Plant hormone cytokinins control cell cycle progression and plastid replication in apicomplexan parasites.

Cytokinins are plant hormones that are involved in regulation of cell proliferation, cell cycle progression, and cell and plastid development. Here, we show that the apicomplexan parasites Toxoplasma gondii and Plasmodium berghei, an opportunistic human pathogen and a rodent malaria agent, respectively, produce cytokinins via a biosynthetic pathway similar to that in plants. Cytokinins regulate the growth and cell cycle progression of T. gondii by mediating expression of the cyclin gene TgCYC4. A natural form of cytokinin, trans-zeatin (t-zeatin), upregulated expression of this cyclin, while a synthetic cytokinin, thidiazuron, downregulated its expression. Immunofluorescence microscopy and quantitative PCR analysis showed that t-zeatin increased the genome-copy number of apicoplast, which are non-photosynthetic plastid, in the parasite, while thidiazuron led to their disappearance. Thidiazuron inhibited growth of T. gondii and Plasmodium falciparum, a human malaria parasite, suggesting that thidiazuron has therapeutic potential as an inhibitor of apicomplexan parasites.

Plasmodium vivax and Plasmodium falciparum at the crossroads of exchange among islands in Vanuatu: implications for malaria elimination strategies.

Understanding the transmission and movement of Plasmodium parasites is crucial for malaria elimination and prevention of resurgence. Located at the limit of malaria transmission in the Pacific, Vanuatu is an ideal candidate for elimination programs due to low endemicity and the isolated nature of its island setting. We analyzed the variation in the merozoite surface protein 1 (msp1) and the circumsporozoite protein (csp) of P. falciparum and P. vivax populations to examine the patterns of gene flow and population structures among seven sites on five islands in Vanuatu. Genetic diversity was in general higher in P. vivax than P. falciparum from the same site. In P. vivax, high genetic diversity was likely maintained by greater extent of gene flow among sites and among islands. Consistent with the different patterns of gene flow, the proportion of genetic variance found among islands was substantially higher in P. falciparum (28.81-31.23%) than in P. vivax (-0.53-3.99%). Our data suggest that the current island-by-island malaria elimination strategy in Vanuatu, while adequate for P. falciparum elimination, might need to be complemented with more centrally integrated measures to control P. vivax movement across islands.

Contrasting infection susceptibility of the Japanese macaques and cynomolgus macaques to closely related malaria parasites, Plasmodium vivax and Plasmodium cynomolgi.

Although the human malaria parasite Plasmodium vivax is closely related to Asian Old World monkey malaria parasites, there are no reports of P. vivax infections in macaques. In this study, we compared the infectivity of P. vivax and Plasmodium cynomolgi in Japanese macaques (Macaca fuscata) and in cynomolgus macaques (Macaca fascicularis). The Japanese macaques were highly susceptible to P. cynomolgi but not to P. vivax, whereas cynomolgus macaques showed mild/limited P. cynomolgi infection and were, also, not susceptible to P. vivax. Serotyping and amino acid sequence comparison of erythrocyte surface Duffy antigen/receptor for chemokines (DARC) indicate that the Japanese macaque DARC sequence is nearly identical to that of rhesus (Macaca mulatta) and cynomolgus macaques. This suggests that the macaques share a common mechanism for preventing P. vivax infection. Comparison of amino acid sequences of the Duffy-binding-like (DBL) domain from several different Plasmodium species suggests that P. vivax DBLs will not bind to macaque DARCs, which can explain the lack of P. vivax infectivity. The DBL sequence analyses also suggest that P. cynomolgi DBLs may target Japanese macaque erythrocytes through a DARC-independent interaction.

Plasmodium falciparum: genetic diversity and complexity of infections in an isolated village in western Thailand.

