An Endo-[beta]-Mannanase Develops Exclusively in the Micropylar Endosperm of Tomato Seeds Prior to Radicle Emergence

A galactomannan-hydrolyzing enzyme that develops pregerminatively in the micropylar region of the endosperm of the tomato (Lycopersicon esculentum [L.] Mill.) seed was characterized. The enzyme was endo-[beta]-mannanase (EC 3.2.1.78), since it hydrolyzed galactomannan into oligosaccharides with no release of galactose and mannose. The mobility of this pregerminative enzyme in sodium dodecyl sulfate and native polyacrylamide gel electrophoresis was not identical to that of any of the three endo-[beta]-mannanases that develop in the same tissue (endosperm) after germination (H. Nonogaki, M. Nomaguchi, Y. Morohashi [1995] Physiol Plant 94: 328-334). There were also some differences in the products of galactomannan hydrolysis between the pregerminative and the postgerminative enzymes, indicating that the action pattern is different between the two types of enzymes. The pregerminative enzyme began to develop in the micropylar region of the endosperm at about 18 h postimbibition and increased up to the time immediately before radicle protrusion (24 h postimbibition). This enzyme was not present in the lateral part of the endosperm at any stage before or after germination. It is proposed that the enzyme develops prior to germination specifically at the micropylar region of the endosperm.     

A Single-Seed Assay for Endo-[beta]-Mannanase Activity from Tomato Endosperm and Radicle Tissues

Completion of germination (radicle emergence) is an all-or-none developmental event for an individual seed. Variation in germination timing among seeds in a population therefore reflects variation among seeds in the rates or extents of physiological or biochemical processes prior to radicle emergence. For tomato (Lycopersicon esculentum Mill.) seeds, correlative evidence suggests that endo-[beta]-mannanase activity weakens the endosperm cap tissue opposite the radicle tip to permit radicle emergence. To test whether endo-[beta]-mannanase activity is causally related to germination rates, we have developed a sensitive assay suitable for use with individual radicle tips or endosperm caps. We show that endo-[beta]-mannanase activity varies at least 100-fold and often more than 1000-fold among individual inbred tomato seeds prior to radicle emergence. Other sources of variation (tissue size and experimental error) were evaluated and cannot account for this range of activity. Endo-[beta]-mannanase activity was generally 10-fold greater in leachates from endosperm caps than from radicle tips. Release of reducing sugars from individual endosperm caps also varied over a considerable (9-fold) range. These extreme biochemical differences among individual tomato seeds prior to radicle emergence indicate that results obtained from bulk samples could be misleading if it is assumed that all seeds exhibit the "average" behavior.     

Endo-[beta]-Mannanase Activity from Individual Tomato Endosperm Caps and Radicle Tips in Relation to Germination Rates

Endo-[beta]-mannanase is hypothesized to be a rate-limiting enzyme in endosperm weakening, which is a prerequisite for radicle emergence from tomato (Lycopersicon esculentum Mill.) seeds. Using a sensitive, single-seed assay, we have measured mannanase activity diffusing from excised tomato endosperm caps following treatments that alter the rate or percentage of radicle emergence. Most striking was the 100- to more than 10,000-fold range of mannanase activity detected among individual seeds of highly inbred tomato lines, which would not be detected in pooled samples. In some cases a threshold-type relationship between mannanase activity and radicle emergence was observed. However, when radicle emergence was delayed or prevented by osmoticum or abscisic acid, the initial increase in mannanase activity was unaffected or even enhanced. Partially dormant seed lots displayed a bimodal distribution of activity, with low activity apparently associated with dormant seeds in the population. Gibberellin- and abscisic acid-deficient mutant seeds exhibited a wide range of mannanase activity, consistent with their variation in hormonal sensitivity. Although the presence of mannanase activity in the endosperm cap is consistently associated with radicle emergence, it is not the sole or limiting factor under all conditions.     

