Pore Direction in Relation to Anisotropy of Mechanical Strength in a Cubic Starch Compact

The purpose of this research was to evaluate the relation between preferential direction of pores and mechanical strength of cubic starch compacts. The preferential pore direction was quantified in SEM images of cross sections of starch compacts using a previously described algorithm for determination of the quotient of transitions (Q). This parameter and the mechanical strength were evaluated in compacts of different porosities. Starch was chosen as a model compound for materials with ductile behaviour of which tablets with low porosities can be made and which shows some elastic recovery after compaction. At medium and high porosity Q was significantly higher in the images providing a side view of the compact than in the images providing a top view (0.973 vs. 0.927 and 0.958 vs. 0.874 at 0 mm from the side of the compact and 0.956 vs. 0.854 and 0.951 vs. 0.862 at 3.5 mm), indicating that the pores were mainly oriented in the direction perpendicular to the direction of compression. This was accompanied by a lower crushing force in this direction. This could be explained by considering the pores as cracks which propagate through the sample during crushing. For both directions the crushing force decreased with increasing porosity. The yield strength of the compacts also decreased with increasing porosity, but this parameter was not dependent on the direction of crushing when the porosity was below 10%. The results show that pore direction significantly influences the crushing force but does not influence the yield strength, at porosities below 10%.     

Biochemical Analysis of Interactions between Outer Membrane Proteins That Contribute to Starch Utilization by Bacteroides thetaiotaomicron

An early step in the utilization of starch by Bacteroides thetaiotaomicron is the binding of starch to the bacterial surface. Four starch-associated outer membrane proteins of B. thetaiotaomicron that have no starch-degrading activity have been identified. Two of these, SusC and SusD, have been shown by genetic analysis to be required for starch binding. In this study, we provide the first biochemical evidence that these two proteins interact physically with each other. Both formaldehyde cross-linking and nondenaturing gel electrophoresis experiments showed that SusC and SusD interact to form a complex. Two other proteins encoded by genes in the same operon, SusE and SusF, proved not to be essential for starch utilization and actually decreased starch binding when they were present along with SusC and SusD. Consistent with this, nondenaturing gel analysis revealed that in a strain producing SusC, SusD, and SusE, the SusCD complex was partially destabilized. The strain producing SusC, SusD, and SusE also grew more slowly on starch than a strain producing SusC, SusD, SusE, and SusF (mu(max), 0.29 and 0.37/h, respectively). Thus, SusE appears to interact with the SusCD complex. SusE also interacts with SusF, because SusE was less susceptible to proteinase K digestion when SusF was present, and nondenaturing gel analysis detected a complex formed by these two proteins. Our results indicate that SusC, SusD, SusE, and SusF form a protein complex in the outer membrane but that SusE and SusF are dispensable members of this complex.     

Differential Lipopolysaccharide Core Capping Leads to Quantitative and Correlated Modifications of Mechanical and Structural Properties in Pseudomonas aeruginosa Biofilms

