STUDIES IN THE BACTERIAL DISEASES OF SUDAN CROPS: II. BACTERIAL LEAF BLIGHT DISEASE OF CASTOR (RICINUS COMMUNIS L.)

The bacterial disease of castor in the Sudan has been reinvestigated. Typically the disease symptoms are brown or black, round or angular lesions on leaves; occasionally the thin succulent branches are attacked, but not the vascular tissue.
The bacterium responsible for the disease was previously thought to be Pseudomonas solanacearum E.F.Sm. but comparative studies show that the Sudan species differs in pathogenicity and physiology from P. solanacearum. It closely resembles Xanthomonas ricinicola (Elliott) Dowson (= Bacterium ricini Yoshi and Takimoto) in characters, host range and pathogenic effects. In culture on solid media the growth of X. ricinicola is typically yellow, while that of the Sudan species is dirty white; but this difference is not considered sufficient to justify the establishment of a new species.
The author proposes the specific name ricini originally adopted by Archibald (1927) and Yoshi & Takimoto (1928), for the species of Xanthomonas causing the leaf blight disease of castor, and considers that the causal organism in the Sudan is an atypical strain of this species.     

STEM INFECTION OF COTTON BY XANTHOMONAS MALVACEARUM (E. F. SM.) DOWSON

Varying the position of stem inoculation, the concentration of inoculum and the age of plant affected the reaction of cotton, Gossypium sp., to infection with Xanthomonas malvacearum (E. F. Sm.) Dowson.
The extent of stem discoloration, internal and external, and the probability of disease ocurring in leaves by bacteria moving within the plant increased ( a ) the nearer the point of stem inoculation was to the apex, and ( b ) the higher the concentration of inoculum. The leaf symptoms were not the angular spots typical of primary leaf infection. Instead, bacteria seemed to lodge in, discolour and blacken sections of leaf veins. Then tissue next to the affected veins became water-soaked and leaf sectors dependent upon these veins died and dried. These symptoms usually developed 14 to 55 days after inoculation in the expanding leaves.
The amounts of stem discoloration and the probabilities of leaf symptoms developing were less when hypocotyls of old plants were inoculated than when hypocotyls of young plants were inoculated. The probabilities of leaf symptoms developing were similar, however, when young tissues in young and old plants were inoculated.
American cotton, Gossypium hirsutum , was less affected by stem inoculation than Egyptian cotton, G. barbadense. Of the resistance factors against primary leaf infection only B_6m gave appreciable stem resistance.     

An Xanthomonas citri pathogenicity gene, pthA, pleiotropically encodes gratuitous avirulence on nonhosts.

The pathogenicity gene, pthA, of Xanthomonas citri is required to elicit symptoms of Asiatic citrus canker disease; introduction of pthA into Xanthomonas strains that are mildly pathogenic or opportunistic on citrus confers the ability to induce cankers on citrus (S. Swarup, R. De Feyter, R. H. Brlansky, and D. W. Gabriel, Phytopathology 81:802-809, 1991). The structure and the function of pthA in other xanthomonads and in X. citri were further investigated. When pthA was introduced into strains of X. phaseoli and X. campestris pv. malvacearum (neither pathogenic to citrus), the transconjugants remained nonpathogenic to citrus and elicited a hypersensitive response (HR) on their respective hosts, bean and cotton. In X. c. pv. malvacearum, pthA conferred cultivar-specific avirulence. Structurally, pthA is highly similar to avrBs3 and avrBsP from X. c. pv. vesicatoria and to avrB4, avrb6, avrb7, avrBIn, avrB101, and avrB102 from X. c. pv. malvacearum. Surprisingly, marker-exchanged pthA::Tn5-gusA mutant B21.2 of X. citri specifically lost the ability to induce the nonhost HR on bean, but retained the ability to induce the nonhost HR on cotton. The loss of the ability of B21.2 to elicit an HR on bean was restored by introduction of cloned pthA, indicating that the genetics of the nonhost HR may be the same as that found in homologous interactions involving specific avr genes. In contrast with expectations of homologous HR reactions, however, elimination of pthA function (resulting in loss of HR) did not result in water-soaking or even moderate levels of growth in planta of X. citri on bean; the nonhost HR, therefore, may not be responsible for the "resistance" of bean to X. citri and may not limit the host range of X. citri on bean. The pleiotropic avirulence function of pthA and the heterologous HR of bean to X. citri are both evidently gratuitous.     

