Numerous yellows-type diseases of plants have been associated with mycoplasmas now renamed phytoplasmas over the last 40 years. The increasing threat of phytoplasma diseases comes both from devastating diseases in emerging agricultural countries and from epidemics and or endemic situations in areas traditionally devoted to agriculture. Phytoplasmas usually do not kill in short time the host plant from which they are strongly metabolic dependent therefore increasing the possibility of epidemic spreading; phytoplasma-infected plants result also more susceptible to infection by other pathogens such as fungi and viruses. Severe disease epidemics have been described worldwide associated with phytoplasma presence; these include coconut lethal yellowing in Africa and in the Caribbean, grapevine yellows in major viticultural areas and different diseases affecting stone and pome fruit plants. Phytoplasma infected plants exhibit symptoms suggesting profound disturbance in the normal balance of growth regulators and also yellow symptoms, but very often symptomatology is not diagnostic. Detection and characterization of phytoplasmas infecting different plant species are now possible with molecular methods based on study of 16S rDNA polymorphisms. Molecular diversity of phytoplasmas is also demonstrated by studying genes coding ribosomal proteins S3, tuf, SecY, amp, imp and other genes. Four phytoplasma genomes have been fully sequenced, including those of two ‘Candidatus Phytoplasma asteris’ strains, and strains of ‘Ca. P. mali’ and ‘Ca. P. australiense’. Three of these genomes contain large amounts of repeated DNA sequence, and the fourth carries multiple copies of almost 100 genes. In the last ten years several molecular biological findings involved in both the plant-phytoplasma and insect-phytoplasma interactions greatly increased the scientific knowledge about these pathogens. It was shown that their encodes very few metabolic functions confirming that phytoplasmas are highly dependent from their host cells. Moreover the presence of two glycolytic gene clusters in some strains suggested that a higher consumption of the carbon source can also be present together with specific genes interacting with plant/insect hormone balance. Considering that phytoplasmas have unusually small genomes, the repeats might be related to their transkingdom habitat and to their pathogenic activity. The phytoplasma surface membrane protein Amp may reflect an interaction between the phytoplasma and the host cytoplasm producing in some strains a complex with insect microfilament proteins that is correlated with the phytoplasma the phytoplasma transmissibility. Another phytoplasma coded protein ‘tengu’ was shown to be associated with the plant witches broom symptoms. Phytoplasma presence is also interfering with life cycle of insect vectors a stable over-expression of protein SAP11 in Arabidopsis increased the fecundity of M. quadrilineatus by on average 25% and also has some lines with severely crinkled leaves and higher number of stems suggesting that it may interact also with plant proteins that regulate plant defense responses to pathogens and pests. The phytoplasma genomes encodes many membrane or secreted proteins whose functions are still under study to elucidate the phytoplasma-plant-insect vector interactions.

Mycoplasmas of plants and insects.

BERTACCINI, ASSUNTA
2010

Abstract

Numerous yellows-type diseases of plants have been associated with mycoplasmas now renamed phytoplasmas over the last 40 years. The increasing threat of phytoplasma diseases comes both from devastating diseases in emerging agricultural countries and from epidemics and or endemic situations in areas traditionally devoted to agriculture. Phytoplasmas usually do not kill in short time the host plant from which they are strongly metabolic dependent therefore increasing the possibility of epidemic spreading; phytoplasma-infected plants result also more susceptible to infection by other pathogens such as fungi and viruses. Severe disease epidemics have been described worldwide associated with phytoplasma presence; these include coconut lethal yellowing in Africa and in the Caribbean, grapevine yellows in major viticultural areas and different diseases affecting stone and pome fruit plants. Phytoplasma infected plants exhibit symptoms suggesting profound disturbance in the normal balance of growth regulators and also yellow symptoms, but very often symptomatology is not diagnostic. Detection and characterization of phytoplasmas infecting different plant species are now possible with molecular methods based on study of 16S rDNA polymorphisms. Molecular diversity of phytoplasmas is also demonstrated by studying genes coding ribosomal proteins S3, tuf, SecY, amp, imp and other genes. Four phytoplasma genomes have been fully sequenced, including those of two ‘Candidatus Phytoplasma asteris’ strains, and strains of ‘Ca. P. mali’ and ‘Ca. P. australiense’. Three of these genomes contain large amounts of repeated DNA sequence, and the fourth carries multiple copies of almost 100 genes. In the last ten years several molecular biological findings involved in both the plant-phytoplasma and insect-phytoplasma interactions greatly increased the scientific knowledge about these pathogens. It was shown that their encodes very few metabolic functions confirming that phytoplasmas are highly dependent from their host cells. Moreover the presence of two glycolytic gene clusters in some strains suggested that a higher consumption of the carbon source can also be present together with specific genes interacting with plant/insect hormone balance. Considering that phytoplasmas have unusually small genomes, the repeats might be related to their transkingdom habitat and to their pathogenic activity. The phytoplasma surface membrane protein Amp may reflect an interaction between the phytoplasma and the host cytoplasm producing in some strains a complex with insect microfilament proteins that is correlated with the phytoplasma the phytoplasma transmissibility. Another phytoplasma coded protein ‘tengu’ was shown to be associated with the plant witches broom symptoms. Phytoplasma presence is also interfering with life cycle of insect vectors a stable over-expression of protein SAP11 in Arabidopsis increased the fecundity of M. quadrilineatus by on average 25% and also has some lines with severely crinkled leaves and higher number of stems suggesting that it may interact also with plant proteins that regulate plant defense responses to pathogens and pests. The phytoplasma genomes encodes many membrane or secreted proteins whose functions are still under study to elucidate the phytoplasma-plant-insect vector interactions.
2nd International Mycosafe Symposium
9
10
Bertaccini A.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11585/98448
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