Early recognition of the importance of roots for drought resistance, and the diversity in rice root architecture, provided a strong foundation for drought research at the International Rice Research Institute (IRRI). IRRI was founded in 1960, and large efforts for research on root growth in response to drought were ongoing by the mid-1970s, with an emphasis on deep root growth, formation of coarse nodal roots, and the root pulling force method. In the 1980s, aeroponic studies on root morphology and anatomy and line-source sprinkler field studies were commonly conducted. The use of crosses to better understand the genetics of root traits started in the 1980s. Further characterization of the genetics behind root traits was conducted in the 1990s, specifically the use of molecular markers to select for root trait QTLs. A shift toward rainfed lowland experiments in addition to upland conditions began in the 1990s, with increased recognition of the different types of drought stress environments and characterization of root water uptake. In the 2000s, drought breeding efforts moved from selection of root traits to direct selection for yield under drought. Today (the 2010s), we have identified two major drought-yield QTLs to be related to root traits, and phenotyping for association mapping of genes related to root traits and functions is underway. After direct selection for yield during the past decade that is now approaching impact at the farm level, we are seeing that root traits are indeed involved in improved yield under drought.

抄録全体を表示

Legume-grass mixed cropping has significant advantages that affect crop yield and soil resources. Generally, grasses grown with legumes take up more nitrogen than those grown under sole cropping. We focused on the effect of cutting hairy vetch during a vigorous growth stage on N uptake in oat under mixed cropping. We evaluated the amounts of N transferred from hairy vetch to oat by using a ¹⁵N dilution method. Cutting hairy vetch increased the number of tillers and dry weight of oat, but total N content was not significantly higher than that under mixed cropping without cutting. In contrast, the amount of N transferred to oat was increased by cutting. Estimated amounts of N transferred to oat were 2.7 g m^-2 with cutting of hairy vetch and 0.8 g m^-2 without cutting. Cutting half of the oats under sole cropping did not indicate the transfer of rhizodeposited N in oat to the residual plants. In addition, cutting hairy vetch increased the amounts of accumulated solar radiation in the middle canopy of the mixed cropping plots. Therefore, in a hairy vetch and oat mixed cropping system, cutting of the hairy vetch might enhance growth of oat due to the transfer of N from hairy vetch and the reduction of light interception to the oat canopy. N fertility enhancement of the soil by cutting of the legume would be valuable for low-input crop production.

抄録全体を表示

Microbial respiration from plant litter decomposition is sensitive to soil water status; however, its response to water status remains ambiguous, particularly in the litter of fine roots. We investigated the effect of fine-root water content on microbial respiration after 468 days of decomposition in forest soil for two diameter classes (0-0.5 and 0.5-2 mm) of Quercus serrata and Ilex pedunculosa in central Japan. Direct measurement of microbial respiration from root litter resulted in a range of 0.015-3.52 nmol CO₂ g^-1 s^-1. Microbial respiration in both diameter classes and species decreased linearly with decreasing root water content. These changing patterns of microbial respiration did not differ significantly between the diameter classes of either species, indicating that microbial respiration was regulated by the moisture of root litter, and not by characteristics associated with diameter class or species. In contrast, the carbon to nitrogen ratio and mass loss of the root litter differed significantly between diameter classes in both species. These findings suggest that along with chemical and morphological properties of fine root litter, the changes in root water content should also be considered as a viable factor in microbial activity variations. Drying-wetting cycles of fine roots could lead to sensitive responses of microorganisms during the short term, leading to variation in the decomposition rate of fine root litter over the long term. This study provided insight into the potential impact of microbial physiological performance on heterotrophic respiration and fine root decomposition under the varying root water content.

