Biochemical characterization and differential gene expression associated with the 'translocated' peach-plum graft-incompatibility

Grafting is an ancient plant propagation technique widely used in horticultural crops, particularly in fruit trees. However, the involvement of two different species in grafting may lead to lack of affinity between the graft components, known as graft-incompatibility. Over the last decades, more attention has been given to the different mechanisms underlying the localized type of graft-incompatibility; whereas the phenylpropanoid-derived compounds and the differential gene expression associated with the translocated peach-plum graft-incompatibility remain unstudied. Therefore, the aim of this study was to shed light on the biochemical and molecular mechanisms involved in the typical „translocated‟ graft-incompatibility of peach/plum rootstocks. In this study, the Summergrand (SG) nectarine cultivar was budded on two plum rootstocks: Adara and Damas GF 1869. Translocated symptoms of incompatibility were shown and biochemically characterized in the case of SG-Damas GF 1869, three years after grafting. The statistical results indicated significant differences between the two graft-combinations, in terms of the non-structural carbohydrates (soluble sugars and starch), phenolic compounds and antioxidant capacity. These compounds accumulated in general above the graft-union in the incompatible graft-combination. Similarly, the enzymatic activities of the antioxidant enzyme peroxidase (POX), the phenylalanine lyase-ammonia (PAL) and polyphenol oxidase (PPO), involved in the phenylpropanoid pathway, were significantly affected by the incidence of the translocated incompatibility, showing higher activities in the scion of the incompatible graft-combination than in the rootstock. Moreover, at the molecular level, the Real-Time qPCR expression revealed an induction of PAL genes (PAL1 and PAL2) in the „translocated‟ incompatibility, where the incompatible graft-combination exhibited a higher expression of both PAL genes than the compatible one. These genes seemed to be involved in the PAL enzymatic activity, a key enzyme in the phenylpropanoid pathway and the main source of phenolic compounds synthesis. Regarding the SG-Adara graft-combination, there were neither external symptoms of translocated incompatibility nor significant differences in biochemical and molecular parameters above and below the graft-union, proving it to be a compatible combination. The differential expression of PAL genes, together with the above mentioned biochemical factors, proved to be good markers to predict the translocated peach/plum graft-incompatibility. This conclusion is also of great importance in the investigation of graft-incompatibility in Prunus species and in Prunus rootstock breeding programs