Relationship of Endo-[beta]-D-Mannanase Activity and Cell Wall Hydrolysis in Tomato Endosperm to Germination Rates

The endosperm tissue enclosing the radicle tip (endosperm cap) governs radicle emergence in tomato (Lycopersicon esculentum Mill.) seeds. Weakening of the endosperm cap has been attributed to hydrolysis of its mannan-rich cell walls by endo-[beta]-D-mannanase. To test this hypothesis, we measured mannanase activity in tomato endosperm caps from seeds allowed to imbibe under conditions of varying germination rates. Over a range of suboptimal temperatures, mannanase activity prior to radicle emergence increased in accordance with accumulated thermal time. Reduced water potential delayed or prevented radicle emergence but enhanced mannanase activity in the endosperm caps. Abscisic acid did not prevent the initial increase in mannanase activity, although radicle emergence was markedly delayed. Sugar composition and percent mannose (Man) content of endosperm cap cell walls did not change prior to radicle emergence under any condition. Man, glucose, and other sugars were released into the incubation solution by endosperm caps isolated from intact seeds during imbibition. Pregerminative release of Man was suppressed and the release of glucose was enhanced when seeds were incubated in osmoticum or abscisic acid; the opposite occurred in the presence of gibberellin. Thus, whereas sugar release patterns were sensitive to environmental and hormonal factors affecting germination, neither assayable endo-[beta]-D-mannanase activity nor changes in cell wall sugar composition of endosperm caps correlated well with tomato seed germination rates under all conditions.     

Endo-1,4-{beta}-Glucanase in the Cell Wall of Stems of Auxin-Treated Pea Seedlings

Endo-1,4-ß-glucanase induced by treatment of pea seedlingswith 2,4-D was extracted from a preparation of the walls ofepicotyl cells. The ß-glucanase was purified by chromatographyon DEAE-cellulose, affinity chromatography on Con A-Sepharoseand SDS-polyacrylamide gel electrophoresis (SDS-PAGE). The activityof ß-glucanase was retained after removal of SDS andextraction from polyacrylamide gels. The band of a protein (46kDa), that corresponded to the activity of endo-1,4-ß-glucanase,was injected directly into mice for preparation of antiserumand the protein was also subjected to amino acid sequencingafter blotting onto a membrane. Western blot analysis showedthat the antiserum obtained bound to a 46-kDa polypeptide andrecognized endo-1,4-ß-glucanase. The N-terminal sequenceof the 46-kDa polypeptide revealed some homology to abscissionendo-1,4-ß-glucanases of bean and avocado fruit. (Received September 29, 1993; Accepted January 20, 1994)     

Changes in the Endosperm Cell Walls of Two Datura Species before Radicle Protrusion

The possibility of an association between changes in cell walls of the micropylar portion of the endosperm and the induction of germination was explored in seeds of Datura ferox and Datura stramonium. The structure of the inner surface of the endosperm was studied by scanning electron microscopy and the composition of cell wall polysaccharides analyzed by gas chromatography and gas chromatography-mass spectrometry. Both scanning electron microscope images and chemical analysis showed changes in the micropylar portion of the endosperm in induced seeds before radicle protrusion. The inner surface of the endosperm appeared eroded, and in some areas, wall material seemed to be missing. The content of the main component of the cell wall polysaccharides, containing predominantly 4-linked mannose, decreased well before the emergence of the radicle through the endosperm. We propose that the degradation of a mannan type polysaccharide is an important factor in the reduction in mechanical strength of the endosperm, thus facilitating germination.     

Class I [beta]-1,3-Glucanases in the Endosperm of Tobacco during Germination

Rupture of the seed coat and rupture of the endosperm are separate events in the germination of Nicotiana tabacum L. cv Havana 425 seeds. Treatment with 10-5 M abscisic acid (ABA) did not appreciably affect seed-coat rupture but greatly delayed subsequent endosperm rupture by more than 100 h and resulted in the formation of a novel structure consisting of the enlarging radicle with a sheath of greatly elongated endosperm tissue. Therefore, ABA appears to act primarily by delaying endosperm rupture and radicle emergence. Measurements of [beta]-1,3-glucanase activity, antigen content, and mRNA accumulation together with reporter gene experiments showed that induction of class I [beta]-1,3-glucanase genes begins just prior to the onset of endosperm rupture but after the completion of seed-coat rupture. This induction was localized exclusively in the micropylar region of the endosperm, where the radicle will penetrate. ABA treatment markedly inhibited the rate of [beta]-1,3-glucanase accumulation but did not delay the onset of induction. Independent of the ABA concentration used, onset of endosperm rupture was correlated with the same [beta]-1,3-glucanase content/seed. These results suggest that ABA-sensitive class I [beta]-1,3-glucanases promote radicle penetration of the endosperm, which is a key limiting step in tobacco seed germination.     

