Mariko Oka

Application of silicon improves salt tolerance through ameliorating osmotic and ionic stresses in the seedling of Sorghum bicolor

Silicon has been widely reported to have a beneficial effect on improving plant tolerance to biotic and abiotic stresses. However, the mechanisms of silicon in mediating stress responses are still poorly understood. Sorghum is classified as a silicon accumulator and is relatively sensitive to salt stress. In this study, we investigated the short-term application of silicon on growth, osmotic adjustment and ion accumulation in sorghum (Sorghum bicolor L. Moench) under salt stress. The application of silicon alone had no effects upon sorghum growth, while it partly reversed the salt-induced reduction in plant growth and photosynthesis. Meanwhile, the osmotic potential was lower and the turgor pressure was higher than that without silicon application under salt stress. The osmolytes, the sucrose and fructose levels, but not the proline, were significantly increased, as well as Na+ concentration was decreased in silicon-treated plants under salt stress. These results suggest that the beneficial effects of silicon on improving salt tolerance under short-term treatment are attributed to the alleviating of salt-induced osmotic stress and as well as ionic stress simultaneously.

Elevated sensitivity to gibberellin by vernalization in the vegetative rosette plants of Eustoma grandiflorum and Arabidopsis thaliana

Changes in the sensitivity to gibberellin (GA) after vernalization were studied in the vegetative rosette of Eustoma grandiflorum and late flowering Arabidopsis thaliana mutant, fca-1. The sensitivity to GA after vernalization was monitored using the bolting rate of plants that were grown on a medium containing GA(3) or ancymidol. The bolting rates were higher in vernalized plants than non-vernalized plants when the same GA(3) concentration was used. There was a positive relationship between the duration of vernalization and the bolting rate in E. grandiflorum. In contrast, a negative relationship between the duration of treatment and bolting rate was found in the non-vernalized plants. In fca-1, the flowering time of vernalized plants was significantly reduced compared with the non-vernalized plants under various concentrations of GA(3) treatment. To elucidate whether this elevated sensitivity relates to the efficiency of GA signal transduction, we measured the transcript amounts of Expansin (Exp), which is up-regulated by GA, and GA20-oxidase (GA20ox) and GA3 beta-hydroxylase (GA3betahy), which are down-regulated by GA. The transcript amounts were estimated using the Taq-Man PCR system based on combinations of primers and probes that specifically detect the genes, and normalized by the transcript amount of ubiquitin gene measured as an internal standard. For each concentration of GA treatments examined, Expansin of both E. grandiflorum and A. thaliana was induced at a higher rate in the vernalized plants than in the non-vernalized plants. The expression of GA20ox and GA3betahy of E. grandiflorum decreased faster in the vernalized plants than the non-vernalized plants. We conclude that vernalization is a critical environmental cue not only for initiating GA biosyntheses in vegetative rosette, but also for elevating the GA sensitivity of the plants via a GA signal transduction pathway.

Sodium tolerance of plants in relation to ionic balance and the absorption ability of microelements

Abstract Salinity stress is a major abiotic problem in arid land agriculture. In particular, Na stress severely limits crop production because it causes Na toxicity and disturbs the homeostasis of essential cations and microelements in crops. The purpose of the present study was to verify the validity of two indexes with regard to Na tolerance in plants: (1) cation balance (([K]+[Ca]+[Mg])/[Na]), (2) absorption ability of microelements. Salicornia bigelobii (highly salt tolerant), beet (tolerant), maize (moderately sensitive) and bean (sensitive) were grown in artificially prepared saline (SA), sodic (SO) and highly sodic (HSO) soils. Salicornia bigelobii showed the best growth in SO soil and accumulated Na in the growing part of the shoot. Beet showed the best growth in SA soil and also needed Na for satisfactory growth. Both species, therefore, can be classified as halophytes. There was no relationship between growth and cation balance in Salicornia bigelobii and beet. The salt treatments suppressed the growth of maize and bean, with more severe suppression in bean. The cation balance of maize was higher than that of bean. Cation balance can, therefore, be an index of Na tolerance in maize and bean, which are glycophytes. Salicornia bigelovii and beet actively absorbed microelements under high Na conditions. In maize and bean, the salt treatments lowered the uptake amount of microelements, more so in bean. Absorption ability of microelements can, therefore, be an index of Na tolerance, irrespective of whether the plants are halophytes or glycophytes.

Abscisic acid substantially inhibits senescence of cucumber plants (Cucumis sativus) grown under low nitrogen conditions.

Low nitrogen (N) availability such as that found in both dry land and tropical regions limits plant growth and development. The relationship between the level of abscisic acid (ABA) in a plant and its growth under low-N conditions was investigated. The level of ABA in cucumber (Cucumis sativus) plants under low-N conditions was significantly higher at 10 and 20 d after transplantation compared with that under sufficient-N conditions. Chlorophyll was preserved in the aerial parts of cucumber plants grown under low-N conditions in the presence of ABA, while there was no significant difference between control plants and ABA-applied plants under sufficient-N conditions. ABA suppressed the reduction of chlorophyll biosynthesis under low-N conditions but not under sufficient-N conditions. On the other hand, ABA decreased the expression of the chlorophyll degradation gene in older cucumber plants grown under both conditions. In addition, transcript and protein levels of a gene encoding a chlorophyll a/b binding protein were positively correlated with ABA concentration under low-N conditions. The chloroplasts in control plants were round, and the stack of thylakoid membranes was reduced compared with that of plants treated with ABA 10(-5) M. These results strongly suggest that ABA is accumulated in cucumber plants grown under low-N conditions and that accumulated ABA promotes chlorophyll biosynthesis and inhibits its degradation in those plants.

