Infection Density Dynamics of the Citrus Greening Bacterium “Candidatus Liberibacter asiaticus” in Field Populations of the Psyllid Diaphorina citri and Its Relevance to the Efficiency of Pathogen Transmission to Citrus Plants

Huanglongbing, or citrus greening, is a devastating disease of citrus plants recently spreading worldwide, which is caused by an uncultivable bacterial pathogen, “Candidatus Liberibacter asiaticus,” and vectored by a phloem-sucking insect, Diaphorina citri. We investigated the infection density dynamics of “Ca. Liberibacter asiaticus” in field populations of D. citri with experiments using field-collected insects to address how “Ca. Liberibacter asiaticus” infection density in the vector insect is relevant to pathogen transmission to citrus plants. Of 500 insects continuously collected from “Ca. Liberibacter asiaticus”-infected citrus trees with pathological symptoms in the spring and autumn of 2009, 497 (99.4%) were “Ca. Liberibacter asiaticus” positive. The infections were systemic across head-thorax and abdomen, ranging from 10³ to 10⁷ bacteria per insect. In spring, the infection densities were low in March, at ∼10³ bacteria per insect, increasing up to 10⁶ to 10⁷ bacteria per insect in April and May, and decreasing to 10⁵ to 10⁶ bacteria per insect in late May, whereas the infection densities were constantly ∼10⁶ to 10⁷ bacteria per insect in autumn. Statistical analysis suggested that several factors, such as insect sex, host trees, and collection dates, may be correlated with “Ca. Liberibacter asiaticus” infection densities in field D. citri populations. Inoculation experiments with citrus seedlings using field-collected “Ca. Liberibacter asiaticus”-infected insects suggested that (i) “Ca. Liberibacter asiaticus”-transmitting insects tend to exhibit higher infection densities than do nontransmitting insects, (ii) a threshold level (∼10⁶ bacteria per insect) of “Ca. Liberibacter asiaticus” density in D. citri is required for successful transmission to citrus plants, and (iii) D. citri attaining the threshold infection level transmits “Ca. Liberibacter asiaticus” to citrus plants in a stochastic manner. These findings provide valuable insights into understanding, predicting, and controlling this notorious citrus pathogen.     

Metabolic Coupling Determines the Activity: Comparison of 11β-Hydroxysteroid Dehydrogenase 1 and Its Coupling between Liver Parenchymal Cells and Testicular Leydig Cells

Background

11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) interconverts active 11β-hydroxyl glucocorticoids and inactive 11keto forms. However, its directionality is determined by availability of NADP+/NADPH. In liver cells, 11β-HSD1 behaves as a primary reductase, while in Leydig cells it acts as a primary oxidase. However, the exact mechanism is not clear. The direction of 11β-HSD1 has been proposed to be regulated by hexose-6-phosphate dehydrogenase (H6PDH), which catalyzes glucose-6-phosphate (G6P) to generate NADPH that drives 11β-HSD1 towards reduction.

Methodology

To examine the coupling between 11β-HSD1 and H6PDH, we added G6P to rat and human liver and testis or Leydig cell microsomes, and 11β-HSD1 activity was measured by radiometry.

Results and Conclusions

G6P stimulated 11β-HSD1 reductase activity in rat (3 fold) or human liver (1.5 fold), but not at all in testis. S3483, a G6P transporter inhibitor, reversed the G6P-mediated increases of 11β-HSD1 reductase activity. We compared the extent to which 11β-HSD1 in rat Leydig and liver cells might be coupled to H6PDH. In order to clarify the location of H6PDH within the testis, we used the Leydig cell toxicant ethane dimethanesulfonate (EDS) to selectively deplete Leydig cells. The depletion of Leydig cells eliminated Hsd11b1 (encoding 11β-HSD1) expression but did not affect the expression of H6pd (encoding H6PDH) and Slc37a4 (encoding G6P transporter). H6pd mRNA level and H6PDH activity were barely detectable in purified rat Leydig cells. In conclusion, the availability of H6PDH determines the different direction of 11β-HSD1 in liver and Leydig cells.     

