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.

中文翻译：

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在韧皮部中表达的番茄毛MERISTEM4是正常芽和果实发育所必需的


毛茸茸的分生组织 （HAM） 基因家族编码植物中的 I 型和 II 型 GRAS 结构域转录调节因子。II 型 HAM 主要在分生组织中表达并受 microRNA171 调节，对于维持未分化的分生组织至关重要，这种作用在各种物种中都是保守的。相反，I 型 HAM 的功能特征较少。在这项研究中，我们研究了 I 型 HAM SlHAM4 在番茄中的作用。CRISPR 诱导的 SlHAM4 功能丧失突变 （slham4CR） 导致芽和果实异常，通过将由其天然启动子驱动的 SlHAM4 重新引入突变背景，这些异常被完全逆转。突变异常包括更简单的叶子和叶子和萼片原基中的花青素色素沉着增加，这让人想起在某些韧皮部受损的拟南芥突变体中观察到的表型。此外，slham4CR 植物产生的果实明显更小，其中一部分形成类似猫脸的疤痕，这归因于坐果果皮中发生的撕裂。使用由天然 SlHAM4 启动子驱动的 GUS 报告基因，我们发现 SlHAM4 主要在韧皮部组织中表达。与此一致，突变花序卵巢的转录组分析揭示了与韧皮部发育和功能有关的基因的特异性下调，特别是在伴侣细胞中表达的基因。然而，组织学分析显示韧皮部脉管系统无明显异常。综上所述，我们的数据表明，SlHAM4 可能通过调节韧皮部功能所必需的基因在芽和果实发育中发挥作用。