Bacterial biofilms are responsible for the majority of all microbial infections and have profound impact on industrial and geochemical processes. While many studies documented phenotypic differentiation and gene regulation of biofilms, the importance of their structural and mechanical properties is poorly understood. Here we investigate how changes in lipopolysaccharide (LPS) core capping in Pseudomonas aeruginosa affect biofilm structure through modification of adhesive, cohesive, and viscoelastic properties at an early stage of biofilm development. Microbead force spectroscopy and atomic force microscopy were used to characterize P. aeruginosa biofilm interactions with either glass substrata or bacterial lawns. Using isogenic migA, wapR, and rmlC mutants with defined LPS characteristics, we observed significant changes in cell mechanical properties among these strains compared to wild-type strain PAO1. Specifically, truncation of core oligosaccharides enhanced both adhesive and cohesive forces by up to 10-fold, whereas changes in instantaneous elasticity were correlated with the presence of O antigen. Using confocal laser scanning microscopy to quantify biofilm structural changes with respect to differences in LPS core capping, we observed that textural parameters varied with adhesion or the inverse of cohesion, while areal and volumetric parameters were linked to adhesion, cohesion, or the balance between them. In conclusion, this report demonstrated for the first time that changes in LPS expression resulted in quantifiable cellular mechanical changes that were correlated with structural changes in bacterial biofilms. Thus, the interplay between architectural and functional properties may be an important contributor to bacterial community survival.Biofilms are sessile microbial communities growing on a surface or at an interface, often enmeshed in polymeric substances. Being the predominant mode of microbial growth in nature, bacterial biofilms are particularly problematic in the context of human health, accounting for up to 80% of all bacterial infections. In industrial processes, bacterial biofilms cause corrosion and biofouling, resulting in considerable loss of productivity. In the natural environment, biofilms play a role in modulating worldwide geochemical cycles. Given the impact of biofilms in these diverse areas, the need for developing effective strategies to control them is of paramount importance. Since bacterial cell surface structures are convenient targets for control agents, their roles in influencing biofilm function and architecture warrant in-depth investigations. To date, most studies of biofilms have focused on genetic regulation, phenotypic differentiation and their contribution to antibiotic resistance. In contrast, the mechanical and structural properties that link the genotypes to phenotypes of bacterial biofilms are not well understood and rarely studied in a quantitative and correlated manner.Pseudomonas aeruginosa is a gram-negative opportunistic pathogen implicated in serious infections in patients with cystic fibrosis and immunocompromised patients. This bacterium has a relatively large genome (6.3 Mb) consistent with its propensity to utilize versatile metabolic pathways, thereby developing antibiotic resistance and producing an arsenal of virulence factors, including lipopolysaccharide (LPS) present on the cell surface. LPS is localized in the outer leaflet of the outer membrane of all gram-negative bacteria, forming the first point of contact between the bacterial cell and any surface that it colonizes or therapeutic agents. The LPS of P. aeruginosa consists of three regions: lipid A, core oligosaccharide (core OS), and O antigen. The O antigen is synthesized as two distinct forms with overlapping pathways: the shorter A-band homopolymer is the so-called “common polysaccharide antigen” among this species and consists of repeating d-rhamnose (d-Rha) subunits, while the longer B-band heteropolymer is composed of repeating tri- to pentasaccharide subunits that vary among the 20 serotypes of P. aeruginosa (42). The core OS is conceptually divided into the highly conserved inner core and the more variable outer core. Depending on the linkage of l-rhamnose (l-Rha) with two distinct d-glucose (d-Glc) residues, two main glycoforms of the core OS exist (see Fig. ​Fig.1A).1A). In the “capped” glycoform, l-Rha is α-1,3 linked to a d-Glc and acts as the acceptor molecule for O antigen, resulting in the production of smooth LPS. In the “uncapped” glycoform, l-Rha is α-1,6 linked to a different d-Glc and is not substituted with O antigen, resulting in the production of rough LPS (39). In addition, the presence or absence of the α-1,6 linked l-Rha substituted with a terminal d-Glc gives rise to the so-called intact or truncated outer core, respectively. The functional significance of this terminal glucose is unclear at present, although a role in host cell binding has been proposed (57).Open in a separate windowFIG. 