TheArabidopsisBEL1-LIKE HOMEODOMAIN Proteins SAW1 and SAW2 Act Redundantly to RegulateKNOXExpression Spatially in Leaf Margins

Abstract

In Arabidopsis thaliana, the BEL1-like TALE homeodomain protein family consists of 13 members that form heterodimeric complexes with the Class 1 KNOX TALE homeodomain proteins, including SHOOTMERISTEMLESS (STM) and BREVIPE-DICELLUS (BP). The BEL1-like protein BELLRINGER (BLR) functions together with STM and BP in the shoot apex to regulate meristem identity and function and to promote correct shoot architecture. We have characterized two additional BEL1-LIKE HOMEODOMAIN (BLH) proteins, SAWTOOTH1 (BLH2/SAW1) and SAWTOOTH2 (BLH4/SAW2) that, in contrast with BLR, are expressed in lateral organs and negatively regulate BP expression. saw1 and saw2 single mutants have no obvious phenotype, but the saw1 saw2 double mutant has increased leaf serrations and revolute margins, indicating that SAW1 and SAW2 act redundantly to limit leaf margin growth. Consistent with this hypothesis, overexpression of SAW1 suppresses overall growth of the plant shoot. BP is ectopically expressed in the leaf serrations of saw1 saw2 double mutants. Ectopic expression of Class 1 KNOX genes in leaves has been observed previously in loss-of-function mutants of ASYMMETRIC LEAVES (AS1). Overexpression of SAW1 in an as1 mutant suppresses the as1 leaf phenotype and reduces ectopic BP leaf expression. Taken together, our data suggest that BLH2/SAW1 and BLH4/SAW2 establish leaf shape by repressing growth in specific subdomains of the leaf at least in part by repressing expression of one or more of the KNOX genes.

KNOXLost theOX: TheArabidopsis KNATMGene Defines a Novel Class of KNOX Transcriptional Regulators Missing the Homeodomain

“…SAW1 and SAW2 BELL genes are expressed in hydathodes, and the double mutant has increased leaf serration (Kumar et al, 2007). These observations suggest that KNATM-B might sequester SAW1 and SAW2 and shape wild-type leaf lamina by controlling free SAW1 SAW2 dosage at the hydathodes.…”

“…In addition, leaf serration observed in 35S-KNATM-B plants resembles the saw1 saw2 double mutant (Kumar et al, 2007). To investigate their genetic interaction, we generated transgenic lines overexpressing both SAW1 and KNATM-B.…”

“…To investigate their genetic interaction, we generated transgenic lines overexpressing both SAW1 and KNATM-B. 35S-SAW1 plants show reduced size and male sterility ( Figure 6B) (Kumar et al, 2007). The double overexpression of KNATM-B and SAW1 fully suppresses both single overexpression phenotypes, resulting in a wild-type-looking plant ( Figure 6).…”

“…ARABI-DOPSIS THALIANA HOMEOBOX1 also controls floral competency by transcriptionally activating the flower repressor gene FLOWERING LOCUS C (Proveniers et al, 2007). SAWTOOTH1 (SAW1) and SAW2 negatively regulate BP, and the saw1 saw2 double mutant has increased leaf serration (Kumar et al, 2007).…”

Population genomics of the eastern cottonwood (Populus deltoides)

“…This gene (Potri.007G032700, POPTR_0007s12170), a Tajima's D outlier in the West‐K2 subpopulation, also was identified as a selection candidate in P. trichocarpa by four scans performed in that species. This gene is orthologous to the gene AT4G36870 that encodes the BEL1‐LIKE HOMEODOMAIN 2 (BLH2) transcription factor also known as SAWTOOTH 1 (SAW1), a negative regulator of growth involved in leaf margin development in A. thaliana (Kumar et al., 2007). Finally, three genes were identified by two selection scans in each species.…”