Effects of strigolactone-biosynthesis inhibitor TIS108 on Arabidopsis

TIS108 is a triazole-type strigolactone (SL)-biosynthesis inhibitor that reduces the level of 2′-epi-5-deoxystrigol (epi-5DS) in rice. Here we report the effects of TIS108 on Arabidopsis. Treatment of TIS108 increased the number of branches and repressed root hair elongation as was observed in SL-deficient mutants, and co-application of GR24, a synthetic SL analog, recovered the TIS108-induced phenotype to that of wild-type. In addition, MAX3 and MAX4 genes in the SL-biosynthesis pathway were upregulated in TIS108-treated Arabidopsis, probably due to feedback regulation caused by SL deficiency. These results indicate that TIS108 is an effective tool for regulating SL production in Arabidopsis.     

Allelopathy of rice (Oryza sativa L.) root exudates and its relations with Orobanche cumana Wallr. and Orobanche minor Sm. germination

Allelopathy has been considered not only as an environmentally friendly approach for weed control but also a potential reason causing autotoxicity in crop production. In this study, the responses of seeds of lettuce, wheat, rice, clover broomrape (CB), and sunflower broomrape (SB) to the root exudates of rice cultivars were studied. Lettuce germination was promoted by root exudates of Yliangyou 3218 and I-Kung-Pao. Wheat seedling growth was inhibited by all nine rice species. I-Kung-Pao and Ganxin 203 exerted greater autotoxicity than other cultivars. Yongyou 15 and I-Kung-Pao induced the highest germination rate of CB, while Yongyou 13, Zhongzao 22, and I-Kung-Pao induced the highest germination rate of SB. A significantly correlation was noted between germination-inducing ability on broomrape seeds and allelopathic effects on target plants. It is suggested that using broomrape seeds germination is a better receptor for the identification of rice allelopathic potential.     

Role of Ethylene in the Germination of the Hemiparasite Striga hermonthica

Seed germination of the hemiparasitic angiosperm Striga hermonthica (Del.) Benth is elicited by compounds present in the root exudates of the host plant. Although a variety of compounds can substitute for the host-derived signal, the mechanism through which these act is unknown. In the present study, an inhibitor of ethylene biosynthesis, aminoethoxyvinyl glycine, was found to inhibit germination. Addition of an intermediate in ethylene biosynthesis, 1-aminocyclopropane-1-carboxylic acid, was found to override this inhibition and to act as a substitute for the host-derived signal. 2,5-Norbornadiene, an inhibitor of ethylene action, was also found to inhibit germination. Ethylene is rapidly produced by Striga seeds after treatment with host root exudates. These results are consistent with a model for Striga seed germination in which host-derived signals and other compounds act by eliciting the synthesis of ethylene and in which ethylene itself initiates the biochemical changes leading to germination.     

Rice seedlings release momilactone B into the environment

Since the growth inhibitor momilactone B was found recently in root exudates of rice (Oryza sativa L.), 3-day-old rice seedlings were transferred to hydroponic culture and the level of momilactone B released into the environment from the seedlings was measured. At day 15 after transfer, the level of momilactone B in the culture solution was 1.8 nmol per seedling compared with endogenous levels of 0.32 and 0.63 nmol per root and shoot, respectively, suggesting that rice seedlings actively releases momilactone B into the culture solution. This release must occur from the roots because only rice roots were immersed in the culture solution. Momilactone B inhibited the growth of ten cress (Lepidium sativum L.) seedlings at concentrations greater than 3 microM. Ten rice seedlings were incubated with ten cress seeds in a Petri dish containing 1 ml of medium, the medium contained 18 nmol of momilactone B, which came to 18 microM. This level of momilactone B was enough to reveal growth inhibition of the cress seedlings. Release level of momilactone B and its effectiveness as a growth inhibitor suggest that it may play an important role in rice allelopathy.     