1.Comparison of LPSs from Pseudomonas aeruginosa wild-type strain PAO1 and mutant strains with LPS core variants. (A) Schematic diagram illustrating the chemical structures of smooth LPS and rough LPS. The gray and black arrows point to the position of outer core truncation and position of O-antigen capping, respectively. (B) Silver-stained SDS-polyacrylamide gels illustrating LPS profiles of planktonic and biofilm cells. The two gray arrows and the black arrows point to the position of O antigen and position of core-plus-one entities, respectively. Planktonic cells (lanes 1 to 4) and biofilm cells (lanes 5 to 8) of strain PAO1 (P), migA mutant (M), wapR mutant (W), and rmlC mutant (R) are shown.Mechanical processes that are important in the biofilm life cycle include bacterial adhesion, cohesion, and viscoelasticity. Bacterial adhesion is a prerequisite for surface colonization and the most important functional determinant in the early stages of biofilm development. Accurate measurement of adhesion is therefore essential for monitoring the tendency of bacteria to attach to surfaces and to switch from a planktonic lifestyle to a biofilm lifestyle. Data accumulated in previous studies suggest that LPS is involved in bacterial cell adhesion to both abiotic (2, 8, 20, 32, 35, 54, 56) and biotic (17, 38, 49, 56, 57) surfaces. Moreover, environmental factors, such as growth temperature, pH, ionic strength, nutrient availability, and oxygen levels, may influence cell adhesion via modification of LPS expression and conformation (16, 36, 46, 47, 53). The effect of LPS on bacterial adhesion to various types of surfaces apparently involves distinct and complex mechanisms that remain to be elucidated.Bacterial cohesion, herein defined as cell-to-cell adherence, is crucial to the formation of microbial flocs and the growth and detachment of established bacterial biofilms. Quantification of cohesion is important for understanding biofilm biology, and such data are crucial for modeling and forecasting biofilm development so that better control strategies can be developed (55). Previous studies of biofilm cohesiveness have characterized it as highly stratified (3, 4, 18, 43), influenced by ionic strength (14, 34), proportional to shear rate (37), and often variable over 3 orders of magnitude (40, 50). Although an earlier study by Spiers and Rainey (48) provided semiquantitative measurements of the role of LPS on bacterial cohesion within a biofilm, a truly quantitative account of the effect of LPS on biofilm cohesion has not been demonstrated.Bacterial viscoelasticity refers to the combined liquid-like and solid-like characteristics in the behavior of polymeric systems such that when deformed under stress, their strain can increase over time (i.e., creep) and their original shape may be only partially restored upon stress relief (19). Although earlier reports suggested that LPS modulates bacterial cell compressibility and helps prevent catastrophic structural failure due to mechanical stress (1, 52), no direct physical evidence of its involvement in these processes has yet been presented. Therefore, monitoring biofilm viscoelasticity is crucial for demonstrating how well biofilms resist stresses, due to, for instance, fluid shear and antimicrobial peptides (5, 6). To date, quantitative data on how LPS affects viscoelastic properties of biofilms are lacking, and existing studies have merely focused on elasticity measurements (7, 52). Recently, our group has developed an atomic force microscopy (AFM)-based technique called microbead force spectroscopy (MBFS) to measure the adhesive forces and viscoelastic properties of cells within bacterial biofilms (28). In this study, we expand the application of this MBFS method to measure cohesive forces between cells at an early stage of biofilm development.Biofilm structure refers to the distribution of biomass or carbonaceous materials associated with cells (including all viable and nonviable cells and their extracellular polymeric substances) within the space occupied by a biofilm. It is known to be very heterogeneous and highly stratified, typically composed of a cohesive basal layer and a relatively fragile top layer (15, 18, 43). Using confocal laser scanning microscopy (CLSM), biofilm structure can be quantitatively described in terms of textural and volumetric parameters (11, 29). Textural parameters characterize the pattern of cell clusters and interstitial voids in a biofilm, whereas volumetric parameters describe the morphological characteristics of bacterial biofilms in three dimensions (3-D) (11). Biomass distribution is affected by the surrounding environment and may reflect fundamental processes occurring within biofilms, such as nutrient transport, accumulation rate, microbial physiology, and mechanical behavior (29). Therefore, quantifying biofilm structure by CLSM will allow us to understand the underlying processes and the relationship between biofilm architecture and behavior (15).To examine the effects of differential LPS core capping on the mechanical properties of early biofilms (here defined as confluent bacterial lawns that have just begun to develop into full-fledged biofilms) and the structural properties of mature biofilms, we compare P. aeruginosa wild-type strain PAO1 with those of its isogenic migA, wapR, and rmlC mutant strains with defects in the respective genes affecting LPS core biosynthesis (22, 31, 39, 41). The migA gene (PA0705) encodes the putative α-1,6-rhamnosyltransferase necessary for the attachment of the terminal d-Glc to the outer core (39). The wapR gene (PA5000) encodes the putative α-1,3-rhamnosyltransferase crucial to the capping of the core with O antigen (39). The rmlC gene (PA5164) encodes a dTDP-4-dehydrorhamnose 3,5-epimerase essential in the biosynthesis of TDP-l-Rha, which is the precursor for the l-Rha in the LPS core (41). Defects in migA, wapR, and rmlC mutants result in the expression of different LPS phenotypes, including a truncated outer core and/or a lack of capping by O antigen (Table ​(Table1).1). In this study, we test the hypothesis that LPS contributes to biofilm function and architecture through modulation of cellular mechanics and microcolony structures, thereby contributing to bacterial community survival. By correlating quantitative mechanical changes in early P. aeruginosa biofilms and structural changes in mature biofilms due to differences in LPS chemistry, we aim to elucidate how the properties of these important bacterial cell surface molecules can alter the physical nature of biofilms.

