Position dependent expression of GL2-type homeobox gene, Roc1: significance for protoderm differentiation and radial pattern formation in early rice embryogenesis

In early plant embryogenesis, the determination of cell fate in the protodermal cell layer is considered to be the earliest event in radial pattern formation. To elucidate the mechanisms of epidermal cell fate determination and radial pattern formation in early rice embryogenesis, we have isolated a GL2-type homeobox gene Roc1 (Rice outermost cell-specific gene1), which is specifically expressed in the protoderm (epidermis). In early rice embryogenesis, cell division occurs randomly and the morphologically distinct layer structure of the protoderm cannot be observed until the embryo reaches more than 100 microm in length. Nonetheless, in situ hybridization analyses revealed that specific expression of Roc1 in the outermost cells is established shortly after fertilization, much earlier than protoderm differentiation. In the regeneration process from callus, the Roc1 gene is also expressed in the outermost cells of callus in advance of tissue and organ differentiation, and occurs independently of whether the cells will differentiate into epidermis in the future or not. Furthermore, this cell-specific Roc1 expression could be induced flexibly in the newly produced outermost cells when we cut the callus. These findings suggest that the expression of Roc1 in the outermost cells may be dependent on the positional information of cells in the embryo or callus prior to the cell fate determination of the protoderm (epidermis). Furthermore, the Roc1 expression is downregulated in the inner cells of ligule, which have previously been determined as protodermal cells, also suggesting that the Roc1 expression is position dependent and that this position dependent Roc1 expression is important also in post-embryonic protoderm (epidermis) differentiation.

Study on the mechanism of cerium nitrate effects on germination of aged rice seed

Attempts were made to promote germination of naturally and artificially aged rice seeds by treating them with cerium nitrate. The germination rate, germination index, and vigor index of aged rice seed were significantly increased by cerium. It was because the treatments of aged rice seed with cerium nitrate enhanced respiratory rate and activities of superoxide dismutase, catalase, and peroxidase, and decreased superoxide O2- and malondialdehyde contents that plasma membrane permeability was reduced. It was suggested that cerium be used for the seed treatment before sowing.

Rice bicoid-related cDNA sequence and its expression during early embryogenesis

Bicoid is one of the important Drosophila maternal genes involved in the control of embryo polarity and larvae segmentation. To clone and characterize the rice bicoid-related genes, one cDNA clone, Rb24 (EMBL accession number: AJ2771380), was isolated by screening of rice unmature seed cDNA library. Sequence analysis indicates that Rb24 contains a putative amino acid sequence, which is homologous to unique 8 amino acids sequence within Drosophila bicoid homeodomain (50% identity, 75% similarity) and involves a lys-9 in putative helix 3. Northern blot analysis of rice RNA has shown that this sequence is expressed in a tissue-specific manner. The transcript was detected strongly in young panicles, but less in young leaves and roots. This results are further confirmed with paraffin section in situ hybridization. The signal is intensive in rice globular embryo and located at the apical tip of the embryo, then, along with the development of embryo, the signal is getting reduced and transfers into both sides of embryo. The existence of bicoid-related sequence in rice embryo and the similarity of polar distribution of bicoid and Rb24 mRNA in early embryo development may implicates a conserved maternal regulation mechanism of body axis presents in Drosophila and in rice.

A role for the rice homeobox gene Oshox1 in provascular cell fate commitment

The vascular tissues of plants form a network of interconnected cell files throughout the plant body. The transition from a genetically totipotent meristematic precursor to different stages of a committed procambial cell, and its subsequent differentiation into a mature vascular element, involves developmental events whose molecular nature is still mostly unknown. The rice protein Oshox1 is a member of the homeodomain leucine zipper family of transcription factors. Here we show that the strikingly precise onset of Oshox1 gene expression marks critical, early stages of provascular ontogenesis in which the developmental fate of procambial cells is specified but not yet stably determined. This suggests that the Oshox1 gene may be involved in the establishment of the conditions required to restrict the developmental potential of procambial cells. In support of this hypothesis, ectopic expression of Oshox1 in provascular cells that normally do not yet express this gene results in anticipation of procambial cell fate commitment, eventually culminating in premature vascular differentiation. Oshox1 represents the first example of a transcription factor whose function can be linked to specification events mediating provascular cell fate commitment.