抄録全体を表示

Flavonoids are widespread throughout the plant kingdom and present in different parts of plants. Tea (Camellia sinensis) is well known for very high content of flavonoids especially flavan-3-ols antioxidants and is an aluminium (Al) accumulator plant. Dihydroflavonol 4-reductase (DFR) and anthocyanidin reductase (ANR) are known to be regulatory enzymes of flavonoid biosynthetic pathway. In this study, cDNA encoding DFR (CsDFR) and ANR (CsANR) from tea were overexpressed individually in tobacco to check their influence on accumulation of flavonoid contents and antioxidant potential in roots of transgenic tobacco. Root morphological features, such as total volume and the number of lateral roots were improved in CsDFR and CsANR overexpressing tobacco plants relative to control tobacco plants. Both types of transgenic showed higher content of flavonoids and proanthocyanidins and lower content of anthocyanins in the roots compared to roots of control tobacco. Among flavan-3-ols, only epigallocatechin was observed in the roots and its content was higher in CsDFR and CsANR overexpressing tobacco as compared to control tobacco. Expression of genes encoding various other enzymes of flavonoid pathway like Phenylalanine ammonia-lyase, Chalcone isomerase, Flavanol synthase and Anthocyanin synthase was increased in roots of CsDFR and CsANR overexpressing tobacco plants as compared to control tobacco. The antioxidant potential of root portion of CsDFR and CsANR transgenic tobacco plants was found to be increased as indicated by enhanced total free radical scavenging activity and tolerance against Al toxicity. Taken together, these changes in roots of CsDFR and CsANR transgenic tobacco provided tolerance to aluminum toxicity.

抄録全体を表示

Auxin regulates almost every aspect of plant growth and development. Its intracellular concentration is controlled by biosynthesis and degradation. In addition, there is an “auxin pool” that consists of the conjugates with sugars, amino acids, and peptides. Some of the conjugates reversely release auxin, enabling alternative methods to regulate auxin concentrations. Auxin concentrations are also affected by transport. Besides the long distance delivery through the phloem, auxin is transported across the cell by influx and efflux carriers, from the shoot to root with maximum concentration at the root tip. At the root tip, the auxin flow reverses, and shootward auxin transport occurs. An auxin gradient formed this way is indispensable for proper development, maintenance of the meristem, and cell identity. The formation of root hairs is auxin-dependent. Auxin controls not only the initiation of root hairs but also regulates their elongation. In Arabidopsis thaliana, auxin accumulates in atrichoblasts and it is supplied to trichoblasts. Before hair initiation, randomization of cortical microtubule arrays is observed in lettuce seedlings. This action is promoted by auxin and is indispensable to hair formation. Furthermore, light promotes CMT randomization and root hair initiation via auxin signaling. Ethylene is another promoter of root hair formation. Ethylene affects auxin signaling and vice versa. Interactions between these hormones are synergistic for root growth inhibition but antagonistic for lateral root formation. Reactive oxygen species also regulate various responses in plants. They play an important role during root hair elongation, although their precise relationship with auxin is yet not clear.

抄録全体を表示

Plant protection against soil-borne diseases has been a challenge in horticultural production of Ericaceae for a long time. We tested the hypothesis that ericoid mycorrhizal fungi (ERMF) control root pathogens. Isolates of ERMF previously obtained from Calluna vulgaris and Rhododendron hirsutum were selected based on growth inhibiting activity against Oomycetes in dual agar plate tests. In addition, we assessed their impact on economically important Pythium spec. and Phytophthora cinnamomi in sterile Rhododendron plantlets in a liquid culture system, which is suitable for continuous observations of the infection process. For this purpose, rooted cuttings of micro-propagated Rhododendron plantlets were transferred to a mineral solution and subsequently inoculated with either Oidiodendron maius or the ERMF isolates. Before the root pathogens were applied to the experimental system, the symbioses were established over a four-week period. Mycorrhizal development, pathogen infection sites and development in mycorrhizal plants and non-mycorrhizal control plants were assessed microscopically. The root-colonising intensity of the tested ERMF differed considerably, but each of the applied ERMF impaired external pathogen mycelium and reduced pathogenic infections. A complete reduction was achieved at higher ERMF colonisation levels. The failure of symbiosis formation allowed pathogens to infect and spread. The quantification data concerning mycorrhiza frequencies and extramatrical hyphal nets provided details for a discussion on the suppressive effects of ERMF on the pathogens. The tested ERMF confer at least a localized protection from pathogen attack through suppression prior to infection. It is now to prove, whether these biocontrol effects will also be expressed in pot experiments.

