Unique Tissue‐Specific Cell Cycle in Physcomitrella

Abstract

Abstract: The moss Physcomitrella patens (Hedw.) B.S.G. is a novel tool in plant functional genomics as it has an inimitable high gene targeting efficiency facilitating the establishment of gene/function relationships.Here we report, based on flow cytrometric (FCM) data, that the basic nuclear DNA content per cell of Physcomitrella is 0.53 pg, equating to a genome size of 1 C = 511 Mbp. Furthermore, we describe a unique tissue‐specific cell cycle change in this plant. Young plants consisting of only one cell type (chloronema) displayed one single peak of fluorescence in FCM analyses. As soon as the second cell type (caulonema) developed from chloronema, a second peak of fluorescence at half the intensity of the previous one became detectable, indicating that caulonema cells were predominantly at the G1/S transition, whereas chloronema cells were mainly accumulating at the G2/M transition. This conclusion was validated by further evidence: i) The addition of ammonium tartrate arrested Physcomitrella in the chloronema state and in G2/M. ii) Two different developmental mutants, known to be arrested in the chloronema/caulonema transition, remained in G2/M, regardless of age and treatment. iii) The addition of auxin or cytokinin induced the formation of caulonema, as well as decreasing the amount of cells in G2/M phase. Additionally, plant growth regulators promoted endopolyploidisation.Thus, cell cycle and cell differentiation are closely linked in Physcomitrella and effects of plant hormones and environmental factors on both processes can be analysed in a straight forward way. We speculate that this unique tissue‐specific cell cycle arrest may be the reason for the uniquely high rate of homologous recombination found in the Physcomitrella nuclear DNA.