菌根真菌对白及种子萌发和幼苗生根的作用

自然条件下,兰科植物需要依赖菌根真菌获得营养才能萌发。本研究对白及根和原球茎中分离的4株菌根真菌（WQ17-33、WQ17-43、JST-3和SL15-7）进行分子鉴定,并评价光照和黑暗条件下不同菌株促白及种子萌发和幼苗生根的效果。结果表明,4个菌株分别隶属于鬼伞属Coprinus、胶膜菌属Tulasnella、腊壳菌属Sebacina和Serendipita。在种子萌发前期（未形成叶子）进行暗培养较光照对菌株JST-13和SL15-7处理组原球茎阶段萌发具有显著的促进作用。不同菌株共生萌发效果不同,菌株SL15-7较其他处理原球茎和幼苗发育阶段的萌发率高。菌株JST-13和SL15-7处理组形成的幼苗较其他处理组强壮,定殖的菌丝团也较多,其幼苗生根效果也较对照组好。该研究表明白及可与多种不同类群的菌根真菌菌株形成共生关系,这些真菌在促进白及种子萌发和生根能力方面存在差异。     

Biosynthesis, accumulation and secretion of isoflavonoids during germination and development of white lupin (Lupinus albus L.)

A combination of ¹⁴C labelling experiments of white lupin seedlings(Lupinus albus L.) and high pressure liquid chromatography oftissue extracts indicated active biosynthesis of isoflavonoidsduring the first 2–4 d of seed germination. These weresynthesized mostly as glucosides and to a lesser extent as aglycones,with trace amounts of prenylated derivatives. There was a generaldecrease in total isoflavonoids during later stages of germination(c. until d 13), which may be ascribed to their turnover and/orexudation into the rhizosphere. In addition, exudation of isoflavonoidsby lupin seeds germinated under sterile conditions continuesfor 12 d with a preferential release of monoprenylated compounds.The relationship between the specific developmental events duringseed germination and the accumulation of certain groups of isoflavonoidsis discussed in relation to their possible role in the growthprocesses of lupin. Key words: Isoflavonoids, glucosides, aglycones, prenylated derivatives, root exudates, white lupin     

Barnyard grass-induced rice allelopathy and momilactone B

Here, we investigated chemical-mediated interaction between crop and weeds. Allelopathic activity of rice seedlings exhibited 5.3-6.3-fold increases when rice and barnyard grass seedlings were grown together, where there may be the competitive interference between rice and barnyard grass for nutrients. Barnyard grass is one of the most noxious weeds in rice cultivation. The momilactone B concentration in rice seedlings incubated with barnyard grass seedlings was 6.9-fold greater than that in rice seedlings incubated independently. Low nutrient growth conditions also increased allelopathic activity and momilactone B concentrations in rice seedlings. However, the increases in the low nutrient-induced allelopathic activity and momilactone B concentration were much lower than those in barnyard grass-induced allelopathic activity and momilactone B concentration. Root exudates of barnyard grass seedlings increased allelopathic activity and momilactone B concentration in rice seedlings at concentrations greater than 30 mg/L of the root exudates, and increasing the exudate concentration increased the activity and momilactone B concentration. Therefore, barnyard grass-induced allelopathic activity of rice seedlings may be caused not only by nutrient competition between two species, but also by components in barnyard grass root exudates. As momilactone B shows strong allelopathic activities, barnyard grass-induced allelopathic activity of rice may be due to the increased concentration of momilactone B in rice seedlings. The present research suggests that rice may respond to the presence of neighboring barnyard grass by sensing the components in barnyard grass root exudates and increasing allelopathic activity by production of elevated concentration of momilactone B. Thus, rice allelopathy may be one of the inducible defense mechanisms by chemical-mediated plant interaction between rice and barnyard grass, and the induced-allelopathy may provide a competitive advantage for rice through suppression of the growth of barnyard grass.     