Convenient Detection of the Citrus Greening (Huanglongbing) Bacterium ‘Candidatus Liberibacter asiaticus’ by Direct PCR from the Midrib Extract

A phloem-limited bacterium, ‘Candidatus Liberibacter asiaticus’ (Las) is a major pathogen of citrus greening (huanglongbing), one of the most destructive citrus diseases worldwide. The rapid identification and culling of infected trees and budwoods in quarantine are the most important control measures. DNA amplification including conventional polymerase chain reaction (PCR) has commonly been used for rapid detection and identification. However, long and laborious procedures for DNA extraction have greatly reduced the applicability of this method. In this study, we found that the Las bacterial cells in the midribs of infected leaves were extracted rapidly and easily by pulverization and centrifugation with mini homogenization tubes. We also found that the Las bacterial cells in the midrib extract were suitable for highly sensitive direct PCR. The performance of direct PCR using this extraction method was not inferior to that of conventional PCR. Thus, the direct PCR method described herein is characterized by its simplicity, sensitivity, and robustness, and is applicable to quarantine testing.     

Nationalism remixed? The politics of cultural flows between the South Asian diaspora and ‘homeland’

This article examines ‘Asian electronic music’, a generally progressive diasporic South Asian scene which fuses electronic dance music beats with instruments/sounds traditionally associated with the subcontinent, and how it became embedded into ‘majoritarian’ Indian nationalism. In India, the music's perceived ‘fusion’ aesthetic became emblematic of an emergent India which was economically prosperous while ‘respecting’ its cultural heritage. Using the case of an album which remixed India's national song, Vande Mataram, this article explores the convergences and divergences between Asian electronic musicians in Delhi and Hindu nationalists. The article concludes that the musicians in Delhi did not lend to Hindu nationalism. However, they perhaps gave secular Indian nationalism a ‘cool’ gloss. Ultimately, the production and consumption of Asian electronic music in Delhi raises significant questions regarding the scene's relationship to Indian nationalisms.     

The ‘success’ image of Asian Americans: Its validity,and its practical and theoretical implications 1

Abstract

The success stereotype of Asian Americans is examined in terms of three major research questions. (1) What factors account for the vicissitudes of the dominant group's perception of Asian Americans since 1850? (2) To what extent is the current success image of Asian Americans true? (3) How would such a model minority image affect Asian Americans, the other minorities, and majority Americans? The first question is addressed by historical data, and the second and third questions are answered by analyses of census data supplemented by other empirical data on Asian Americans. Factors accounting for the changes of the dominant group's image of Asian Americans are found to be primarily situational, and our analyses in the light of the principle of earnings equity indicate that the success image is largely a myth due to labour market disadvantages and other related social problems. Practical and theoretical implications are discussed in the conclusion.     

Phenotypic Effect of “Candidatus Rickettsiella viridis,” a Facultative Symbiont of the Pea Aphid (Acyrthosiphon pisum), and Its Interaction with a Coexisting Symbiont

A gammaproteobacterial facultative symbiont of the genus Rickettsiella was recently identified in the pea aphid, Acyrthosiphon pisum. Infection with this symbiont altered the color of the aphid body from red to green, potentially affecting the host''s ecological characteristics, such as attractiveness to different natural enemies. In European populations of A. pisum, the majority of Rickettsiella-infected aphids also harbor another facultative symbiont, of the genus Hamiltonella. We investigated this Rickettsiella symbiont for its interactions with the coinfecting Hamiltonella symbiont, its phenotypic effects on A. pisum with and without Hamiltonella coinfection, and its infection prevalence in A. pisum populations. Histological analyses revealed that coinfecting Rickettsiella and Hamiltonella exhibited overlapping localizations in secondary bacteriocytes, sheath cells, and hemolymph, while Rickettsiella-specific localization was found in oenocytes. Rickettsiella infections consistently altered hosts'' body color from red to green, where the greenish hue was affected by both host and symbiont genotypes. Rickettsiella-Hamiltonella coinfections also changed red aphids to green; this greenish hue tended to be enhanced by Hamiltonella coinfection. With different host genotypes, Rickettsiella infection exhibited either weakly beneficial or nearly neutral effects on host fitness, whereas Hamiltonella infection and Rickettsiella-Hamiltonella coinfection had negative effects. Despite considerable frequencies of Rickettsiella infection in European and North American A. pisum populations, no Rickettsiella infection was detected among 1,093 insects collected from 14 sites in Japan. On the basis of these results, we discuss possible mechanisms for the interaction of Rickettsiella with other facultative symbionts, their effects on their hosts'' phenotypes, and their persistence in natural host populations. We propose the designation “Candidatus Rickettsiella viridis” for the symbiont.     

