Pérez-López, Edel

A novel phytoplasma species, ′Candidatus Phytoplasma zeae′, is proposed based on ecological distinctiveness, vector specificity, whole-genome comparisons, and community consensus. This phytoplasma is associated with maize bushy stunt (MBS) disease in corn (Zea mays) and is transmitted exclusively by Dalbulus maidis and D. elimatus, two leafhopper species endemic to the Americas. , and has been reported in Brazil, Colombia, Mexico, Peru, and several U.S. states. Here we sequenced and assembled the genome of MBS phytoplasma strains from Brazil, and U.S. to describe and propose this new species. Although the 16S rRNA gene sequence of the proposed reference strain, MBSP–BRRS, shares >99% identity with that of ′Ca. Phytoplasma asteris′, key nucleotide polymorphisms distinguish ′Ca. P. zeae′ from other 16SrI-related phytoplasma species. Average nucleotide identity (ANI) and average amino acid identity (AAI) values between ′Ca. P. zeae′ and ′Ca. P. asteris′ are 97.70–98.00% and 96.65–96.88%, respectively, both near the established species delineation thresholds. Comparative genomic analyses revealed unique gene clusters in ′Ca. P. zeae′ associated with amino acid transport, defence mechanisms, and protein turnover, which may contribute to its specialization in corn. The ecological profile of ′Ca. P. zeae′, including its narrow host range and restricted geographic distribution, supports its recognition as a novel species under Rule c of the IRPCM guidelines. The designation ′Candidatus Phytoplasma zeae′ is therefore proposed by members of the research community who have studied this pathogen for over a decade, with the MBSP-BrazilRS strain serving as the reference.

Abstract‘CandidatusLiberibacter asiaticus’ (CLas) is an unculturable, Gram-negative, phloem restricted plant pathogenic bacterium associated with a very serious disease of citrus worldwide known as Citrus Huanglongbing (HLB). CLas is widely spread in the Americas. In Cuba, CLas has been associated with HLB symptoms and has seriously affected the Cuban citrus industry. In this short communication we discuss the identification of CLas-infected sour orange in urban areas of Mayabeque Province in Cuba, an area previously unexplored for the presence of HLB, and a host widely cultivated in gardens and yards along Cuba. We used for the first time the bacteria molecular barcode chaperonin-60 universal target (cpn60UT) to identify and to detect CLas in HLB-symptomatic host plants.

In recent decades, ‘Candidatus Liberibacter spp.’ have emerged as a versatile group of psyllid-vectored plant pathogens and endophytes capable of infecting a wide range of economically important plant hosts. The most notable example is ‘Candidatus Liberibacter asiaticus’ (CLas) associated with Huanglongbing (HLB) in several major citrus-producing areas of the world. CLas is a phloem-limited α-proteobacterium that is primarily vectored and transmitted among citrus species by the Asian citrus psyllid (ACP) Diaphorina citri. HLB was first detected in North America in Florida (USA) in 2005, following introduction of the ACP to the State in 1998. HLB rapidly spread to all citrus growing regions of Florida within three years, with severe economic consequences to growers and considerable expense to taxpayers of the state and nation. Inability to establish CLas in culture (except transiently) remains a significant scientific challenge toward effective HLB management. Lack of axenic cultures has restricted functional genomic analyses, transfer of CLas to either insect or plant hosts for fulfillment of Koch’s postulates, characterization of host-pathogen interactions and effective screening of antibacterial compounds. In the last decade, substantial progress has been made toward CLas culturing: (i) three reports of transient CLas cultures were published, (ii) a new species of Liberibacter was identified and axenically cultured from diseased mountain papaya (Liberibacter crescens strain BT-1), (iii) psyllid hemolymph and citrus phloem sap were biochemically characterized, (iv) CLas phages were identified and lytic genes possibly affecting CLas growth were described, and (v) genomic sequences of 15 CLas strains were made available. In addition, development of L. crescens as a surrogate host for functional analyses of CLas genes, has provided valuable insights into CLas pathogenesis and its physiological dependence on the host cell. In this review we summarize the conclusions from these important studies.