Plant organ size is sensitive to environmental conditions, but is also limited by hardwired genetic constraints. In Arabidopsis, a few organ size regulators have been identified. Among them, the BIG BROTHER (BB) gene has a prominent role in the determination of flower organ and leaf size. BB loss-of-function mutations result in a prolonged proliferation phase during leaf(-like) organ formation, and consequently larger leaves, petals and sepals. Whether BB has a similar role in root growth is unknown. Here we describe a novel bb allele which carries a P235L point mutation in the BB RING finger domain. This allele behaves similarly to described bb loss-of-function alleles and displays increased root meristem size due to a higher number of dividing, meristematic cells. In contrast, mature cell length is unaffected. The increased meristematic activity does not, however, translate into overall enhanced root elongation, possibly because bb mutation also results in an increased number of cell files in the vascular cylinder. These extra formative divisions might offset any growth acceleration by extra meristematic divisions. Thus, although BB dampens root cell proliferation, the consequences on macroscopic root growth are minor. However, bb mutation accelerates overall root growth when introduced into sensitized backgrounds. For example, it partially rescues the short root phenotypes of the brevis radix and octopus mutants, but does not complement their phloem differentiation or transport defects. In summary, we provide evidence that BB acts conceptually similarly in leaf(-like) organs and the primary root, and uncouples cell proliferation from elongation in the root meristem.
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