Cytokinin overproducing ove mutants of Physcomitrella patens show increased riboside to base conversion

Ove mutants in the moss Physcomitrella patens can arise from different rece ssive mutations. These mutants display a much larger number of buds than th e wild type (wt) due to a dramatic overproduction of cytokinins (Cks), whic h are released into the culture medium (T.L. Wang, R. Horgan, D.J. Cove [19 81] Plant Physiol 68: 735-738). The amounts of isopentenyladenine (iP) and isopentenyladenosine ([9R]iP) produced by chloronema of different ove mutan ts were measured. Levels of the major Ck iP in the culture medium of the mu tants oveA78, oveA201, oveC200, and oveB300 (cultured at 21 degreesC) were 4-fold (oveA78) to 22-fold (oveB300) higher than for the wt. A new temperat ure-sensitive ave strain oiieST25, which exhibits a strong ove phenotype at 25 degreesC, was also studied. It produced about 260 times more iP than th e thiamine auxotrophic wt from which it was derived. To contribute to the p hysiological understanding of Ck overproduction, in vivo labeling experimen ts with H-3-[9R]iP were performed. In all ove mutants analyzed, the rate of H-3-[9R]iP conversion to H-3-iP was higher as compared with the wt. In ove ST25, the 3-fold increased riboside to base conversion was temperature indu cible and correlated with the iP production. Analysis of Ck catabolism reve aled no major differences between ove mutants and wt, thus indicating that ove mutants are unlikely to be degradation mutants. The data suggest that i n ove mutants the increased riboside to base conversion is part of a genera lly up-regulated Ck biosynthetic pathway and may play an important role for the enhanced release of iP into the medium.