SciCombinator

Discover the most talked about and latest scientific content & concepts.

Journal: Journal of experimental botany

172

Miscanthus sacchariflorus is a fast-growing C(4) perennial grass that can naturally hybridize with M. sinensis to produce interspecific hybrids, such as the sterile triploid M.× giganteus. The creation of such hybrids is essential for the rapid domestication of this novel bioenergy crop. However, progress has been hindered by poor understanding of the environmental cues promoting floral transition in M. sacchariflorus, which flowers less readily than M. sinensis. The purpose of this work was to identify the flowering requirements of M. sacchariflorus genotypes in order to expedite the introduction of new germplasm optimized to different environments. Six M. sacchariflorus accessions collected from a range of latitudes were grown under controlled photoperiod and temperature conditions, and flowering, biomass, and morphological phenotypic data were captured. Results indicated that M. sacchariflorus, irrespective of origin, is a quantitative short-day plant. Flowering under static long days (15.3h daylength), compared with shorter photoperiods, was delayed by an average 61 d, with an average associated increase of 52% of above-ground biomass (DM plant(-1)). Timing of floral initiation occurred between photoperiods of 14.2h and 12.1h, and accumulated temperatures of 553-1157 °C above a base temperature of 10 °C. Miscanthus sacchariflorus flowering phenology closely resembles that of Sorghum and Saccharum, indicating potentially similar floral pathways and suggesting that determination of the underlying genetic mechanisms will be facilitated by the syntenic relationships existing between these important C(4) grasses.

Concepts: Evolution, Poaceae, Maize, Flower, Biofuel, Photoperiodism, Biomass, Miscanthus giganteus

170

Fully drought-resistant crop plants would be beneficial, but selection breeding has not produced them. Genetic modification of species by introduction of very many genes is claimed, predominantly, to have given drought resistance. This review analyses the physiological responses of genetically modified (GM) plants to water deficits, the mechanisms, and the consequences. The GM literature neglects physiology and is unspecific in definitions, which are considered here, together with methods of assessment and the type of drought resistance resulting. Experiments in soil with cessation of watering demonstrate drought resistance in GM plants as later stress development than in wild-type (WT) plants. This is caused by slower total water loss from the GM plants which have (or may have-morphology is often poorly defined) smaller total leaf area (LA) and/or decreased stomatal conductance (g(s)), associated with thicker laminae (denser mesophyll and smaller cells). Non-linear soil water characteristics result in extreme stress symptoms in WT before GM plants. Then, WT and GM plants are rewatered: faster and better recovery of GM plants is taken to show their greater drought resistance. Mechanisms targeted in genetic modification are then, incorrectly, considered responsible for the drought resistance. However, this is not valid as the initial conditions in WT and GM plants are not comparable. GM plants exhibit a form of ‘drought resistance’ for which the term ‘delayed stress onset’ is introduced. Claims that specific alterations to metabolism give drought resistance [for which the term ‘constitutive metabolic dehydration tolerance’ (CMDT) is suggested] are not critically demonstrated, and experimental tests are suggested. Small LA and g(s) may not decrease productivity in well-watered plants under laboratory conditions but may in the field. Optimization of GM traits to environment has not been analysed critically and is required in field trials, for example of recently released oilseed rape and maize which show ‘drought resistance’, probably due to delayed stress onset. Current evidence is that GM plants may not be better able to cope with drought than selection-bred cultivars.

Concepts: DNA, Gene, Water, Biotechnology, Francis Crick, James D. Watson, Genetically modified food, Genetic engineering

