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Summary Since the myrtle rust pathogen (Austropuccinia psidii) was first reported (as Puccinia psidii) in Brazil on guava (Psidium guajava) in 1884, it has been found infecting diverse myrtaceous species. Because A. psidii has recently spread rapidly worldwide with an extensive host range, genetic and genotypic diversities were evaluated within and among A. psidii populations in its putative native range and other areas of myrtle rust emergence in the Americas and Hawaii. Microsatellite markers revealed several unique multilocus genotypes (MLGs), which grouped isolates into nine distinct genetic clusters [C1–C9 comprising C1: from diverse hosts from Costa Rica, Jamaica, Mexico, Puerto Rico, and USA‐Hawaii, and USA‐California; C2: from eucalypts (Eucalyptus spp.) in Brazil/Uruguay and rose apple (Syzygium jambos) in Brazil; C3: from eucalypts in Brazil; C4: from diverse hosts in USA‐Florida; C5: from Java plum (Syzygium cumini) in Brazil; C6: from guava and Brazilian guava (Psidium guineense) in Brazil; C7: from pitanga (Eugenia uniflora) in Brazil; C8: from allspice (Pimenta dioica) in Jamaica and sweet flower (Myrrhinium atropurpureum) in Uruguay; C9: from jabuticaba (Myrciaria cauliflora) in Brazil]. The C1 cluster, which included a single MLG infecting diverse host in many geographic regions, and the closely related C4 cluster are considered as a “Pandemic biotype,” associated with myrtle rust emergence in Central America, the Caribbean, USA‐Florida, USA‐Hawaii, Australia, China‐Hainan, New Caledonia, Indonesia and Colombia. Based on 19 bioclimatic variables and documented occurrences of A. psidii contrasted with reduced sets of specific genetic clusters (subnetworks, considered as biotypes), maximum entropy bioclimatic modelling was used to predict geographic locations with suitable climate for A. psidii which are at risk from invasion. The genetic diversity of A. psidii throughout the Americas and Hawaii demonstrates the importance of recognizing biotypes when assessing the invasive threats posed by A. psidii around the globe.

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A reproducible and effective biolistic method for transforming papaya (Carica papaya L.) was developed with a transformation-regeneration system that targeted a thin layer of embryogenic tissue. The key factors in this protocol included: 1) spreading of young somatic embryo tissue that arose directly from excised immature zygotic embryos, followed by another spreading of the actively growing embryogenic tissue 3 d before biolistic transformation; 2) removal of kanamycin selection from all subsequent steps after kanamycin-resistant clusters were first isolated from induction media containing kanamycin; 3) transfer of embryos with finger-like extensions to maturation medium; and 4) transferring explants from germination to the root development medium only after the explants had elongating root initials, had at least two green true leaves, and were about 0.5 to 1.0 cm tall. A total of 83 transgenic papaya lines expressing the nontranslatable coat protein gene of papaya ringspot virus (PRSV) were obtained from somatic embryo clusters that originated from 63 immature zygotic embryos. The transformation efficiency was very high: 100% of the bombarded plates produced transgenic plants. This also represents an average of 55 transgenic lines per gram fresh weight, or 1.3 transgenic lines per embryo cluster that was spread. We validated this procedure in our laboratory by visiting researchers who did four independent projects to transform seven papaya cuhivars with coat protein gene constructs of PRSV strains from four different countries. The method is described in detail and should be useful for the routine transformation and regeneration of papaya.

BACKGROUND: Papaya (Carica papaya L.) production is limited by over 20 viruses, the most damaging of which is papaya ringspot virus (PRSV). Owing to a lack of suitable PRSV-resistant sources in Carica germplasm, transgenic resistance using the coat protein (cp) gene of a local PRSV strain is being developed to manage the disease in Jamaica. For assurance of food safety, the nutritional and antinutritional composition of transgenic papayas during ripening was compared with that of unmodified control samples.

Papaya rinsgpot virus type P (PRSV), a member of the genus Potyvirus in the family Potyviridae, is primarily transmitted by aphids in a nonpersistent manner (2). The virus is geographically widespread but has a narrow host range within the plant families Caricaceae, Chenopodiaceae, and Cucurbitaceae (2). The first reported epidemic of PRSV in Jamaica was during the late 1980s (1). Since then, the virus has spread across the island and is recognized as a potential problem for continued production of papaya (Carica papaya L.). In the summers of 1999 and 2000, prominent vein clearing symptoms were observed on leaves of a common weed, cerasee (Momordica charantia L.), in papaya orchards of western Jamaica. This weed, a climbing annual in the Cucurbitaceae family used in a variety of local herbal preparations, was found to be growing on fences or the ground along the periphery of the orchards. Leaf samples were collected and tested for PRSV by double-antibody sandwich (DAS)-ELISA with polyclonal antibodies (Agdia Inc, Elkhart, IN). In addition, crude sap extracts from 12 cerasee leaf samples that were diluted 1:20 were mechanically inoculated onto six plants each of cerasee and papaya. Within 2 weeks, vein clearing symptoms were observed on cerasee and symptoms (vein clearing followed by mosaic development and leaf distortions) typical of PRSV infection were obtained on papaya (2). All original leaf samples and inoculated plants tested positive in DAS-ELISA. In subsequent vector transmission tests, 10 healthy cerasee or papaya seedlings were inoculated with aphids (Aphis gossypii) that were previously permitted to feed on PRSV-infected papaya or cerasee. High rates of virus transmission were achieved in three tests from cerasee to papaya (77 to 83%), papaya to cerasee (90 to 93%), and cerasee to cerasee (60 to 70%). Total RNA from papaya samples was subjected to reverse transcriptase-PCR using primers to the capsid protein gene (3). A single fragment of the expected size (approximately 996 bp) was amplified and sequenced and showed high nucleotide identity (90.3 to 91.4%) with previously reported PRSV type P from Jamaica (GenBank Accession No. DQ104823), Cuba (GenBank Accession No. DQ089482), Florida (GenBank Accession No. AF196839), Brazil (GenBank Accession No. AF344650), and Hawaii (GenBank Accession No. S46722). To our knowledge, this is the first report of the natural occurrence of PRSV on a weed host in Jamaica. Because of its widespread distribution and potential of serving as a reservoir of PRSV, cerasee may play a role in the epidemiology of PRSV. References: (1) M. Chin et al. Jam. J. Sci. Technol. 14:58, 2003. (2) D. Purcifull et al. No 292 in: Descriptions of Plant Viruses. CMI/AAB, Surrey, England, 1984. (3) J. Slightom. Gene 100:251, 1991.

Transgenic papayas (Carica papaya) containing translatable coat protein (CPT) or nontranslatable coat protein (CPNT) gene constructs were evaluated over two generations for field resistance to Papaya ringspot virus in a commercial papaya growing area in Jamaica. Reactions of R0 CPT transgenic lines included no symptoms and mild or severe leaf and fruit symptoms. All three reactions were observed in one line and among different lines. Trees of most CPNT lines exhibited severe symptoms of infection, and some also showed mild symptoms. R1 offspring showed reactions previously observed with parental R0 trees; however, reactions not previously observed or a lower incidence of the reaction were also obtained. The transgenic lines appear to possess virus disease resistance that can be manipulated in subsequent generations for the development of a product with acceptable commercial performance.