Metabolic Syndrome and Nephrolithiasis Risk: Should the Medical Management of Nephrolithiasis Include the Treatment of Metabolic Syndrome?

This article reviews the relationship between metabolic syndrome (MetS) and nephrolithiasis, as well as the clinical implications for patients with this dual diagnosis. MetS, estimated to affect 25% of adults in the United States, is associated with a fivefold increase in the risk of developing diabetes, a doubling of the risk of acquiring cardiovascular disease, and an increase in overall mortality. Defined as a syndrome, MetS is recognized clinically by numerous constitutive traits, including abdominal obesity, hypertension, dyslipidemia (elevated triglycerides, low high-density lipoprotein cholesterol), and hyperglycemia. Urologic complications of MetS include a 30% higher risk of nephrolithiasis, with an increased percentage of uric acid nephrolithiasis in the setting of hyperuricemia, hyperuricosuria, low urine pH, and low urinary volume. Current American Urological Association and European Association of Urology guidelines suggest investigating the etiology of nephrolithiasis in affected individuals; however, there is no specific goal of treating MetS as part of the medical management. Weight loss and exercise, the main lifestyle treatments of MetS, counter abdominal obesity and insulin resistance and reduce the incidence of cardiovascular events and the development of diabetes. These recommendations may offer a beneficial adjunctive treatment option for nephrolithiasis complicated by MetS. Although definitive therapeutic recommendations must await further studies, it seems both reasonable and justifiable for the urologist, as part of a multidisciplinary team, to recommend these important lifestyle changes to patients with both conditions. These recommendations should accompany the currently accepted management of nephrolithiasis.Key words: Nephrolithiasis, Metabolic syndrome, Uric acid nephrolithiasisMetabolic syndrome (MetS), as defined by the National Cholesterol Education Program and the Adult Treatment Panel III in 2001 (and updated in 2005), represents a growing medical problem affecting more than 22% of US adults.^1–4 It is associated with an almost fivefold increase in the risk of developing diabetes and a doubling of the risk of acquiring cardiovascular disease.⁵ MetS is a clinical disorder defined by the presence of at least three of the following criteria: central obesity (abdominal girth > 102 cm [40 in] men and > 88 cm [35 in] women), low high-density lipoprotein (HDL) cholesterol (< 40 mg/dL in men and < 50 mg/dL in women), hypertriglyceridemia (> 150 mg/dL), hypertension (blood pressure > 130/85 mm Hg), and elevated fasting glucose (> 100 mg/dL).^2,4 The development of MetS appears to result from a complex interaction of genetics, phenotypic visceral fat accumulation (central obesity), insulin resistance, and sedentary behavior.^5,6 Beyond cardiometabolic risks, MetS has a wide range of long-term complications, including nonalcoholic fatty liver disease, polycystic ovarian syndrome, obstructive sleep apnea, hypogonadism, lipodystrophy, microvascular disease, and chronic renal disease.⁶ An important urologic complication of MetS, not routinely cited, is nephrolithiasis.^6–8Nephrolithiasis continues to be a major cause of morbidity and healthcare spending.⁹ A history of kidney stones is approximately twice as common in individuals with three criteria for MetS and three times as common in those with five criteria for MetS, as compared with those with none.¹⁰ These trends were confirmed in a large-scale, nationwide study of 30,448 Japanese patients with urolithiasis, who demonstrated that MetS was associated with a significantly increased risk of hypercalciuria, hyperuricosuria, hyperoxaluria, and hypocitraturia, independent of age and sex.¹¹ Additionally, patients with a history of nephrolithiasis are significantly more likely to have multiple risk factors for cardiovascular disease, premature atherosclerosis, and cardiovascular events.^10,12,13 It is unknown whether this is primarily a reflection of factors associated with nephrolithiasis, such as obesity, hypertension, or glucose intolerance/diabetes, or due to components of MetS, such as insulin resistance.¹⁴ The current American Urological Association (AUA) guidelines on the medical management of kidney stones suggests a need for future research on advising patients to exercise and lose weight, but does not make definitive recommendations on these lifestyle changes.¹⁵     

Mutations and addiction to EGFR: the Achilles ‘heal’ of lung cancers?

The epidermal growth factor receptor (EGFR) gene product is a receptor tyrosine kinase (TK) that affects many important downstream pathways. The recent finding that mutations in EGFR predict the response of lung cancers to therapies that target the TK domain of the gene product has generated considerable interest. The mutations are associated with adenocarcinoma histology, oriental origin, female gender and never-smoker status. Most mutations target structures in the TK domain that appear to be essential for the phosphorylation function of the gene. Cancer cells with mutant EGFR genes might become physiologically dependent on the continued activity of the gene for the maintenance of their malignant phenotype; however, this might also be a target for therapy.     