Differential Activity of Striga hermonthica Seed Germination Stimulants and Gigaspora rosea Hyphal Branching Factors in Rice and Their Contribution to Underground Communication

Strigolactones (SLs) trigger germination of parasitic plant seeds and hyphal branching of symbiotic arbuscular mycorrhizal (AM) fungi. There is extensive structural variation in SLs and plants usually produce blends of different SLs. The structural variation among natural SLs has been shown to impact their biological activity as hyphal branching and parasitic plant seed germination stimulants. In this study, rice root exudates were fractioned by HPLC. The resulting fractions were analyzed by MRM-LC-MS to investigate the presence of SLs and tested using bioassays to assess their Striga hermonthica seed germination and Gigaspora rosea hyphal branching stimulatory activities. A substantial number of active fractions were revealed often with very different effect on seed germination and hyphal branching. Fractions containing (−)−orobanchol and ent-2''-epi-5-deoxystrigol contributed little to the induction of S. hermonthica seed germination but strongly stimulated AM fungal hyphal branching. Three SLs in one fraction, putative methoxy-5-deoxystrigol isomers, had moderate seed germination and hyphal branching inducing activity. Two fractions contained strong germination stimulants but displayed only modest hyphal branching activity. We provide evidence that these stimulants are likely SLs although no SL-representative masses could be detected using MRM-LC-MS. Our results show that seed germination and hyphal branching are induced to very different extents by the various SLs (or other stimulants) present in rice root exudates. We propose that the development of rice varieties with different SL composition is a promising strategy to reduce parasitic plant infestation while maintaining symbiosis with AM fungi.     

Isolation and identification of a potent allelopathic substance in rice root exudates

A search for growth inhibitors in rice root exudates was undertaken in order to clarify the allelopathic system in rice ( Oryza sativa L.). Rice seedlings inhibited the growth of cress ( Lepidium sativum L.) and lettuce ( Lactuca sativa L.) seedlings when the cress and lettuce were grown with rice seedlings. The putative compound causing the inhibitory effect of rice seedlings was isolated from their culture solution, and the chemical structure of the inhibitor was determined by spectral data as momilactone B. Momilactone B inhibited the growth of cress and lettuce seedlings at concentrations greater than 3 and 30 µ M , respectively. The concentration of momilactone B was 3.4 and 1.1 nmol per seedling in the culture solutions of husked and non-husked rice seedlings, respectively. These results suggest that rice seedlings may release momilactone B into the environment and the stress caused by the husk-treatment may increase the amount of momilactone B released. Thus, momilactone B may play an important role in rice allelopathy.     

Photomodulation of strigolactone biosynthesis and accumulation during sunflower seedling growth

Present investigations report the presence of strigolactones (SLs) and photomodulation of their biosynthesis in sunflower seedlings (roots, cotyledons and first pair of leaves) during early phase of seedling development. Qualitative analyses and characterization by HPLC, ESI-MS and FT-IR revealed the presence of more than one type of SLs. Orobanchyl acetate was detected both in roots and leaves. Five-deoxystrigol, sorgolactone and orobanchol were exclusively detected in seedling roots. Sorgomol was detectable only in leaves. HPLC eluted fraction from seedling roots and leaves co-chromatographing with GR24 (a synthetic SL) could also bring about germination in Orobanche cernua (a weed) seeds, which are established to exhibit SL – mediated germination, thereby indicating the SL identity of the eluates using this bioassay. SLs accumulation was always more in the roots of light-grown seedlings, it being maximum at 4 d stage. Although significant activity of carotenoid cleavage dioxygenase (CCD, the enzyme critical for SL biosynthesis) was detected in 2 d old seedling roots, SLs remained undetectable in cotyledons at all stages of development and also in the roots of 2 d old light and dark-grown seedlings. Roots of light-grown seedlings showed maximum CCD activity during early (2 d) stage of development, thereby confirming photomodulation of enzyme activity. These observations indicate the migration of a probable light-sensitized signaling molecule (yet to be identified) or a SL precursor from light exposed aerial parts to the seedling roots maintained in dark. Thus, a photomodulation and migration of SL precursor/s is evident from the present work.     

