Abstract
Chinese cabbage plants go through seedling and rosette stages before forming their leafy head. Chinese cabbage plants resemble pak-choi plants at their seedling stage, but in their rosette stage the leaves of Chinese cabbage differentiate, as they increase in size with shorter petioles. In order to understand the molecular pathways that play a role in leafy head formation, transcript abundance of young emerging leaves was profiled during development of two Chinese cabbage genotypes and a single pak-choi genotype. The two Chinese cabbages differed in many aspects, among others earliness, leaf size and shape, leaf numbers, and leafy head shape. Genome-wide transcriptome analysis clearly separated the seedling stages of all three genotypes together with the later stages from pak-choi, from the later developmental stages of both Chinese cabbages (rosette, folding, and heading). Weighted correlation network analysis and hierarchical clustering using Euclidean distances resulted in gene clusters with transcript abundance patterns distinguishing the two Chinese cabbages from pak-choi. Three clusters included genes with transcript abundance affected by both genotype and developmental stage, whereas two clusters showed only genotype effects. This included a genotype by developmental stage cluster highly enriched with the MapMan category photosynthesis, with high expression during rosette and folding in Chinese cabbages and low expression in the heading inner leaves that are not exposed to light. The other clusters contained many genes in the MapMan categories Cell, showing again differences between pak-choi and both Chinese cabbages. We discuss how this relates to the differences in leaf blade growth between Chinese cabbage and pak-choi, especially at the rosette stage. Overall, comparison of the transcriptome between leaves of two very different Chinese cabbages with pak-choi during plant development allowed the identification of specific gene categories associated with leafy head formation.
Highlights
Chinese cabbage (CC; Brassica rapa ssp. pekinensis) is a widely cultivated and economically important vegetable in Asia, composed of a large number of tightly wrapped heading leaves (HLs) surrounding the shoot apexes[1]
In week 1 and week 2, plants were in the seedling stage (S), followed by the rosette stage (R), which is characterized by the onset of upward curving of leaves in CC-Z16 with younger emerging leaves having increased leaf angles
CC-A03 develops slower, with a longer rosette stage, which is characterized by increasing leaf angles, around weeks 7 till 9 the folding stage, where leaves curve upwards forming a mold for the later head, and around week 10–11 the heading stage with further growth of the leafy head
Summary
Chinese cabbage (CC; Brassica rapa ssp. pekinensis) is a widely cultivated and economically important vegetable in Asia, composed of a large number of tightly wrapped heading leaves (HLs) surrounding the shoot apexes[1]. The leafy head is the storage organ of CC and is formed after the rosette stage[2]. Rosette leaves (RLs) become large and round with short petioles, which begin to fold upward. The incurving process of the leaves continues until HLs are arranged tightly around each other to form leafy heads as storage organs for nutrients. The HLs are wrinkled with an upward curvature with broad midveins. Both the size, Sun et al Horticulture Research (2019)6:130 shape, and degree of incurvature of HLs define the final head size and shape. PC does not form leafy heads and the flat green leaves with fleshy petioles that grow as a rosette represent the consumed part of this leafy vegetable. The inner leaves remain green during PC development
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