Accumulation of sorghum phytoalexins induced by Colletotrichum graminicola at the infection site

Microspectrophotometry was performed on intact, pigmented vesicle-like inclusions within living sorghum cells that were accumulating phytoalexins as a response to attempted fungal infection. The results indicate that the deoxyanthocyanidin phytoalexins are present in inclusions. Moreover, the phytoalexin concentration within a single inclusion, based on luteolinidin, was calculated to be 0·15 m. The amounts of luteolinidin and apigeninidin in cells involved in the phytoalexin response at individual infection sites were also determined. The data showed that luteolinidin accumulated to levels of 0·48–1·20 ng/cell whereas apigeninidin accumulated to levels of 0·24–0·91 ng/cell. The results of both analyses confirmed that at the infection site the deoxyanthocyanidins accumulate to levels in substantial excess of those required for inhibition of the fungus Colletotrichum graminicola.     

Excision of the Candystripe1 transposon from a hyper-mutable Y1-cs allele shows that the sorghumY1 gene controls the biosynthesis of both 3-deoxyanthocyanidin phytoalexins and phlobaphene pigments

The 3-deoxyanthocyanidin phytoalexins produced in sorghum leaves in response to Colletotrichum sublineolum have chemical structure similarities to the 3-deoxy flavonoids that are precursors of phlobaphene pigments. Phlobaphenes are commonly observed in the pericarp of mature sorghum grains, while synthesis of 3-deoxyanthocyanidin phytoalexins is a site-specific response to infection with C. sublineolum. We have taken a genetic approach to investigate the possible overlap between the two sub-branches of flavonoid biosynthesis in sorghum that lead to phlobaphenes and 3-deoxyanthocyanidin phytoalexins. A sorghum line with a functional y1 gene synthesizes 3-deoxyanthocyanidins as well as phlobaphenes. However, a progenitor line with the mutable Y1-candystripe (Y1-cs) allele shows variable levels of biosynthesis of these compounds. The Y1-cs allele carries a copy of the Candystripe1 (Cs1) transposable element in the y1 gene. We demonstrate here that the variability in the expression of 3-deoxyanthocyanidins produced in individual mesocotyls of hyper-mutable Y1-cs plants is a function of the activity of the y1 gene. TheCs1 insertion in the Y1-cs allele blocks y1 function, while excision of Cs1 out of they1 locus restores the gene to a functional state. The combined molecular and biochemical characterization of sibling plants confirms that the allelic state of the y1 gene is completely correlated with the production of phytoalexins in response to fungal infection. These results provide support for the idea that the y1 gene regulates the biosynthesis of both 3-deoxyanthocyanidin phytoalexins and phlobaphene pigments in sorghum.     

Synthesis of C-glycosyl flavonoid phytoalexins as a site-specific response to fungal penetration in cucumber

Leaf tissue harvested from cucumber plants (Cucumis sativus L.) expressing induced resistance against the powdery mildew fungus Podosphaera xanthii (syn. Sphaerotheca fuliginea, Castagne; Braun and Shishkoff) was extracted and analyzed for phytoalexin compounds. Fluorescence microscopy was then used to observe the production of these compounds in planta, and laser scanning confocal microscopy observations were made to locate the subcellular sites of phytoalexin accumulation. Phytochemical analyses and fluorescence microscopy observations revealed the production of autofluorescent C-glycosyl flavonoid phytoalexins within the epidermal tissues of disease-resistant plants undergoing fungal ingress. Phytoalexin production was triggered by the combination of an eliciting/inoculation treatment, and tissue autofluorescence of color characteristic of the phytoalexins reached a maximum 48 h after elicitation prior to subsiding following the collapse of the pathogen. After a second eliciting treatment, disease-resistant plants produced phytoalexins more rapidly in response to fungal challenge. At the cellular level, autofluorescent C-glycosyl flavonoid phytoalexins were observed associated with the plasma membrane of infected epidermal cells immediately following elicitation. In the hours that preceded the collapse of conidial chains, phytoalexins accumulated inside the haustorial complexes of the pathogen within the epidermal cells of disease-resistant plants. Taken together, the results of this study show the timely synthesis of C-glycosyl flavonoid phytoalexins at precise subcellular locations as a key defense reaction used by cucumber to create incompatible interactions with powdery mildew.     