Dehydrocostus lactone is exuded from sunflower roots and stimulates germination of the root parasite Orobanche cumana

The germination of the obligate root parasites of the Orobanchaceae depends on the perception of chemical stimuli from host roots. Several compounds, collectively termed strigolactones, stimulate the germination of the various Orobanche species, but do not significantly elicit germination of Orobanche cumana, a specific parasite of sunflower.Phosphate starvation markedly decreased the stimulatory activity of sunflower root exudates toward O. cumana, and fluridone - an inhibitor of the carotenoid biosynthesis pathway - did not inhibit the production of the germination stimulant in both shoots and roots of young sunflower plants, indicating that the stimulant is not a strigolactone.We identified the natural germination stimulant from sunflower root exudates by bioassay-driven purification. Its chemical structure was elucidated as the guaianolide sesquiterpene lactone dehydrocostus lactone (DCL). Low DCL concentrations effectively stimulate the germination of O. cumana seeds but not of Phelipanche aegyptiaca (syn. Orobanche aegyptiaca). DCL and other sesquiterpene lactones were found in various plant organs, but were previously not known to be exuded to the rhizosphere where they can interact with other organisms.     

大豆和水稻对铝胁迫响应的生理机制

通过水培方式,研究了铝处理对双子叶植物大豆和单子叶植物水稻根系生长、养分吸收、根系内含物及根分泌物的影响.结果表明,低铝 (10 μmol·L^-1)浸种刺激大豆种子萌发和根系的生长,对水稻无明显促进作用.铝处理增加了两种作物根系对P的吸收,降低K、Ca、Mg的吸收.水稻比大豆根系积累较少的Al和更多的P.铝胁迫条件下,大豆和水稻根系内源可溶性蛋白含量升高、可溶性酚含量下降、可溶性糖含量先上升后下降,且大豆根系内源柠檬酸含量下降明显.与大豆相比,水稻积累较低的柠檬酸和较高的可溶性蛋白、可溶性酚,但两者可溶性糖没有差异.铝处理增加大豆根系柠檬酸、可溶性蛋白、可溶性酚、可溶性糖的分泌量,且大豆分泌量显著高于水稻.在铝处理条件下,大豆根系具有较高的阳离子交换量(CEC),而水稻的CEC较低.这说明大豆和水稻对铝胁迫具有不同的生理反应,水稻的高耐铝性可能与其较高的磷吸收和较低的CEC有关,而与其根系分泌物的关系不大.     

Allelopathic effects of root exudates from watermelon and rice plants on Fusarium oxysporum f.sp. niveum

Root exudates have a key role in communication between plants and microbes in the rhizosphere. Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum (Fusarium oxysporum), drastically reduces watermelon yields in continuous cultivation systems, but it can be significantly alleviated using watermelon/aerobic rice intercropping system as shown by the research carried out in this laboratory. It is important to evaluate the interaction between root exudates from the two crops and the pathogen and thus to clarify the mechanism of disease suppressiveness in the intercropping system. The effects of phenolic acids, sugars and free amino acids in root exudates from watermelon (REW) and rice (RER) on the growth of Fusarium oxysporum were studied. The results obtained are listed as follows: (1) REW significantly increased spore germination and sporulation, whereas RER had inhibitory effects on those two parameters. (2) HPLC analysis showed that salicylic acid, p-hydroxybenzoic acid and phthalic acid were identified in exudates from both plants, but p-coumaric acid was only detected in rice and ferulic acid only in watermelon. Moreover, of the total rice exudates a high proportion (37.9 %) of p-coumaric acid was detected and the total amount of phenolic acids was 1.4-fold as high as that in watermelon. (3) Considerable differences in the components and contents of both sugars and amino acids were found between REW and RER exudates. (4) Exogenously applied alanine (Ala) increased spore germination and sporulation. In contrast, addition of exogenous p-coumaric acid reduced spore germination and sporulation, relative to controls. It was concluded that the rice root exudates had anti-fungal properties while that from watermelon promoted pathogen growth. This discovery provided a scientific basis for practicing watermelon/aerobic rice intercropping to control Fusarium wilt in watermelon.     

