| Anther development, and therefore male fertility, relies on the coordinated differentiation of several adjacent cell types. Establishment of these cell layers likely involves cell-to-cell communication pathways as plants rely more on positional cues than strict cell lineage for their development. Currently, only a few genes are known to be involved in Arabidopsis early anther development, particularly in the establishment of these different cell layers. The SPOROCYTELESS/NOZZLE ( SPL/NZZ) gene functions in early anther development to promote formation of the sporogenous cell type. Subsequently the EXCESS MICROSPOROCYES1/EXTRA SPOROGENOUS CELLS (EMS1/EXS) gene specifies the formation of the tapetum. SPL/NZZ encodes a putative transcription factor, while EMS1/EXS encodes a leucine-rich repeat receptor-like kinase (LRR-RLK) and is thought to function with other known LRR-RLKs to mediate tapetum differentiation. CLAVATA1 (CLV1) belongs to a four member clade of LRR-RLKs including BAM1 (for BARELY ANY MERISTEM1), BAM2 and BAM3. The bam1 bam2 and bam1 bam2 bam3 mutants are male sterile, suggesting that BAM1, BAM2 and BAM3 are important for normal anther development. In addition, recent work has shown that loss of the three ERECTA (ER) genes, ER, ERL1 and ERL2, results in underdeveloped and presumably sterile anthers, indicating that this gene family may also play a role in anther development. The objective of this research was to understand the function of the BAM and ER genes in Arabidopsis anther development. Also, the relationship between SPL/NZZ, EMS1/EXS and the BAM genes in specifying anther cell differentiation was examined. Analyses using molecular markers and cytological techniques shows that bam1 bam2 anthers lack the normal anther somatic cell layers and produce only pollen mother-like cells (PMLs), indicating a very early defect in cell fate specification. Furthermore the bam1 bam2 bam3 triple mutant anther can produce the somatic cell types and BAM3 expression is drastically increased in the bam1 bam2 mutant flowers. This indicates that BAM1/2 mediate a signal to negatively regulate BAM3 expression in order to properly specify the parietal cell type. In addition, the meiotic cells of the bam1 bam2 double and bam1 bam2 bam3 triple mutants degrade, suggesting these cells are defective and BAM1, BAM2 and BAM3 may be important for normal meiosis. Mutant analysis also demonstrates that SPL/NZZ is epistatic to BAM1/2, and BAM1/2 are in turn epistatic to EMS1/EXS. Finally, phenotypic analysis of the ER-family mutants indicates that these genes act redundantly to promote cell proliferation during anther development and to direct normal growth and differentiation of anther cell layers, particularly the tapetum. |
