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 degradation at night. Starch is the major energy reserve in Arabidopsis. It is broken down at night to maintain growth and metabolism of the plant, when photosynthesis is not possible. The rate of starch degradation follows a linear pattern and is matched to the length of the night period such that almost all starch is exhausted by dawn. The mechanisms and the proteins involved in controlling starch degradation rates are largely unknown. With my work I wanted to identify components involved in the control of starch degradation rates. Using a forward genetic screen, I discovered several mutants with new starch degradation phenotypes. One of them was affected in the circadian clock component EARLY FLOWERING 3 (ELF3). It degraded its starch slower than wildtype plants and in a non-linear way. Two mutants degraded their starch at a much faster rate than wild-type plants and exhausted their reserves before dawn. One of them lacks a novel protein, which was named EXCESS STARCH TURNOVER 1 (EST1). This protein is required for normal starch degradation rates, but its function is still unknown. The second mutant is affected in BETA-AMYLASE 1 (BAM1) and produces aberrant BAM1 protein containing a serine to asparagine amino acid substitution in position 132. Faster starch degradation rates in this mutant depend on the presence of another protein of the pathway, LIKE SEX FOUR 1 (LSF1). The data indicate that modulation of BAM1 activity can strongly affect the rates of starch degradation, although starch is degraded normally in absence of BAM1. In a second approach, I analysed which known components of the starch degradation pathway are necessary for the adjustment of starch degradation rates. I found that PHOSPHOGLUCAN WATER DIKINASE (PWD) and BAM3 might play a role in adjusting starch degradation rates in response to an unexpectedly early night. In summary, in this thesis I introduce a novel protein necessary for normal starch degradation rates in Arabidopsis leaves and provide insights into proteins and mechanisms which might control starch degradation rates in response to an early night