The control of starch degradation in Arabidopsis thaliana leaves at night

The aim of this work was to understand how Arabidopsis thaliana plants control starch \ndegradation at night. Starch is the major energy reserve in Arabidopsis. It is broken \ndown at night to maintain growth and metabolism of the plant, when photosynthesis is \nnot possible. The rate of starch degradation follows a linear pattern and is matched to \nthe length of the night period such that almost all starch is exhausted by dawn. The \nmechanisms and the proteins involved in controlling starch degradation rates are largely \nunknown. \nWith my work I wanted to identify components involved in the control of starch \ndegradation rates. Using a forward genetic screen, I discovered several mutants with \nnew starch degradation phenotypes. One of them was affected in the circadian clock \ncomponent EARLY FLOWERING 3 (ELF3). It degraded its starch slower than wildtype \nplants and in a non-linear way. Two mutants degraded their starch at a much faster \nrate than wild-type plants and exhausted their reserves before dawn. One of them lacks \na novel protein, which was named EXCESS STARCH TURNOVER 1 (EST1). This \nprotein is required for normal starch degradation rates, but its function is still unknown. \nThe second mutant is affected in BETA-AMYLASE 1 (BAM1) and produces aberrant \nBAM1 protein containing a serine to asparagine amino acid substitution in position 132. \nFaster starch degradation rates in this mutant depend on the presence of another protein \nof the pathway, LIKE SEX FOUR 1 (LSF1). The data indicate that modulation of \nBAM1 activity can strongly affect the rates of starch degradation, although starch is \ndegraded normally in absence of BAM1. In a second approach, I analysed which known \ncomponents of the starch degradation pathway are necessary for the adjustment of \nstarch degradation rates. I found that PHOSPHOGLUCAN WATER DIKINASE \n(PWD) and BAM3 might play a role in adjusting starch degradation rates in response to \nan unexpectedly early night. \nIn summary, in this thesis I introduce a novel protein necessary for normal starch \ndegradation rates in Arabidopsis leaves and provide insights into proteins and \nmechanisms which might control starch degradation rates in response to an early night.