Glucose modulates the abscisic acid-inducible Rab16A gene in cereal embryos

Glucose effects on the expression of the abscisic acid-inducible Rab16A gene were examined in rice and barley embryos. Glucose feeding to rice embryos negatively affects the endogenous abscisic acid content and represses the promoter activity of the Rab16A gene. Glucose repression of the Rab16A gene takes place both at a transcriptional and a post-transcriptional level. Modulation of the abscisic acid content in rice embryos triggered by glucose did not directly influence the expression of the rice alpha-amylase gene RAmy3D, which is known to be under glucose control. The possible interaction between the glucose and abscisic acid signaling pathway is discussed.

Promoter strength influences polyamine metabolism and morphogenic capacity in transgenic rice tissues expressing the oat adc cDNA constitutively

We analyzed molecularly and biochemically a series of transgenic rice lines expressing the oat adc (arginine decarboxylase) cDNA under the control of the constitutive maize ubiquitin 1 promoter. We established baseline biochemical parameters to elucidate the role of polyamines (PAs) during morphogenesis. We measured mRNA levels, ADC enzyme activity and cellular PAs in dedifferentiated callus. Polyamine levels were also quantified in two subsequent developmental stages--regenerating tissue and differentiated shoots. We observed significant (P < 0.05) differences in the levels of individual PAs at the three developmental stages. The amounts of putrescine (Put) and spermidine (Spd) in dedifferentiated transgenic callus were lower than those in the wild type or in hpt (hygromycin resistant)-controls, whereas the amount of spermine (Spm) was increased up to two-fold. In regenerating tissue, this trend was reversed, with significantly higher levels of Put and Spd (P < 0.05), and lower levels of Spm (P < 0.05) compared to non-transformed or hpt-control tissues at the same developmental stage. In differentiated shoots, there was a general increase in PA levels, with significant increases in Put, Spd, and Spm (P < 0.05); on occasion reaching six times the level observed in wild type and hpt-control tissues. These results contrast those we reported previously using the weaker CaMV 35S promoter driving adc expression. mRNA measurements and ADC enzyme activity were consistently higher (P < 0.01) in all tissues expressing pUbiadcs compared to equivalent tissues engineered with 35Sadc. Our findings are consistent with a threshold model which postulates that high adc expression leading to production of Put above a basal level is necessary to generate a big enough metabolic pool to trigger PA flux through the pathway leading to an increase in the concentration of Spd and Spm. This can be best accomplished by a strong constitutive promoter driving adc. We discuss our results in the context of flux through the PA pathway and its impact on morphogenesis.

Regional expression of the rice KN1-type homeobox gene family during embryo, shoot, and flower development

We report the isolation, sequence, and pattern of gene expression of members of the KNOTTED1 (KN1)-type class 1 homeobox gene family from rice. Phylogenetic analysis and mapping of the rice genome revealed that all of the rice homeobox genes that we have isolated have one or two direct homologs in maize. Of the homeobox genes that we tested, all exhibited expression in a restricted region of the embryo that defines the position at which the shoot apical meristem (SAM) would eventually develop, prior to visible organ formation. Several distinct spatial and temporal expression patterns were observed for the different genes in this region. After shoot formation, the expression patterns of these homeobox genes were variable in the region of the SAM. These results suggest that the rice KN1-type class 1 homeobox genes function cooperatively to establish the SAM before shoot formation and that after shoot formation, their functions differ.

Cloning of a wheat puroindoline gene promoter by IPCR and analysis of promoter regions required for tissue-specific expression in transgenic rice seeds

A genomic DNA fragment containing the 5'-upstream sequence and part of the open reading frame corresponding to Triticum aestivum puroindoline-b cDNA, was isolated by inverse PCR. Promoter fragments extending to -1068, -388, -210 or -124 upstream of the translation initiation ATG codon and the sequence coding for the first 13 amino acids of the puroindoline-b, were translationally fused to the uidA reporter gene encoding beta-glucuronidase and transferred to rice calli via particle bombardment-mediated transformation. The 1068 bp and 124 bp promoters were also transcriptionally fused to the uidA reporter gene. Out of the 196 plants regenerated from transformed rice calli, 118 plants set seeds. No GUS activity was detectable in the stems, roots, leaves or pollen of the transgenic rice which had integrated the puroindoline-b promoter or its deletions; GUS activity was detected only in seeds, except in those having integrated the 124 bp promoter. Within seeds, histological localisation showed GUS activity as being restricted to the endosperm, aleurone cells and pericarp cell layers; no GUS activity was detected in the embryonic axis. Analysis of 5' promoter deletions identified the region between -388 and -210 as essential for endosperm expression, and the region between -210 and -124 as essential for expression in the epithelium of the scutellum. No difference of expression was observed between the translational and transcriptional fusion genes.

