Morphological and genetic characterization of the leafy head of Chinese cabbage (Brassica rapa)

Abstract

The leafy head is the harvested organ of Chinese cabbage and is composed of a large number of tightly wrapped heading leaves surrounding the shoot apex. For both farmers and consumers, the Chinese cabbage leafy head trait has clear advantages, related to yield, stress resistance and storability. The leafy head is an obvious doemstication trait, also found in cababges (B. oleracea ssp. Capitata), lettuce (Lactuca sativa L.) and radicchio of Treviso (Cychorium intibus L.). The formation of the leafy head is genetically determined, however affected by both internal and external factors. The developmental and genetic mechanisms underlying leafy head formation are however poorly understood. In recent years, scientists started to study this domestication trait, which resulted in identification of domestication signals and QTL for leafy head formation. This thesis link morphological observations (plant, tissue and cell level) to genetic regulation (by population genetic analysis, trancriptome analysis) and gene functional experiments to explain how leafy heads form in Chinese cabbage. The relationship between variation in rosette-leaves and leafy heads by phenotyping 152 Chinese cabbage accessions was determined and QTL explaining heading and heading related traits were identified. Genome wide transcriptome analysis and the morphological differences at cellular level between heading and non-heading B. rapa pak choi and Chinese cabbage were studied to help us explain how Chinese cabbage heads form. Furthermore, the function of leafy head formation associated gene BrARF3.1 was studied. Taken together, research described in this thesis links the morphological observations on leaf and leafy head development to molecular genetic results, both at the population and cellular levels.