Resident population of Xanthomonas campestris pv. malvacearum on cotton leaves: a source of inoculum for bacterial blight

Xanthomonas campestris pv. malvacearum was transmitted from infested seed to the cotyledons of cotton cv. Deltapine 61 seedlings at 28°C and relative humidities (RH) of 90% or 73%. A resident population was present on the first and second true leaves but not on the third true leaf of plants at either RH. There were smaller numbers of resident bacteria on fewer leaves of plants at the lower RH than on plants at the higher RH. Cotton plants grown from infested seed at 25°C and 30°C and incubated at 100% RH at different stages of growth developed bacterial blight on leaves that were in bud or partly expanded when incubated. Resident cells of this pathogen can thus invade susceptible leaves when conditions are favourable for infection. Bacterial blight developed on more plants at 30°C than at 25°C. In a field trial, X. campestris pv. malvacearum transmitted from seed was present as resident bacteria on the third leaf from the growing point during the vegetative development of the plant. Resident bacteria, which infected young leaves during rainy periods, were isolated from the bacterial blight lesions which subsequently developed.     

Interference Between Two Strains of Bean Common Mosaic Virus is Accompanied by Suppression of Symptoms Without Affecting Replication of the Challenging Virus

An unusual type of interference between two strains of bean common mosaic virus, viz. NY15 and NL3, infecting bean plants, was investigatred. Wdhen a primary leaf was inoculated with NY15 as inducer and 1–8 days later, the opposite leaf with NL3 as challenger, the plant did not develop symptoms characteristic of NL3, i.e. systemic necrosis in the top, stem and wilting and withering of the youngest trifoloclate leaves. A7-h interval between the inoculations with inducer and challenger already sufficed to reduce the number of plants showing NL3 symptoms. Surprisingly, the amount of NL3 in the challenge-inoculated leaf was always than that in the singly infected control, Furthermore, NY15 could not be detected in the opposite leaf until 8 days post-inoculation and appered enen later in that leaf when challenge-inoculated. Histological studies showed that NL3 appered later in the xylem of bothe the petiole and stem between primary leaves and first trifoliolate leaf, as compared with the singly inoculated control.
The results suggest that suppression of Nl3 symptoms by Ny15 is not caused by impeding its multiplication, but by delaying the transport of NL3 to the xylem of petiole and stem.     

Distribution and Races of Xanthomonas axonopodis pv. malvacearum on Cotton (Gossypium hirsutum) in Uganda

Surveys in 1995 and 1996 showed that bacterial blight caused by Xanthomonas axonopodis pv. malvacearum occurs throughout the main cotton growing areas of Uganda, causing seedling blight, angular leaf spot and bacterial boll rot. During the vegetative and early fruiting stages of crop growth, severe symptoms of `blackarm' spread from leaves to the stem, causing loss of fruiting branches. A set of Upland cotton cultivars ( Gossypium hirsutum ) were then used to determine the races of the blight bacterium present in Uganda. Many of the isolates induced moderate to severe symptoms on all the test hosts except 101–102B, indicating infection with race 10 or 18. The next most common isolate was race 7. Races 16 and 6 were also identified and 23% of isolates caused symptoms on all the differential cultivars including 101–102B, results indicating the presence of a race of the pathogen which may be the same as that identified in countries neighbouring Uganda and designated as race 20.     