Silicon-mediated accumulation of flavonoid phytoalexins in cucumber

ABSTRACT The controversial role of silicon in plant disease resistance, described mostly as a passive mechanical protection, has been addressed. Conclusive evidence is presented that silicon is involved in the increased resistance of cucumber to powdery mildew by enhancing the antifungal activity of infected leaves. This antifungal activity was attributable to the presence of low-molecular-weight metabolites. One of these metabolites, described here as a phytoalexin, was identified as a flavonol aglycone rhamnetin (3,5,3',4'-tetrahydroxy-7-O-methoxyflavone). This is the first report of a phytoalexin for this chemical group in the plant kingdom and of a flavonol phytoalexin in cucumber, a chemical defense long believed to be nonexistent in the family Cucurbitaceae. The antifungal activity of leaf extracts was better expressed after acid hydrolysis, extending to another plant species the concept that some phytoalexins are synthesized as glycosylated phytoalexins or their precursors.     

Degradation of stilbene-type phytoalexins in relation to the pathogenicity of Botrytis cinerea to grapevines

The ability of eight isolates of Botrytis cinerea to degrade the stilbene phytoalexins, resveratrol and pterostilbene, was compared with their pathogenicity to grapevines. All strains which degraded resveratrol and pterostilbene were highly or moderately pathogenic to in vitro cultures of grapevines ( Vitis rupestris ) after inoculation with agar disks containing mycelium, while those which were unable to degrade phytoalexins were non-pathogenic. In all cases, the hydroxystilbene-degrading activity was related to the presence of laccase activity in the culture filtrates, as shown by using syringaldazine as substrate. The role of laccase-mediated degradation of phytoalexins in relation to pathogenicity of B. cinerea to grapevines in discussed.     

Control of Solatium karsensis in grain sorghum

Solatium karsensis Symon, an Australian native perennial, has become an important weed in irrigated summer crops in far western New South Wales. A screening trial of 14 herbicides showed that atrazine was the most effective treatment, though 2,4-D also suppressed S. karsensis in grain sorghum (Sorghum bicolor L. cv. Rico) for the duration of the crop. Grain sorghum yields were higher in plots treated with atrazine at 2–5 kg (a.i.) ha^?1 than for any other herbicide treatment or the untreated control. Because the root system remained viable annual herbicide application would be necessary for the continued control of 5. karsensis.     

Formation of phytoalexins within and outside lesions of Botrytis cinerea in French bean leaves

Rapidly spreading lesions and lesions restricted in size developed in primary leaves of French bean (Phaseolus vulgaris) in response to infection byBotrytis cinera isolates BC-1 and BC-5, respectively. These isolates caused similar differential lesions in leaves of cucumber, flax, lettuce and tomato. To determine whether phytoalexin accumulation was correlated with the resistant reaction in bean leaves, accumulation of phytoalexins was examined in necrotic areas of both types of lesions and in their surrounding green tissues. Phaseollin was the predominant phytoalexin, both inside and outside lesions, whereas phaseollidin and sometimes also phaseollinisoflavan were always present in lower concentrations. Phaseollin accumulated earlier and to higher levels within and around lesions of isolate BC-5 than of isolate BC-1. Relatively low concentrations of phaseollin were detected in the more remote green areas, including the petiole, of leaves bearing a spreading lesion. The phaseollin metabolite, 6a-hydroxyphaseollin, was found only inside lesions and in a narrow zone around lesions of both types. The authors consider the possibility that the differing concentrations of phytoalexins in the infected tissues are not a determining factor for the differential interactions betweenB. cinerea and bean leaves, but are rather the result of it.     

Diallate inhibition of gibberellin biosynthesis in sorghum coleoptiles

Diallate [S-(2,3-dichloroallyl)diisopropylthiocarbamate] incorporated into sand significantly inhibited sorghum (Sorghum bicolor (L.) Moench cv Funks G 522DR) growth of 14-day-old seed lings. Inhibition was competitively reversed by exogenous giberellic acid (GA₃) (0.1 and 10 ppmw). Diallate inhibited gibberellin (GA) precursor biosynthesis in a cell-free enzyme preparation from unruptured, etiolated sorghum coleoptiles. Diallate (10 μM) inhibited kaurene oxidation 40% with a 2.7 × increase in kauren-ol and a 50% decrease in kaurenoic acid. The GA biosynthesis inhibition correlates with symptom phenology and field use application concentrations. Geranylgeranyl pyrophosphate accumulated 5 × at 0.1 μM diallate concentrations but concomitant kaurene concentration decreases did not occur. At 10 μM diallate, kaurene synthetase was inhibited 33%.     

