The conserved cysteine-rich domain of a tesmin/TSO1-like protein binds zinc in vitro and TSO1 is required for both male and female fertility in Arabidopsis thaliana

Development of reproductive tissue and control of cell division are common challenges to all sexually reproducing eukaryotes. The Arabidopsis thaliana TSO1 gene is involved in both these processes. Mild tso1 mutant alleles influence only ovule development, whereas strong alleles have an effect on all floral tissues and cause cell division defects. The tso1 mutants described so far carry point mutations in a conserved cysteine-rich domain, the CRC domain, but the reason for the range of phenotypes observed is poorly understood. In the present study, the tesmin/TSO1-like CXC (TCX) proteins are characterized at the biochemical, genomic, transcriptomic, and functional level to address this question. It is shown that the CRC domain binds zinc, offering an explanation for the severity of tso1 alleles where cysteine residues are affected. In addition, the phylogenetic and expression analysis of the TCX genes suggested an overlap in function between AtTSO1 and the related gene AtTCX2. Their expression ratios indicated that pollen, in addition to ovules, would be sensitive to loss of TSO1 function. This was confirmed by analysis of novel tso1 T-DNA insertion alleles where the development of both pollen and ovules was affected.