Molecular Soil Biology 2025, Vol.16, No.6, 287-296 http://bioscipublisher.com/index.php/msb 290 focused on root hair traits, including root hair length and density. This analysis identified 18 new genomic regions. These regions mainly influence root hair density. They do this by controlling epidermal cell differentiation, rather than changing cell size. Most of the candidate genes are involved in basic developmental processes. Many of these genes have not been reported before in studies on rice or Arabidopsis (Hanlon et al., 2023). 4 Hormonal and Signaling Pathways Linking Roots to Soil Conditions 4.1 Auxin-mediated signaling pathways in root growth and soil sensing Under low phosphorus supply conditions, using the DR5 reporter system and IAA content measurement, the auxin response in the root hair zone was significantly enhanced; the auxin signal originates from the root tip and is transported to the differentiation zone via OsAUX1, driving root hair elongation to increase surface area and phosphorus acquisition from the topsoil layer. The osaux1 mutant showed significantly shorter root hairs and reduced phosphorus absorption under low phosphorus conditions (Giri et al., 2018). In the IAA13 dominant inhibitory mutant, both the number of lateral roots and the aerenchyma formed by cortical cell lysis were significantly reduced; further yeast two-hybrid and ChIP analyses showed that IAA13 interacts with ARF19, which directly activates downstream genes such as LBD1-8, which are highly expressed in the cortex and lateral root primordia. Inactivating mutations or inhibiting auxin transport simultaneously reduced the frequency of aerenchyma and lateral root formation (Yamauchi et al., 2019). 4.2 Roles of ABA, cytokinin, and ethylene in soil stress-responsive root development Using ABA biosynthesis mutants and chemical inhibitors, it was found that ethylene-induced radial expansion of the root cortex was significantly reduced in the absence of ABA, and the root tip became thinner and more easily penetrated compacted layers; exogenous ABA treatment restored cortical expansion without changing ethylene levels (Huang et al., 2022). At low concentrations, ABA treatment can maintain the size of the root apical meristem to some extent, while at high concentrations or in compacted environments where endogenous ABA levels are elevated, it inhibits cell elongation and promotes radial expansion, leading to shorter main roots and a "swollen root" phenotype (Qin et al., 2022). The bZIP transcription factor OsbZIP46, downstream of the ABA signal, directly binds to the OsYUC8 promoter to activate its transcription. After blocking auxin synthesis, the inhibitory effect of ABA on main root elongation was significantly weakened, and the root diameter decreased. The roots of mhz4 and other ABA-deficient mutants are largely insensitive to ethylene treatment, with the primary root maintaining a length close to that of the control in the presence of ethylene; however, supplementing with ABA restores the inhibitory effect of ethylene on root elongation. Ethylene treatment induces the transcription of ABA synthesis genes such as MHZ4 and increases ABA content in the roots, forming an "ethylene → ABA → root growth inhibition" signaling cascade (Ma et al., 2014). Under drought conditions, CRISPR/Cas9 knockout of OsERA1 enhances ABA signaling sensitivity, leading to stronger expression of ABA-induced genes in the roots during osmotic stress treatment, but these mutants have longer primary roots under non-stress conditions (Ogata et al., 2020). 4.3 Crosstalk between hormonal signaling and transcriptional regulation of root development genes Abscisic acid (ABA) and ethylene both affect the auxin synthesis gene OsYUC8, which is also called REIN7. ABA controls OsYUC8 through OsbZIP46. Ethylene works through OsEIL1. Because of this regulation, the level of indole-3-acetic acid (IAA) increases in the root tip. When root growth is inhibited, the osyuc8 mutant does not respond to either ABA or ethylene. Even when the amount of ABA or ethylene is very low, plants that overexpress OsYUC8 still develop short and thick roots (Huang et al., 2022). Ethylene also affects crown root formation through gene regulation. It activates WOX11 expression by OsEIL1. After that, WOX11 combines with the demethylase JMJ706. This complex is recruited to the LBD16 promoter. There, it removes the H3K9me2 mark and turns on LBD16 expression, which helps crown roots form. When LBD16 protein builds up, it binds back to WOX11. This binding prevents WOX11 from forming a complex with JMJ706, creating a negative feedback loop (Dabravolski and Isayenkov, 2025; Das et al., 2025). The shi1 mutant shows a typical root system with low auxin levels and is more sensitive to ABA. OsSHI1 promotes the production of auxin and brassinosteroids. It does this by activating OsYUCCA genes and the BR synthesis gene D11. At the same time, OsSHI1 induces OsNAC2, which suppresses ABA signaling. In this way, OsSHI1 helps balance root growth and drought resistance. This
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