Phytoplasmas are plant pathogenic bacteria belonging to the class Mollicutes
]. They are cell wall-free, and both their cell size (0.1-0.8 μm in diameter) and genome size (0.5-1.3 Mbp) are the smallest among bacteria. They are transmitted from plant to plant by sap-feeding insect vectors and they propagate within the cytoplasm of both insects and plants. In plants they exclusively inhabit nutrient-rich phloem tissues, where they have been documented by electron microscopy in sieve elements, companion cells and phloem parenchyma cells [2
]. Phytoplasmas have a broad range of plant hosts among monocots and dicots and diseases of many important crops have already been associated with these pathogens [1
]. Since the discovery of phytoplasmas four decades ago, attempts to culture them in cell-free media have failed, making it difficult to determine the taxonomic status of phytoplasmas by the traditional methods applied to cultured prokaryotes. Although satisfaction of Koch postulates has not yet been achieved, in order to start formal classification of phytoplasmas the 'Candidatus
Phytoplasma' genus has been proposed and adopted [3
]. However, some phytoplasmas have had an informal description, but they have not been validly described. For them the practice of naming a phytoplasma on the basis of the associated plant syndrome is still valid [3
Recent progress in the sequencing of four phytoplasma genomes and their comparative analysis [4
] have increased our understanding of phytoplasma genetics and revealed that these plant pathogens experienced substantial evolutionary gene decay, gene loss, and disruption or loss of important biosynthetic pathways [8
]. These genomic changes may account for their complete reliance on the host plant or insect cells for survival.
As no phytoplasma cultures are available, not all the approaches can be applied and studies on phytoplasma interactions with host plants are limited. However, the latter have been mostly studied in periwinkle (Cataranthus roseus
). Phytoplasmas can be transmitted to periwinkle via graft inoculation or via their respective insect vectors [9
]. Periwinkle is the first choice also because phytoplasmas concentration can reach very high level and infected plants are easy to propagate. Considering the extreme difficulties associated with the establishment of an accurate and reliable experimental system, very little is also known about the mechanisms of phytoplasma phytopathogenicity [1
]. This is especially true for phytoplasmal diseases of major crops, including woody grapevine (Vitis vinifera
L.), which is, on a global basis, both the most widely cultivated and economically important fruit crop.
In grapevine, phytoplasma cause grapevine yellows (GY) diseases, which were identified in the majority of grapevine growing countries worldwide. Several molecularly distinct phytoplasma groups which cause the GY were identified; however, a phytoplasma from the stolbur group/16SrXII-A [3
], associated with the GY disease 'Bois noir', is very common. The name for this phytoplasma 'Ca
. P. solani' has been proposed at the X International Congress of the International Organization of Mycoplasmology (1994), but the phytoplasma has not been formally described yet. Accordingly it is referred to as 'Bois noir' ('BN') phytoplasma. Typical symptoms of all phytoplasma infection in grapevine are leaf curling and discoloration of leaf veins and laminas, interveinal yellowing or reddening, according to the variety, uneven or total lack of lignification of canes, flower abortion and berry withering. Leaves on the affected shoots often have a hard, brittle texture.
In order to demonstrate the potential of grapevine plants infected with 'BN' phytoplasma to interact with their pathogen, we present here the qualitative and quantitative changes in the global gene expression profiles of the healthy and infected plants of cv. 'Chardonnay'. The experiment was carried out in a production vineyard over two successive growing seasons. Whereas studies of plant responses to pathogen infection conducted under controlled laboratory or glasshouse conditions can focus on the impact of individual factors, it is the resulting changes in the transcript profile of the field established system that potentially uncovers the complexity of signaling networks involved in the pathogenicity [11
]. A complementary experimental system for analyzing gene expression profiling in the field grown 'BN' phytoplasma infected grapevine has been described very recently [12
]. The present work is an extension of our recent study on three genes encoding sucrose synthase, alcohol dehydrogenase 1 and Hsp70 that are differentially expressed in different grapevine cultivars infected with phytoplasmas [13
], to a high throughput transcriptome analysis of the infected cv. 'Chardonnay'. We analyzed the traits of infection and further evaluated the potential contribution of several metabolic/signaling host pathways to the infection status. Based on the assumption that phytoplasmas mobilize phosphorylated hexoses from the companion cells of phloem [2
] and according to accumulating evidence that levels of reducing sugars and sucrose are generally higher in the source leaves of plants, which are infected with different phytoplasma species [14
], our focus was on the gene expression level of plant enzymes involved in carbohydrate metabolism and photosynthesis. Presumably its changes in infected plants are related to the requirements of phytoplasma for energy and growth. Our collective findings contribute towards understanding the potential main steps in phytoplasma pathogenicity pathway.