Down-Regulating the Expression of 53 Soybean Transcription Factor Genes Uncovers a Role for SPEECHLESS in Initiating Stomatal Cell Lineages during Embryo Development  

Abstract

We used an RNA interference screen to assay the function of 53 transcription factor messenger RNAs (mRNAs) that accumulate specifically within soybean (Glycine max) seed regions, subregions, and tissues during development. We show that basic helix-loophelix (bHLH) transcription factor genes represented by Glyma04g41710 and its paralogs are required for the formation of stoma in leaves and stomatal precursor complexes in mature embryo cotyledons. Phylogenetic analysis indicates that these bHLH transcription factor genes are orthologous to Arabidopsis (Arabidopsis thaliana) SPEECHLESS (SPCH) that initiate asymmetric cell divisions in the leaf protoderm layer and establish stomatal cell lineages. Soybean SPCH (GmSPCH) mRNAs accumulate primarily in embryo, seedling, and leaf epidermal layers. Expression of Glyma04g41710 under the control of the SPCH promoter rescues the Arabidopsis spch mutant, indicating that Glyma04g41710 is a functional ortholog of SPCH. Developing soybean embryos do not form mature stoma, and stomatal differentiation is arrested at the guard mother cell stage. We analyzed the accumulation of GmSPCH mRNAs during soybean seed development and mRNAs orthologous to MUTE, FAMA, and INDUCER OF C-REPEAT/DEHYDRATION RESPONSIVE ELEMENT-BINDING FACTOR EXPRESSION1/SCREAM2 that are required for stoma formation in Arabidopsis. The mRNA accumulation patterns provide a potential explanation for guard mother cell dormancy in soybean embryos. Our results suggest that variation in the timing of bHLH transcription factor gene expression can explain the diversity of stomatal forms observed during plant development.

Leveraging RNA-Seq to Characterize Resistance to Brown Stem Rot and the Rbs3 Locus in Soybean

“…Glyma.05G189400, homologous to AtTSK, is required for cell cycle progression (Suzuki et al 2005). Glyma.04G238400 and Glyma.06G125500, homologous to AtSPCH, initiate asymmetric cell divisions in leaves and are essential for stomatal development (Danzer et al 2015). AtCHR17, AtDDM1, AtTSK, and AtSPCH are all repressed in leaves in T7.…”

Does Molecular and Structural Evolution Shape the Speedy Grass Stomata?

“…Overexpression of aquaporin in tomatoes (Sade et al, 2010) and grapevines (Perrone et al, 2012) increased WUE under both optimal and water stress conditions. For stomatal development, it was shown that bHLHs are required for the development of soybean stomata, revealing the relationship between the accumulation sequence of GmSMFs and the initial growth stage of mature GCs (Danzer et al, 2015). Despite the success in breeding drought tolerant crops, most horticultural crops and cash crops are eudicots that do not have stomata similar to those in the crops of the grass family.…”

Too many faces for TOO MANY MOUTHS?

“…Stomatal lineage pausing takes place in various natural contexts, either developmentally programmed (as in soybean (Glycine max) seed cotyledons; Danzer et al, 2015) or in response to environmental cues (such as osmotic stress; Skirycz et al, 2011), or both (as described for A. thaliana seedlings kept in the dark; Kang et al, 2009). These findings suggest that stomatal lineage initiation (the still poorly understood selection of MMCs from the protoderm and their first asymmetric divisions) occurs easily, perhaps under a permissive or even a default programme, and that it does not seem to require TMM because it takes place in tmm mutants.…”