Nuclear DNA content and the control of chloroplast replication in wheat leaves

Profiling of spatial metabolite distributions in wheat leaves under normal and nitrate limiting conditions

The control and interaction between nitrogen and carbon assimilatory pathways is essential in both photosynthetic and non-photosynthetic tissue in order to support metabolic processes without compromising growth. Physiological differences between the basal and mature region of wheat (Triticum aestivum) primary leaves confirmed that there was a change from heterotrophic to autotrophic metabolism. Fourier Transform Infrared (FT-IR) spectroscopy confirmed the suitability and phenotypic reproducibility of the leaf growth conditions. Principal Component-Discriminant Function Analysis (PC-DFA) revealed distinct clustering between base, and tip sections of the developing wheat leaf, and from plants grown in the presence or absence of nitrate. Gas Chromatography-Time of Flight/Mass Spectrometry (GC-TOF/MS) combined with multivariate and univariate analyses, and Bayesian network (BN) analysis, distinguished different tissues and confirmed the physiological switch from high rates of respiration to photosynthesis along the leaf. The operation of nitrogen metabolism impacted on the levels and distribution of amino acids, organic acids and carbohydrates within the wheat leaf. In plants grown in the presence of nitrate there was reduced levels of a number of sugar metabolites in the leaf base and an increase in maltose levels, possibly reflecting an increase in starch turnover. The value of using this combined metabolomics analysis for further functional investigations in the future are discussed.

Chlorophyll accumulation is enhanced by osmotic stress in graminaceous chlorophyllic cells

We have developed a new chlorophyllic cell line ('TADH-XO') from the highly water stress tolerant grass Bouteloua gracilis (blue grama). When grown under normal (non-stress) conditions, this new cell line accumulates higher levels of chlorophyll (up to 368.1 microg total chlorophyll g(-1) FW) than a previously obtained cell line ('TIANSJ98'). Both cell lines are capable of developing substantially higher amounts of chlorophyll when subjected to osmotic stress. In order to explain these changes in the chlorophyll kinetics of the chlorophyllic cells, we analyzed the following population variables in cells subjected to polyethylene glycol 8000-induced osmotic stress: growth, viability, chlorophyll (total, 'a' and 'b'), cell size, percentage of green cells and chloroplast (number and size). Although previous studies in some chlorophyllic cells of dicots have already reported that chlorophyll increases under saline stress, in this report we show that, at least in this graminaceous cell line, the increase in chlorophyll is an immediate and proportional response to the osmotic stress applied and not the result of a progressive adaptation process. Consistent with previous studies, the increase in chlorophyll accumulation could be the result of chloroplast development (increased thylakoid number per chloroplast). On the basis of our results, the increases in chlorophyll accumulation previously observed in salt-adapted dicot cells may be the result of the osmotic shock (water deficit), rather than the ionic effect of salt on the physiology of chlorophyllic cells of dicots. Under the cell population experimental approach we followed, our study provides important insights related to the physiological behavior of chlorophyllic cells subjected to osmotic stress.

A large population of small chloroplasts in tobacco leaf cells allows more effective chloroplast movement than a few enlarged chloroplasts

We generated transgenic tobacco (Nicotiana tabacum cv Xanthi) plants that contained only one to three enlarged chloroplasts per leaf mesophyll cell by introducing NtFtsZ1-2, a cDNA for plastid division. These plants were used to investigate the advantages of having a large population of small chloroplasts rather than a few enlarged chloroplasts in a leaf mesophyll cell. Despite the similarities in photosynthetic components and ultrastructure of photosynthetic machinery between wild-type and transgenic plants, the overall growth of transgenic plants under low- and high-light conditions was retarded. In wild-type plants, the chloroplasts moved toward the face position under low light and toward the profile position under high-light conditions. However, chloroplast rearrangement in transgenic plants in response to light conditions was not evident. In addition, transgenic plant leaves showed greatly diminished changes in leaf transmittance values under both light conditions, indicating that chloroplast rearrangement was severely retarded. Therefore, under low-light conditions the incomplete face position of the enlarged chloroplasts results in decreased absorbance of light energy. This, in turn, reduces plant growth. Under high-light conditions, the amount of absorbed light exceeds the photosynthetic utilization capacity due to the incomplete profile position of the enlarged chloroplasts, resulting in photodamage to the photosynthetic machinery, and decreased growth. The presence of a large number of small and/or rapidly moving chloroplasts in the cells of higher land plants permits more effective chloroplast phototaxis and, hence, allows more efficient utilization of low-incident photon flux densities. The photosynthetic apparatus is, consequently, protected from damage under high-incident photon flux densities.

Estimating position-time relationships in steady-state, one-dimensional growth zones

Two methods are described for estimating position-time relationships (pathlines) in steady, one-dimensional growth zones. Pathlines can be used to provide a time base for spatial data in developmental studies. The methods apply within extension-only zones (zones of growth without cell division) and require data for cell-number densities, or cumulative cell numbers, or mean cell lengths, and for the overall elongation rate of an organ. The first method ("continuous-pathline" method) can be used to estimate spatial velocity fields within extension-only zones and pathlines can then be obtained by integration of the velocity data. This method is based on the continuity equation for cell-number densities. Pathlines can also be estimated using a simple graphical version of this method. The second method ("pathline-coordinate" method) is based on the approximation that a cell of mean length remains of mean length as it moves through the extension-only zone, and can be used to estimate the coordinates of wall pathlines at discrete intervals. The methods are illustrated using published data from studies of apical growth in Zea mays L. roots and of intercalary growth in Triticum aestivum L. leaves.