Endo-1,4-[beta]-Glucanase,Xyloglucanase, and Xyloglucan Endo-Transglycosylase Activities Versus Potential Substrates in Ripening Tomatoes

In ripening fruits of tomato (Lycopersicon esculentum L. var 83-G-38), the amounts of cellulose and xyloglucan (XG) remained constant during tissue softening, but the relative molecular weight (Mr) of XG decreased markedly and the Mr of cellulose declined slightly. These changes could have been due to activities of non-specific endo-1,4-[beta]-glucanases and/or buffer-soluble XG endo-transglycosylase, both of which increased when tissue firmness declined most rapidly. Tomato extracts also reduced the viscosity of XG solutions, especially in the presence of added XG oligosac-charides. This depolymerizing (XGase) capacity differed from [beta]-glucanase and XG transglycosylase activity (a) by being almost entirely buffer insoluble, and (b) by declining precipitously during fruit softening. Although it disappeared from ripe fruit, XGase may have functioned in promoting wall loosening at earlier stages of fruit development when its activity was highest. By contrast, during aging of fruit in the ripening-inhibited mutant rin there was no change in Mr of XG or cellulose, and activities of [beta]-glucanases and XG transglycosylase were lower than in wild-type tomato. Nevertheless, some softening of the fruit did take place over time and XG amounts declined, possibly because high XGase activity was maintained in the mutant, unlike in wild-type fruit.     

A Requirement for Polyamines during the Cell Division Phase of Radicle Emergence in Seeds of Acer saccharum

When stratified sugar maple seeds were germinated at 5C, celldivision did not contribute to radicle emergence or to earlygrowth of the embryonic axis. Putrescine, spermidine and sperminecontents remained low throughout stratification, but followinggermination their levels rose gradually for several days andthen entered a phase of rapid accumulation concurrent with initiationof rapid cell division. When inhibitors of putrescine and polyaminebiosynthesis were applied to newly germinated seeds, levelsof the amines and cell division were markedly reduced, but cellelongation, as evidenced by growth of hypocotyls, was not affected. ¹ Present address: Department of Chemistry and Biochemistry,University of Guelph, Guelph, Ontario, Canada N1G 2W1.     

Kinetics of Phenylalanine Ammonia-Lyase and the Effect of L-{alpha}-aminooxy-{beta}-phenylpropionic Acid on Enzyme Activity and Radicle Growth in Germinating Lettuce Seeds

The effects of different concentrations of L--aminooxy-ß-phenyIpropionicacid (AOPP), an analog of L-phenylalanine, on the activity ofphenylalanine ammonia-lyase (PAL, EC 4.3.1.5 [EC] ) and the growthof radicles in 24 h old germinating lettuce (Lactuca salivaL.) seeds were investigated. AOPP causes a significant inhibitionof PAL activity in the seeds (85% inhibition at 10⁴ M). It alsocauses a stimulation of radicle growth at that concentration.The results show that the inhibition of PAL by AOPP may be dueto an irreversible binding of the inhibitor to the enzyme leadingto its inactivation. AOPP also inhibits ethylene biosynthesisin germinating lettuce seeds which could probably explain thestimulation of radicle growth in these seeds. The enzyme shows typical Michaelis-Menten kinetics. The K_m forL-phenylalanine is 4.2 x 10⁵ M. The enzyme does not show anytyrosine ammonia-lyase activity. Various substrate analogs suchas D-phenylalanine, p-fluorophenylalanine, ß-phenyllacticacid, tryptophan and the product of the enzyme reaction, trans-cinnamicacid, inhibit the enzyme competitively. A number of intermediatesand endproducts of the phenylpropanpid pathway, except chlorogenicacid, do not show any inhibition. ¹Scientific contribution number 1423 from the New HampshireAgricultural Experiment Station. (Received May 9, 1986; Accepted September 8, 1986)     

Differential Expression of Two Endo-1,4-[beta]-Glucanase Genes in Pericarp and Locules of Wild-Type and Mutant Tomato Fruit