The role of phytoalexins in resistance to fusarium wilt in pigeon pea (Cajanus cajan)

Mechanisms of resistance to fusarium wilt (Fusarium udum) were investigated in pigeon pea cultivars from Malawi. Wilt-susceptible (Malawi local) and wilt resistant (ICP 9145) plants were stem-inoculated with a spore suspension containing 2·10⁶ conidia/ml of the pathogen. Occlusion of a small proportion of infected vessels was observed, but the resistant reaction appeared to depend mainly on rapid phytoalexin synthesis. Four fungitoxic isoflavonoid phytoalexins—hydroxygenistein, genistein, cajanin and cajanol-were isolated from plants 15 days after inoculation. Cajanol was identified as the main antifungal compound. The concentration of cajanol was 329·4 μg/g in the resistant cultivar as against 88·6 μg/g in the susceptible cultivar 15 days after inoculation. Crude extract from the resistant plants sampled at 24 h after inoculation contained 34·8 μg ml of cajanol. The LD₅₀ value of cajanol for spore germination was determined as 35μg/ml. The cajanol content of fungus-infected ICP 9145 10 days after inoculation totally inhibited conidial germination of F. udum.     

Silicon enhances the accumulation of diterpenoid phytoalexins in rice: a potential mechanism for blast resistance

ABSTRACT Although several reports underscore the importance of silicon (Si) in controlling Magnaporthe grisea on rice, no study has associated this beneficial effect with specific mechanisms of host defense responses against this fungal attack. In this study, however, we provide evidence that higher levels of momilactone phytoalexins were found in leaf extracts from plants inoculated with M. grisea and amended with silicon (Si(+)) than in leaf extracts from inoculated plants not amended with silicon (Si(-) ) or noninoculated Si(+) and Si(-) plants. On this basis, the more efficient stimulation of the terpenoid pathway in Si(+) plants and, consequently, the increase in the levels of momilactones appears to be a factor contributing to enhanced rice resistance to blast. This may explain the lower level of blast severity observed on leaves of Si(+) plants at 96 h after inoculation with M. grisea. The results of this study strongly suggest that Si plays an active role in the resistance of rice to blast rather than the formation of a physical barrier to penetration by M. grisea.     

CDAA inhibition of Kaurene oxidation in etiolated sorghum coleoptiles

N,N-Diallyl-2-chloroacetamide (CDAA) (0.25 ppmw; I μM) inhibited growth of 14-day-old sorghum (Sorghum bicolor (L) Moench. cv Funks G 522DR) when the herbicide was incorporated into sand. Kaurene oxidation was inhibited in a cell-free enzyme preparation from 4-day-old unruptured, etiolated coleoptiles. CDAA (1 μM) inhibited incorporation of [¹⁴C]mevalonic acid into kauren-19-oic acid (50%), with resultant increases in concentration of precursors. Thus, inhibition of gibberellin precursor biosynthesis was demonstrated, and this activity would explain many of the morphogenic and biochemical responses of grasses to CDAA.     

山西高平高粱丝黑穗病菌致病力研究

 高粱丝黑穗病[Sporisorium reilianum(Kühn) Langdon et Full]是世界高粱产区的主要病害之一,国内外许多研究表明,高粱丝黑穗病菌有明显的生理分化现象^[1~3]。据报道,美国高粱丝黑穗病菌有4个生理小种,其主要鉴别寄主为Tx7078、SA281、Tx414和TAM2571;墨西哥有3个丝黑穗病菌小种。     