Genetic diversity of Plasmodium falciparum is intimately associated with morbidity, mortality and malaria control strategies. It is therefore imperative to study genetic makeup and population structure of this parasite in endemic areas. In Kong Mong Tha, an isolated village in western Thailand, the majority of P. falciparum infections are asymptomatic. In this study we investigated complexity of infections and single nucleotide polymorphisms (SNPs) in the P. falciparum population of Kong Mong Tha, and compared results with those previously obtained from Mae Sod, in northwestern Thailand, where the majority of infections were symptomatic. Using PCR-based determination of the 5' merozoite surface protein 1 gene (msp1) recombinant types, we found that 39% of 59 P. falciparum isolates from Kong Mong Tha had multiple 5' recombinant types with a mean number of 1.54. These values were much lower than those obtained from Mae Sod: 96% for multiple infections and with a mean number of 3.61. Analysis of full-length sequences of two housekeeping genes, the P-type Ca(2+)-transporting ATPase gene (n=33) plus adenylosuccinate lyase gene (n=33), and three vaccine candidate antigen genes, msp1 (n=26), the circumsporozoite protein gene, csp (n=30) and the apical membrane antigen 1 gene, ama 1 (n=32), revealed that in all of these genes within-population SNP diversity was at similar levels between Kong Mong Tha and Mae Sod, suggesting that the extent of MOI and clinical manifestations of malaria are not strongly associated with genetic diversity. Additionally, we did not detect significant genetic differentiation between the two parasite populations, as estimated by the Wright's fixation index of inter-population variance in allele frequencies, suggesting that gene flow prevented the formation of population structuring. Thus, this study highlights unique features of P. falciparum populations in Thailand. The implications of these finding are discussed.

Science insights.

"Below is an essay by Prof. Tanabe originally written in Japanese. It gives an insight to Prof. Tanabe's inquiring mind and his approach to science. He also seek, as always, to inspire and nudge the young to scientific discovery".

Epidemiology of malaria in a village in the Rufiji River Delta, Tanzania: declining transmission over 25 years revealed by different parasitological metrics.

BACKGROUND: Assessments of the epidemiology of malaria over time are needed to understand changes in transmission and guide control and elimination strategies. METHODS: A longitudinal population study was established in 1985 in Nyamisati village in the Rufiji River Delta, Tanzania. A physician and research team lived in the village 1984-2000. Parasite prevalence by microscopy and two PCR methods, spleen rates and haemoglobin levels were measured in repeated cross-sectional surveys between 1985 and 2010. Passive surveillance of malaria cases was maintained until end 1999. Bed nets were distributed after the surveys 1993, 1999 and 2010. RESULTS: In 1985, overall parasite prevalence by microscopy was 70% (90% in children ages two to nine years). The prevalence decreased gradually by microscopy (38.9% 1994, 26.7% 1999) and msp2-PCR (58.7% 1994, 44.8% 1999), whereas real-time PCR prevalence remained higher throughout the 1990s (69.4% 1994, 64.8% 1999). In 2010, parasite prevalence was 17.8% by real-time PCR and 16.3% by msp2-PCR, and estimated to 4.8% by microscopy. Spleen rates in children ages two to nine years decreased earlier than parasite prevalence, from >75 to 42% in the 1980s, to nil during the 1990s. The prevalence of severe and moderate anaemia decreased from 41.1 to 13.1%. No deaths at the time of acute malaria were recorded when the research team lived in the village. CONCLUSIONS: A marked decline in malaria transmission was observed over 25 years. The decrease was detected after the arrival of the research team and continued gradually both before and after distribution of bed nets. Spleen rates and microscopy identified early changes when transmission was still intense, whereas real-time PCR was a more sensitive metric when transmission was reduced. The study provides historical data on malaria within a closely monitored rural village and contributes to the understanding of changing epidemiology in sub-Saharan Africa.

Generation of rodent malaria parasites with a high mutation rate by destructing proofreading activity of DNA polymerase δ.

Plasmodium falciparum malaria imposes a serious public health concern throughout the tropics. Although genetic tools are principally important to fully investigate malaria parasites, currently available forward and reverse tools are fairly limited. It is expected that parasites with a high mutation rate can readily acquire novel phenotypes/traits; however, they remain an untapped tool for malaria biology. Here, we generated a mutator malaria parasite (hereinafter called a 'malaria mutator'), using site-directed mutagenesis and gene transfection techniques. A mutator Plasmodium berghei line with a defective proofreading 3' → 5' exonuclease activity in DNA polymerase δ (referred to as PbMut) and a control P. berghei line with wild-type DNA polymerase δ (referred to as PbCtl) were maintained by weekly passage in ddY mice for 122 weeks. High-throughput genome sequencing analysis revealed that two PbMut lines had 175-178 mutations and a 86- to 90-fold higher mutation rate than that of a PbCtl line. PbMut, PbCtl, and their parent strain, PbWT, showed similar course of infection. Interestingly, PbMut lost the ability to form gametocytes during serial passages. We believe that the malaria mutator system could provide a novel and useful tool to investigate malaria biology.

