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

## TL;DR

This study shows that the tomato gene SlHAM4, active in phloem tissues, is important for normal shoot and fruit development.

## Contribution

The study identifies SlHAM4 as a Type I HAM gene involved in phloem-related development in tomato.

## Key findings

- SlHAM4 loss-of-function mutations caused leaf and fruit abnormalities.
- SlHAM4 is predominantly expressed in phloem tissues.
- Mutant plants showed downregulation of phloem-related genes.

## 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.