK2?.Previous study showed that AtIPK2? is a stress responsive gene.Ectopic expression of AtIPK2? in yeast or tobacco confers to improved resistance to diverse abiotic stresses.AtIPK2? is also inducible by auxin and involved in axillary shoot branching.Recently,AtIPK2a and AtIPK2? have been reported to play a redundant and essential role in pollen development,pollen tube guidance and embryogenesis in a kinase activity-dependent manner.Jasmonate(JA)is a well-known stress hormone that not only regulates defense responses to biotic stress,but also plays important roles in plant growth and development by signaling cascade.JAZ are important transcriptional repressor family proteins binding to diverse transcriptional factors to regulate plant growth and development such as root growth and anthocyanin biosynthesis.The bioactive jasmonate conjugate JA-Ile initiates the signaling cascade by promoting the interaction of JAZ with CORONATINE INSENSITIVE1(COI1)to form co-receptors which leads to poly-ubiquitination of JAZ,and subsequently degradation by 26S-proteasome.Upon the perception of JA signal,JAZ repressors are depleted to relieve these transcriptional factors,which further activate JA-responsive gene expression and physiological responses.Recent reports have showed that Ins(1,4,5,6)P4,InsPs isomers,InsP6 and inositol pyrophosphates can function as co-factors of COI1-JAZ co-receptor complex.These results implicated a possible role of inositol polyphosphates in JA response.In this study,we reported that AtIPK2? is a JA-responsive gene and plays an important role in JA response.Compared with wild type,atipk2? mutant exhibits insensitivity to JA-induced root growth inhibition.The expression of 3-kinase activity reduced or kinase-dead variants of AtIPK2? could not restore the altered JA response in atipk2? mutant.Moreover,protein degradation assay showed that AtIPK2? affects JA-induced degradation of JASMONATE ZIM DOMAIN 1(JAZ1)repressor.These findings suggested that AtIPK2?-mediated inositol polyphosphate metabolism is required for JA-regulated physiological responses.OsIPK2 is the only inositol polyphosphate kinase gene in rice,but there were no reports about molecular or physiological functions of OsIPK2.To understand the physiologic functions of OsIPK2,we generated OsIPK2 overexpression transgenic plants by introducing pCAMBIA1302-OsIPK2 and pER10-OsIPK2 constructs into ZH11 and the growth of these transgenic plants were inhibited.In addition,ectopic expression of OsIPK2 showed retarded growth phenotype.We also gained the polyclonal antibody of OsIPK2 for further functional analysis.Besides,we have identified a potential OsIPK2 interacted protein,OsMPK2,which may reveal a new regulatory mechanism in plant development.