LEAF INFECTION OF COTTON BY XANTHOMONAS MALVACEARUM (E.F.SM.) DOWSON

Xanthomonas malvacearum spread more rapidly along vascular tissue than into mesophyll when inoculated to the main veins of susceptible cotton leaves. The extent of spread varied with the concentrations of inocula, tissue age and cotton variety.
Increasing concentrations of inocula accelerated the initial spread of disease.
Bacteria spread more rapidly in young leaves than in old—increasing age greatly decreased disease in the mesophyll. The initial invasion was quicker in young leaves of young plants than in young leaves of old plants.
Three types of behaviour, according to the host's reaction, distinguish Knight's resistance factors: ( a ) where X. malvacearum spread extensively along veins and into mesophyll of plants containing factors B₃ and B₅; ( b ) where it was restricted to the point of inoculation in plants containing B ₄, B₉ and combinations with B _6m; and ( c ) where it spread along veins but not appreciably into mesophyll in varieties containing B ₂ and B ₂ B ₃.
From this and four other different types of tests, factors B ₂ and B ₃ seem to increase mesophyll resistance but only B ₂ gives appreciable vascular resistance. Further, the vascular bundles in varieties with B ₂ seem to be surrounded by an additional 'barrier' which resists X. malvacearum.     

Contribution of corticular photosynthesis to bud development in African baobab (Adansonia digitata L.) and Castor bean (Ricinus communis L.) seedlings

The African baobab (Adansonia digitata L.) and castor bean (Ricinus communis L.) are drought resistant green-stemmed succulent plants which grow in the arid and semi-arid regions of Africa. Photosynthesis in the stems of green-stemmed plants is known to contribute to plant carbon gain especially during leafless periods. To study the contribution of stem photosynthesis in stem succulent plants, the height and stem diameter of baobab and castor bean plants grown in the greenhouse were measured. The plants were completely defoliated and subjected to different treatments: Watered with open stems (WO), watered and stems covered with aluminium foil (WC) to achieve 100% light exclusion, drought and open (DO) and drought and covered (DC). Stem coverage with aluminium foil resulted in a higher stem height and diameter during drought for baobab with similar trends seen in castor bean. Light exclusion resulted in a significantly lower bud DW production and enrichment in ¹³C in bud dry matter of castor bean and in stem dry matter of baobab. These show that corticular photosynthesis contributes in carbon gain in these species.     

马铃薯离体再生及人工种子研究

马铃薯３个品种虎头,克４和Ｆａｖｏｒｉｔａ的茎叶外植体在ＭＳ＋１ｍｇ／ＬＮＡＡ＋１ｍｇ／ＬＢＡ培养基上形成愈伤组织。在ＭＳ＋０２ｍｇ／ＬＮＡＡ＋１ｍｇ／ＬＢＡ培养基上,愈伤组织分化产生不定芽。在ＭＳ＋００５ｍｇ／ＬＮＡＡ培养基上,不定芽生根形成再生完整植株。０２～０３ｃｍ大小的不定芽反复继代可持续增殖。有３个以上叶片的不定芽在ＭＳ＋５ｍｇ／ＬＢＡ＋００５ｍｇ／ＬＩＢＡ培养基上和黑暗条件下,在侧芽或顶芽部位形成微型薯。用４％海藻酸钠和２％氯化钙溶液包裹０２～０３ｃｍ大小的不定芽或直径为０２～０３ｃｍ的微型薯制成微芽人工种子和微薯人工种子。在４℃下贮存２个月后,微芽人工种子和微薯人工种子在有菌腐殖土壤中播种２１ｄ的萌发率分别是１５７％和９６２％。     

A morphological study of the development of the second inflorescences in strawberry (Fragaria×ananassa Duch.)

