A sweetpotato (Ipomoea batatas cv. ‘Jinhongmi’) MADS-box protein cDNA (SRD1) has been isolated from an early stage storage root cDNA library. The role of the SRD1 gene in the formation of the storage root in sweetpotato was investigated by an expression pattern analysis and characterization of SRD1-overexpressing (ox) transgenic sweetpotato plants. Transcripts of SRD1 were detected only in root tissues, with the fibrous root having low levels of the transcript and the young storage root showing relatively higher transcript levels. SRD1 mRNA was mainly found in the actively dividing cells, including the vascular and cambium cells of the young storage root. The transcript level of SRD1 in the fibrous roots increased in response to 1000 μM indole-3-acetic acid (IAA) applied exogenously. During the early stage of storage root development, the endogenous IAA content and SRD1 transcript level increased concomitantly, suggesting an involvement of SRD1 during the early stage of the auxin-dependent development of the storage root. SRD1-ox sweetpotato plants cultured in vitro produced thicker and shorter fibrous roots than wild-type plants. The metaxylem and cambium cells of the fibrous roots of SRD1-ox plants showed markedly enhanced proliferation, resulting in the fibrous roots of these plants showing an earlier thickening growth than those of wild-type plants. Taken together, these results demonstrate that SRD1 plays a role in the formation of storage roots by activating the proliferation of cambium and metaxylem cells to induce the initial thickening growth of storage roots in an auxin-dependent manner.

Squalene synthase (SQS) catalyzes the biosynthesis of squalene by condensing two molecules of farnesyl pyrophosphate (FPP), a key precursor in sterol and triterpene biosynthesis. Previously, we reported that PgSS1 overexpression results in the enhanced biosynthesis of both phytosterols and triterpene saponins in Panax ginseng. Here, cDNAs encoding two new SQS homologs (PgSS2 and PgSS3) from a P. ginseng expressed sequence tag (EST) library are described. Functional complementation analysis revealed that ectopic expression of PgSS1, PgSS2 and PgSS3 in the yeast erg9 mutant strain 2C1 lacking SQS activity restored ergosterol prototrophy. The recombinant mutant yeast produced squalene, squalene epoxide and ergosterol. PgSS1 (mRNA) was highly transcribed in all organs, whereas PgSS2 and PgSS3 (mRNAs) were only transcribed in specific organs. All three genes were activated positively by an elicitor (methyl jasmonate), but their transcriptional patterns were different. In situ hybridization analysis revealed that both PgSS1 and PgSS3 transcripts were preferentially accumulated near conducting tissue in the petiole. The PgSS1 and PgSS3 promoters were isolated, and the tissue- and organ-specific regulation of PgSS genes was examined. Transgenic ginseng was constructed by introducing PgSS1 and PgSS3 promoters fused to the β-glucuronidase (GUS) gene. GUS expression driven by the PgSS1 promoter was found in both roots and shoots, but PgSS3-driven GUS was only found in shoots. These results suggest that the three SQS genes are differently expressed and that all three SQS enzymes are involved in squalene production in P. ginseng.

A nonamer motif (CATCCAACG) that is one of the cis-acting elements identified in the proximal promoter region of some wheat histone genes is included in the region that interacts with the wheat DNA-binding protein, HBP (histone gene-binding protein)-2. To obtain structural and functional information about this DNA-binding protein, we attempted to isolate a cDNA clone encoding HBP-2 on the basis of its ability to bind to a nonamer-containing 38-bp DNA fragment. Southwestem screening of a wheat cDNA library with concatenated 38-residue oligonucleotides as the probe produced one candidate clone. Nucleotide sequence analyses of this cDNA clone and the corresponding genomic clone showed that the protein deduced from the nucleotide sequence consisted of 261 amino acids and contained a set of zinc-finger motifs similar to those found in many eukaryotic transcription factors. The protein, named WZFl (wheat zinc-finger protein l), which was expressed from the cDNA in Escherichia coli cells, bound specifically and metal-ion-dependently to the nonamer-containing oligonucleotide. The WZFl mRNA was highly expressed in the root apexes of wheat seedlings, but less so in the proximal portion of young leaves; whereas, histone H3 mRNA was highly expressed in both tissues. The expression patterns of the WZFl and histone H3 genes in the early stages of germination differed, expression of the WZFl gene being almost constant but not that of the H3 gene. The relationship of WZFl and HBP-2 and the possible role of WZFl in the histone gene expression were discussed.Histones, major protein components of chromatin, function in the assembling of DNA into nucleosomes, and their subtypes are encoded by multigene families. Results of studies of histone gene expression in animal and yeast systems suggest that the subtype histone genes are expressed coordinatedly with nuclear DNA synthesis during the cell cycle and that they are mainly regulated at the transcriptional level. The mechanism by which they are expressed in the S phase has yet to be clarified, however (for reviews, Schumperli, 1986;Heintz, 1991 ;Osley, 1991). Plant histone genes have now been cloned from a number of higher plants (see and references therein), but little research has been done on their regulatory mechanisms.In our studies of the molecular mechanisms that control transcriptional regulation in wheat histone genes (for reviews, Mikami and Iwabuchi, 1993;Takase and Iwabuchi, 1993), we recently showed that the proximal promoter region (within -185 relaCorrespondence to M. Iwabuchi,

The expression of genes encoding five histones (H1, H2A, H2B, H3 and H4) and the putative transcription factors HBP-1a (17) and HBP-1b (c38) was examined during early germination and in various tissues of young wheat seedlings. The steady-state levels of core histone (H2A, H2B, H3 and H4) mRNAs were coordinately cell cycle-dependent and paralleled the rate of DNA synthesis during early germination, whereas the expression pattern of the linker histone (H1) genes differed. The five subclass histone genes were actively expressed in the meristematic tissues of young seedlings. Moreover, H1 genes were expressed in leaves that consist mostly of non-proliferating cells, in which core histone genes showed little expression. Quantitative alterations to the mRNAs of the putative transcription factors HBP-1a (17) and HBP-1b (c38) of wheat histone genes were similar to those of the core histone mRNAs, suggesting that both factors function in the cell cycle-dependent expression of wheat core histone genes.

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