TABLE 1.

Pseudomonas aeruginosa strains used in this study

Interactions of Botryococcus braunii Cultures with Bacterial Biofilms

Unicellular microalgae generally grow in the presence of bacteria, particularly when they are farmed massively. This study analyzes the bacteria associated with mass culture of Botryococcus braunii: both the planktonic bacteria in the water column and those forming biofilms adhered to the surface of the microalgal cells (∼10⁷–10⁸ culturable cells per gram microalgae). Furthermore, we identified the culturable bacteria forming a biofilm in the microalgal cells by 16S rDNA sequencing. At least eight different culturable species of bacteria were detected in the biofilm and were evaluated for the presence of quorum-sensing signals in these bacteria. Few studies have considered the implications of this phenomenon as regards the interaction between bacteria and microalgae. Production of C4-AHL and C6-AHL were detected in two species, Pseudomonas sp. and Rhizobium sp., which are present in the bacterial biofilm associated with B. braunii. This type of signal was not detected in the planktonic bacteria isolated from the water. We also noted that the bacterium, Rhizobium sp., acted as a probiotic bacterium and significantly encouraged the growth of B. braunii. A direct application of these beneficial bacteria associated with B. braunii could be, to use them like inoculants for large-scale microalgal cultures. They could optimize biomass production by enhancing growth, particularly in this microalga that has a low growth rate.     

Seed Proteins of Peas in Relation to Nitrogen Fixation

The influence of the source of nitrogen on the amount and compositionof the seed protein has been examined. The plants were grownin sterile compost and those that had received their nitrogenas ammonium nitrate compared with those in which nitrogen fixationoccurred in the root nodules. The absence of nodules in theformer and the presence of nitrogen fixation in the latter wasconfirmed. The source of nitrogen did not affect the amountsof extractable seed protein, the proportions of the differentfractions or their composition.     

Effect of Envelope Proteins on the Mechanical Properties of Influenza Virus

The envelope of the influenza virus undergoes extensive structural change during the viral life cycle. However, it is unknown how lipid and protein components of the viral envelope contribute to its mechanical properties. Using atomic force microscopy, here we show that the lipid envelope of spherical influenza virions is ∼10 times softer (∼0.05 nanonewton nm⁻¹) than a viral protein-capsid coat and sustains deformations of one-third of the virion''s diameter. Compared with phosphatidylcholine liposomes, it is twice as stiff, due to membrane-attached protein components. We found that virus indentation resulted in a biphasic force-indentation response. We propose that the first phase, including a stepwise reduction in stiffness at ∼10-nm indentation and ∼100 piconewtons of force, is due to mobilization of membrane proteins by the indenting atomic force microscope tip, consistent with the glycoprotein ectodomains protruding ∼13 nm from the bilayer surface. This phase was obliterated for bromelain-treated virions with the ectodomains removed. Following pH 5 treatment, virions were as soft as pure liposomes, consistent with reinforcing proteins detaching from the lipid bilayer. We propose that the soft, pH-dependent mechanical properties of the envelope are critical for the pH-regulated life cycle and support the persistence of the virus inside and outside the host.     

小麦地方品种淀粉颗粒结合蛋白及淀粉含量差异研究

淀粉颗粒结合蛋白包含了多种淀粉生物合成的关键酶,对作物淀粉品质有重要影响。本研究利用1D-SDS-PAGE,分离了74份四川、西藏及云南毗邻地区小麦的淀粉颗粒结合蛋白,对突变材料进行了分子标记检测,对总淀粉和直链淀粉含量差异进行了比较。发现供试材料中,在分子量57～130 kDa区域共有9种不同的蛋白条带。其中,2个条带可能为新的淀粉颗粒结合蛋白;存在12份Wx-B1缺失的自然突变体和3份稀有的SGP-B1缺失突变体;筛选到直链淀粉含量超过30%的材料2份,直链淀粉含量为15%左右的材料10份。这些材料为小麦淀粉品质改良及淀粉生化合成机理研究提供了基础。     

Role of Force-Sensitive Amyloid-Like Interactions in Fungal Catch Bonding and Biofilms