The control of chloroplast number in wheat mesophyll cells

Chloroplast number per cell and mesophyll cell plan area were determined in populations of separated cells from the primary leaves of different wheat species representing three levels of ploidy. Mean chloroplast number per cell increases with ploidy level as mean cell size increases. But in addition the analysis of individual cells clearly shows that cells of a similar size but from species of different ploidies have similar numbers of chloroplasts. We conclude that the number of chloroplasts within a cell is closely correlated (P<0.001) with the size of the cell and this relationship is consistent for species of different ploidies over a wide range of cell sizes. These results are discussed in relation to the hypothesis that chloroplast number in leaf mesophyll cells is determined by the size of the cell.

Copy numbers of chloroplast and nuclear genomes are proportional in mature mesophyll cells of Triticum and Aegilops species

The possibility of estimating the proportion of chloroplast DNA (ctDNA) and nuclear DNA (nDNA) in nucleic-acid extracts by selective digestion with the methylation-sensitive restriction enzyme PstI, was tested using leaf extracts from Spinacia oleracea and Triticum aestivum. Values of ctDNA as percentage nDNA were estimated to be 14.58%±0.56 (SE) in S. oleracea leaves and 4.97%±0.36 (SE) in T. aestivum leaves. These estimates agree well with those already reported for the same type of leaf material. Selective digestion and quantitative dot-blot hybridisation were used to determine ctDNA as percentage nDNA in expanded leaf tissue from species of Triticum and Aegilops representing three levels of nuclear ploidy and six types of cytoplasm. No significant differences in leaf ctDNA content were detected: in the diploids the leaf ctDNA percentage ranged between 3.8% and 5.1%, and in the polyploids between 3.5% and 4.9%. Consequently, nuclear ploidy and nDNA amount were proportional to ctDNA amount (r(19)=0.935, P>0.01) and hence to ctDNA copy number in the mature mesophyll cells of these species. There was a slight increase in ctDNA copy numbers per chloroplast at higher ploidy levels. The balance between numbers of nuclear and chloroplast genomes is discussed in relation to polyploidisation and to the nuclear control of ctDNA replication.

Variation in cellular ribulose-1,5-bisphosphate-carboxylase content in leaves of Triticum genotypes at three levels of ploidy

Ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 1.1.39) (RuBPCase) was quantified using polyacrylamide-gel electrophoresis in whole 9-d-old first leaves of 14 genotypes of Triticum, and cellular RuBPCase levels calculated. Diploids, tetraploids and hexaploids were analysed and it was confirmed that the RuBPCase level per cell is closely related to ploidy in wheat. Inter-genotypic variation in RuBPCase levels per cell and per leaf were surveyed. It was found that the interactions between leaf size, cell size and RuBPCase levels result in small variations in RuBPCase levels per unit leaf area between genotypes.

Cell size and chloroplast size in relation to chloroplast replication in light-grown wheat leaves

As part of an investigation into the control of chloroplast replication the number and size of chloroplasts in mesophyll cells was examined in relation to the size of the cells. In first leaves of Triticum aestivum L. and T. monococcum L. the number of chloroplasts in fully expanded mesophyll cells is positively correlated with the plan area of the cells. The linear relationship between chloroplast number per cell and cell plan area is also consistent over a fivefold range of cell size in isogenic diploid and tetraploid T. monococcum. In T. aestivum the chloroplast number per unit cell plan area varies among cells in relation to the size of the chloroplasts. Those cells containing chloroplasts with a relatively small face area have a correspondingly higher density of chloroplasts, and consequently, the total chloroplast area per unit cell plan area is very similar in all the cells. The results indicate that the proportion of the cell surface area covered by chloroplasts is precisely regulated, and that this is achieved during cell development by growth and replication of the chloroplasts.

The appearance of photosynthetic proteins in developing maize leaves

The appearance of photosynthetic proteins was directly measured in developing maize (Zea mays L.) leaves. Third leaves of 10-14-d-old seedlings were dissected into six successive sections from the basal meristem to the tip of the leaf. The membrane and soluble proteins were separated by polyacrylamide gel electrophoresis and then transferred onto cyanogen bromide paper. After transfer of membrane proteins the paper was reacted with antisera raised against the light-harvesting chlorophyll a/b protein of photosystem II, the chlorophyll a-binding protein of reaction center P-700 of photosystem I and the α-subunit of chloroplast-coupling factor 1. The blots of soluble proteins were reacted with antisera raised against the electron-transport proteins plastocyanin and Fe-NADP reductase (EC 1.6.7.1), the carbon-fixing enzymes ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) and phosphoenolpyruvate carboxylase (EC 4.1.1.31), as well as pyruvate orthophosphate dikinase (EC 2.7.9.1). The schedule of appearance of proteins shows that the light-harvesting and ATP-generating proteins are present in the most immature segments at the leaf base and accumulate rapidly as the cells mature. The carbon-reducing enzymes, however, appear only in tissue that has differentiated into mesophyll and bundle-sheath cells.