Hormone variation in Robinia pseudoacacia L. (Fabaceae) leaves during gall formation by Obolodiplosis robiniae (Haldeman) (Diptera: Cecidomyiidae)

Gall-inducing insects modify plant metabolism to convert host tissue cells into new forms of galls. The black locust gall midge Obolodiplosis robiniae (Haldeman) forms marginal rolling gall on black locust leaves (Robinia pseudoacacia L.). We investigated the hormonal changes in host plants after infection and gall growth to senescence. A wide range of phytohormones was analysed for the first time in gall tissue. Hierarchical cluster analysis demonstrated that young gall (YGall) composition corresponds with non-galled leaflets control (NGLC) and non-galled leaflets of galled leaves (NGLG). A second cluster was formed by mature gall (MGall) and senescent gall (SGall). Through all stages of gall formation the level of hormones increased ABA, GA3, auxins (IAA and PAA), different types of brassinosteroids, 7-oxalactone (BL, EBL, and HBL), 6-oxo type (CS, ECS, TY, and CT), and 6-deoxo type (6dTY) as well as different types of cytokinins, such as free bases (cZ, tZ, DHZ, and iP), N-glucosides (tZ7G, tZ9G, DHZ7G, DHZ9G, and iP7G), O-glucosides (cZOG, cZROG, tZOG, tZROG, DHZOG, and DHZOGR), and ribosides (cZR, tZR, iPR, and DHZR), and salicylic acid (SA). There was no reduction in the examined hormones. The senescence galls present the highest fluctuation and we also noticed changes in NGLC and NGLG. We noted the highest levels of HBL and DHZOG and the lowest contents of ABA, auxins, brassinosteroids, cytokinins, GA3, and SA. MGall and SGall presented changes in opposite directions. The level of cytokinins in gall tissue was higher than NGLC and NGLG, indicating that infection modulates the levels of these phytohormones in whole compound leaves. DHZOG, tZROG level increased in NGLG variant suggests that they are crucial after infection. Brassinosteroids (HBL) level decreased during YGall formation according to NGLC. Results of wide hormone profiling suggest that hormonal changes occur in sequence.