169

Vitis vinifera scions are commonly grafted onto rootstocks of other grape species to influence scion vigour and provide resistance to soil-borne pests and abiotic stress; however, the mechanisms by which rootstocks affect scion physiology remain unknown. This study characterized the hydraulic physiology of Vitis rootstocks that vary in vigour classification by investigating aquaporin (VvPIP) gene expression, fine-root hydraulic conductivity (Lp®), % aquaporin contribution to Lp®, scion transpiration, and the size of root systems. Expression of several VvPIP genes was consistently greater in higher-vigour rootstocks under favourable growing conditions in a variety of media and in root tips compared to mature fine roots. Similar to VvPIP expression patterns, fine-root Lp® and % aquaporin contribution to Lp® determined under both osmotic (Lp®(Osm)) and hydrostatic (Lp®(Hyd)) pressure gradients were consistently greater in high-vigour rootstocks. Interestingly, the % aquaporin contribution was nearly identical for Lp®(Osm) and Lp®(Hyd) even though a hydrostatic gradient would induce a predominant flow across the apoplastic pathway. In common scion greenhouse experiments, leaf area-specific transpiration (E) and total leaf area increased with rootstock vigour and were positively correlated with fine-root Lp®. These results suggest that increased canopy water demands for scion grafted onto high-vigour rootstocks are matched by adjustments in root-system hydraulic conductivity through the combination of fine-root Lp® and increased root surface area.

Concepts: Gene, Grafting, Root, Plant reproduction, Grape, Viticulture, Phylloxera, Rootstock

168

The effects of exposure to increasing manganese concentrations (50-1500 µM) from the start of the experiment on the functional performance of photosystem II (PSII) and photosystem I (PSI) and photosynthetic apparatus composition of Arabidopsis thaliana were compared. In agreement with earlier studies, excess Mn caused minimal changes in the PSII photochemical efficiency measured as F(v)/F(m), although the characteristic peak temperature of the S(2/3)Q(B)(-) charge recombinations was shifted to lower temperatures at the highest Mn concentration. SDS-PAGE and immunoblot analyses also did not exhibit any significant change in the relative abundance of PSII-associated polypeptides: PSII reaction centre protein D1, Lhcb1 (major light-harvesting protein of LHCII complex), and PsbO (OEC33, a 33kDa protein of the oxygen-evolving complex). In addition, the abundance of Rubisco also did not change with Mn treatments. However, plants grown under excess Mn exhibited increased susceptibility to PSII photoinhibition. In contrast, in vivo measurements of the redox transients of PSI reaction centre (P700) showed a considerable gradual decrease in the extent of P700 photooxidation (P700(+)) under increased Mn concentrations compared to control. This was accompanied by a slower rate of P700(+) re-reduction indicating a downregulation of the PSI-dependent cyclic electron flow. The abundance of PSI reaction centre polypeptides (PsaA and PsaB) in plants under the highest Mn concentration was also significantly lower compared to the control. The results demonstrate for the first time that PSI is the major target of Mn toxicity within the photosynthetic apparatus of Arabidopsis plants. The possible involvement mechanisms of Mn toxicity targeting specifically PSI are discussed.

Concepts: Photosynthesis, Arabidopsis, Photosystem, Light reactions, Photosystem I, Photosystem II, Oxygen evolution, Light-dependent reactions

167

Flowering time of the short-day plant Chrysanthemum morifolium is largely dependent upon daylength, but it is also distinctly influenced by other environmental factors. Flowering is delayed by summer heat. Here, the underlying basis for this phenomenon was investigated. Heat-induced flowering retardation occurred similarly in C. morifolium and C. seticuspe, a wild-type diploid chrysanthemum. In both plants, this flowering retardation occurred mainly because of inhibition of capitulum development. Concurrently, expression of flowering-related genes in the shoot tip was delayed under high temperature conditions. In chrysanthemums, FLOWERING LOCUS T-like 3 (FTL3) has been identified as a floral inducer produced in the leaves after short-day stimuli and transported to the shoot tip. In C. seticuspe, heat-induced flowering retardation was accompanied by a reduction in FTL3 expression in the leaves. Two C. morifolium cultivars with flowering times that are differently affected by growth temperature were also examined. High temperature-induced FTL3 repression was observed in the leaves of both cultivars, although the degree of repression was greater in the heat-sensitive cultivar than in the heat-tolerant cultivar. When a scion of the heat-sensitive cultivar was grafted onto the stock of the heat-tolerant cultivar, flowering in the shoot tip was less sensitive to heat. Conversely, a scion of the heat-tolerant cultivar grafted onto the heat-sensitive cultivar showed increased heat sensitivity. Thus, several lines of evidence suggest that the reduction of FTL3 signalling from the leaves to the shoot tip at high temperatures is involved in flowering retardation in chrysanthemums.