Response of Vegetable Crops to Salinity and Sodicity in Relation to Ionic Balance and Ability to Absorb Microelements

Abstract The effect of salinity and sodicity on the growth of and the absorption of metal elements by asparagus, tomato, and bean was investigated. The plants were grown in Tottori sand dune soil with four soil treatments consisting of original soil (exchangeable sodium percentage (ESP) = − 5.8 : Control), saline soil (ESP = 8.8 : Saline), saline-sodic soil (ESP = 16.2 : Sodic 1), and sodic soil (ESP = 40.8 : Sodic 2). Shoot dry weight (DW) of the three species was smaller with increasing ESP. Growth of asparagus and tomato was suppressed in the Saline soil and strongly suppressed in both Sodic soils. Bean could not survive under Saline and Sodic conditions. The Na concentration in the shoots of the three species was higher with higher ESP. The suppressed growth of asparagus in the Sodic soils was partly due to high concentration ratios of Na to essential cations in the shoots. The imbalance between Na and essential cations in the shoots in the Sodic soils which was more pronounced in tomato at the first harvest, improved afterwards. Asparagus and tomato showed a lower ability to absorb low-available microelements in both Sodic soils than in the Saline soil, except for Fe in asparagus. Higher ability of bean to absorb microelements compared with asparagus and tomato, except for Mn, could not improve growth, due to the inadequate ionic balance in the shoots. We conclude that ionic balance in the shoots and the ability to absorb low-available microelements control the tolerance to salinity and sodicity of vegetable crops as in the case of grain crops.

Cloning of allene oxide cyclase gene from Leymus mollis and analysis of its expression in wheat-Leymus chromosome addition lines.

Leymus mollis (Triticeae; Poaceae) is a useful genetic resource for wheat (Triticum aestivum L.) breeding via wide hybridization to introduce its chromosomes and integrate its useful traits into wheat. Leymus mollis is highly tolerant to abiotic stresses such as drought and salinity and resistant to various diseases, but the genetic mechanisms controlling its physiological tolerance remain largely unexplored. We identified and cloned an allene oxide cyclase (AOC) gene from L. mollis that was strongly expressed under salt stress. AOC is involved in biosynthesis of jasmonic acid, an important signaling compound that mediates a wide range of adaptive responses. LmAOC cDNA consisted of 717 bp, coding for a protein with 238 amino acids that was highly similar to AOCs from barley (Hordeum vulgare) and other monocots. Subcellular localization using Nicotiana benthamiana confirmed it as a chloroplast-localized protein. LmAOC was found to be a multiple-copy gene, and that some copies were conserved and efficiently expressed in wheat–Leymus chromosome addition lines. LmAOC expression was upregulated under drought, heat, cold and wounding stresses, and by jasmonic acid and abscisic acid. Our results suggest that LmAOC plays an important role in L. mollis adaptation to abiotic stresses and it could be useful for wheat improvement.

Molecular Biology of the Metabolism and Signal Transduction of Gibberellins, and Possible Applications to Crop Improvement

Abstract The hormones gibberellins (GAs) have profound biological effects on several aspects of plant growth. The genes encoding enzymes of GA metabolism and proteins of GA signaling have been cloned, and their functions well characterized. Notably, progress has been made in our understanding of how GAs are synthesized and decomposed in response to various environmental cues. The homeostasis of endogenous levels of GAs is maintained at the transcriptional level of the genes by feed-back and feed-forward regulatory mechanisms through the signaling pathway. Mutations of each gene result in dwarf or succulent growth phenotypes. Some mutations cause agriculturally important phenotypes, such as ‘green revolution’ semi-dwarf in cereal crops. This indicates that the genes involved in the metabolism and signaling of GAs have the potential to improve economically important crops through the manipulation of their functions in transgenic plants.

Procedures for chemical fixation in immunohistochemical analyses of PIN proteins regulating polar auxin transport: Relevance to spaceflight experiments

The mechanism by which gravity controls the polar transport of auxin, a plant hormone regulating multiple physiological processes in higher plants, remains unclear, although an important role of PIN proteins as efflux carriers/facilitators in polar auxin transport is suggested. We are going to study the effect of microgravity on the polar transport of auxin, focusing on the cellular localization of its efflux carrier, PsPIN1 in etiolated pea seedlings and ZmPIN1a in etiolated maize seedlings grown under microgravity conditions on the International Space Station (ISS) using immunohistochemical analyses according to space experimental plans (Ueda, 2016). To obtain adequate results regarding the cellular localization of functional proteins, prolonged chemical fixation processes as well as chemical fixatives should be well-matched to the properties of functional proteins as antigens since experimental analyses will be performed on the ground after keeping samples for a long duration on the ISS. As a result of ground verification, clear detection of the cellular localization of PsPIN1 and ZmPIN1a immunohistochemically was successful based on the results of several kinds of chemical fixation tested, even when etiolated pea and maize seedlings were fixed by immersion in chemical fixative for a long duration. The addition of 0.1% (w/v) Nonidet P-40 to chemical fixative composed of 50% (v/v) ethanol and 5% (v/v) acetic acid or that of 50% (v/v) methanol and 5% (v/v) acetic acid has led to a significant improvement in the immunohistochemical detection of PsPIN1 or ZmPIN1a. These chemical fixatives were also shown to be storage-stable for a long time before use. In this study, adequate chemical fixatives and fixation protocols were developed, which can be used to detect localization of PsPIN1 and ZmPIN1a proteins in young etiolated pea and maize seedlings, respectively, using anti PsPIN1 and ZmPIN1a antibodies. These protocols can be used in spaceflight experiments to investigate the effects of the microgravity environment on the ISS on PIN protein localization in pea and maize seedlings.