The non-tandem CCCH zinc finger protein AtC3H26 modulates salt stress response and phosphate starvation response through its ribonuclease activity in Arabidopsis

CCCH-type zinc finger (ZF) proteins comprise a diverse family of RNA- and DNA-binding regulators that play crucial roles in plant growth, development, and stress responses. Although tandem CCCH ZF (TZF) proteins have been extensively studied, their molecular functions remain poorly understood. In this study, Arabidopsis thaliana AtC3H26, a non-TZF CCCH protein and paralog of AtC3H3, was characterized, and its role as a ribonuclease (RNase) in salt tolerance and phosphate (Pi) homeostasis was demonstrated. AtC3H26 expression was rapidly induced by salt, mannitol, and abscisic acid (ABA), and its overexpression (OX) lines exhibited enhanced salt tolerance but not drought resistance. Moreover, AtC3H26 modulated both ABA-dependent and -independent stress marker genes, which suggests that both ABA-dependent and -independent pathways contribute to salt tolerance in AtC3H26 OX lines. In vitro assays confirmed that AtC3H26 degraded RNA substrates in a dose-dependent manner and mRNA-sequencing of OX lines revealed substantial upregulation of Pi starvation-responsive genes, including SPX1, PS2/PECP2, and SRG3/GDPD1. In contrast, small RNA-sequencing identified the downregulation of miR399 and miR827, which are involved in the Pi starvation response. AtC3H26 OX plants accumulated higher Pi levels than those in wild-type plants, thereby suggesting that AtC3H26 modulates Pi accumulation. Collectively, these findings establish AtC3H26 as a novel cytoplasmic RNase in Arabidopsis that coordinates RNA turnover with stress and nutrient signaling.