Functional Analysis Of BolOST1, An Ortholog Of Open Stomata1Involved In Plant Drought Responses In Brassica Oleraceal

Cabbage (Brassica oleracea) are most common vegetable, playing an importantposition in the supply of vegetables in china. However, various kind of environmentalstresses (such as drought stress and high-salt) seriously affect the yield and quality of cabbage.ABA (Abscisic Acid), one of the most important environmental stress phytohormones, playscrucial roles in plant growth and development as well as in the response to several types ofenvironmental stresses. Previous research indicate that improvement of plant stre ss tolerancecan be achieved via moldulation of plant ABA signaling. The recently reported regulatorymechanism in the core module of early ABA signaling denotes that sucrose non-fermenting1-related protein kinase2(SnRK2) is very important. Open Stomata1(OST1)/SnRK2.6/SnRK2E, an ABA-activated SnRK2-type protien kinase, is critical for ABA signaling,drought stress and high salinity in plant. Kinase activity of OST1is closely correlated withstress tolerance impromvement.In the current study two fragments of BolOST1, designated BolOST1-1andBolOST1-2(GenBank Accession Nos. KF577724and KF577725), were isolated fromcabbage (Brassica oleracea).The full length of cDNA is1089nt and960nt,and encodepredicted proteins with362and319amino acids, respectively. The calculated molecularmasses of BolOST1-1and BolOST1-2are40.6kDa and35.6kDa, and the predicted pIvalues are4.72and4.49, respectively. Sequence analysis revealed that BolOST1-1andBolOST1-2contain nearly all of the conserved regions of OST1, including the kinase domainat the N-terminus and the ABA box at the C-terminus. Both BolOST1-1and BolOST1-2arenuclear-and cytoplasmic-localized proteins.A high degree of homology was observed amongBolOST1and Arabidopsis OST1. qRT-PCR (Real-time quantitative PCR) analysis of theexpression pattern of BolOST1in various tissues and various environmental stress thatBolOST1is mainly expressed in young leaves and seed,induced by drought and salt. Ecotopicexpression of BolOST1rescues the lack of OST1function in drought stress signaling.Recombinants BolOST1-1and BolOST1-2are active protein kinases, which are capable ofphosphorylating the N-terminus fragment of ABI5and MBP. Protein-Protein interactions are observed between BolOST1and homolog of the cabbage bZIP transcription factor BolABI5and/or Group A PP2C BolABI1.Overall, these results indicate that BolOST1is a functional SnRK2-type proteinkinase, and participate in positive moldulation of plant drought responses. BolOST1interactswith BolABI5and BolABI1and phosphorylates ABI5-N.The activation mechanism ofSnRK2-related kinases via the inhibition of ABI1-like PP2C proteins and the phosphorylationdependent activation of ABI5-like bZIP transcription factors by SnRK2-related kinases ispossibly conserved between Arabidopsis and cabbage.