In Arabidopsis, a mutant in carpel morphogenesis named CRABS CLAW (CRC), causes the gynoecium to develop into a wider but shorter structure, and the two carpels are unfused at the apex (Alvarez et al., 1999).  The CRC protein was found to belong to a small family of C2C2 type transcription factors with a conserved C2C2 domain (PF04690) and that are plant specific (Siegfried et al., 1999).

In maize, the CRC ortholog indeterminate floral apex1 (ifa1, ZmYAB2, Zm00001eb008680) regulates meristem determinacy. The defect found in ifa1 mutants is specific to meristems and does not affect lateral organs (Laudencia-Chingcuanco et al., 2002).  Later it was found that ifa1 is allelic with drooping leaf1 (drl1) (Strable et al., 2017). The co-orthologs drooping leaf1 (drl1, ZmYAB2, Zm00001eb008680) and drl2 (ZmYAB7, Zm00001eb400130) are required for development of ear and tassel florets (Strable et al., 2019).  Pistillate florets of drl1 ears are sterile with unfused carpels that fail to enclose an expanded nucellus-like structure. Staminate florets of drl1 tassels have extra stamens and fertile anthers. Gene regulatory network inference, genetic interaction and expression analyses suggest that DRL1 and ZAG1 target each other and a common set of downstream genes that function during floret development, thus defining a regulatory module that fine-tunes floret patterning and FM determinacy (Strable et al., 2019).

During the domestication of maize from its wild ancestor (teosinte), an allelic substitution at teosinte branched (tb1) caused changes in both plant and ear architecture. The effects of tb1 on phenotype were shown to depend on multiple background loci, including one called enhancer of tb1.2 (etb1.2). etb1.2 was mapped to a YABBY class transcription factor (ZmYAB2.1, GRMZM2G085873, ZmYAB12) (Yang et al., 2016). Tb1 and Etb1.2 interact epistatically to control the length of internodes within the maize ear, which affects how densely the kernels are packed on the ear. The interaction effect is also observed at the level of gene expression, with TB1 acting as a repressor of ZmYAB2.1 expression.

In Arabidopsis, Lateral organs produced by shoot apical and flower meristems exhibit a fundamental abaxial-adaxial asymmetry. At least three members of the YABBY gene family that bear homology to CRC are expressed in a polar manner in all lateral organ primordia produced from the apical and flower meristems. Their expression of these genes is precisely correlated with abaxial cell fate in mutants in which abaxial cell fates are found ectopically reduced or eliminated. Members of this gene family are responsible for the specification of abaxial cell fate in lateral organs of Arabidopsis. Furthermore ectopic expression studies suggest that ubiquitous abaxial cell fate and maintenance of a functional apical meristem are incompatible (Siegfried et al., 1999).

Last updated June 2023 by John Gray

References:

Alvarez J, Smyth DR. CRABS CLAW and SPATULA, two Arabidopsis genes that control carpel development in parallel with AGAMOUS. Development. 1999 Jun;126(11):2377-86. doi: 10.1242/dev.126.11.2377. PMID: 10225997.

Siegfried KR, Eshed Y, Baum SF, Otsuga D, Drews GN, Bowman JL. Members of the YABBY gene family specify abaxial cell fate in Arabidopsis. Development. 1999 Sep;126(18):4117-28. doi: 10.1242/dev.126.18.4117. PMID: 10457020.

Bowman JL, Eshed Y, Baum SF. Establishment of polarity in angiosperm lateral organs. Trends Genet. 2002 Mar;18(3):134-41. doi: 10.1016/s0168-9525(01)02601-4. PMID: 11858837.

Laudencia-Chingcuanco D, Hake S. The indeterminate floral apex1 gene regulates meristem determinacy and identity in the maize inflorescence. Development. 2002 Jun;129(11):2629-38. doi: 10.1242/dev.129.11.2629. PMID: 12015291.

Strable J, Wallace JG, Unger-Wallace E, Briggs S, Bradbury PJ, Buckler ES, Vollbrecht E. Maize YABBY Genes drooping leaf1 and drooping leaf2 Regulate Plant Architecture. Plant Cell. 2017 Jul;29(7):1622-1641. doi: 10.1105/tpc.16.00477. Epub 2017 Jul 11. PMID: 28698237; PMCID: PMC5559738.

Strable J, Vollbrecht E. Maize YABBY genes drooping leaf1 and drooping leaf2 regulate floret development and floral meristem determinacy. Development. 2019 Mar 26;146(6):dev171181. doi: 10.1242/dev.171181. PMID: 30858227.

Yang CJ, Kursel LE, Studer AJ, Bartlett ME, Whipple CJ, Doebley JF. A Gene for Genetic Background in Zea mays: Fine-Mapping enhancer of teosinte branched1.2 to a YABBY Class Transcription Factor. Genetics. 2016 Dec;204(4):1573-1585. doi: 10.1534/genetics.116.194928. Epub 2016 Oct 11. PMID: 27729422; PMCID: PMC5161286.