Characteristic age distribution of Plasmodium vivax infections after malaria elimination on Aneityum Island, Vanuatu.

Resurgence is a major concern after malaria elimination. After the initiation of the elimination program on Aneityum Island in 1991, microscopy showed that Plasmodium falciparum disappeared immediately, whereas P. vivax disappeared from 1996 onward, until P. vivax cases were reported in January 2002. By conducting malariometric surveys of the entire population of Aneityum, we investigated the age distribution of individuals with parasites during this epidemic in the context of antimalarial antibody levels and parasite antigen diversity. In July 2002, P. vivax infections were detected by microscopy in 22/759 individuals: 20/298 born after the beginning of the elimination program in 1991, 2/126 born between 1982 and 1991, and none of 335 born before 1982. PCR increased the number of infections detected to 77, distributed among all age groups. Prevalences were 12.1%, 16.7%, and 6.0%, respectively (P < 0.001). In November, a similar age pattern was found, but with fewer infections: 6/746 and 39/741 individuals were found to be infected by microscopy and PCR, respectively. The frequencies of antibody responses to P. vivax were significantly higher in individuals born before 1991 than in younger age groups and were similar to those on Malakula Island, an area of endemicity. Remarkably low antigen diversity (h, 0.15) of P. vivax infections was observed on Aneityum compared with the other islands (h, 0.89 to 1.0). A P. vivax resurgence was observed among children and teenagers on Aneityum, an age distribution similar to those before elimination and on islands where P. vivax is endemic, suggesting that in the absence of significant exposure, immunity may persist, limiting infection levels in adults. The limited parasite gene pool on islands may contribute to this protection.

Ordered accumulation of mutations conferring resistance to sulfadoxine-pyrimethamine in the Plasmodium falciparum parasite.

BACKGROUND: Monitoring the prevalence of drug resistant Plasmodium falciparum is essential for effective malaria control. Resistance to pyrimethamine and sulfadoxine increases as mutations accumulate in the parasite genes encoding dihydrofolate reductase (dhfr) and dihydropteroate synthase (dhps), respectively. Although parasites are exposed to these antifolate drugs simultaneously, it remains virtually unknown whether dhfr and dhps mutations accumulate along interrelated paths. METHODS: We investigated the order of step-wise accumulation in dhfr and dhps by cumulative analyses using binomial tests in 575 P. falciparum isolates obtained from 7 countries in Asia and Melanesia. RESULTS: An initial step in the accumulation of mutations preferentially occurred in dhfr (2 mutations), followed by 1 mutation in dhps. In a subsequent step, mutations were estimated separately for 5 dhfr/dhps-resistant lineages identified using 12 microsatellites flanking dhfr and dhps. Among these lineages, we found 3 major mutational paths, each of which follows a unique stepwise trajectory to produce the most highly resistant form with 4 mutations in dhfr and 3 in dhps. CONCLUSIONS: The ordered accumulation of mutations in dhfr and dhps elucidated here will assist in predicting the status and progression of antifolate resistance in malaria-endemic regions where antifolate drugs are used for intermittent preventive treatment.

Plasmodium falciparum mitochondrial genetic diversity exhibits isolation-by-distance patterns supporting a sub-Saharan African origin.

The geographical distribution of single nucleotide polymorphism (SNP) in the mitochondrial genome of the human malaria parasite Plasmodium falciparum was investigated. We identified 88 SNPs in 516 isolates from seven parasite populations in Africa, Southeast Asia and Oceania. Analysis of the SNPs postulated a sub-Saharan African origin and recovered a strong negative correlation between within-population SNP diversity and geographic distance from the putative African origin over Southeast Asia and Oceania. These results are consistent with those previously obtained for nuclear genome-encoded housekeeping genes, indicating that the pattern of inheritance does not substantially affect the geographical distribution of SNPs.

Diversity of mitochondrial genome structure in the phylum Apicomplexa.

Mitochondria are ubiquitous organelles in all eukaryotes that are essential for a range of cellular processes and cellular signaling. Nearly all mitochondria have their own DNA or mitochondrial (mt) genome, which varies considerably in size, structure and organization. The phylum Apicomplexa includes a variety of unicellular eukaryotes, some of which are parasites of clinical or economic importance. Recent studies have demonstrated that apicomplexan mt genomes, which include the smallest 6 kb genome of the malaria parasites, exhibit remarkably diverse structures. Apicomplexan parasites are interesting model organisms in order to understand the evolution of mt genomes. This review summarizes the structure of apicomplexan mt genomes and highlights the unique features and the evolution of the mt genome.