Expression of novel homeobox genes in early embryogenesis in rice

We isolated four novel cDNA clones of rice (Oryza sativa L.), which encode predicted proteins with a KN1-like homeodomain. In situ hybridization and RT-PCR analysis with solid cDNA libraries as templates showed that these genes are expressed in distinct patterns during the early stages of rice embryogenesis.

Characterization of the KNOX class homeobox genes Oskn2 and Oskn3 identified in a collection of cDNA libraries covering the early stages of rice embryogenesis

For identification of genes involved in embryogenesis in the model cereal rice, we have constructed a collection of cDNA libraries of well-defined stages of embryo development before, during and after organ differentiation. Here, we focus on the possible role of KNOX (maize Knotted1-like) class homeobox genes in regulation of rice embryogenesis. Three types of KNOX clones were identified in libraries of early zygotic embryos. Two of these, Oskn2 and Oskn3, encode newly described KNOX genes, whereas the third (Oskn1) corresponds to the previously described OSH1 gene. In situ hybridizations showed that during the early stages of embryo development, all three KNOX genes are expressed in the region where the shoot apical meristem (SAM) is organizing, suggesting that these genes are involved in regulating SAM formation. Whereas OSH1 was previously proposed to function also in SAM maintenance, Oskn3 may be involved in patterning organ positions, as its expression was found to mark the boundaries of different embryonic organs following SAM formation. The expression pattern of Oskn2 suggested an additional role in scutellum and epiblast development. Transgenic expression of Oskn2 and Oskn3 in tobacco further supported their involvement in cell fate determination, like previously reported for Knotted1 and OSH1 ectopic expression. Whereas Oskn3 transformants showed the most pronounced phenotypic effects during vegetative development, Oskn2 transformants showed relatively mild alterations in the vegetative phase but a more severely affected flower morphology. The observation that the KNOX genes produce similar though distinct phenotypic reponses in tobacco, indicates that their gene products act on overlapping but different sets of target genes, or that cell-type specific factors determine their precise action.

Plastid promoter utilization in a rice embryogenic cell culture

Plastid promoter utilization was characterized in rice by mapping transcript 5'-ends in samples derived from leaves and cultured embryogenic cells. We have found that rbcL, atpB and the rRNA operon are transcribed by the plastid-encoded plastid RNA polymerase (PEP), while clpP is transcribed by the nucleus-encoded plastid RNA polymerase (NEP) in both chloroplasts and the non-green plastids of embryogenic cultured cells. This finding is in contrast to reports on BY2 tobacco, in which NEP promoter activity in cultured cells was enhanced relative to leaves, facilitating identification of NEP promoters which are undetectable in chloroplasts. Therefore, it appears that activation of plastid NEP promoters in rice is not essential for adaptation to cell culture.

Sugar sensing and alpha-amylase gene repression in rice embryos

We used a transient expression system to study the mechanism by which carbohydrates repress a rice (Oryza sativa L.) alpha-amylase (EC 3.2.1.1) gene. Exogenously fed metabolizable carbohydrates are able to elicit repression of the alpha-amylase gene RAmy3D in the rice embryo, and our results indicate that repression is also triggered efficiently by endogenous carbohydrates. Glucose analogs that are taken up by plant cells but not phosphorylated by hexokinase are unable to repress the alpha-amylase gene studied, while 2-deoxyglucose, which is phosphorylable but not further metabolized, down-regulates RAmy3D promoter activity, indicating a role for hexokinase in the sugar-sensing mechanism triggering repression of the RAmy3D gene. We tested two different hexokinase inhibitors, mannoheptulose and glucosamine, but only the latter was able to relieve RAmy3D promoter activity from repression by endogenous carbohydrates. This correlates with the higher ability of glucosamine to inhibit the activity of rice hexokinases in vitro. The glucosamine-mediated relief of RAmy3D promoter activity from repression by endogenous carbohydrates does not correlate with a reduced rate of carbohydrate utilization.

