Physiological properties and chlorophyll biosynthesis in a Pak-choi (Brassica rapa L. ssp. chinensis) yellow leaf mutant, pylm

The chlorophyll (Chl)-deficient mutant pylm is a double haploid (DH) line of Pak-choi (Brassica rapa L. ssp. chinensis) that was obtained from Pak-choi variety ‘Huaguan’ by isolated microspore culture. In this study, the aim was to examine the physiological properties and Chl biosynthesis in the pylm mutant. Compared to the control line ‘CK-51’, pylm is slender, with an earlier flowering phenotype and obviously elongated hypocotyls. Normally grown pylm seedlings had yellow leaves with reduced total Chl content but an increased Chl a/b ratio. Ultrastructural analysis showed that the pylm mutant has a loose grana lamellae structure and few thylakoid stacks, resulting in impaired chloroplast development. Measurement of photosynthetic gas exchange and Chl fluorescence parameters indicated that the pylm mutant has lower photosynthetic activity and photochemical conversion efficiency than ‘CK-51’ plants. We investigated the biochemical mechanism of Chl deficiency in pylm by examining Chl biosynthesis precursors in this mutant, showing that Chl biosynthesis is blocked in the mutant at the Chl a production step. In addition, the expression of a number of genes related with Chl biosynthesis is down-regulated in pylm. Therefore, these results provide a basis for understanding the physiological and biochemical causes of the mutant phenotype of pylm.