Tomato Mutants Reveal Root and Shoot Strigolactone Involvement in Branching and Broomrape Resistance

The phytohormones strigolactones (SLs) control root and shoot branching and are exuded from roots into the rhizosphere to stimulate interaction with mycorrhizal fungi. The exuded SLs serve as signaling molecules for the germination of parasitic plants. The broomrape is a widespread noxious weed in various crop plants, including tomato ( ). We have isolated three mutants that impair SL functioning in the tomato variety M82: ( ) and ( ), which abolish SL biosynthesis, and ( ), which impairs SL perception. The over-branching phenotype of the mutants resulted in a severe yield loss. The isogenic property of the mutations in a determinate growth variety enabled the quantitative evaluation of the contribution of SL to yield under field conditions. As expected, the mutants and were completely resistant to infection by due to the lack of SL in the roots. In contrast, was more susceptible to . than the wild-type M82. The SL concentration in roots of the was two-fold higher than in the wild type due to the upregulation of the transcription of SL biosynthesis genes. This phenomenon suggests that the steady-state level of root SLs is regulated by a feedback mechanism that involves the SL signaling pathway. Surprisingly, grafting wild-type varieties on and rootstocks eliminated the branching phenotype and yield loss, indicating that SL synthesized in the shoots is sufficient to control shoot branching. Moreover, commercial tomato varieties grafted on were protected from . infection without significant yield loss, offering a practical solution to the broomrape crisis.