Cloning of genes specifically expressed in rice embryogenic cells

We have examined differences in gene expression pattern between embryogenic callus (EC) and nonembryogenic callus (NEC) derived from mature seed embryo of rice (Oryza sativa L. cv Donggin). Three EC-specific transcripts were identified by differential display of amplified cDNAs. Specific expression of two partial cDNAs, designated as REC1 and REC2, respectively, was confirmed by a northern blot analysis. Partial nucleotide sequence of the clone REC1 showed no homology with any known genes, but partial amino acid sequence deduced from the clone REC2 exhibited 55-82% homology with nickel-cobalt-resistant proteins identified from a bacterium, Alcaligenes eutrophus CH34.

Functional dissection of a sugar-repressed alpha-amylase gene (RAmy1 A) promoter in rice embryos

The gibberellin-inducible rice alpha-amylase gene, RAmy1 A, was demonstrated to be sugar repressed in rice embryos and functional dissection of the promoter of RAmy1 A in relation of its sugar-modulated expression was performed. Gibberellin-response cis-elements of GARE (TAACAAA) and pyrimidine box (CCTTTT) were partially involved in the sugar repression.

Plant regeneration of indica rice (Oryza sativa) cultivars from mature embryo-derived calli

Plant regeneration from seven-week-old callus cultures derived from mature embryos of several indica rice cultivars was achieved with frequencies of morphogenic calli from 10 to 47%. Three media were tested both for callogenesis and plant regeneration. For 3 of the 7 genotypes examined, the best combination of media for plant regeneration was Murashige & Skoog basal medium: MSC (callogenesis) and MSR (regeneration). The rates of callogenesis were not related to the capacity for plant regeneration. Two genotypes CR-1113 and CR-5272 produced the highest number of regenerated green plants. The results of this study suggest that genetic differences could be directly linked to the ability to regenerate in these plant cultivars.

Rice embryo globulins: amino-terminal amino acid sequences, cDNA cloning and expression

A globulin fraction prepared from rice embryos contained polypeptides or polypeptide groups of 49 kDa (designated REG1), 46 kDa (designated REG2), about 35 kDa, 32 kDa and 25 kDa. The amino-terminal sequences of REG1 and the major polypeptide in the 35-kDa group were identical, suggesting that the REG1 polypeptide undergoes partial proteolytic processing that removes a carboxy-terminal region. A cDNA clone, designated pcREG2, encoding REG2 was isolated, and its nucleotide sequence was determined. The deduced amino acid sequence of REG2 was found to be 68% identical to that of the maize GLB2 globulin. Reg2 mRNA was present at high levels during embryo development for up to 14 days after flowering (DAF). Lower levels were found 20 DAF when the maturation of embryos was almost completed, and at the dry mature stage. Reg2 mRNA almost disappeared upon imbibition of isolated dry mature embryos but it was re-induced at a low level by further treatment with ABA. The expression of Reg2 was not induced by ABA in suspension-cultured cells, unlike that of Osem, one of the late embryogenesis abundant protein (LEA) genes.

Characterization of common cis-regulatory elements responsible for the endosperm-specific expression of members of the rice glutelin multigene family

Glutelin is the most abundant storage protein in rice, which is expressed specifically in the endosperm of maturing seed. Glutelin is encoded by about 10 genes per haploid genome, which are clearly divided into two subfamilies (GluA and GluB). Most of them are coordinately expressed during seed maturation in spite of the remarkable divergence in the 5'-flanking regions between members of two subfamilies. In order to identify the common regulatory mechanisms responsible for the endosperm-specific expression, various cis-regulatory elements in the 5'-flanking region of the glutelin GluB-1 gene were characterized by studying the expression of chimeric genes that consisted of the sequentially deleted or mutagenized promoter and a beta-glucuronidase (GUS) reporter gene in transgenic tobacco seeds. The essential cis-regulatory elements governing the spatially and temporally specific expression of the glutelin gene expression were located within the first 245 bp of the promoter region of the GluB-1 gene from the site of initiation of transcription. The AACA motif between positions -73 and -61 common to all the six genes for glutelin sequenced to date and is repeated between positions -212 and -200 is implicated in the seed-specific expression. The GCN4 motif between positions -165 and -158 and between positions -96 and -92 that is conserved at homologous sites in all the members of glutelin gene family is also involved in the seed-specific regulation. However, both are required for the high level of seed-specific expression, because deletion of the region containing one set of both elements or substitution mutation of the AACA or GCN4 motif substantially reduced the activity. As a whole, our results suggest the combinatorial interaction of the elements in regulation of the glutelin gene expression.