The mRNA accumulation of two endo-1,4-[beta]-D-glucanase genes, Cel1 and Cel2, was examined in the pericarp and locules throughout the development of normal tomato (Lycopersicon esculentum) fruit and the ripening-impaired mutants rin and Nr. Both Cel1 and Cel2 were expressed transiently at the earliest stages of fruit development during a period corresponding to cell division and early cell expansion. In the pericarp, the mRNA abundance of both genes increased markedly at the breaker stage; the level of Cel1 mRNA decreased later in ripening, and that of Cel2 increased progressively. Cel2 mRNA levels also increased at the breaker stage in locules but after initial locule liquefaction was already complete. In rin fruit mRNA abundance of Cel1 was reduced and Cel2 was virtually absent, whereas in Nr Cel1 was expressed at wild-type levels and Cel2 was reduced. In wild-type fruit ethylene treatment slightly promoted the mRNA accumulation of both genes. In rin fruit ethylene treatment strongly increased the mRNA abundance of Cel1 to an extent greater than in wild-type fruit, but Cel2 mRNA was absent even after ethylene treatment. These two endo-1,4-[beta]-D-glucanase genes, therefore, do not show coordinated expression during fruit development and are subject to distinct regulatory control. These results suggest that the product of the Cel2 gene contributes to ripening-associated cell-wall changes.     

Regulation of [beta]-Methylcrotonyl-Coenzyme A Carboxylase Activity by Biotinylation of the Apoenzyme

Regulation of the expression of the gene(s) coding for the 78-kD, biotin-containing subunit of [beta]-methylcrotonyl-coenzyme A carboxylase (MCCase) was investigated in different organs of tomato (Lycopersicon esculantus) plants. The specific activity of MCCase is highest in extracts from roots, followed in descending order by ripe and ripening fruits, stems, and leaves. The specific activity is 10-fold higher in roots than in leaves. However, the steady-state levels of the 78-kD subunit of MCCase and its mRNA are approximately equal in both roots and leaves. Instead, the difference in MCCase activity between these two organs is directly correlated to the biotinylation status of the enzyme's biotin-containing subunit. Thus, the lower activity of MCCase in leaves is attributed to the reduced biotinylation of the biotin-containing subunit of the enzyme. Consistent with this model, a pool of nonbiotinylated enzyme is present in leaves, whereas the nonbiotinylated enzyme is undetectable in roots. The underbiotinylation of MCCase in leaves is not due to a lack of biotin in this organ, since the biotin concentration is 4- to 5-fold higher in leaves than in roots. These observations indicate that the posttranslational biotinylation of the biotin-containing sub-unit of MCCase is an important mechanism for regulating the organ-specific expression of MCCase activity.     

Aleurones from a Barley with Low [alpha]-Amylase Activity Become Highly Responsive to Gibberellin When Detached from the Starchy Endosperm

The physiological and molecular bases for contrasting [alpha]-amylase phenotypes were examined in germinating seeds of two barley (Hordeum vulgare L.) cultivars, Morex and Steptoe. Morex is a high-quality malting barley that develops high [alpha]-amylase activity soon after germination. Steptoe is a feed barley that develops only low [alpha]-amylase activity levels during this period. The expression of all high- and low-isoelectric point (pl) [alpha]-amylase isozymes is reduced in Steptoe. The amount of [alpha]-amylase mRNA per gram of seedling tissue is correspondingly lower in Steptoe. Southern blot analysis revealed that the cultivars have the same copy number and organization for most high- and low-pl genes. Steptoe seedlings or embryoless half-seeds produce little [alpha]-amylase in response to exogenous applications of gibberellic acid (GA3) compared with Morex. However, when isolated aleurones of both cultivars are treated with GA3, they produce similar amounts of high- and low-pl [alpha]-amylase RNAs. This suggests that a factor in the starchy endosperm is responsible for lowered [alpha]-amylase response in Steptoe. The factor is probably not abscisic acid (ABA), since the two cultivars have similar concentrations of ABA during germination.     

Inhibition of the Coleoptile Growth in Avena sativa by Endosperm Removal--Changes in Auxin, Osmotica and Cell Wall