The Candida albicans Als adhesin Als5p has an amyloid-forming sequence that is required for aggregation and formation of model biofilms on polystyrene. Because amyloid formation can be triggered by force, we investigated whether laminar flow could activate amyloid formation and increase binding to surfaces. Shearing Saccharomyces cerevisiae cells expressing Als5p or C. albicans at 0.8 dyne/cm² increased the quantity and strength of cell-to-surface and cell-to-cell binding compared to that at 0.02 dyne/cm². Thioflavin T fluorescence showed that the laminar flow also induced adhesin aggregation into surface amyloid nanodomains in Als5p-expressing cells. Inhibitory concentrations of the amyloid dyes thioflavin S and Congo red or a sequence-specific anti-amyloid peptide decreased binding and biofilm formation under flow. Shear-induced binding also led to formation of robust biofilms. There was less shear-activated increase in adhesion, thioflavin fluorescence, and biofilm formation in cells expressing the amyloid-impaired V326N-substituted Als5p. Similarly, S. cerevisiae cells expressing Flo1p or Flo11p flocculins also showed shear-dependent binding, amyloid formation, biofilm formation, and inhibition by anti-amyloid compounds. Together, these results show that laminar flow activated amyloid formation and led to enhanced adhesion of yeast cells to surfaces and to biofilm formation.     

Competitive Interactions in Mixed-Species Biofilms Containing the Marine Bacterium Pseudoalteromonas tunicata

Pseudoalteromonas tunicata is a biofilm-forming marine bacterium that is often found in association with the surface of eukaryotic organisms. It produces a range of extracellular inhibitory compounds, including an antibacterial protein (AlpP) thought to be beneficial for P. tunicata during competition for space and nutrients on surfaces. As part of our studies on the interactions between P. tunicata and the epiphytic bacterial community on the marine plant Ulva lactuca, we investigated the hypothesis that P. tunicata is a superior competitor compared with other bacteria isolated from the plant. A number of U. lactuca bacterial isolates were (i) identified by 16S rRNA gene sequencing, (ii) characterized for the production of or sensitivity to extracellular antibacterial proteins, and (iii) labeled with a fluorescent color tag (either the red fluorescent protein DsRed or green fluorescent protein). We then grew single- and mixed-species bacterial biofilms containing P. tunicata in glass flow cell reactors. In pure culture, all the marine isolates formed biofilms containing microcolony structures within 72 h. However, in mixed-species biofilms, P. tunicata removed the competing strain unless its competitor was relatively insensitive to AlpP (Pseudoalteromonas gracilis) or produced strong inhibitory activity against P. tunicata (Roseobacter gallaeciensis). Moreover, biofilm studies conducted with an AlpP⁻ mutant of P. tunicata indicated that the mutant was less competitive when it was introduced into preestablished biofilms, suggesting that AlpP has a role during competitive biofilm formation. When single-species biofilms were allowed to form microcolonies before the introduction of a competitor, these microcolonies coexisted with P. tunicata for extended periods of time before they were removed. Two marine bacteria (R. gallaeciensis and P. tunicata) were superior competitors in this study. Our data suggest that this dominance can be attributed to the ability of these organisms to rapidly form microcolonies and their ability to produce extracellular antibacterial compounds.     

Metal Interactions with Microbial Biofilms in Acidic and Neutral pH Environments

Microbial biofilms were grown on strips of epoxy-impregnated filter paper submerged at four sites in water contaminated with metals from mine wastes. At two sample stations, the water was acidic (pH 3.1); the other sites were in a lake restored to a near neutral pH level by application of a crushed limestone slurry. During a 17-week study period, planktonic bacterial counts increased from 10¹ to 10³ CFU/ml at all sites. Biofilm counts increased rapidly over the first 5 weeks and then leveled to 10⁴ CFU/cm² in the neutral pH system and 10³ CFU/cm² at the acidic sites. In each case, the biofilms bound Mn, Fe, Ni, and Cu in excess of the amounts adsorbed by control strips covered with nylon filters (pore size, 0.22 μm) to exclude microbial growth; Co bound under neutral conditions but not under acidic conditions. Conditional adsorption capacity constants, obtained graphically from the data, showed that biofilm metal uptake at a neutral pH level was enhanced by up to 12 orders of magnitude over acidic conditions. Similarly, adsorption strength values were usually higher at elevated pH levels. In thin sections of the biofilms, encapsulated bacterial cells were commonly found enmeshed together in microcolonies. The extracellular polymers often contained iron oxide precipitates which generated weak electron diffraction patterns with characteristic reflections for ferrihydrite (Fe₂O₃ · H₂O) at d equaling 0.15 and 0.25 nm. At neutral pH levels, these deposits incorporated trace amounts of Si and exhibited a granular morphology, whereas acicular crystalloids containing S developed under acidic conditions.     

