Arabidopsis NF‐YB subunits LEC1 and LEC1‐LIKE activate transcription by interacting with seed‐specific ABRE‐binding factors

SUMMARYLEAFY COTYLEDON 1 (LEC1) plays vital roles in the regulation of seed maturation in Arabidopsis. LEC1encodesahomologofyeastHAP3ormammalianNF-YB/CBF-AsubunitoftrimericCCAATbindingfactor(CBF).Among the nine paralogs of NF-YB in Arabidopsis, LEC1-LIKE (L1L) is most closely related to LEC1, and cancomplement the lec1 mutation when expressed under the control of the LEC1 promoter. Although the natureof the B3-type seed maturation regulators as transcription factors have been investigated, knowledge of themolecular action of LEC1 is limited. When co-expressed with NF-YC2 in the presence of ABA, we found thatLEC1 or L1L, but not other NF-YBs, activated the promoter of CRUCIFERIN C (CRC), which encodes a seedstorage protein. However, additional expression of an NF-YA subunit interfered with the activation. The LEC1/L1L-[NF-YC2]activationdependedonABA-responseelementspresentinthepromoter,whichledtothefindingthatLEC1/L1L-[NF-YC2]canstronglyactivatetheCRCpromoterintheabsenceofABAwhenco-expressedwitha seed-specific ABA-response element (ABRE)-binding factor, bZIP67. Functional coupling of LEC1/L1L-[AtNF-YC2] and bZIP67 was also observed in the regulation of sucrose synthase 2 (SUS2). ImmunoprecipitationexperimentsrevealedthatL1LandbZIP67formedaproteincomplexinvivo.Theseresultsdemonstrateanovelplant-specific mechanism for NF-Y subunit function that enables LEC1 and L1L to regulate a defineddevelopmental network.Keywords: CCAAT-binding factor,abscisicacidresponseelement,LEAFYCOTYLEDON1,basic-leucinezipper,storage protein.INTRODUCTIONEmbryogenesis in higher plants is a developmental processthat not only establishes the basic body plan, but also pro-duces seed, a strategy that secures progeny by maintaininga quiescent state under unfavorable conditions, andresuming growth under favorable conditions. Seed charac-teristics, such as storage of proteins and other nutrients,desiccation tolerance and dormancy, are conferred duringthe maturation phase, which corresponds to the latter half ofembryogenesis after the completion of morphogenesis. InArabidopsis, four genes, LEAFY COTYLEDON 1 (LEC1),LEC2, FUSCA 3 (FUS3) and ABSCISIC ACID INSENSITIVE 3(ABI3), have been found to play central roles in the regula-tion of embryo maturation (Giraudat et al., 1992; Keith et al.,1994; Lotan et al., 1998; Luerssen et al., 1998; Meinke, 1992;Meinke et al., 1994; Stone et al., 2001; West et al., 1994).Mutation in any of these genes leads to defects in broadaspects of the maturation process, and thus they areregarded as master regulators of seed maturation; notably,however, these genes do not function exclusively duringmaturation (Vicente-Carbajosa and Carbonero, 2005). Thelec1, lec2 and fus3 mutants, which are grouped as lec-typemutants, differ morphologically from the abi3 mutant in thatthey exhibit leafy traits in the cotyledons, which are typicallyrecognized by the presence of trichomes on the cotyledons(Baumlein et al., 1994; Keith et al., 1994; Meinke, 1992;Meinke et al., 1994; West et al., 1994). abi3 mutant seedsalso display unique phenotypes, not observed for the lec-type mutants, such as failure in chlorophyll degradation and