Concepts: Gene, Temperature, Thermodynamics, Grafting, Flower, Chrysanthemum, Asteraceae, Cultivar

166

Although the mechanisms of nodule N(2) fixation in legumes are now well documented, some uncertainty remains on the metabolic consequences of water deficit. In most cases, little consideration is given to other organs and, therefore, the coordinated changes in metabolism in leaves, roots, and nodules are not well known. Here, the effect of water restriction on exclusively N(2)-fixing alfalfa (Medicago sativa L.) plants was investigated, and proteomic, metabolomic, and physiological analyses were carried out. It is shown that the inhibition of nitrogenase activity caused by water restriction was accompanied by concerted alterations in metabolic pathways in nodules, leaves, and roots. The data suggest that nodule metabolism and metabolic exchange between plant organs nearly reached homeostasis in asparagine synthesis and partitioning, as well as the N demand from leaves. Typically, there was (i) a stimulation of the anaplerotic pathway to sustain the provision of C skeletons for amino acid (e.g. glutamate and proline) synthesis; (ii) re-allocation of glycolytic products to alanine and serine/glycine; and (iii) subtle changes in redox metabolites suggesting the implication of a slight oxidative stress. Furthermore, water restriction caused little change in both photosynthetic efficiency and respiratory cost of N(2) fixation by nodules. In other words, the results suggest that under water stress, nodule metabolism follows a compromise between physiological imperatives (N demand, oxidative stress) and the lower input to sustain catabolism.

Concepts: Photosynthesis, Carbon dioxide, Amino acid, Metabolism, Adenosine triphosphate, Enzyme, Oxidative phosphorylation, Cellular respiration

164

In subfamily Salsoloideae (family Chenopodiaceae) most species are C4 plants having terete leaves with Salsoloid Kranz anatomy characterized by a continuous dual chlorenchyma layer of Kranz cells (KCs) and mesophyll (M) cells, surrounding water storage and vascular tissue. From section Coccosalsola sensu Botschantzev, leaf structural and photosynthetic features were analysed on selected species of Salsola which are not performing C4 based on leaf carbon isotope composition. The results infer the following progression in distinct functional and structural forms from C3 to intermediate to C4 photosynthesis with increased leaf succulence without changes in vein density: From species performing C3 photosynthesis with Sympegmoid anatomy with two equivalent layers of elongated M cells, with few organelles in a discontinuous layer of bundle sheath (BS) cells (S. genistoides, S. masenderanica, S. webbii) > development of proto-Kranz BS cells having mitochondria in a centripetal position and increased chloroplast number (S. montana) > functional C3-C4 intermediates having intermediate CO2 compensation points with refixation of photorespired CO2, development of Kranz-like anatomy with reduction in the outer M cell layer to hypodermal-like cells, and increased specialization (but not size) of a Kranz-like inner layer of cells with increased cell wall thickness, organelle number, and selective expression of mitochondrial glycine decarboxylase (Kranz-like Sympegmoid, S. arbusculiformis; and Kranz-like Salsoloid, S. divaricata) > selective expression of enzymes between the two cell types for performing C4 with Salsoloid-type anatomy. Phylogenetic analysis of tribe Salsoleae shows the occurrence of C3 and intermediates in several clades, and lineages of interest for studying different forms of anatomy.

