Unraveling the complexity of melon-pumpkin graft collapse: Hormonal imbalances and stress responses in incompatible combinations
image: Comparison of leaf metabolites between graft combinations and between treatments, i.e. fruiting and fruit removed.
Credit: Horticulture Research
Grafting melon (Cucumis melon L.) onto pumpkin (Cucurbita maxima Duch. × Cucurbita moschata Duch.) rootstock is a method of improving yield, but is challenged by the scion-rootstock incompatibility, leading to plant collapse attributed to various physiological and biochemical factors. Although initial beliefs of blockages in graft zones causing late-stage collapses, recent studies have shown that specific metabolites and hormone imbalances, particularly cytokinins and auxin (IAA), are more likely culprits. These findings indicate a complex interplay of stress response and hormone regulation that disrupts the putative blockage mechanism and pointing towards metabolic and hormonal imbalances as the main factors inducing plant collapse, thus necessitating further research into the underlying physiological pathways.
In May 2022, Horticulture Research published a research article entitled by “Impairment of root auxin–cytokinins homeostasis induces collapse of incompatible melon grafts during fruit ripening ”.
In this study, researchers conducted a series of hormone and metabolomic profiling, gene expression monitoring, and biochemical analyses of compatible and incompatible melon-pumpkin grafts under fruiting and fruitless conditions to understand the impact of grafting melon onto pumpkin rootstocks. They found significant differences emerged in the accumulation of certain leaf metabolites according to plant physiological stages, suggesting the rootstock genotype plays a role in metabolite accumulation.
This study revealed that the rate of collapse increased during fruit ripening due to early root senescence of incompatible grafts roots and increased hydrogen peroxide content. Hormone profiling showed elevated levels of IAA, 2-oxoindole-3-acetic acid (IAA catabolite), indole-3-acetylaspartic acid (IAA conjugate), and cis-zeatin-type cytokinins but lower levels of trans-zeatin-type cytokinins in the roots of incompatible combinations of plants during fruit ripening. This suggested a disrupted hormone balance and potential blockage at the graft junction. Additionally, expression of the YUCCA gene, YUC2, YUC6 and YUC11 (required for auxin biosynthesis), the GRETCHEN-HAGEN3 gene (required for auxin conjugation), and the cytokinin oxidase/dehydrogenase 7 (CKX7) gene (which regulates the irreversible degradation of cytokinins) was enhanced in roots of incompatible combinations of plants during fruit ripening..
Further studies showed that the level of cytokinin oxidase/dehydrogenase, which degrades cytokinins, were higher in the roots of incompatible grafts. Moreover, high levels of IAA in the roots of the incompatible grafts during fruiting disrupts the balance between IAA and cytokinins, leading to oxidative stress and a reduction in photoassimilate transport from scion to rootstock. H2O2 and MDA contents increased and antioxidant enzyme activities decreased in the roots of the incompatible grafts during fruit ripening. The results suggest that the collapse of incompatible graft combinations during fruit ripening is closely associated with a dramatic accumulation of IAA in the root system, which may cause oxidative damage and disrupt the balance of IAA and cytokinins that is essential for the compatibility of melon-pumpkin grafts.
Overall, this study points to a complex interplay of hormonal imbalances, oxidative stress, and metabolite changes that leads to the collapse of incompatible melon-pumpkin grafts, especially during fruit ripening. This suggests the need for further research to fully understand the mechanisms behind graft incompatibility and to develop strategies to mitigate these effects for more successful grafting practices.
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References
Authors
Maria Dolores Camalle1, Aleš Pěnčík2, Ondřej Novák2, Lina Zhao1, Udi Zurgil3, Aaron Fait3 and Noemi Tel-Zur3,*
Affiliations
1The Albert Katz International School for Desert Studies, The Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus 8499000, Israel
2Laboratory of Growth Regulators, Faculty of Science, The Czech Academy of Sciences, Palacký University & Institute of Experimental Botany, Slechtitelu 27, CZ-783 71 Olomouc, Czech Republic
3French Associates Institute for Agriculture and Biotechnology of Drylands, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, P.O.B. 653, Beer Sheva 84104000, Israel
About Noemi Tel-Zur
She is an associate professor at the French Associates Institute for Agriculture and Biotechnology of Drylands, Blaustein Institutes for Desert Research.