Biochemical Characterization and Differential Expression of PAL Genes Associated With 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 and severe disorders between the graft components, known as graft-incompatibility. This complex agronomic trait is traditionally classified into two categories: localized (weak graft unions with breaks in cambial and vascular continuity at the graft interface and absence of visual symptoms in scion leaves and shoots) and translocated (degeneration of the sieve tubes and phloem companion cells at the graft interface causing translocation problems in neighboring tissues, and reddening/yellowing of scion leaves). Over the 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 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 graft-combinations. 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 graft-combination, 3 years after grafting. Non-structural carbohydrates (soluble sugars and starch), phenolic compounds and antioxidant activity, were significantly enhanced in the incompatible graft-combination scion. Similarly, the enzymatic activities of the antioxidant enzyme peroxidase, the phenylalanine ammonia-lyase (PAL) and polyphenol oxidase involved in the phenylpropanoid pathway were significantly affected by the incompatible rootstock Damas GF 1869, inducing higher activities in the scion than those induced by the compatible rootstock Adara. In addition, a positive and strong correlation was obtained between total phenol content, antioxidant capacity and the expression of the key genes involved in the phenylpropanoid pathway, PAL1 and PAL2. Regarding the SG/Adara graft-combination, there were neither external symptoms of translocated incompatibility nor significant differences in the biochemical and molecular parameters between scion and rootstock, proving it to be a compatible combination. The differential expression of PAL genes together with the biochemical factors cited above could be good markers for the translocated peach/plum graft-incompatibility.

Automated summary

Grafting, an ancient plant propagation technique widely used in horticultural crops like fruit trees, may lead to graft-incompatibility when involving different species. This incompatibility can be classified into localized and translocated categories, with more emphasis on understanding the mechanisms of the latter type in peach/plum graft-combinations. The study highlighted biochemical and molecular factors involved in translocated graft-incompatibility.