Cytogenetic characterization of induced tetraploids in medicinal plant (Platycodon grandiflorus)

Platycodon grandiflorus is an important medicinal plant in China and its root has been used as medicine or food for centuries. Polyploids are usually valuable because they exhibit increased biomass and contain effective medicinal compounds. Polyploidy induction was successfully achieved in this plant in a previous study by apical shoot tip treatment of young seedlings for 72 h using semi-solid 0.05% colchicine. Polyploidy was further confirmed by meiosis behavior of pollen mother cells in this study. The meiosis of diploid controls is regular, with normal chromosome pairing, and diploids were identified with 2n = 2x = 18 chromosomes at metaphaseI, nine bivalents at diakinesis and 9-9 separation at meiosis I, then differentiating into tetrads that subsequently give rise to mature pollen grains. Induced tetraploids were cytogenetically distinguished from diploids by the occurrence of 36 chromosomes at diakinesis with different combinations of univalents, bivalents, trivalents, quadrivalents, and multivalents at metaphase I and insynchronous chromosome segregation at anaphase II. Other abnormalities were observed at anaphase I in induced tetraploids as laggard chromosomes and some chromosome bridges. The univalents, unmatched chromosomes and lagging chromosomes meant that induced tetraploids were a meiotically unstable species with low pollen viability; this gives further proof of their polyploidy. Therefore, it is helpful to confirm the ploidy level by studying the meiosis of pollen mother cells.