The role of strigolactone and auxin in the regulation of shoot branching

Shoot branching is an essential ecological and agronomical trait regulated by the complex interaction of many signals, of which phytohormones are among the main actors. In this thesis I investigate the role of strigolactone and auxin in controlling bud growth dynamics and their effect on plant branching levels and development more generally. Firstly, I present a novel molecular mechanism for strigolactone perception, developed in collaboration with Prof. Ning Zheng’s group; I then link strigolactone signal release to cellular and organismal phenotypes. Secondly, I introduce novel methods to acquire and analyse bud growth dynamics data. I then exploit these methods to investigate how strigolactone and auxin treatments affect bud growth and bud-bud competition in a variety of key mutant lines. Following this, I present my work on the interaction between strigolactone perception and PIN patterning, focusing on PIN endocytosis and polarity. I propose a model according to which strigolactone regulates the levels and relative polarity of specific PIN proteins at the plasma membrane of xylem parenchyma cells and I assess the implications of PIN SL- responsiveness for branching phenotypes and plant development more generally. Finally, I assess the importance of SL-mediated transcriptional regulation during axillary bud activation. Using transcriptomics, I identify a set of genes that are associated with bud activation and I assess how the expression of these sets of genes is affected in mutant lines lacking the expression of key genes involved in strigolactone signal transduction. In conclusion, my work identifies several novel aspects of strigolactone and auxin biology, linking molecular events in the nucleus and cellular/tissue-level patterning of PIN proteins to developmental phenotypes at the organismal level.