ASYMMETRIC LEAVES1 reveals knox gene redundancy in Arabidopsis

The shoot apical meristem comprises undifferentiated stem cells and their derivatives, which include founder cells for lateral organs such as leaves. Meristem maintenance and lateral organ specification are regulated in part by negative interactions between the myb domain transcription factor ASYMMETRIC LEAVES1, which is expressed in lateral organ primordia, and homeobox transcription factors which are expressed in the shoot apical meristem (knox genes). The knox gene SHOOT MERISTEMLESS (STM) negatively regulates ASYMMETRIC LEAVES1 (AS1) which, in turn, negatively regulates other knox genes including KNAT1 and KNAT2, and positively regulates the novel gene LATERAL ORGAN BOUNDARIES (LOB). Genetic interactions with a second gene, ASYMMETRIC LEAVES2 (AS2), indicate it acts at the same position in this hierarchy as AS1. We have used a second-site suppressor screen to isolate mutations in KNAT1 and we show that KNAT1 is partially redundant with STM in regulating stem cell function. Mutations in KNAT2 show no such interaction. We discuss the regulation and evolution of redundancy among knox genes.