Changes in Cell Number and Cell Division Activity during Endosperm Development in Allohexaploid Wheat, Triticum aestivum L.

Nuclear or cell number, and the mitotic index, were recordedin endosperms of Triticum aestivum cv. Mardler to test if aparticular stage of endosperm development was critical in determiningthe final grain weight. The basal four florets of emasculatedspikelets (controls), and the third and fourth florets of spikeletswhere the two basal ovaries were removed (ovary-removed), weresampled at various times up to 360 h after hand-pollination.The coenocytic phase in endosperms ended about 84 h after pollinationregardless of both grain position and the treatment. The onsetof the cellular stage was characterized by the final large fluctuationsin the mitotic index reflecting the culmination of the synchronousnuclear division of the coenocytic stage. Thereafter, the mitoticindex fluctuated with smaller amplitudes and, by 216 h afterpollination, was < 1%. Neither floret position in the spikeletnor the treatment affected the pattern of alteration to themitotic index. However, ovary removal from first and secondflorets resulted in significantly heavier grains and higherendosperm cell number in the 3rd and 4th florets compared withthe controls. In all florets, mean endosperm cell number peakedat 280 h but decreased by 360 h after pollination. At this time,the mean cell numbers in endosperms of the 3rd and 4th floretsof ovary-removed spikelets were significantly higher than inthe corresponding endosperms in the controls. Thus, a key contributoryfactor in determining the final endosperm cell number may bethe number of cells which are lost during the late period ofthe cellular stage of endosperm development. Key words: Endosperm cell number, florets, grain weight, mitotic index, Triticum aestivum     

A Study of Floret Development in Wheat (Triticum aestivum L.)

Plants of wheat (Triticum aestlvum L.) cv. Aotea were grownat high or low nitrogen levels and in a natural photoperiodor continuous light. Starting 17–21 days from the double-ridgestage, eight plants from each treatment were sampled every 3days until anthesis, and the two basal, the sixth, and the terminalspikelets were sectioned longitudinally. A developmental scorewas assigned to each floret and rates of development calculated.Continuous light hastened development but reduced the numberof spikelets per ear, while high nitrogen delayed developmentbut increased spikelet numbers. The number of florets initiatedin each spikelet varied within narrow limits, but grain settingdepended strongly on spikelet position and on treatment. Althoughflorets were initiated in acropetal succession, the rate ofdevelopment tended to increase up to floret 4 but then declinedmarkedly. As a result grain setting was confined to basal floretpositions, although the two basal spikelets developed so slowlythat they contributed relatively little to grain yield. Distalflorets degenerated almost simultaneously at or before ear emergence,but those in intermediate positions continued to develop untilafter fertilization in the lower florets. It is argued thatthe spikelet is an integrated system in which correlative mechanismsplay a part throughout the development of the florets.     

Free Polyamines in Senescing Wheat(Triticum aestivum L.)

The free polyamine content of flag leaves, peduncles, rachis,glumes, and grains of wheat (Triticum aestivum L., cv. Castell)plants, ripening under field conditions, has been investigatedduring three consecutive growing seasons. Putrescine was quantitativelythe most important of all polyamines detected in these organs.Concentrations were highest in the grains, glumes and flag leaves.No correlation was found between polyamine content and the onsetof senescence of flag leaves and other organs. Excised primaryleaves, however, showed a decrease in polyamine content in thedark and also in light/dark cycles, but in the latter case onlyafter an initial increase. Sink removal of otherwise intactwheat plants caused an accumulation of putrescine in flag leavesat the later stages of senescence, whereas removal of all otherleaves was without any significant effect. Putrescine was alsorecovered in phloem-exudate samples collected throughout theperiod of grain development. In both grains and glumes, peakconcentrations of polyamines were found early during seed development. Key words: Triticum aestivum, polyamines, ripening, senescence     

The Mechanics of Root Lodging in Winter Wheat, Triticum aestivum L.

The anchorage of winter wheat, Triticum aestivum L., is providedby a cone of rigid coronal roots which emerge from around thestem base. During root lodging this cone rotates at its windwardedge below the soil surface, the soil inside the cone movingas a block and compressing the soil beneath. A theoretical modelof anchorage suggested that lodging resistance should be dependenton the diameter of the root-soil cone, coronal root bendingstrength and soil shear strength. We tested the predictions of the anchorage model by carryingout two series of experiments. In the first, varieties of contrastinglodging resistances were artificially lodged. The moment requiredto rotate plants into the soil, the diameter of the root-soilcone, and the bending strength of the coronal roots were recorded.The lodging moment was correlated with the size of the soilcone, as predicted. Generally, differences in anchorage strengthbetween varieties were due to differences in root-soil conediameter, although coronal root strength was also important. A second series of tests was carried out using model plantsanchored by plastic discs. The behaviour of the models duringartificial lodging supported the anchorage model; the forceresisting lodging was similar to that of plants with root-soilcones of the same size and the resisting force was dependenton the soil strength. These results suggest that root lodging resistance might beimproved by increasing both the angle of spread and the bendingstrength of the coronal roots. Key words: Anchorage, root-soil cone, coronal roots, lodging, wheat     

Colchicine-induced Paracrystals in Root Cells of Wheat (Triticum aestivum L.)