Alternations of Metabolic Profile and Kynurenine Metabolism in the Plasma of Parkinson’s Disease

The pathogenesis of Parkinson’s disease (PD) remains to be elucidated. Metabolomic analysis has the potential to identify biochemical pathways and metabolic profiles that are involved in PD pathogenesis. Here, we performed a targeted metabolomics to quantify the plasma levels of 184 metabolites in a discovery cohort including 82 PD patients and 82 normal controls (NCs) and found two up-regulated (dopamine, putrescine/ornithine ratio) and four down-regulated (octadecadienylcarnitine C18:2, asymmetric dimethylarginine, tryptophan, and kynurenine (KYN)) metabolites in the plasma of PD patients. We then measured the plasma levels of a panel of metabolic products of KYN pathway in an independent validation cohort including 118 PD patients, 22 Huntington’s disease (HD) patients, and 37 NCs. Lower kynurenic acid (KA)/KYN ratio, higher quinolinic acid (QA) level, and QA/KA ratio were observed in PD patients compared to HD patients and NCs. PD patients at advanced stage (Hoehn-Yahr stage >?2) showed lower KA and KA/KYN ratio, as well as higher QA and QA/KA ratio compared to PD patients at early stage (Hoehn-Yahr stage ≤?2) and NCs. Levels of KA and QA, as well as the ratios of KA/KYN and QA/KA between PD patients with and without psychiatric symptoms, dementia, or levodopa-induced dyskinesia in the advanced PD were similar. This metabolomic analyses demonstrate a number of plasma biomarker candidates for PD, suggesting a shift toward neurotoxic QA synthesis and away from neuroprotective KA production in KYN pathway.     

Pathogenicity of ‘take-all’ fungus to oats: Its relationship to the concentration and detoxification of the four avenacins

Data for inhibition of the growth of Gaeumannomyces graminis var. tritici (Ggt) and var. avenae (Gga), Phialophora radicicola and Fusarium avenaceum, caused by avenacins, are presented. The avenacins found in all oat species examined are sufficient in quantity to totally suppress growth of wheat ‘take-all’ (Ggt), even old roots containing 25 μg/g (fr. wt). Fungal variants that can also attack oats [var. avenae (Gga)] show considerable variations in their tolerance to avenacin A-1, ec₅₀ values being 5–80 μg/ml. Nevertheless, all Gga isolates maintained some growth at avenacin A-1 concentrations as high as 200 μg/ml and it is this ability to grow, albeit slowly, at high concentrations that is the critical difference between Gga and Ggt strains. The pathogenicity towards oats of a range of isolates of Gga is related to the fungicidal activity of avenacins. Gga pathogenicity is shown to increase with poor nutrition of the oat hosts (poor illumination, lack of minerals). Fungal detoxification of avenacins produces mono-deglucosylavenacin A-1, bis-deglucosylavenacin A-1 and, in one case, tris-deglycosylavenacin A-1. Ggt strains left avenacin A-1 almost unaffected giving only traces of mono-deglucosyl product. Gga strains bring about mono- and bis-deglucosylation whilst Fusarium avenaceum causes mainly bis-deglucosylation. Mono-deglucosylavenacin is shown to be less inhibitory to Gga than is avenacin A-1, whilst the bis-deglucosyl compound is still less inhibitory.     

Sticholysin I–membrane interaction: An interplay between the presence of sphingomyelin and membrane fluidity

Sticholysin I (St I) is a pore-forming toxin (PFT) produced by the Caribbean Sea anemone Stichodactyla helianthus belonging to the actinoporin protein family, a unique class of eukaryotic PFT exclusively found in sea anemones. As for actinoporins, it has been proposed that the presence of sphingomyelin (SM) and the coexistence of lipid phases increase binding to the target membrane. However, little is known about the role of membrane structure and dynamics (phase state, fluidity, presence of lipid domains) on actinoporins' activity or which regions of the membrane are the most favorable platforms for protein insertion. To gain insight into the role of SM on the interaction of St I to lipid membranes we studied their binding to monolayers of phosphatidylcholine (PC) and SM in different proportions. Additionally, the effect of acyl chain length and unsaturation, two features related to membrane fluidity, was evaluated on St I binding to monolayers. This study revealed that St I binds and penetrates preferentially and with a faster kinetic to liquid-expanded films with high lateral mobility and moderately enriched in SM. A high content of SM induces a lower lateral diffusion and/or liquid-condensed phases, which hinder St I binding and penetration to the lipid monolayer. Furthermore, the presence of lipid domain borders does not appear as an important factor for St I binding to the lipid monolayer.     