Concepts: Photosynthesis, Cell, Bacteria, Eukaryote, Mitochondrion, Organelle, Plant physiology, C4 carbon fixation

162

In several taxa, increasing leaf succulence has been associated with decreasing mesophyll conductance (g M) and an increasing dependence on Crassulacean acid metabolism (CAM). However, in succulent Aizoaceae, the photosynthetic tissues are adjacent to the leaf surfaces with an internal achlorophyllous hydrenchyma. It was hypothesized that this arrangement increases g M, obviating a strong dependence on CAM, while the hydrenchyma stores water and nutrients, both of which would only be sporadically available in highly episodic environments. These predictions were tested with species from the Aizoaceae with a 5-fold variation in leaf succulence. It was shown that g M values, derived from the response of photosynthesis to intercellular CO2 concentration (A:C i), were independent of succulence, and that foliar photosynthate δ(13)C values were typical of C3, but not CAM photosynthesis. Under water stress, the degree of leaf succulence was positively correlated with an increasing ability to buffer photosynthetic capacity over several hours and to maintain light reaction integrity over several days. This was associated with decreased rates of water loss, rather than tolerance of lower leaf water contents. Additionally, the hydrenchyma contained ~26% of the leaf nitrogen content, possibly providing a nutrient reservoir. Thus the intermittent use of C3 photosynthesis interspersed with periods of no positive carbon assimilation is an alternative strategy to CAM for succulent taxa (Crassulaceae and Aizoaceae) which occur sympatrically in the Cape Floristic Region of South Africa.

Concepts: Photosynthesis, Carbon dioxide, Leaf, RuBisCO, C4 carbon fixation, C3 carbon fixation, Crassulacean acid metabolism, Carbon fixation

149

Flavonoid compounds play important roles in the modern diet, and pear fruits are an excellent dietary source of these metabolites. However, information on the regulatory network of flavonoid biosynthesis in pear fruits is rare. In this work, 18 putative flavonoid-related MYB transcription factors (TFs) were screened by phylogenetic analysis and four of them were correlated with flavonoid biosynthesis patterns in pear fruits. Among these MYB-like genes, the specific functions of two novel MYB TFs, designated as PbMYB10b and PbMYB9, were further verified by both overexpression and RNAi transient assays. PbMYB10b, a PAP-type MYB TF with atypical motifs in its conserved region, regulated the anthocyanin and proanthocyanidin pathways by inducing the expression of PbDFR, but its function could be complemented by other MYB TFs. PbMYB9, a TT2-type MYB, not only acted as the specific activator of the proanthocyanidin pathway by activating the PbANR promoter, but also induced the synthesis of anthocyanins and flavonols by binding the PbUFGT1 promoter in pear fruits. The MYBCORE-like element has been identified in both the PbUFGT1 promoter and ANR promoters in most species, but it was not found in UFGT promoters isolated from other species. This finding was also supported by a yeast one-hybrid assay and thus enhanced the likelihood of the interaction between PbMYB9 and the PbUFGT1 promoter.

Concepts: DNA, Gene, Gene expression, Promoter, Transcription, Transcription factor, RNA polymerase, Pear

148

Mosses are among the earliest branching embryophytes and probably originated not later than the early Ordovician when atmospheric CO2 was higher and O2 was lower than today. The C3 biochemistry and physiology of their photosynthesis suggests, by analogy with tracheophytes, that growth of extant bryophytes in high CO2 approximating Ordovician values would increase the growth rate. This occurs for many mosses, including Physcomitrella patens in suspension culture, although recently published transcriptomic data on this species at high CO2 and present-day CO2 show down-regulation of the transcription of several genes related to photosynthesis. It would be useful if transcriptomic (and proteomic) data comparing growth conditions are linked to measurements of growth and physiology on the same, or parallel, cultures. Mosses (like later-originating embryophytes) have been subject to changes in bulk atmospheric CO2 and O2 throughout their existence, with evidence, albeit limited, for positive selection of moss Rubisco. Extant mosses are subject to a large range of CO2 and O2 concentrations in their immediate environments, especially aquatic mosses, and mosses are particularly influenced by CO2 generated by, and O2 consumed by, soil chemoorganotrophy from organic C produced by tracheophytes (if present) and bryophytes.

Concepts: Carbon dioxide, Plant, Bryophyte, Embryophyte, Moss, Marchantiophyta, Physcomitrella patens, Mosses