Genetic Analysis of Chinese Cabbage Reveals Correlation Between Rosette Leaf and Leafy Head Variation

Abstract

To understand the genetic regulation of the domestication trait leafy-head formation of Chinese cabbages, we exploit the diversity within Brassica rapa. To improve our understanding of the relationship between variation in rosette-leaves and leafy heads, we phenotyped a diversity set of 152 Chinese cabbages. This showed correlation between rosette-leaf traits and both head traits and heading capacity. Interestingly, the leaf number of the mature head is not correlated to heading degree nor head shape. We then chose a non-heading pak choi genotype to cross to a Chinese cabbage to generate populations segregating for the leafy head traits. Both a large F2 (485 plants) and a smaller Doubled Haploid (88 lines) mapping population were generated. A high density DH-88 genetic map using the Brassica SNP array and an F2 map with a subset of these SNPs and InDel markers was used for quantitative trait locus (QTL) analysis. Thirty-one quantitative trait loci (QTLs) were identified for phenotypes of rosette-leaves in time and both heading degree and several heading traits. On chromosome A06 in both DH-88 and F2-485 QTLs for rosette leaf length and petiole length at different developmental days and an F2 QTL for head height co-located. Variation in head height, width and weight all correlate with variation in heading degree with co-locating QTLs, respectively, on chromosome A03, A05, and A08 in F2-485. The correlation between rosette-leaf and heading traits provides not only insight in the leaf requirements to form a head, but also can be used for selection by Chinese cabbage breeders.

Genetic analysis of the “head top shape” quality trait of Chinese cabbage and its association with rosette leaf variation

“…At the QTL level, we detected QTLs for rosette leaf-, outer heading leaf-, and leafy head-related traits in this study. We did not detect the same QTLs for rosette LL and width or rosette LPL compared to the previous QTL study 9 . In addition to environmental variation, in these studies, the timing of phenotypic data collection differed: we measured the rosette leaf traits at the heading stage in this study, but phenotyped the growing rosette leaves at rosette stages (30, 34, 37, 41, 44, and 48 days after sowing (DAS)) in a previous QTL study.…”

“…However, in Chinese cabbage, rosette leaves remain active until the leafy heads mature, likely performing photosynthesis in the fast-growing leafy head 18 . Both this study and previous studies showed a correlation between the size and shape of rosette leaves and leafy head shape 6 , 9 . HTS is clearly associated with outer rosette LL and leaf petiole length (LPL) (this study).…”

“…In this paper, they also identified that allelic variation in the TCP4 gene in the miR319a recognition sequence modulated head shape by differentially arresting cell division in different leaves. Sun et al 9 also showed the correlation between rosette leaf traits, such as leaf length (LL), width, and midvein length, with leafy head traits in a diverse set of 152 Chinese cabbages. Although the phenotypic correlation between rosette leaves and leafy heads has been described, it is unclear how rosette leaves influence leafy head growth and how this is regulated at the molecular level.…”

“…These Chinese cabbage rosette leaves provide the basis for plant growth via photosynthesis 8 . A number of recent studies have shown that rosette leaves supply products for leafy head formation and that their size and shape are related to the leafy head shape or size 6 , 9 , 10 . In a study by Mao et al 6 , the correlation between rosette leaves and head shape was demonstrated by using 150 recombinant inbred lines (RILs): round heads with flat rosette leaves, cylindrical heads with rosette leaves with wavy margins, oblong heads with shrinking rosette leaves, and cone-like heads with incurved rosette leaves.…”

“…Genetic information from quantitative trait loci (QTLs), sequencing‐based bulked segregant analysis (Seq‐BSA), and RNA-sequencing approaches provides clues for understanding Chinese cabbage leaf and leafy head traits. QTLs have been identified for Chinese cabbage heading traits, such as head diameter, head height, and HWe, which are all important components of head yield and quality 9 , 11 – 13 . Seq-BSA revealed genomic regions enriched with candidate genes related to plant hormones associated with Chinese cabbage heading type and leafy head formation 14 , 15 .…”