CRABS CLAW, a gene that regulates carpel and nectary development in Arabidopsis, encodes a novel protein with zinc finger and helix-loop-helix domains

Studies of plants with mutations in the CRABS CLAW gene indicate that it is involved in suppressing early radial growth of the gynoecium and in promoting its later elongation. It is also required for the initiation of nectary development. To gain further insight, the gene was cloned by chromosome walking. CRABS CLAW encodes a putative transcription factor containing a zinc finger and a helix-loop-helix domain. The latter resembles the first two helices of the HMG box, known to bind DNA. At least five other genes of Arabidopsis carry the same combination of domains, and we have named them the yabby family. The new helix-loop-helix domain itself we call the yabby domain. Consistent with the mutant phenotype, CRABS CLAW expression is mostly limited to carpels and nectaries. It is expressed in gynoecial primordia from their inception, firstly in lateral sectors where it may inhibit radial growth, and later in the epidermis and in four internal strips. The internal expression may be sufficient to support longitudinal growth, as carpels are longer in a crabs claw promoter mutant where expression is now confined to these regions. The patterns of expression of CRABS CLAW in ectopic carpels of floral homeotic mutants suggest that it is negatively regulated by the A and B organ identity functions, but largely independent of C function. CRABS CLAW expression occurs in nectaries throughout their growth and maturation. It is also expressed in their presumptive anlagen so it may specify cells that will later develop as nectaries. Nectaries arise from the floral receptacle at normal positions in all A, B and C organ identity mutants examined, and CRABS CLAW is always expressed within them. Thus CRABS CLAW expression is regulated independently in carpels and nectaries.