Effects of long duration space flight on rice seed (or embryo) radiation sensitivity and element microlocalizations

In long duration space experiments Rice caryopses and embryos, which are able to remain alive 10 years (or more) and tolerate extreme physical conditions (temperature, few water content) during irradiation and post-irradiation storage, were used (8, 40, 201 and 457 days on board of Salyut 7, 2107 days on LDEF). In certain experiments (Salyut 7), samples were irradiated either before or after the flight. Effects of the flight and radiosensitivity were observed in Rice seedlings cultivated in in vitro conditions. Statistical results indicate an increase in radiosensitivity when irradiations occur before the flight. Microanalyses were made in different parts of one caryopsis and of one embryo, and the results compared with those of control samples. With caryopses and embryos of the same Rice varieties, but from LDEF, we made the same kinds of experiments to compare results.

Investigation on rice embryos and seeds after the LDEF flight: electronic spin resonance identification

Rice caryopsis of Cigalon variety with short grain of the LDEF mission can develop and grow as well as those of the laboratory control. Rice caryopsis of Delta variety with long grain did not develop while a small number of excised embryos can develop and grow as well as the control group. A preliminary study of the Electron Spin Resonance (ESR) spectra of Rice embryos and seeds recorded several month after the flight on flight samples and on control ones has been carried out. All these samples had the same storage time. During storage the radical concentration which usually decreases, now depends on irradiation doses and on whether or not they were delivered in presence of oxygen. The signal variations are smaller than those usually observed in the different parts of the starch. An estimation of a "gamma-equivalent-dose" can be reached.

cDNA cloning of regeneration-specific genes in rice by differential screening of randomly amplified cDNAs using RAPD primers

A simple method for differential screening of randomly amplified cDNAs using primers for detection of randomly amplified polymorphic DNA (RAPD) has been developed. To detect and clone differentially expressed genes during regeneration, we compared mRNAs from rice calli before the induction of regeneration, 7 days after induction of organogenesis, and 7 days after induction of embryogenesis. The cDNAs were amplified by the polymerase chain reaction (PCR) with a single RAPD primer and were separated by agarose gel electrophoresis. A number of differentially amplified bands were detected. Five of the specific bands were cloned and their expression was analyzed by Northern hybridization. We isolated a cDNA clone which is specific to organogenesis, two clones which are specific to embryogenesis, and two clones which are common to organogenesis and embryogenesis but not present in unorganized calli. Two of the isolated clones are expressed at low levels. Thus, this method is useful for cloning of differentially expressed genes whose transcripts are of low abundance. Expression of one of the embryogenesis-specific cDNA clones, pCRE2, was analyzed in detail. The pCRE2 transcript accumulates transiently in calli after the induction of embryogenesis, and its accumulation in planta was specific to zygotic embryos.

Interactions among four subunits of elongation factor 1 from rice embryo

To establish the subunit construction of elongation factor EF-1, interactions among four non-identical subunits of rice embryo EF-1 (alpha, beta, beta', and gamma) were analyzed with polyacrylamide gel electrophoresis. Complexes beta beta', alpha beta, alpha beta', and beta gamma were formed by mixing the two respective subunits. However, no complex was formed between EF-1 beta' and EF-1 gamma. Complexes containing three subunits like alpha beta beta', alpha beta gamma, and beta beta' gamma, were formed by mixing the three respective subunits. EF-1 was reconstructed when each subunit was added in the following order, beta, beta', gamma, and alpha. The affinity of EF-1 alpha for other subunits was as follows, beta beta' gamma > beta beta' > beta not equal to beta'. Likewise, the affinity of EF-1 gamma for other subunits was: beta beta' gamma > beta >> beta'. Phe-tRNA binding activity of the reconstructed EF-1 was about 90% of that of the native EF-1. From these results, we concluded that rice embryo EF-1 is constructed of equimolar amount of four subunits, alpha, beta, beta' and gamma.

Role of media constituents and proline in callus growth, somatic embryogenesis and regeneration of Oryza sativa cv Indica

Conditions were optimised for efficient callus induction from seeds of four local indica rice cultivars, GR-3, GR-102, Jaya and Te-Tep. Addition of 2,4-D to MS medium at 2.5 mg/l resulted in 100% callus induction. N6 medium was superior to MS medium for callus growth, formation of embryogenic callus as well as regeneration. Cultivar Te-Tep showed highest callus growth while GR-102 the least. Addition of casein hydrolysate enhanced growth of callus but did not yield more embryogenic calli. Supplementation of MS or N6 media with proline, not only increased callus growth but also showed an increase in embryogenic callus formation. GR-102 callus was most embryogenic followed by Te-Tep, GR-3 and Jaya. Histological observation of embryogenic calli revealed the presence of pro-embryo like structures. It was also observed that calli induced on N6 medium supplemented with proline could maintain regeneration potential for a longer period as compared to other media. Cytokinins like BAP or kinetin alone could not initiate shoot formation. Regeneration frequency, and the number of shoots formed per callus increased significantly. Cultivar Te-Tep gave the best response for regeneration followed by Jaya, GR-3 and GR-102.

