MS23, a master basic helix-loop-helix factor, regulates the specification and development of tapetum in maize

Abstract

Successful male gametogenesis involves orchestration of sequential gene regulation for somatic differentiation in pre-meiotic anthers. We report here the cloning of Male Sterile23 (Ms23), encoding an antherspecific predicted basic helix-loop-helix (bHLH) transcription factor required for tapetal differentiation; transcripts localize initially to the precursor secondary parietal cells then predominantly to daughter tapetal cells. In knockout ms23-ref mutant anthers, five instead of the normal four wall layers are observed. Microarray transcript profiling demonstrates a more severe developmental disruption in ms23-ref than in ms32 anthers, which possess a different bHLH defect. RNA-seq and proteomics data together with yeast two-hybrid assays suggest that MS23 along with MS32, bHLH122 and bHLH51 act sequentially as either homo-or heterodimers to choreograph tapetal development. Among them, MS23 is the earliest-acting factor, upstream of bHLH51 and bHLH122, controlling tapetal specification and maturation. By contrast, MS32 is constitutive and independently regulated and is required later than MS23 in tapetal differentiation.

What Does the Molecular Genetics of Different Types of Restorer-of-Fertility Genes Imply?

“…The identity of the S-orf in CMS-C mitochondrial genomes is unknown, although three candidates have been found (discussed by Allen et al [50]), making the mechanism for how Rf4 restores pollen fertility more elusive. Maize Rf4 is an allele of Ms23 that was originally identified as playing an essential role in the differentiation of tapetal tissue in anthers [51]. Null alleles of ms23 cause male sterility even in N cytoplasm plants, whereas the rf4 allele (inducing male sterility in CMS-C but male fertile in N cytoplasm) has an amino acid substitution that is potentially harmful to the stability of a heterodimer composed of RF4 and another protein bHLH51 [43].…”

Molecular Cloning of Genic Male-Sterility Genes and Their Applications for Plant Heterosis via Biotechnology-based Male-sterility Systems

“…TIP2 is the rice ortholog of Ms23, whereas maize bHLH122 is the ortholog of rice EAT1. Maize bHLH51, rice TDR, and Arabidopsis AMS fall in the same clade, while maize Ms32, rice UDT1, and Arabidopsis DYT1 fall in the same clade [15]. These results not only confirm the function of Ms23 and Ms32 in regulating male sterility but also predict that their paralogs bHLH122 and bHLH51 may be involved in male sterility, and this needs to be confirmed by targeted mutagenesis analysis and/or other strategies.…”

Cloning, molecular evolution and functional characterization of ZmbHLHl6, the maize ortholog of OsTIP2 (OsbHLH142)

“…Coincidently, it was recently reported that ZmbHLH16 was a candidate gene for the ms23 mutant (Nan et al, 2017). The tapetal layer of the ms23 mutant undergoes abnormal periclinal division instead of tapetal differentiation (Chaubal et al, 2000).…”

“…These results strongly support the hypothesis that ZmbHLH16 is involved in tapetal specification and maturation. In Nan’s paper (Nan et al, 2017), the researchers mainly focused on the abortion mechanism in the ms23 mutant, combining RNA-seq with proteomics data. These authors also confirmed the interaction between ZmbHLH16 and ZmbHLH51, similar to our result.…”