Growth-limiting Proteins in Relation to Auxin-induced Elongation in Lupin Hypocotyls

The role of protein synthesis in auxin-induced cell elongation in lupin hypocotyl segments was studied using cycloheximide. Cycloheximide inhibited protein synthesis by 9 minutes. Experiments adding cycloheximide at various times before and after indolyl-3-acetic acid are reported. Estimates of the relative amounts of growth-limiting protein(s), and a first order rate constant for the apparent turnover of the growth-limiting protein(s) were made. It was shown that there is a sizeable growth promotion by auxin after protein synthesis has essentially ceased. It is concluded that the initial phases of auxin action do not require protein synthesis but that its action depends on the existing pool of growth-limiting proteins which is rapidly depleted, and protein synthesis is then required for continued elongation.     

Mechanical Resistance in Unstructured Proteins

Single-molecule pulling experiments on unstructured proteins linked to neurodegenerative diseases have measured rupture forces comparable to those for stable folded proteins. To investigate the structural mechanisms of this unexpected force resistance, we perform pulling simulations of the amyloid β-peptide (Aβ) and α-synuclein (αS), starting from simulated conformational ensembles for the free monomers. For both proteins, the simulations yield a set of rupture events that agree well with the experimental data. By analyzing the conformations occurring shortly before rupture in each event, we find that the mechanically resistant structures share a common architecture, with similarities to the folds adopted by Aβ and αS in amyloid fibrils. The disease-linked Arctic mutation of Aβ is found to increase the occurrence of highly force-resistant structures. Our study suggests that the high rupture forces observed in Aβ and αS pulling experiments are caused by structures that might have a key role in amyloid formation.     

Mechanical Resistance in Unstructured Proteins

Single-molecule pulling experiments on unstructured proteins linked to neurodegenerative diseases have measured rupture forces comparable to those for stable folded proteins. To investigate the structural mechanisms of this unexpected force resistance, we perform pulling simulations of the amyloid β-peptide (Aβ) and α-synuclein (αS), starting from simulated conformational ensembles for the free monomers. For both proteins, the simulations yield a set of rupture events that agree well with the experimental data. By analyzing the conformations occurring shortly before rupture in each event, we find that the mechanically resistant structures share a common architecture, with similarities to the folds adopted by Aβ and αS in amyloid fibrils. The disease-linked Arctic mutation of Aβ is found to increase the occurrence of highly force-resistant structures. Our study suggests that the high rupture forces observed in Aβ and αS pulling experiments are caused by structures that might have a key role in amyloid formation.     

Wild Oat/Barley Interactions: Varietal Differences in Competitiveness in Relation to K+ Supply

Eight cvs of barley (Hordeum vulgare L.) were separately plantedwith Wild Oats (Avena fatua L., genetically pure line CS40)in a sand culture with two external K⁺ concentrations. Substantialdifferences were observed among barley cvs in their abilityto compete with wild oat. The variety Fergus was highly competitiveat both high and low [K⁺]_e, whereas Steptoe was competitiveonly at high [K⁺]_e, and Compana was only weakly competitivewith wild oat. The differences between barley cvs were relatedto their previously reported efficiencies of K⁺ uptake and utilization. Hordeum vulgare L., Avena fatua L., barley, wild oat, competition, K⁺ nutrition, utilization efficiency     

玉米籽粒胚乳细胞增殖及其与淀粉充实的关系

用纤维素酶解离胚乳、滤膜法统计玉米胚乳细胞的数目,进一步借助Logistic方程模拟胚乳细胞增殖动态的结果表明,整个灌浆期间胚乳细胞增殖呈现“慢-快-慢”的变化趋势。授粉15d后,不同类型胚乳的细胞数目依序为普通玉米〉糯玉米〉甜玉米〉爆裂玉米;胚乳细胞数目主要取决于细胞的增殖速率,并与淀粉充实和粒重关系密切。胚乳发育前期以胚乳细胞增殖为主,后期以淀粉积累为主。