Herbicide-resistant Indica rice plants from IRRI breeding line IR72 after PEG-mediated transformation of protoplasts

The commercially important Indica rice cultivar Oryza sativa cv. IR72 has been transformed using direct gene transfer to protoplasts. PEG-mediated transformation was done with two plasmid constructs containing either a CaMV 35S promoter/HPH chimaeric gene conferring resistance to hygromycin (Hg) or a CaMV 35S promoter/BAR chimaeric gene conferring resistance to a commercial herbicide (Basta) containing phosphinothricin (PPT). We have obtained so far 92 Hgr and 170 PPTr IR72 plants from protoplasts through selection. 31 Hgr and 70 PPTr plants are being grown in the greenhouse to maturity. Data from Southern analysis and enzyme assays proved that the transgene was stably integrated into the host genome and expressed. Transgenic plants showed complete resistance to high doses of the commercial formulations of PPT.

Isolation and characterization of nuclei from rice embryos

A method has been developed to isolate pure preparations of nuclei in high yield from commercially available viable rice embryos (germ), employing extraction with buffer solution containing glycerol (without detergent) and polyamine, followed by centrifugation on a 30% Percoll cushion. The intactness of the isolated nuclei was confirmed by light microscopy as well as electron microscopy. The protein profiles of both whole nuclei and nuclear extracts obtained by SDS-PAGE, organellar marker enzyme activities, DNA and RNA analyses, and in vitro RNA synthesis, all indicate that the highly purified nuclei are isolated from rice embryos.

Spatial patterns of histone mRNA expression during grain development and germination in rice

The pattern of distribution of histone H3 mRNA during the development of the rice grain and its germination was monitored by in situ hybridization and confirmed by Northern blot analysis. In ovaries sampled before and after fertilization, a 3H-labeled histone RNA probe was localized in the cells of the pericarp, outer integument and nucellus but binding of the probe decreased as these tissues senesced. In the developing embryo, the histone message was first detected in the scutellum; later, all parts of the embryo except the shoot apex, newly formed leaf primordia and the quiescent center of the root, revealed the presence of transcripts. Considerable binding of the probe also occurred in the endosperm as its cells began to accumulate starch. Cells of the embryo and endosperm of mature grains displayed very little or no histone mRNA, although during germination, these transcripts reappeared in the cells of both embryo and endosperm. There was a good correlation between the presence or absence of binding to the 32P-labeled histone gene DNA by RNA extracted from grains of different stages of development and germination, as revealed by Northern blot analysis and by spatial localization following in situ hybridization.

Expression of rice lectin is governed by two temporally and spatially regulated mRNAs in developing embryos

Two cDNA clones encoding rice lectin have been isolated and characterized to investigate the expression of rice lectin at the molecular and cellular levels. The two cDNA clones code for an identical 23-kilodalton protein which is processed to the mature polypeptide of 18 kilodaltons by co-translational cleavage of a 2.6-kilodalton signal sequence and selective removal of a 2.7-kilodalton COOH-terminal peptide which contains a potential N-linked glycosylation site. In addition, the mature 18-kilodalton lectin is post-translationally cleaved between residues 94 and 95 to yield polypeptides of 10 kilodaltons and 8 kilodaltons, corresponding to the NH2- and COOH-terminal portions of the mature subunit, respectively. RNA gel blot analysis established that rice lectin is encoded by two mRNA transcripts (0.9 kilobase and 1.1 kilobase). On DNA gel blots, the rice lectin cDNAs hybridize specifically to a single restriction fragment. In situ hybridization showed localization of the 1.1-kilobase rice lectin mRNA in root caps and specific cell layers of the radicle, coleorhiza, scutellum, and coleoptile. RNA gel blot analysis demonstrated that both the 0.9-kilobase and 1.1-kilobase mRNAs are present in developing rice embryos. The two lectin mRNAs are differentially expressed temporally such that the 1.1-kilobase lectin mRNA accumulates to levels twofold higher than the 0.9-kilobase mRNA.