Paenibacillus polymyxa SQR-21 systemically affects root exudates of watermelon to decrease the conidial germination of Fusarium oxysporum f.sp. niveum

Paenibacillus polymyxa SQR-21 has been identified as a potential agent for the biocontrol of Fusarium wilt in watermelon, which is caused by the pathogenic fungus Fusarium oxysporum f.sp. niveum (FON). In the present study, the effects of root exudates from watermelon plants inoculated or non-inoculated with either SQR-21 or FON on conidial germination of FON were investigated. Compared to the control, conidial germination was decreased with root exudates from SQR-21-inoculated plants, but conidial germination was enhanced by root exudates from FON-inoculated plants. Maximal germination was found with root exudates from FON-inoculated plants after 30 d, which was 1.35 times more germination than the control. A split-root system was designed to verify that the alterations of the exudation pattern in SQR-21- inoculated or FON-inoculated watermelon roots were not only local, but also systemic. Cinnamic acid was found in the watermelon root exudates. An assay to test the effects of cinnamic acid on conidial germination of FON revealed that the stimulation of conidial germination was observed from cinnamic acid concentrations ranging from 0 to 30 μg/ml. In conclusion, both of SQR-21 and FON systemically affects watermelon root exudates. These results will help to the better understanding of the plant-microbe communication and will guide to improve the biocontrol strategies against Fusarium wilt of watermelon plants.     

Root exudates of rice seedlings. The influence of one variety on another

Summary In the present study attempts have been made to elucidate the composition and physiological actions of root exudates from CB-1 Rupsail and the mixed population of these two varieties. The chemical nature of the different components of root exudates was studied by means of paper chromatography, bioassay and other biochemical procedures. It was noted that exudates from CB-1 and Rupsail seedlings inhibited root/shoot growth of test seedlings of both varieties. But when these two varieties were grown together root/shoot growth of both varieties were markedly promoted. Both qualitative and quantitative differences in amino acids, carbohydrates and growth substances contents of root exudates of individual and mixed population of rice varieties were noted.     

Examination of two lowland rice cultivars reveals that gibberellin-dependent early response to submergence is not necessarily mediated by ethylene

Using two lowland rice (Oryza sativa L.) cultivars we found that in both cases submerged-induced elongation early after germination depends on gibberellins (GAs). Submergence increases the content of the active GA₁ by enhancing the expression of GA biosynthesis genes, thus facilitating the seedlings to escape from the water and preventing asphyxiation. However, the two cultivars differ in their response to ethylene. The cultivar Senia (short), by contrast to cultivar Bomba (tall), does not elongate after ethylene application, and submerged-induced elongation is not negated by an inhibitor of ethylene perception. Also, while ethylene emanation in Senia is not altered by submergence, Bomba seedlings emanate more ethylene upon desubmergence, associated with enhanced expression of the ethylene biosynthesis gene OsACS5. The cultivar Senia thus allows the possibility of clarifying the role of ethylene and other factors as triggers of GA biosynthesis enhancement in rice seedlings under submergence.Key words: gibberellins, ethylene, submergence, lowland rice, Oryza sativa     

New branching inhibitors and their potential as strigolactone mimics in rice

Strigolactones (SLs) are rhizosphere communication chemicals. Recent studies of highly branched mutants revealed that SL or its metabolites work as a phytohormone to inhibit shoot branching. When SLs are exogenously applied to the rice d10-1 mutant that has a highly branched phenotype caused by a defect in the SL biosynthesis gene (CCD8), they inhibit tiller bud outgrowth (branching in rice) of the mutant. We focused our attention on the SL function as a phytohormone and tried to find new chemicals mimicking the hormonal action of SL by screening chemicals that inhibit branching of rice d10-1 mutant. Fortunately, we found 5-(4-chlorophenoxy)-3-methylfuran-2(5H)-one (3a) as a new chemical possessing SL-like activity against the rice d10-1 mutant. Then, we prepared several derivatives of 3a (3b-3k) to examine their ability to inhibit shoot branching of rice d10-1. These derivatives were synthesized by a one-pot coupling reaction between phenols and halo butenolide to give 5-phenoxy 3-methylfuran-2(5H)-one (3) derivatives, which possess a common substructure with SLs. Some of the derivatives showed SL-like activity more potently than GR24, a typical SL derivative, in a rice assay. As SLs also show activity by inducing seed germination of root parasitic plants, the induction activity of these derivatives was also evaluated. Here we report the structure-activity relationships of these compounds.     