To clarify the timing of the differentiation of the first and second inflorescences in strawberry (Fragaria × ananassa Duch.), morphological changes on shoot apices during short day and low night temperature treatments were observed by scanning electron microscopy (SEM) and optical microscopy. Axillary buds just below the first inflorescence (axillary bud 1) became visible when sepal primordia of the primary flower were differentiated. By this time, other axillary buds had already developed. Axillary bud 1 developed four leaf primordia, and then a differentiated inflorescence at its summit. The phase transition of shoot apices from the vegetative to the reproductive phase may therefore trigger the differentiation of axillary bud 1 which is destined to develop into extension crowns.     

Flowering Habit and Vegetative Behaviour in Tea (Camellia sinensis L) Seed Trees in North-East India

Apical growth of a tea shoot occurs by a succession of flushesseparated by short periods of rest. This paper describes theexternal morphology of flowering, fruiting, and abscission ofleaves of the tea plant in north-east India in relation to thephasic activity of shoot apices. All shoots on a tree make leafy growth when a new cycle of growthbegins in the spring, but terminal buds apparently become dormantas the season advances. Apparently dormant terminal buds shedbud scales, leaving on the stem a considerable number of scars,representing leafless cataphyllary flushes. These cataphyllaryflushes are produced at the same time as the leafy flushes onother shoots. A flower is formed only in the axil of a bud scale. Flowerswhich appear to develop in leaf axils are in fact inserted inthe axils of bud scales of the axillary buds. A distal leafy flush is without flowers. Flowers appear in itsleaf axils only when the terminal bud starts growth for thenext higher flush. A distal floriferous cataphyllary flush appearsas a terminal cluster of flowers. Thus, there is an acropetalsuccession of flowers, flush by flush on a caulome, determinedby the phasic activity of the apical bud. The main crop of flowers exposes anthers from the end of thethird flush (late September to early October) until the endof the winter period of growth (late January to early February).In some plants a second, minor crop of flowers appears in thespring between the end of the first and beginning of the secondflushes. In spite of considerable time lag between anthesis,the fruits produced by these two crops of flowers mature anddehisce at the same time during October to November. Abscission of leaves is also dependent upon the phasic activityof the apical buds. Only the top two flushes of a shoot possessleaves. Resumption of apical growth for a third flush, leafyor cataphyllary, causes the abscission of leaves on the lowermostof the three flushes. Two cataphyllary flushes therefore resultin the loss of all leaves on a shoot.     

Floral-stimulus activity in tobacco stem pieces

The growth patterns of axillary buds of dayneutral tobacco (Nicotiana tabacum L. cv. Wisconsin 38) plants were assessed by using expiants of single buds attached to leafless stem cuttings and allowing the buds to grow to flowering without additional manipulation. Buds located 5, 10 and 15 nodes below the inflorescence were employed. For a given bud position, when a cutting had few internodes the growth pattern of a bud tended to fall into one of two groups: buds that produced few-noded shoots and buds that produced many-noded shoots. For example, in a group of 13 cuttings composed of bud 5 with 2 associated internodes, 11 buds produced 14.2 nodes (range, 11–17) and 2 buds produced 32.0 nodes (range, 30–34). As the number of internodes on the cutting increased, the number of buds producing few-noded shoots increased and the number of nodes produced decreased (e.g. in contrast to the data above, all 5th buds with 6 internodes produced 12.8 nodes; range 11–15). When cuttings from the 3 positions had the same number of internodes, the more apical cuttings had buds that produced fewer nodes (e.g. for cuttings with 6 internodes all 5th buds produced 12.8 nodes, all 10th buds produced 15.5 nodes and 85% of 15th buds produced few-noded shoots with 19.3 nodes). The number of nodes produced by a bud was a function of the original position of the stem piece and not the original position of the bud. That is, bud 5 associated with the 6 internodes below it produced 12.8 nodes and bud 10 associated with essentially the same 6 internodes (i.e. the 6 above it) produced 12.9 nodes while bud 10 associated with the 6 internodes below it produced 15.5 nodes. Thus, the number of nodes produced by a bud was dependent upon the original main-axis position of the cutting as well as the number of internodes on the cutting. Buds forced to grow out in situ on main axes devoid of leaves produced substantially more nodes than similar buds on cuttings. Buds isolated without associated internodes produced many-noded plants with a number of nodes similar to that of plants grown from seed. The simplest interpretation of these data is that stem pieces contain floral-stimulus activity and that this activity is present in a gradient with the highest activity being located in the apical part of the stem.We thank Susan Smith and Harry Roy (Rensselaer) for comments, and the National Science Foundation for financial support (IBN-9003739 to C.N.M.).     