Finding an “Achilles’ heel” of cancer: The role of glucose and glutamine metabolism in the survival of transformed cells

Multiple lines of evidence indicate that the process of tumorigenesis is often associated with altered metabolism of two major nutrients, glucose and glutamine. These two nutrients are engaged in multiple metabolic pathways that can be required for cell viability. The roles of glucose and glutamine in the survival of transformed cells both in vitro and in vivo have been separately evaluated in various cell systems, and glucose as the major cellular energy source has received most of the attention. At the same time, data suggests that the inclusion of glucose and glutamine into specific metabolic pathways and cellular sensitivity to the availability of either of these nutrients depends on the cell origin and the combination and nature of transforming events. Exploiting cell metabolism to develop selective cancer therapeutics requires consideration of these factors and evaluation of the requirement of glucose and glutamine metabolism for survival of different transformed cells. Here we discuss possible molecular mechanisms underlying oncogene-induced sensitivity to deprivation of these nutrients.     

Molecular Population Genetics and Evolution of the Chagas’ Disease Vector Triatoma infestans (Hemiptera: Reduviidae)

Triatoma infestans (Klug) is the main vector of Chagas’ disease in the Southern Cone of Latin America between the latitudes 10° S and 46° S. The long-term effectiveness of the control campaigns is greatly dependent upon the vector population structure. Mitochondrial DNA (mtDNA) genes have been used in a number of T. infestans population genetic analyses. However, the maternally inherited markers as well as nuclear ribosomal DNA analyzed until the present exhibited low or limited levels of variation. Analyses based on microsatellite markers strongly supported the existence of some type of stratification in T. infestans populations and supported the hypothesis of vector population recovery from survivors of the insecticide-treated areas, highlighting the value of population genetic analyses in assessing the effectiveness of Chagas’ disease vector control programmes. Although phylogeographic studies have generally suggested a Bolivian Andean origin of T. infestans, they recovered two reciprocal monophyletic groups of T. infestans and Bolivian populations who were not basal as expected for an ancestral group. In addition, a non-Andean origin could not be excluded by mtDNA genealogies that included sylvatic bugs from Gran Chaco. On the other side, mitochondrial and microsatellite markers supported the hypothesis of two independent migration events of colonization and secondary contacts in southern South America. Since the phylogenetic analyses remain inconclusive, more sequences, not only from mitochondrial genes but also from nuclear genes, need to be examined.     

‘An island of socialism in a capitalist country’: Postsocialist Russian science and the culture of the state

Through a comparison of privatization programs in two physics institutes, this article explores the ways in which scientists in the Siberian science city of Akademgorodok adapted to the low levels of state funding available to them in the 1990s. Scientists transformed structures that were available under socialism into hybrid state-private ventures. Rather than ‘freeing’ Russian science from its former dependence on the state, however, these changes have reconfigured, and in some cases even strengthened, the relationship between state power and the production of knowledge. Seeing ‘the state’ as it is constituted in Russian scientists' discourse challenges Western models of the autonomy of science.     

Primary Care Physicians’ Attitudes and Beliefs towards Chronic Low Back Pain: An Asian Study

Background

Chronic low back pain is a serious global health problem. There is substantial evidence that physicians’ attitudes towards and beliefs about chronic low back pain can influence their subsequent management of the condition.

Objectives

(1) to evaluate the attitudes and beliefs towards chronic low back pain among primary care physicians in Asia; (2) to study the cultural differences and other factors that are associated with these attitudes and beliefs.

Method

A cross sectional online survey was sent to primary care physicians who are members of the Hong Kong College of Family Physician (HKCFP). The Pain Attitudes and Beliefs Scale for Physiotherapist (PABS-PT) was used as the questionnaire to determine the biomedical and biopsychosocial orientation of the participants.

Results

The mean Biomedical (BM) score was 34.8+/-6.1; the mean biopsychosocial (BPS) score was 35.6 (+/- 4.8). Both scores were higher than those of European doctors. Family medicine specialists had a lower biomedical score than General practitioners. Physicians working in the public sector tended to have low BM and low BPS scores; whereas physicians working in private practice tended to have high BM and high BPS scores.

