Tomato HAIRY MERISTEM4, expressed in the phloem, is required for proper shoot and fruit development

Abstract

The HAIRY MERISTEM (HAM) gene family encodes Type I and II GRAS domain transcriptional regulators in plants. Type II HAMs, predominantly expressed in meristems and regulated by microRNA171, are essential for maintaining undifferentiated meristems, a role conserved across various species. Conversely, the functions of Type I HAMs have been less characterized. In this study, we investigated the role of SlHAM4, a Type I HAM in tomato. CRISPR-induced SlHAM4 loss-of-function mutations (slham4CR) resulted in shoot and fruit abnormalities, which were fully reversed by reintroducing SlHAM4, driven by its native promoter, into the mutant background. Mutant abnormalities included simpler leaves and increased anthocyanin pigmentation in the leaf and sepal primordia, reminiscent of phenotypes observed in certain Arabidopsis mutants with compromised phloem. In addition, slham4CR plants produced significantly smaller fruits with a subset developing catface-like scars, attributed to tears that occurred in the pericarp of setting fruits. Using a GUS reporter gene driven by the native SlHAM4 promoter, we found that SlHAM4 is predominantly expressed in phloem tissues. Consistent with this, transcriptome analysis of mutant anthesis ovaries revealed specific downregulation of genes implicated in phloem development and function, particularly those expressed in companion cells. However, histological analysis showed no obvious abnormalities in phloem vasculature. Taken together, our data suggest that SlHAM4 plays a role in shoot and fruit development likely by regulating genes essential for phloem function.

Acknowledgements

We would like to thank Prof. Shmuel Wolf for providing the seeds of the AtSUC2::GFP tomato line. We also extend our gratitude to Dr Amir Sherman and Dr Ada Rozen for their assistance in genotyping the slham4^CR mutants, and to Dr Adi Faigenboim for her help in analyzing the RNA-seq results.This work was supported by the ISF research grant 939/12 to T.A.

Author contributions

A.P.V., O.G., S.C., and J.K., characterized the slham4^CR mutants. A.P.V. generated and characterized the 35S:SlHAM4 transgenic plants. S.K.G. preformed the RNA-seq. J.K. generated and characterized the SlHAM4pro::GUS and SlHAM4pro::SlHAM4-GFP transgenic plants and assisted in manuscript writing. T.A., Conceptualization, planned the experiments, supervised the work, annotated the RNA-seq data and wrote the manuscript.

Data availability

The RNA-seq data is available from the SRA database under the accession number PRJNA682014. The candidate DEGs and SlHAM4 expression profiles in pericarp cell types tissue across the stages (5DPA to RR) of tomato fruit were retrieved from the Tomato Expression Atlas database (http://tea.solgenomics.net/) [47].

Conflict of interest statement

The authors declare no conflicts of interest.

Supplementary data

Supplementary data is available at Horticulture Research Journal online.

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