Cytokinin action and its interaction with other plant hormones in the control of branching in tomato (Solanum lycopersicum L.)

Abstract

Branching of plants determines its overall architecture and affects fundamental aspects such as nutrient partitioning, height, light gathering efficiency and visibility to pollinators. Branching occurs by two successive steps: i) initiation and ii) growth of axillary buds. Mutants affecting growth of axillary buds have been identified in several species: ramosus (rms) in pea, more axillary growth (max) in Arabidopsis, decreased apical dominance (dad) in petunia and dwarf (d) in rice. Few mutants have been described affecting initiation of axillary buds, such as lateral suppresser (ls) and blind (bl) in tomato. While it is well established that the aforementioned mutants affecting axillary buds are related to synthesis and signaling of the hormone strigolactone, little is known about the impact of plant hormones in ls and bl mutations. Cytokinin, auxin, and gibberellins are the major hormone groups associated with control of branching in plants. Auxins and cytokinins have antagonistic effect in the control of branching. Auxin derived from the stem apex may regulate the levels of cytokinin in the stem, inducing its degradation by regulating the expression of cytokinin oxidase (CKX) as well as by suppressing the biosynthesis of cytokinin, through regulation of IPT genes expression. Strigolactones comprise another hormone class that negatively controls the development of axillary buds. The expression of at least two of the genes responsible for strigolactone biosynthesis is regulated by auxin. Additionally, cytokinins and strigolactones act antagonistically in the development of the bud. Gibberellins interact with cytokinins in the control of the stem apical meristem activity, both being influenced by KNOX family of regulatory genes. However, little is known about the interaction between cytokinins and gibberellins in the control of branching. Apices of plants homozygous for the defective ls allele contain higher levels of auxin and gibberellic acid, while cytokinin levels are reduced in comparison to wild type plants. Another major gap in our knowledge is the lack of an explanation for the increased branching of plants overexpressing the CKX gene, which leads to reduced cytokinin levels. This project aims at exploring the role of cytokinin as well as its interaction with auxin, gibberellin and strigolactone in control of branching. With this goal, transgenic plants overexpressing CKX2 will be characterized and crossed to hormone-related mutants to obtain homozygous double transgenic/mutant lines. Vegetative and reproductive traits and responses to cytokinin, auxin, gibberellin and strigolactone will be evaluated. (AU)