Within-population genetic diversity of Plasmodium falciparum vaccine candidate antigens reveals geographic distance from a Central sub-Saharan African origin.

Populations of Plasmodium falciparum, the most virulent human malaria parasite, are diverse owing to wide levels of transmission and endemicity of infection. Genetic diversity of P. falciparum antigens, within and between parasite populations, remains a confounding factor in malaria pathogenesis as well as clinical trials of vaccine candidates. Variation of target antigens in parasite populations may arise from immune pressure depending on the levels of acquired immunity. Alternatively, similar to our study in housekeeping genes [Tanabe et al. Curr Biol 2010;70:1-7], within-population genetic diversity of vaccine candidate antigens may also be determined by geographical distance from a postulated origin in Central sub-Saharan Africa. To address this question, we obtained full-length sequences of P. falciparum genes, apical membrane antigen 1 (ama1) (n=459), circumsporozoite protein (csp) (n=472) and merozoite surface protein 1 (msp1) (n=389) from seven geographically diverse parasite populations in Africa, Southeast Asia and Oceania; and, together with previously determined sequences (n=13 and 15 for csp and msp1, respectively) analyzed within-population single nucleotide polymorphism (SNP) diversity. The three antigen genes showed SNP diversity that supports a model of isolation-by-distance. The standardized number of polymorphic sites per site, expressed as θ(S), indicates that 77-83% can be attributed by geographic distance from the African origin, suggesting that geographic distance plays a significant role in variation in target vaccine candidate antigens. Furthermore, we observed that a large proportion of SNPs in the antigen genes were shared between African and non-African parasite populations, demonstrating long term persistence of those SNPs. Our results provide important implications for developing effective malaria vaccines and better understanding of acquired immunity against falciparum malaria.

Evolution of Plasmodium falciparum drug resistance: implications for the development and containment of artemisinin resistance.

Malaria is a protozoan disease transmitted by the bite of the Anopheles mosquito. Among five species that can infect humans, Plasmodium falciparum is responsible for the most severe human malaria. Resistance of P. falciparum to chloroquine and pyrimethamine/sulfadoxine, conventionally used antimalarial drugs, is already widely distributed in many endemic areas. As a result, artemisinin-based combination therapies have been rapidly and widely adopted as first-line antimalarial treatments since the mid-2000s. Recent population and evolutionary genetic analyses have proven that the geographic origins of parasite lineages resistant to the conventional drugs are considerably limited. Almost all resistance emerged from either Southeast Asia or South America. The Greater Mekong subregion in Southeast Asia is probably the most alarming source of resistance, from which P. falciparum resistant to chloroquine and pyrimethamine/sulfadoxine dispersed to Africa. The emergence of artemisinin resistance has also recently been confirmed in the Greater Mekong. The WHO Global Malaria Programme has recently launched a "Global Plan for Artemisinin Resistance Containment," which aims to prevent the spread of artemisinin resistance while also stopping the emergence of novel resistance. However, an inadequate understanding of a mechanism of artemisinin resistance and the lack of reliable genetic markers to monitor artemisinin resistance make it difficult to survey the spread of resistance. Elucidation of such markers would substantially contribute to the design of an effective policy for the containment of artemisinin resistance.

Novel type of linear mitochondrial genomes with dual flip-flop inversion system in apicomplexan parasites, Babesia microti and Babesia rodhaini.

BACKGROUND: Mitochondrial (mt) genomes vary considerably in size, structure and gene content. The mt genomes of the phylum Apicomplexa, which includes important human pathogens such as the malaria parasite Plasmodium, also show marked diversity of structure. Plasmodium has a concatenated linear mt genome of the smallest size (6-kb); Babesia and Theileria have a linear monomeric mt genome (6.5-kb to 8.2-kb) with terminal inverted repeats; Eimeria, which is distantly related to Plasmodium and Babesia/Theileria, possesses a mt genome (6.2-kb) with a concatemeric form similar to that of Plasmodium; Cryptosporidium, the earliest branching lineage within the phylum Apicomplexa, has no mt genome. We are interested in the evolutionary origin of linear mt genomes of Babesia/Theileria, and have investigated mt genome structures in members of archaeopiroplasmid, a lineage branched off earlier from Babesia/Theileria. RESULTS: The complete mt genomes of archaeopiroplasmid parasites, Babesia microti and Babesia rodhaini, were sequenced. The mt genomes of B. microti (11.1-kb) and B. rodhaini (6.9-kb) possess two pairs of unique inverted repeats, IR-A and IR-B. Flip-flop inversions between two IR-As and between two IR-Bs appear to generate four distinct genome structures that are present at an equi-molar ratio. An individual parasite contained multiple mt genome structures, with 20 copies and 2 - 3 copies per haploid nuclear genome in B. microti and B. rodhaini, respectively. CONCLUSION: We found a novel linear monomeric mt genome structure of B. microti and B. rhodhaini equipped with dual flip-flop inversion system, by which four distinct genome structures are readily generated. To our knowledge, this study is the first to report the presence of two pairs of distinct IR sequences within a monomeric linear mt genome. The present finding provides insight into further understanding of evolution of mt genome structure.

