Minisymposium 22: Development

Abs # 36004: The developmental role of genes that are expressed between the shoot apical meristem and lateral organ primordia

Presenter:	Springer, Patricia S., patricia.springer@ucr.edu
Authors	Springer, Patricia S. (A)   Lin, Wan-ching  (A)   Shuai, Bin  (A)   Geisler, Matt  (A)   Lee, Dong-keun  (A)
Affiliations:	(A): University of California, Riverside

The normal development of leaves and other lateral organs requires the simultaneous repression of meristem-specific genes and the activation of organ-specific genes. This process must therefore involve communication between the shoot apical meristem (SAM) and initiating lateral organ primordia. We have identified several Arabidopsis genes that are expressed in a domain between the SAM and initiating lateral organs, in a pattern suggestive of a role in communication between the two groups of cells. The expression of one of these genes, LATERAL ORGAN BOUNDARIES (LOB) is positively regulated both by KNOX genes in the SAM and by genes that are expressed in lateral organ primordia. These observations are consistent with the hypothesis that LOB plays a role in communication between the two domains. LOB encodes a novel, plant-specific protein that defines a large gene family in Arabidopsis. Loss-of-function mutations in LOB do not result in morphological phenotypes, so as yet, the biological role of LOB is unknown. ASYMMETRIC LEAVES2 (AS2), a gene that is required both for the development of normal leaf shape and for the repression of KNOX genes in the leaf, encodes a LOB-domain protein that is closely related to LOB. Transgenic plants that ubiquitously express AS2 under the control of the 35S promoter produce cotyledons and leaves that curl upward (toward the adaxial side), develop outgrowths on the abaxial surface, and undergo reduced blade expansion. 35S:AS2 leaves have defects in vascular patterning and exhibit abnormal abaxial-adaxial polarity. KNOX genes are also down-regulated in 35S:AS2 plants. The related gene LOB DOMAIN36 is also expressed at the adaxial base of lateral organs, and ectopic expression causes spectrum of phenotypes similar to 35S:AS2 plants, including the down-regulation of KNOX genes. These data suggest that a sub-class of LOB-DOMAIN genes function to regulate KNOX gene expression and adaxial-abaxial polarity in lateral organ development. We have also identified two MYB-domain genes that are expressed in a "LOB-like" pattern. Both MYBs are negatively regulated by the KNOX gene BREVIPEDICELLUS. Ectopic expression of one of the MYB genes results in floral organ defects such as incomplete carpel fusion and the production of ectopic ovules, phenotypes that suggest polarity defects. Together our results provide evidence that genes expressed between the SAM and lateral organs play a role in the regulation of adaxial-abaxial polarity.