Removal of the endosperm from 84-h-old etiolated oat seedlingsstrongly retarded the subsequent growth of coleoptiles. Thecontribution of the endosperm to coleoptile growth was studied.Endosperm removal was found to: (1) decrease the endogenouslevel of indole-3-acetic acid (IAA) in the coleoptile tip. IAAapplied to the coleoptile tip stimulated coleoptile growth inseedlings with and without the endosperm. The sensitivity ofthe coleoptile to a suboptimal concentration of IAA was higherin seedlings without the endosperm than in intact ones. At theoptimal concentration of IAA, however, the final length of thecoleoptile was larger in intact seedlings than in those withoutthe endosperm. (2) decrease the concentration of the solublesugars and amino acids in the cell sap. (3) retard the increasein the amount of polysaccharides in the cell wall of the coleoptile,particularly noncellulosic ones. (4) make the cell wall mechanicallyrigid according to stress-relaxation analysis of the cell wall.(5) induce an increase in the osmotic potential of the coleoptilecell sap. From these results, it was concluded that the endosperm suppliesthe coleoptile with IAA, sugars and amino acids, thus promotingcoleoptile growth. (Received September 24, 1987; Accepted February 3, 1988)     

Control by pH of Cell Wall Peroxidase Activity Involved in Lignification

pH affected both conyferyl alcohol (CA) oxidation and NADH-dependentH₂O₂ formation catalysed by cell wall-bound peroxidase fromlupin. The kinetics of CA oxidation shown at pH 5.0 was of theMichaelian-type, independently of the physical state (boundor soluble) of the enzyme in the cell wall, whereas at pH 8.0,it was cooperative. The affinity of the enzyme to H₂O₂and therate of H₂O₂ formation were higher at pH 8.0 than at pH 5.0.The kinetics of H₂O₂ formation at pH 8.0 was strongly cooperative.These results are discussed with respect to the important roleof pH in the regulation of cell wall peroxidase activities involvedin lignification. (Received December 12, 1988; Accepted December 7, 1988)     

Tomato Fruit Cell Wall Synthesis during Development and Senescence : In Vivo Radiolabeling of Wall Fractions Using [C]Sucrose

The pedicel of tomato fruit (Lycopersicon esculentum Mill., cv `Rutgers') of different developmental stages from immature-green (IG) to red was injected on the vine with 7 microcuries [¹⁴C(U)]sucrose and harvested after 18 hours. Cell walls were isolated from outer pericarp and further fractionated yielding ionically associated pectin, covalently bound pectin, hemicellulosic fraction I, hemicellulosic fraction II, and cellulosic fraction II. The dry weight of the total cell wall and of each cell wall fraction per gram fresh weight of pericarp tissue decreased after the mature-green (MG) stage of development. Incorporation of radiolabeled sugars into each fraction decreased from the IG to MG3 (locules jellied but still green) stage. Incorporation in all fractions increased from MG3 to breaker and turning (T) and then decreased from T to red. Data indicate that cell wall synthesis continues throughout ripening and increases transiently from MG4 (locules jellied and yellow to pink in color) to T, corresponding to the peak in respiration and ethylene synthesis during the climacteric. Synthesis continued at a time when total cell wall fraction dry weight decreased indicating the occurrence of cell wall turnover. Synthesis and insertion of a modified polymer with removal of other polymers may produce a less rigid cell wall and allow softening of the tissue integrity during ripening.     

beta]-Glucan Synthesis in the Cotton Fiber (I. Identification of [beta]-1,4- and [beta]-1,3-Glucans Synthesized in Vitro)

In vitro [beta]-glucan products were synthesized by digitonin-solubilized enzyme preparations from plasma membrane-enriched fractions of cotton (Gossypium hirsutum) fiber cells. The reaction mixture favoring [beta]-1,4-glucan synthesis included the following effectors: Mg2+, Ca2+, cellobiose, cyclic-3[prime]:5[prime]-GMP, and digitonin. The ethanol insoluble fraction from this reaction contained [beta]-1,4-glucan and [beta]-1,3-glucan in an approximate ratio of 25:69. Approximately 16% of the [beta]-1,4-glucan was resistant to the acetic/nitric acid reagent. The x-ray diffraction pattern of the treated product favoring [beta]-1,4-glucan synthesis strongly resembled that of cellulose II. On the basis of methylation analysis, the acetic/nitric acid reagent-insoluble glucan product was found to be exclusively [beta]-1,4-linked. Enzymic hydrolysis confirmed that the product was hydrolyzed only by cellobiohydrolase I. Autoradiography proved that the product was synthesized in vitro. The degree of polymerization (DP) of the in vitro product was estimated by nitration and size exclusion chromatography; there were two average DPs of 59 (70%) and 396 (30%) for the [beta]-1,3-glucanase-treated sample, and an average DP of 141 for the acetic/nitric acid reagent-insoluble product. On the basis of product analysis, the positive identification of in vitro-synthesized cellulose was established.