Strain-specific protective effect of the immunity induced by live malarial sporozoites under chloroquine cover.

The efficacy of a whole-sporozoite malaria vaccine would partly be determined by the strain-specificity of the protective responses against malarial sporozoites and liver-stage parasites. Evidence from previous reports were inconsistent, where some studies have shown that the protective immunity induced by irradiated or live sporozoites in rodents or humans were cross-protective and in others strain-specific. In the present work, we have studied the strain-specificity of live sporozoite-induced immunity using two genetically and immunologically different strains of Plasmodium cynomolgi, Pc746 and PcCeylon, in toque monkeys. Two groups of monkeys were immunized against live sporozoites of either the Pc746 (n = 5), or the PcCeylon (n = 4) strain, by the bites of 2-4 sporozoite-infected Anopheles tessellates mosquitoes per monkey under concurrent treatments with chloroquine and primaquine to abrogate detectable blood infections. Subsequently, a group of non-immunized monkeys (n = 4), and the two groups of immunized monkeys were challenged with a mixture of sporozoites of the two strains by the bites of 2-5 infective mosquitoes from each strain per monkey. In order to determine the strain-specificity of the protective immunity, the proportions of parasites of the two strains in the challenge infections were quantified using an allele quantification assay, Pyrosequencing™, based on a single nucleotide polymorphism (SNP) in the parasites' circumsporozoite protein gene. The Pyrosequencing™ data showed that a significant reduction of parasites of the immunizing strain in each group of strain-specifically immunized monkeys had occurred, indicating a stronger killing effect on parasites of the immunizing strain. Thus, the protective immunity developed following a single, live sporozoite/chloroquine immunization, acted specifically against the immunizing strain and was, therefore, strain-specific. As our experiment does not allow us to determine the parasite stage at which the strain-specific protective immunity is directed, it is possible that the target of this immunity could be either the pre-erythrocytic stage, or the blood-stage, or both.

Plasmodium cynomolgi genome sequences provide insight into Plasmodium vivax and the monkey malaria clade.

P. cynomolgi, a malaria-causing parasite of Asian Old World monkeys, is the sister taxon of P. vivax, the most prevalent malaria-causing species in humans outside of Africa. Because P. cynomolgi shares many phenotypic, biological and genetic characteristics with P. vivax, we generated draft genome sequences for three P. cynomolgi strains and performed genomic analysis comparing them with the P. vivax genome, as well as with the genome of a third previously sequenced simian parasite, Plasmodium knowlesi. Here, we show that genomes of the monkey malaria clade can be characterized by copy-number variants (CNVs) in multigene families involved in evasion of the human immune system and invasion of host erythrocytes. We identify genome-wide SNPs, microsatellites and CNVs in the P. cynomolgi genome, providing a map of genetic variation that can be used to map parasite traits and study parasite populations. The sequencing of the P. cynomolgi genome is a critical step in developing a model system for P. vivax research and in counteracting the neglect of P. vivax.

Age of the last common ancestor of extant Plasmodium parasite lineages.

Parasites of the genus Plasmodium infect all classes of amniotes (mammals, birds and reptiles) and display host specificity in their infections. It is therefore generally believed that Plasmodium parasites co-evolved intimately with their hosts. Here, we report that based on an evolutionary analysis using 22 genes in the nuclear genome, extant lineages of Plasmodium parasites originated roughly in the Oligocene epoch after the emergence of their hosts. This timing on the age of the common ancestor of extant Plasmodium parasites suggest the importance of host switches and lends support to the evolutionary scenario of a "malaria big bang" that was proposed based on the evolutionary analysis using the mitochondrial genome.