Tubulin conformations other than microtubules in the meristematiccells of wheat roots grown in the presence of 2 mM colchicinesolution were investigated by immunofluorescence and electronmicroscopy. In the affected cells microtubules disappeared andwere replaced by tubulin fluorescent strands that occurred inthe cortical cytoplasm. With increasing time of exposure tocolchicine the tubulin strands became better organized and occurredalso in the subcortical cytoplasm and finally they were restrictedto the area around the nucleus. In prophase and preprophasecells thick strands occupied the cortical cytoplasmic zone wherein normal cells a preprophase microtubule band (PMB) was expectedto be assembled. In the colchicine-treated cells electron microscopy revealedan accumulation of paracrystalline aggregates, which initiallyoccurred along the cell wall and later deeper in the cytoplasm,in the perinuclear regions and the cytoplasmic invaginationsof the nucleus. In transverse planes the paracrystalline strandsappear to consist of hexagonal subunits in a 'honeycomb' arrangement,while in longitudinal and oblique sections they exhibit variableimages. Since their distribution coincides with that of thetubulin strands visualized by immunofluorescence, they are consideredto be the same structure. Therefore, the paracrystals consistof, or at least contain, tubulin. They are most likely to bepolymers of tubulin-colchicine complexes.Copyright 1995, 1999Academic Press Wheat roots, colchicine, immunofluorescence, electron microscopy, tubulin paracrystals, Triticum aestivum L     

Changes in Abscisic Acid Levels in Developing Grains of Wheat (Triticum aestivum L.

ABA has been determined in wheat grains during their development. The maximum level of ABA occurred approximately 7 weeks afterear emergence (40 d after peak anthesis). This level subsequentlyfell sharply as a result of metabolism of the ABA by the maturinggrains.     

Floret Initiation and Development in Spring Wheat (Triticum aestivum L.)

The number and developmental stages of florets were determinedin each spikelet of the spike in the main shoots of spring wheat.Samples were taken frequently from plants grown in a phytotronand in a nitrogen application field-test. Ten stages of development,from floret initiation until anthesis, were recognized and described. Inter-spikelet variation in the total number of initiated floretswas rather small. However, the number of florets at advancedstages of development, as well as the number of grains, washighest in the central spikelets in which florets initiatedfirst. Floret initiation did not proceed beyond spike emergence,whereafter the distal florets and the spikelet apex degenerated.Grain-set was restricted to florets which had developed at leastto the stage of visible anther lobes at spike emergence. Thenumber of these florets was increased significantly by nitrogenapplication. Wheat, Triticum aestivum L., spikelet, floret, grain set, nitrogen     

中国春双端二体中端体的配对研究

21个中国春双端二体与中国春品种杂交,观察了杂种F_1减数分裂中期Ⅰ端体与相应染色体的配对。结果表明:14个组合的中期Ⅰ具有1个异形三价体的PMC的百分数超过90%;有3个组合(6A、5B和6D)分别为89.87%、83.56%和85.71%;2个组合(4A和1B)低于80%。在4A、1B和5B的3个组合中,具有1个异形二价体和1个单价端体的PMC超过15%。在4A、1B和4D的组合中,还有一定频率的PMC具有2个单价端体。用21个组合中端体配对频率计算的二价体频率与中国春品种中期Ⅰ构型频率基本一致。这些结果表明,染色体臂作为端体时和它作为双臂染色体的一部分时,是以同样的方便程度配对的。     

Intracellular Localization of Peptide Hydrolases in Wheat (Triticum aestivum L.) Leaves

Protoplasts from 8- to 9-day-old wheat (Triticum aestivum L.) leaves were used to isolate organelles which were examined for their contents of peptide hydrolase enzymes and, in the case of vacuoles, other acid hydrolases. High yields of intact chloroplasts were obtained using both equilibrium density gradient centrifugation and velocity sedimentation centrifugation on sucrose-sorbitol gradients. Aminopeptidase activity was found to be distributed, in approximately equal proportions, between the chloroplasts and cytoplasm. Leucyltyrosine dipeptidase was mainly found in the cytoplasm, although about 27% was associated with the chloroplasts. Vacuoles shown to be free from Cellulysin contamination contained all of the protoplast carboxypeptidase and hemoglobin-degrading activities. The acid hydrolases, phosphodiesterase, acid phosphatase, α-mannosidase, and β-N-acetylglucosamidase were found in the vacuole to varying degrees, but no β-glucosidase was localized in the vacuole.     