化感水稻PI312777苗期根系分泌物中化学成分分析

在接近自然栽培的田间土壤条件下,以无种植水稻秧苗的田间土壤为对照,采用循环法分别收集化感水稻品种PI312777(PI)苗期(3～4叶)根系分泌物和对照土壤溶液,经乙醚萃取,GC-MS分析和仪器谱图库(NIST98 & WILEY)检索.结果表明,在水稻PI根系分泌物的乙醚萃取物中检测到3个化合物,其中萜类9个(峰面积10.97%)、酚醌类8个(5.87%)、酯类个(10.8%)、醛酮类3个(1.44%)、杂环类4个(8.04%)、醇类2个(1.23%)、醚类2个(0.57%)和其他物质2个(1.20%),萜类之间、酚醌类之间的化合物结构具有明显的相似性.在对照土壤溶液的乙醚萃取物中检测到39个化合物,其中有7个化合物与水稻PI根系分泌物中化合物相同.此外,还讨论了水稻根系分泌物与对照土壤溶液之间物质的差异和各类物质的化感作用机理.     

Evaluation of bakanae disease progression caused by Fusarium fujikuroi in Oryza sativa L.

Bakanae disease caused by Fusarium fujikuroi is an important fungal disease in rice. Among the seven strains isolated from symptomatic rice grains in this study, one strain, FfB14, triggered severe root growth inhibition and decay in the crown and root of rice seedlings. The remaining six strains caused typical Bakanae symptoms such as etiolation and abnormal succulent rice growth. To reveal the relationship between mycelial growth in the infected tissues and Bakanae disease progression, we have established a reliable quantification method using real time PCR that employs a primer pair and dual-labeled probe specific to a unigene encoding F. fujikuroi PNG1 (FfPNG1), which is located upstream of the fumonisin biosynthesis gene cluster. Plotting the crossing point (CP) values from the infected tissue DNAs on a standard curve revealed the active fungal growth of FfB14 in the root and crown of rice seedlings, while the growth rate of FfB20 in rice was more than 4 times lower than FfB14. Massive infective mycelial growth of FfB14 was evident in rice stems and crown; however, FfB20 did not exhibit vigorous growth. Our quantitative evaluation system is applicable for the identification of fungal virulence factors other than gibberellin.     

Transcriptomic Profiling of Rice Seedlings Inoculated with the Symbiotic Fungus Trichoderma asperellum SL2

The fungal species Trichoderma is reported to have a significant impact on the growth and physiological performance of rice plants. However, the molecular mechanisms that induce these effects remain unspecified. Using next-generation sequencing technology, this study compared the differential expression of genes in rice seedlings that had been inoculated with Trichoderma asperellum SL2 with the gene expression in seedlings that had no such inoculation. The study showed that many genes related to plant growth enhancement and physiological functioning are differentially expressed in seedlings which have been symbiotically colonized by T. asperellum SL2. In these seedlings, specific genes related to photosynthesis, RNA activity, stomatal activity, and root development were found to be up-regulated as others were down-regulated. Although the exact causal mechanisms at the molecular level remain to be identified, the presence of Trichoderma versus its absence was associated with almost ten times more significant up-regulations than down-regulations for specific genes that have been identified from previous genomic mapping. Such analysis at the molecular level can help to explain observed phenotypic effects at the organismic level, and it begins to illuminate the observed beneficial relationships expressed phenotypically between crop plants and certain symbiotic microbes.