Large-scale survey for novel genotypes of Plasmodium falciparum chloroquine-resistance gene pfcrt.

BACKGROUND: In Plasmodium falciparum, resistance to chloroquine (CQ) is conferred by a K to T mutation at amino acid position 76 (K76T) in the P. falciparum CQ transporter (PfCRT). To date, at least 15 pfcrt genotypes, which are represented by combinations of five amino acids at positions 72-76, have been described in field isolates from various endemic regions. To identify novel mutant pfcrt genotypes and to reveal the genetic relatedness of pfcrt genotypes, a large-scale survey over a wide geographic area was performed. METHODS: Sequences for exon 2 in pfcrt, including known polymorphic sites at amino acid positions 72, 74, 75 and 76, were obtained from 256 P. falciparum isolates collected from eight endemic countries in Asia (Bangladesh, Cambodia, Lao P.D.R., the Philippines and Thailand), Melanesia (Papua New Guinea and Vanuatu) and Africa (Ghana). A haplotype network was constructed based on six microsatellite markers located -29 kb to 24 kb from pfcrt in order to examine the genetic relatedness among mutant pfcrt genotypes. RESULTS: In addition to wild type (CVMNK at positions 72-76), four mutant pfcrt were identified; CVIET, CVIDT, SVMNT and CVMNT (mutated amino acids underlined). Haplotype network revealed that there were only three mutant pfcrt lineages, originating in Indochina, Philippines and Melanesia. Importantly, the Indochina lineage contained two mutant pfcrt genotypes, CVIET (n = 95) and CVIDT (n = 14), indicating that CVIDT shares a common origin with CVIET. Similarly, one major haplotype in the Melanesian lineage contained two pfcrt genotypes; SVMNT (n = 71) and CVMNT (n = 3). In Africa, all mutant pfcrt genotypes were the CVIET of the Indochina lineage, probably resulting from the intercontinental migration of CQ resistance from Southeast Asia. CONCLUSIONS: The number of CQ-mutant lineages observed in this study was identical to that found in previous studies. This supports the hypothesis that the emergence of novel CQ resistance is rare. However, in the mutant pfcrt genotypes, amino acid changes at positions 72, 74 and 75 appear to have recently been generated at least several times, producing distinct pfcrt mutant genotypes. The occurrence of new mutations flanking K76T may yield stronger resistance to CQ and/or a higher fitness than the original pfcrt mutant.

The Plasmodium apicoplast genome: conserved structure and close relationship of P. ovale to rodent malaria parasites.

Apicoplast, a nonphotosynthetic plastid derived from secondary symbiotic origin, is essential for the survival of malaria parasites of the genus Plasmodium. Elucidation of the evolution of the apicoplast genome in Plasmodium species is important to better understand the functions of the organelle. However, the complete apicoplast genome is available for only the most virulent human malaria parasite, Plasmodium falciparum. Here, we obtained the near-complete apicoplast genome sequences from eight Plasmodium species that infect a wide variety of vertebrate hosts and performed structural and phylogenetic analyses. We found that gene repertoire, gene arrangement, and other structural attributes were highly conserved. Phylogenetic reconstruction using 30 protein-coding genes of the apicoplast genome inferred, for the first time, a close relationship between P. ovale and rodent parasites. This close relatedness was robustly supported using multiple evolutionary assumptions and models. The finding suggests that an ancestral host switch occurred between rodent and human Plasmodium parasites.

Gibberellin biosynthetic inhibitors make human malaria parasite Plasmodium falciparum cells swell and rupture to death.

Malaria remains as one of the most devastating infectious disease, and continues to exact an enormous toll in medical cost and days of labor lost especially in the tropics. Effective malaria control and eventual eradication remain a huge challenge, with efficacious antimalarials as important intervention/management tool. Clearly new alternative drugs that are more affordable and with fewer side effects are desirable. After preliminary in vitro assays with plant growth regulators and inhibitors, here, we focus on biosynthetic inhibitors of gibberellin, a plant hormone with many important roles in plant growth, and show their inhibitory effect on the growth of both apicomplexa, Plasmodium falciparum and Toxoplasma gondii. Treatment of P. falciparum cultures with the gibberellin biosynthetic inhibitors resulted in marked morphological changes that can be reversed to a certain degree under hyperosmotic environment. These unique observations suggest that changes in the parasite membrane permeability may explain the pleiotropic effects observed within the intracellular parasites.