Abstract
Grapevine can be severely affected by phytoplasmas, which are phytopathogenic Mollicutes invading the sieve elements of the host plant. The biochemical and molecular relationships between phytoplasmas and their hosts remain largely unexplored. Equally unknown is an interesting aspect of the pathogen–plant interaction called “recovery,” which is a spontaneous remission of symptoms in previously symptomatic plants. Recovered plants develop resistance mechanisms correlated with ultrastructural and biochemical changes in the sieve elements. Callose as well as sugars are involved in several plant defense processes and signaling. In the present work we have examined the possible involvement of callose, as well as callose synthase, sugar transporter, and cell wall invertase genes, during the infection and after “recovery” of grapevine from bois noir (BN). Ultrastructural investigation of leaf tissue showed that callose accumulated in the sieve elements of diseased grapevine; moreover, two genes encoding for callose synthase were up-regulated in the infected leaves. Regarding sucrose, expression analysis showed that sucrose transport and cleavage were severely affected by BN phytoplasma, which induced the establishment of a carbohydrate sink in the source leaf, and was analogous to other obligate biotrophs that acquire most of their nutrients from the host plant. Interestingly, whereas in recovered plants the transcript level of sucrose synthase was similar to healthy plants, sucrose transporters as well as cell wall invertase were expressed to a greater degree in recovered leaves than in healthy ones. Recovered plants seem to acquire structural and molecular changes leading to increases in sucrose transport ability and defense signaling.
Highlights
Phytoplasmas have been associated with several hundred diseases affecting economically important crops, such as ornamentals, vegetables, fruit trees, and grapevines (Lee et al, 2000)
In this work we studied the responses of callose during phytoplasma infection in grapevines affected by Bois noir (BN) and their possible role in the establishment of “recovery.” the expression of sucrose metabolism-related genes such as sucrose transporters, sucrose synthase (SUS), and invertase genes were investigated in leaves of recovered plants compared to infected and healthy plants
For the first time we have investigated and reported modifications occurring in the sieve elements of grapevine during BN infection and“recovery.”An integrated approach has been adopted through the combined use of ultrastructural and gene expression analyses of leaf tissues
Summary
Phytoplasmas have been associated with several hundred diseases affecting economically important crops, such as ornamentals, vegetables, fruit trees, and grapevines (Lee et al, 2000). Phytoplasmas are plant-pathogenic prokaryotes belonging to the class Mollicutes, a group of wall-less microorganisms phylogenetically related to low G+C, Gram-positive bacteria (Weisburg et al, 1989). In host plants, they are restricted to the sieve elements of phloem and are transmitted among plants by phloem sap feeding leafhoppers or psyllids in a persistent manner. BN causes symptoms such as abnormal lignification of canes, short internodes, flower abortion, and curling and discoloration of leaves with intervein yellowing or reddening, and these are accompanied by a dramatic reduction in yield (Osler et al, 1993) These symptoms have been related to stoma closure, reduced photosynthesis rate, and anomalous accumulation of carbohydrates in leaves (Bertamini et al, 2002; Musetti et al, 2007; Endeshaw et al, 2012)
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