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Changes in cell and tissue organization and in peroxidase activity have been analyzed to find early markers to predict graft incompatibility occurrence in peach/plum combinations (Prunus persica/Prunus spp.) at 5 months after grafting in the dormancy period. Different compatible and incompatible peach/plum grafts were grown for 5 months in a nursery. The cellular study of the graft interface revealed structural changes associated with graft incompatibility symptoms. The main structural features were cambium cell disorganization, less differentiation of vascular tissues, degeneration of phloem and xylem cells, and accumulation of phenols at the graft interface after 5 months of graft development. The peroxidase study was performed during dormancy and the vegetative growth period, and revealed a significant increase in peroxidase activity in the incompatible unions, with significant differences between compatible and incompatible grafts. Analysis of gel profiles of nonbudded rootstocks and scions revealed an anodal isoperoxidase band [relative front (Rf) = 0.48] present in scions and compatible rootstocks, and another isoperoxidase band (Rf = 0.53) only present in the incompatible rootstocks. Our results show that the analysis of cell organization to detect early structural events and the evaluation of peroxidase activity at graft unions constituted feasible and convenient methods for early diagnosis of graft incompatibility. Also, it was suggested that the presence of band Rf = 0.48 in plum rootstocks and peach cultivars could be used as a marker to predict graft compatibility for peach scions and plum rootstocks.

In order to localize at EM level the sites of transcription of both pre-mRNA and pre-rRNA, we have detected the DNA/RNA hybrid molecules and m3Gcapped structures by means of specific antibodies after short bromo-uridine (BrU) incorporation. In addition, the sections have been stained by a selective RNA stain, terbium citrate. Our data indicate that perichromatin fibrils incorporate BrU and are labeled by the anti-hybrid probe; this supports the idea that they are the pre-mRNA transcription sites. On the contrary, interchromatin granules do not incorporate BrU after short pulses and are not labeled by the anti-hybrid probe. Concerning the nucleolus, anti-hybrid and anti-BrdU antibodies colocalize only on the dense fibrillar component, suggesting that this is the site of rRNA transcription. Interestingly, the dense fibrillar component and the granular component, after specific RNA staining, show remarkable structural similarities, both containing fibrogranular RNA structures.

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