In Vitro Synthesis of Wheat (Triticum aestivum L.) Storage Proteins

Free and membrane-associated polysomes were isolated in approximately equal amounts from endosperm of wheat kernels harvested 20 days after anthesis. The presence of heparin in the homogenizing buffer minimized polysome degradation. Ribonucleic acid from the isolated polysomes, when translated in vitro in a wheat germ system, yielded products ranging in size from about 12,000 to about 80,000 daltons, including at least two polypeptides that co-migrated with seed extract proteins in sodium dodecyl sulfate polyacrylamide gel electrophoresis. The nature of the translation products of free and membrane-associated RNA are distinctly different, with membrane-associated RNA yielding a higher proportion of polypeptides in the size range of 30,000 to 37,000 daltons. Analysis of membrane-associated 3′-terminal polyadenylyl-containing RNA in vitro translation products, by solubility in 70% ethanol and by immunoprecipitation, indicates that the 33,000- to 37,000-dalton polypeptides contain gliadins, and the analysis provides evidence that these proteins are synthesized in association with membranous cell organelles. Gliadin polypeptides synthesized in vitro are larger than authentic gliadins and probably are precursors which, in vivo, undergo modification to yield the smaller final products.     

An Assessment of Heavy Ion Irradiation Mutagenesis for Reverse Genetics in Wheat (Triticum aestivum L.)

Reverse genetic techniques harnessing mutational approaches are powerful tools that can provide substantial insight into gene function in plants. However, as compared to diploid species, reverse genetic analyses in polyploid plants such as bread wheat can present substantial challenges associated with high levels of sequence and functional similarity amongst homoeologous loci. We previously developed a high-throughput method to identify deletions of genes within a physically mutagenized wheat population. Here we describe our efforts to combine multiple homoeologous deletions of three candidate disease susceptibility genes (TaWRKY11, TaPFT1 and TaPLDß1). We were able to produce lines featuring homozygous deletions at two of the three homoeoloci for all genes, but this was dependent on the individual mutants used in crossing. Intriguingly, despite extensive efforts, viable lines possessing homozygous deletions at all three homoeoloci could not be produced for any of the candidate genes. To investigate deletion size as a possible reason for this phenomenon, we developed an amplicon sequencing approach based on synteny to Brachypodium distachyon to assess the size of the deletions removing one candidate gene (TaPFT1) in our mutants. These analyses revealed that genomic deletions removing the locus are relatively large, resulting in the loss of multiple additional genes. The implications of this work for the use of heavy ion mutagenesis for reverse genetic analyses in wheat are discussed.     

Resistance to Meloidogyne spp. in Allohexaploid Wheat Derived from Triticum turgidum and Aegilops squarrosa

Expression of resistance to Meloidogyne incognita and M. javanica from Aegilops squarrosa was studied in a synthetic allohexaploid produced from Triticum turgidum var. durum cv. Produra and Ae. squarrosa G 3489. The reproductive rate of different races of M. incognita and M. javanica, expressed in eggs per gram of fresh root, was low (P < 0.05) on the synthetic allohexaploid and the resistant parent, Ae. squarrosa G 3489, compared with different bread and durum wheat cultivars. Reproduction of race 2 and race 3 of M. incognita and an isolate of M. javanica was studied on the synthetic allohexaploid and seven cultivars of T. aestivum: Anza, Coker 747, Coker 68-15, Delta Queen, Double Crop, McNair 1813, and Southern Bell. The latter six cultivars are grown in the southeastern United States and reportedly were resistant to M. incognita. Significant differences (P < 0.05) were detected in nematode reproduction on the seven bread wheat cultivars. Reproduction of M. incognita race 3 and M. javanica was highest on Anza. Reproductive rates on the six southeastern United States bread wheat cultivars varied both within and among nematode isolates. The lowest reproductive rates of the three root-knot isolates were detected in the synthetic allohexaploid.     

Gibberellic Acid Regulates Cell Wall Extensibility in Wheat (Triticum aestivum L.)

Mutations (Rht genes) blocking sensitivity to gibberellic acid (GA) were used to examine phytohormone mediated cell wall expansion in wheat (Triticum aestivum L.). Irreversible extensibility of immature leaf segments, as determined by stress/strain (instron) measurements, declined with Rht gene dose. Exogenous GA₃ significantly increased wall extensibility in the nonmutant controls but had no effect on the near-isogenic GA-insensitive genotypes. Furthermore, ancymidol, an inhibitor of gibberellin biosynthesis, diminished wall extensibility in the nonmutant control. Extensibility of immature segments was highly correlated with mature leaf sheath length (R = +0.95). The results indicate that wall yielding properties of expanding wheat leaves are associated with leaf cell expansion potential and that GA is involved in the determination of those properties.     