Conclusion

The lack of concordance in the pain explanatory models used by private and public sector may have a detrimental effect on patients who are under the care of both parties. The uncertain treatment orientation may have a negative influence on patients’ attitudes and beliefs, thus contributing to the tension and, perhaps, even ailing mental state of a person with chronic LBP.     

The study of the association between genotype and phenotypic manifestations of the Huntington’s chorea pathogenesis

Direct molecular-genetic analysis of the region containing CAG and CCG repeats of the IT15 gene from 37 patients with the clinical diagnosis of Huntington’s chorea was carried out. The allele with the expansion of CAG repeats in the state of heterozygosity was found in 33 patients; DNA analysis did not confirm a clinical diagnosis in four cases. Twenty probable asymptomatic carriers were examined; 11 of them inherited a mutant chromosome. A disequilibrium in the linkage between the (CGG)₁₀ allele and the alleles with the expansion of CAG repeats in the IT15 gene in a group of patients from Ukraine was found. Significant differences in character of instability of CAG repeats that depends on paternal or maternal inheritance were revealed. The genetic factors that are associated with age variability of onset of the disease were studied.     

Metabolic and hedonic drives in the neural control of appetite: who is the boss?

Obesity is on the rise in all developed countries, and a large part of this epidemic has been attributed to excess caloric intake, induced by ever present food cues and the easy availability of energy dense foods in an environment of plenty. Clearly, there are strong homeostatic regulatory mechanisms keeping body weight of many individuals exposed to this environment remarkably stable over their adult life. Other individuals, however, seem to eat not only because of metabolic need, but also because of excessive hedonic drive to make them feel better and relieve stress. In the extreme, some individuals exhibit addiction-like behavior toward food, and parallels have been drawn to drug and alcohol addiction. However, there is an important distinction in that, unlike drugs and alcohol, food is a daily necessity. Considerable advances have been made recently in the identification of neural circuits that represent the interface between the metabolic and hedonic drives of eating. We will cover these new findings by focusing first on the capacity of metabolic signals to modulate processing of cognitive and reward functions in cortico-limbic systems (bottom-up) and then on pathways by which the cognitive and emotional brain may override homeostatic regulation (top-down).     

Insect thermal tolerance: what is the role of ontogeny,ageing and senescence?

Temperature has dramatic evolutionary fitness consequences and is therefore a major factor determining the geographic distribution and abundance of ectotherms. However, the role that age might have on insect thermal tolerance is often overlooked in studies of behaviour, ecology, physiology and evolutionary biology. Here, we review the evidence for ontogenetic and ageing effects on traits of high- and low-temperature tolerance in insects and show that these effects are typically pronounced for most taxa in which data are available. We therefore argue that basal thermal tolerance and acclimation responses (i.e. phenotypic plasticity) are strongly influenced by age and/or ontogeny and may confound studies of temperature responses if unaccounted for. We outline three alternative hypotheses which can be distinguished to propose why development affects thermal tolerance in insects. At present no studies have been undertaken to directly address these options. The implications of these age-related changes in thermal biology are discussed and, most significantly, suggest that the temperature tolerance of insects should be defined within the age-demographics of a particular population or species. Although we conclude that age is a source of variation that should be carefully controlled for in thermal biology, we also suggest that it can be used as a valuable tool for testing evolutionary theories of ageing and the cellular and genetic basis of thermal tolerance.     

Phloem unloading and ‘sink strength’: The parallel between the site of attachment of Cuscuta and developing legume seeds

Sink regions play a central role in determining assimilate distribution patterns. Two factors are discussed which have a strong effect on the sink strength of a sink, viz. phloem unloading and turgor-sensitive transport. Sink strength may be defined as the capacity of phloem in the sink region to import assimilates from other parts of the plants and to release the imported substances into the sink apoplast.A stem parasitized by Cuscuta represents a very strong sink. A review is presented of data on enhanced phloem unloading, at the site of attachment of Custuta. Recent data on metabolically controlled sucrose and amino acid unloading into the seed coat apoplast of developing legume seeds show a remarkable parallel with phloem unloading in a parasitized Vicia faba stem. Data on turgor-sensitive sucrose and amino acid transport into developing seeds are presented, which throw new light on the pressure flow theory of phloem transport.