Hormone autotrophic growth and differentiation identifies mutant lines of Arabidopsis with altered cytokinin and auxin content or signaling

We describe mutant tissue lines of Arabidopsis that are able to grow in vit ro as callus on hormone-free medium. The 14 lines presented here show diffe rent hormone autotrophic differentiation behaviors that can be classified i nto three categories: (a) forming roots (rooty callus), (b) forming shoots or shoot-like structures (shooty callus), or (c) growing without organ form ation (callus). Three fast-growing lines showed altered steady-state mRNA l evels of the Cdc2 and CycD3 cell cycle genes. Three of the six rooty callus lines contained about 20- to 30-fold higher levels of auxins than wild-typ e callus. These and two other lines with normal auxin content showed an inc reased steady-state level of IAA1 and IAA2 transcripts in the absence of ex ogenous auxin. Five of the six shooty callus lines had increased steady-sta te mRNA levels of the CK11 gene and/or of the homeobox genes KNAT1 and STM, suggesting that the phenotype is linked to altered cytokinin signaling. Al so, one cytokinin-overproducing line with only 5% of wild-type cytokinin ox idase activity was identified. These results indicate that screening for ho rmone-autonomous growth identifies mutants with altered hormone content or signaling.