Assessment of resurgence of bacterial blight and its effect on cotton yield in northern Nigeria

Abstract

Cotton production in Nigeria causes fluctuating socio-economic and biotic factors. Bacterial blight induced by Xanthomonas campestris pv malvacearum causes the greatest yield loss annually. A study was therefore carried out to investigate the resurgence of the disease on 10 different cotton genotypes in Dowaya, Kem and Ngurore which are the major cotton areas of Adamawa state of Nigeria under field conditions. Other objectives were to determine the relationship between the different manifestations of bacterial blight and the yield of seed cotton and to identify resistance in the selected genotypes to the disease manifestations. Results revealed the presence of angular leaf spot, vein and boll rot manifestations of the disease in the study areas. The absence of vein blight symptoms in the Kem location did not result in higher yield because the severity of the leaf spot for this location was relatively higher than for the other two locations. Results also revealed that despite the high severity of angular leaf spot, high boll rot and vein blight incidences observed particularly on SAMCOT-11, SAMCOT-13 and ex-Benin in these locations with a high yield of seed cotton was recorded. The three multi-adversity resistant genotypes (MAR), TX-CDP37HH-1-83, TAMCOT SP-21S and TAMCOT CAMD-E recorded relatively lower severity and incidences in both cases. There was, however, a significantly negative correlation between angular leaf spot severity, boll rot incidence and yield at Kem location, significant negative correlation between angular leaf spot severity, boll rot incidence, vein blight and yield at Dowaya location, as well as significant negative correlation between angular leaf spot severity, vein blight and yield at Ngurore location. The tolerant and high yielding SAMCOT-11, SAMCOT-13 and ex-Benin should through breeding work be improved to be used for production in this area and its surroundings.     

Use of cloned DNA methylase genes to increase the frequency of transfer of foreign genes into Xanthomonas campestris pv. malvacearum.

In vitro-packaged cosmid libraries of DNA from the bacterium Xanthomonas campestris pv. malvacearum were restricted 200- to 1,000-fold when introduced into Mcr+ strains of Escherichia coli compared with restriction in the Mcr- strain HB101. Restriction was predominantly associated with the mcrBC+ gene in E. coli. A plasmid (pUFR052) encoding the XmaI and XmaIII DNA methylases was isolated from an X. campestris pv. malvacearum library by a screening procedure utilizing Mcr+ and Mcr- E. coli strains. Transfer of plasmids from E. coli strains to X. campestris pv. malvacearum by conjugation was enhanced by up to five orders of magnitude when the donor cells contained pUFR052 as well as the plasmid to be transferred. Subcloning of pUFR052 revealed that at least two regions of the plasmid were required for full modification activity. Use of such modifier plasmids is a simple, novel method that may allow the efficient introduction of genes into any organism in which restriction systems provide a potent barrier to such gene transfer.     