抄録全体を表示

Electric fields can determine changes at morphological and physiological levels in plants. In this study, seedlings of Solanum lycopersicum L., grown hydroponically in a floating system, were exposed to a DC 12.0 V m^-1 electric field (EF). Root morphology was strongly affected by the electric field applied and a significant variation in root growth rate was observed along the gradient. The tomato plants grown on the hand of the positive electrode showed a pronounced length, root hairs’ development and root branching, compared to the plants grown at the central area of the container and on the hand of the negative electrode. Root growth of the control plants not exposed to the EF resembled that of EF-exposed plants taken in the central area. Hypotheses according to which the different growth patterns observed could be related to a chemiosmotic-induced activity and/or the distribution of plasma membrane carriers are discussed. In conclusion, the root growth was affected by the positions under application of EF. The results point to a possibility of applying electric fields for controlling tomato root growth.

抄録全体を表示

The higher abundance of microarthropods around plant roots has been considered to result from the release of labile carbon by roots (e.g., root exudation), but the concentration of labile carbon itself has not been measured. We investigated whether fine root distribution affects distribution of Collembola (Folsomia candida) by changing the soil dissolved organic carbon (DOC) concentration, which is supposed to represent rhizodeposits, under both low- and high-light conditions using a sandy soil system with Chamaecyparis obtusa. Fine root biomass and total DOC content were greater under the high-light condition than under the low-light condition, but no significant difference was detected in collembolan abundance. In addition, soil DOC concentration was correlated with fine root biomass, but collembolan distribution was not affected by root or DOC distribution under either light condition. Although it remains unsolved why collembolan distribution did not correspond to the fine-root or DOC distribution, our results indicate that there is the case of no significant correlation between roots and Collembola.

抄録全体を表示

Two well-known methods of root porosity measurement (i.e. buoyancy and cross-sectional) were used to evaluate the porosity of adventitious roots of 3-day hypoxia treated sunflower (Helianthus annus L.) cultivars to address the effect of hypoxia on root porosity as well as advantages and disadvantages of each method. In the first experiment, length of adventitious roots of two sunflower cultivars (CMS14 × R-864 and Hysun-33) was divided to three segments as apical, middle, and basal portions and porosity was measured via both methods. In buoyancy method, root porosities of Hysun-33 and CMS14 × R-864 cultivars were 5.12% and 6.47% in aerated and 11.58% and 15.71% in hypoxic condition, respectively. There is no statistically significant difference between cultivars in aerated condition. In contrast, more than twice additional aerenchyma formation was observed due to hypoxic condition in comparison to aerated condition. Results of cross-sectional method showed that middle portion of adventitious roots is the place that aerenchyma initiates to form, but well-developed aerenchyma was seen at about 80-90 mm behind the root apex (basal portion) under hypoxia. The results of root porosity of basal portion of adventitious roots of four sunflower cultivars (CMS14 × R-864, CMS51 × R-14, CMS19 × R-N₁-118 and Hysun-33) showed higher amounts of root porosity in buoyancy method in comparison to cross-sectional method.

抄録全体を表示

Eggplant (Solanum melongena) seedlings cultured in vitro were excised at the center of the hypocotyl to generate decapitated seedlings with intact roots. This modification of the complete decapitation method (CDM) developed in vivo by Harada et al. (2005) allowed in vitro culture (CDM in vitro; CDMi). As controls, rootless hypocotyl segment explants (approximately 1 cm) and cotyledon explants were cultured on media supplemented with 4.4 μM 6-benzyladenin (BA) and 0.2 μM thidiazuron (TDZ), respectively. Cotyledon explants had formed calli 2 weeks after excision but did not develop adventitious buds, despite the use of optimal conditions reported previously for a different eggplant cultivar. Calli formed at the cut ends of hypocotyls 1 week after excision in both CDMi and hypocotyl cultures, and adventitious buds regenerated 1 week earlier in CDMi. Six weeks after excision, CDMi yielded 11.4 adventitious buds per explant, but only 4.1 formed in hypocotyl culture. Moreover, shoots longer than 1 cm developed 2 weeks earlier in CDMi than in hypocotyl culture. The number of shoots per explant was 8.1 in CDMi, but only 2.4 in hypocotyl culture 6 weeks after cutting. All shoots that developed were rooted on MS medium in CDMi, but only 71% of shoots formed roots in hypocotyl culture. These results indicate that intact roots are important for explant shoot regeneration and development, and CDMi is a simple and efficient method for obtaining multiple shoots without the need to determine optimal concentrations of plant growth regulators and overcome inhibition of rooting in the obtained shoots.

抄録全体を表示