Initiation of Procambial Strands in Leaf Primordia of Bread Wheat, Triticum aestivum L

In Triticum aestivum L. the median and lateral procambial strandsserving the primordia originate independently and in isolationfrom the vascular system of the rest of the plant. The medianstrand is initiated first, followed by a succession of lateralstrands during the next four or so plastochrones. The medianand first lateral strands have their point of origin in theaxis, in the disc of insertion of the primordium. The laterlaterals are initiated up in the primordium. Once initiatedthe procambial strands extend from their point of origin bothacropetally and basipetally, the latter extension eventuallylinking them to strands associated with older leaves. It wouldappear that the materials necessary for the growth of the apicaldome and of the first four leaf primordia are supplied by generaldiffusion and not via direct vascular connexions with the restof the plant.     

Nitrogen and Photosynthesis in the Flag Leaf of Wheat (Triticum aestivum L.)

Wheat (Triticum aestivum L. cv Yecora 70) plants were grown with various concentrations of nitrate nitrogen available to the roots. Sampling of flag leaves began after they had reached full expansion and continued throughout senescence. Rates of gas exchange, ribulose-1,5-bisphosphate (RuP₂) carboxylase activity, and the amounts of chlorophyll, soluble protein, nitrogen, and phosphorus were determined for each flag leaf. Rate of CO₂ assimilation was uniquely related to total leaf nitrogen irrespective of nutrient treatment, season, and leaf age. Assimilation rate increased with leaf nitrogen, but the slope of the relationship declined markedly when leaf nitrogen exceeded 125 millimoles nitrogen per square meter. Chlorophyll content and RuP₂ carboxylase activity were approximately proportional to leaf nitrogen content. As leaves aged, RuP₂ carboxylase activity and calculated Hill activity declined in parallel. With normal ambient partial pressure of CO₂, the intercellular partial pressure of CO₂ was always such that rate of assimilation appeared colimited by RuP₂ carboxylation and RuP₂ regeneration capacity.

The initial slope of rate of CO₂ assimilation against intercellular partial pressure of CO₂ varied nonlinearly with carboxylase activity. It is suggested that this was due to a finite conductance to CO₂ diffusion in the wall and liquid phase which causes a drop in CO₂ partial pressure between the intercellular spaces and the site of carboxylation. A double reciprocal plot was used to obtain an estimate of the transfer conductance.

     

Wheat (Triticum aestivum L.) growth enhancement by Azospirillum sp. under drought stress

Plant growth promoting rhizobacteria (PGPR) can enhance plant growth by alleviating soil stresses. Although previously investigated, some new interesting details are presented regarding the alleviating affects of Azospirillum sp. on wheat growth under drought stress in this research work. We hypothesized that the isolated strains of Azospirillum sp. may alleviate the adverse effects of drought stress on wheat (Triticum aestivum L.) growth. Three different strains of Azospirillum lipoferum (B1, B2 and B3) were used to inoculate wheat seedlings under drought. During the flowering stage the seedlings were subjected to three drought levels with five different time longevity, including control. Pots were water stressed at 80% (S0), 50% (S1) and 25% (S2) of field capacity moisture in a 25 day-period. Soil and plant water properties including water potential and water content, along with their effects on bacterial inoculum and wheat growth, were completely monitored during the experiment. While stress intensity significantly affected bacterial population and wheat growth, stress longevity only affected wheat water potential and water content. Compared to uninoculated treatments strain B3 (fixing and producing the highest amounts of N and auxin, respectively, with P solubilizing and ACC-deaminase activities) increased wheat yield at S1 and S2 by 43 and 109%, respectively. However, strain B2 (producing siderophore) was the most resistant strain under drought stress. The results of this experiment may elucidate the more efficient strains of Azospirillum sp. for wheat inoculation under drought stress and the mechanisms by which they alleviate the stress.     

High Frequency Divisions in Leaf Base Protoplasts of Wheat (Triticum aestivum L.)

Protoplasts were isolated from the basal meristematic region of leaves from 6-day-old seedlings of wheat (Triticum aestivum). Protoplasts divided when cultured on MS medium (as agarose beads) in presence of nurse tissue. Donor seedlings when grown on BAP-supplemented MS medium were found to be considerably superior for protoplast isolation and culture than when grown on MS basal medium, in terms of protoplast viability, cell wall formation and cell division frequency. In addition, reduction of ammonium content of the culture medium, together with a dark Incubation, led to a high protoplast division frequency of about 70%. Microcolonies of 10-to 12-celled stages were obtained in Triticum aestivum, varieties HD 2329, HD 2285, Kalyan Sona, Arjun and CPAN 1676.