醋糟、粉煤灰对高粱苗期土壤微生物数量、酶活性及高粱生长的影响

研究了不同醋糟、粉煤灰处理下高粱苗期土壤微生物数量和土壤酶特性的变化特征及对高粱生长的影响,以探讨醋糟、粉煤灰的施用效果及最佳配比,试验设置4个处理,分别是处理1(醋糟100%)、处理2(醋糟∶粉煤灰=1∶1)、处理3(醋糟∶粉煤灰=2∶1)、处理4(醋糟∶粉煤灰=3∶1)和化肥对照。结果表明:施用醋糟、粉煤灰可以有效增加土壤微生物数量和提高土壤酶活性,提高叶片光合速率和高粱产量。处理1～处理4细菌数量分别比对照提高19.66%、31.34%、15.69%、12.54%,放线菌数量分别比对照提高20.99%、32.37%、17.56%、10.43%,真菌数量分别比对照提高22.78%、34.26%、13.54%、9.56%;土壤磷酸酶活性分别比对照提高17.11%、20.85%、12.29%和19.78%;土壤蔗糖酶活性比对照提高了23. 70%、37. 08%、11. 54%和3. 08%,土壤脲酶活性比对照提高了25. 28%、34.77%、19.82%和13.79%;处理2苗期光合速率分别比对照、处理1、处理3和处理4提高13.8%、6.67%、9.98%和13.30%,拔节期提高3.45%、3.22%、3. 21%和4. 05%,抽穗期提高14. 52%、3. 75%、1. 86%和12. 66%,灌浆期提高了13.08%、4.26%、4.08%和9.36%,成熟期提高了3.8%、1.05%、2.12%和2.20%。醋糟含有大量的有机物质和无机物质,可为土壤微生物的生命活动提供能量和养料,进而促进土壤微生物的繁殖和土壤酶活性。     

Absorption and distribution of flurazole and acetochlor in grain sorghum

The site of uptake, absorption, and distribution of a safener, flurazole [2-chloro-4-(trifluoromethyl)-5-thiazolecarboxylic acid, (phenylmethyl ester)], and a herbicide, acetochlor [2-chloro-N-(ethoxymethyl)-6′-ethyl-O-acetoluidide], in grain sorghum [Sorghum bicolor (L.) Moench “G-522 DR”] were investigated in laboratory and growth chamber studies. Acetochlor was absorbed through shoots while flurazole was taken up primarily by roots. Uptake of [¹⁴C]acetochlor into the plant was rapid, linear, and the ¹⁴C was concentrated in primary roots by 7 days. Absorption of [¹⁴C]flurazole by sorghum was immediate, leveled off at 4 days, and the ¹⁴C was concentrated in primary roots by 7 days. Absorption and distribution of either chemical were not affected by the presence of the other. Flurazole had a slight effect on acetochlor metabolism at 3 days, but by 6 days no differences were noted.     

Control of johnsongrass (sorghum halepense) in cotton fields

Control of johnsongrass (Sorghum halepense (L.) Pers.) in cotton fields was achieved by the combined action of two herbicides: 1.5 kg/ha trifluralin incorporated preplanting prevented emergence of seedlings throughout the cotton growth season and reduced the vigor of rhizomes; and 1.25 kg/ha fluaziflop applied postemergence to the cotton crop selectively inhibited the growth of established rhizomes of the grass and completed the control. Reducing the spray volume for fluazifop from 300 to 200 1/ha resulted in efficient control of johnsongrass with lower rates of the herbicide.     

Induction of phytoalexins in pigeonpea (Cajanus cajan) in response to inoculation with Fusarium udum and other treatments

Cajanol accumulation was monitored in four cultivars of pigeonpea, Cajanus cajan (L) Millsp, after inoculation with 2 x 10(6) conidia ml-1 of Fusarium udum Butler. Rapid accumulation was observed only in wilt-resistant cultivar ICP 9145. Another wilt-resistant cultivar, ICP 8863, had low levels of cajanol similar to those in wilt-susceptible cultivars ICP 2376 and Malawi local, indicating that rapid cajanol accumulation is not positively correlated with resistance to wilt in pigeonpea. A comparison of various inducing agents showed live conidia of the pathogen to cause more rapid accumulation than the other agents.     

高粱链格孢叶斑病菌生物学特性研究

为明确高粱链格孢叶斑病菌Alternaria alternata的生物学特性, 本文采用生长速率法研究了不同营养及环境条件对该病菌生长的影响。试验结果表明:不同培养基上菌丝生长有明显差异, 其中马铃薯蔗糖琼脂培养基(PSA)最适合菌丝生长和孢子产生; 不同碳、氮源条件下菌丝的生长有显著差异, 其中最适宜菌丝生长碳源为葡萄糖, 氮源是蛋白胨; 该病原菌最适宜营养生长的温度为28℃, pH为7; 菌丝的致死温度为47℃; 光照时间对高粱叶斑病菌丝的营养生长没有影响。