Characterization of the damage of Spodoptera eridania (Cramer) and Spodoptera cosmioides (Walker) (Lepidoptera: Noctuidae) to structures of cotton plants

The cotton plant, Gossypium hirsutum, hosts various pests that damage different structures. Among these pests, Spodoptera cosmioides (Walker) and Spodoptera eridania (Cramer) (Lepidoptera: Noctuidae) are considered important. The objectives of this study were to characterize and to quantify the potential damage of S. eridania and S. cosmioides feeding on different structures of cotton plants. For this purpose, newly-hatched larvae were reared on the following plant parts: leaf and flower bud; leaf and boll; flower bud or boll; and leaf, flower bud and boll. The survival of S. cosmioides and S. eridania was greater than 80% and 70% for larvae fed on cotton plant parts offered separately or together, respectively. One larva of S. eridania damaged 1.7 flower buds, but did not damage bolls, while one larva of S. cosmioides damaged 5.2 flower buds and 3.0 cotton bolls. Spodoptera eridania and S. cosmioides can be considered species with potential to cause economic damage to cotton plants because they can occur throughout cotton developmental stages causing defoliation and losses of reproductive structures. Therefore, the results validate field observations that these two species of Spodoptera are potential pests for cotton.     

Water-soaking reaction inhibitory metabolite excreted byXanthomonas campestris pv.malvacearum

The total cultural filtrate (TCF) ofXanthomonas campestris pv.malvacearum (i.e. cultural filtrate after removal of bacterial cells) induced non-persistent watersoaking (WS), on infiltration, in cotton. However diethyl-ether-extracted TCF produced persistent WS reaction. Diethyl-ether-extracted metabolite (EEM) did not induce any reaction but when EEM was co-inoculated with CF (TCF after extraction with diethyl ether), the persistence of WS on cotton was reduced, indicating the presence of an inhibitory metabolite (for WS) in EEM. Analysis of EEM indicated the presence of ammonia, iron and sulfate. Apparently, NH₃, Fe and SO₄ were covalently linked with diethyl-ether-extracted extracellular matrix. Ferrous/ferric ammonium sulfate, on co-inoculation and/or preinoculation toX. campestris in cotton, was shown to inhibit both susceptible (SR) and hypersensitive (HR) reactions. Inactivation of WS inducing factor by ferric/ferrous ammonium sulfate seem to be the cause for inhibition of induction and/or persistence of SR and HR in cotton.     

Identification and Characterization of GATA Gene Family in Castor Bean (Ricinus communis)

GATA proteins are considered to be broadly involved in yield associated biological process, such as photoresponse, chlorophyll biosynthesis, and carbon and nitrogen metabolism. Based on castor bean genome database, a total of 19 GATA genes were identified and classified into 4 subfamilies according to gene structure, protein structure and their phylogenetic relationships. Results exhibited that GATA factors were hydrophilic proteins. Analysis of gene structure and protein structure revealed the conserved structural features of GATA factors between castor bean and Arabidopsis thaliana. The high throughput RNA seq data were used to define the expressional profiles of GATA genes among tissues. The results showed that most of the castor GATA genes preferentially expressed in leaf and root in contrast to their expression in developing seeds. In particular, the expression of GATA genes responding to darkness treatment in leaves was detected using semi quantitative RT PCR. It was shown that expression of three genes was down regulated under darkness treatment, which suggests a role for GATA genes of castor bean in light mediated regulation. These results provide important theoretical basis to the functions identification of castor GATA genes and increase castor yields.     

THE FATE OF THE DWARF SHOOT APEX IN BRISTLECONE PINE (PINUS LONGAEVA)

The fate of the pine dwarf shoot (DS) apex after needle initiation has been controversial. Dwarf shoot primordia of Pinus longaeva were examined to determine the developmental basis for DS with and without interfoliar buds. Interfoliar buds are microscopic buds derived from the original terminal apex of the DS. In October, all the DS primordia are similar in size and appearance. However, as the needles elongate in the following June the apices of more proximal DS decrease in size, such that by July there is a clear diminishing size gradient of apical domes in going from the most distal to the most proximal positions. The distal DSs start to form bud scales in July and have fully formed interfoliar buds by mid-August. In contrast, those DS apices lacking protective bud scales at needle maturity become suberized and can never proliferate into long shoots. The distal placement of interfoliar buds may be due to a group effect, where each developing DS inhibits the more proximal DSs in the long shoot terminal bud.