STRIGOLACTONES - NEW CANDIDATES FOR PLANT HORMONES

Strigolactones, are the new regulators of plant growth and development. The first information about strigolactones came from experiments verifying the interactions between host plants and parasites of the Orobanchaceae family. In these interactions some substances from root exudates play critical role in the induction of parasitic plants seed germination. Isolation and identification of new representatives of these substances revealed their similar chemical structure containing the lactone group. These compounds have been termed to as strigolactones after the name of the parasite Striga sp. and lactone group. The next step in the research carried out on strigolactones was to prove that these regulators act as branching factors for symbiotic arbuscular mycorrhizal fungi (AMF). This fact may explain why plants exude strigolactones that enable them to be located by their enemies. The parasitic plants that evolved later than AMF may then have developed a detection system for strigolactones as cues to find living host roots in their vicinity. The new function of strigolactones is their role in inhibiting shoot branching (discovered in 2008). Analysis of several mutants with increased shoot branching, in which known hormones were not responsible for the bud outgrow, showed that strigolacones play critical role in negatively regulated branching in plants. Strigolactone mutants found in Arabidopsis, pea, rice and tomato always had a similar phenotype these plants were smaller and more-bushy than parental varieties. The new functions of strigolactones (e.g. regulation of mesocotyl elongation or control of the root growth and root hair growth) are described. Analysis of mutants with bushy-like phenotypes enabled the identification of the first genes involved in strigolactone biosynthesis and signaling. The studies of molecular basis of these processes are necessary to understand the mechanisms by which strigolactones regulate plant growth and development and to describe the strigolactone interactions with fitohormones. It was proved that strigolactones control the level of auxin and together with this hormone negatively regulate shoot branching. Moreover, the first step of strigolactones biosynthesis is similar to one of the steps of abscisic acid biosynthesis. The main genes involved in strigolactone biosynthesis - CCD7 and CCD8 were identified in chloroplasts, together with genes responsible for the control of this process IAA12 and IGI1. The first inhibitor of strigolactone synthesis - TIS13 - was also described. Analyses carried out on rice mutants insensitive to synthetic analog of strigolactone reveled proteins that may play a role as strigolactones receptors - FCI, D3 and D14. It is interesting that the amino acid sequence and/or conformation of these proteins are similar to different proteins identified as receptors for fitohormones, such as auxin, jasmonates or gibberellins. Experiments on different species, such as Arabidopsis, pea, rice or tomato, showed that the mechanisms of strigolactone biosynthesis and signaling are conserved in plants. The paper presents a review of the history about strigolactones discovery, molecular basis of RMS/MAX/D pathway and their functions in plants.