Roots or root-like structures are key adaptive traits of plants in terrestrial environments and are essential for plant survival on land. Roots not only mediate many biotic and abiotic interactions, but their structure is also an important factor in determining yield under normal and stressed (especially drought) conditions. Understanding the factors that control root growth is essential for a comprehensive understanding of plant development and adaptive responses. The overall growth rate of roots is determined by the balance of three basic processes: cell proliferation activity in the root apical meristem (RAM), cell differentiation, and cell elongation after leaving the RAM. These processes are known to be regulated by plant hormones, including cytokinins and ethylene. Cytokinins both promote and inhibit the size and activity of the root apical meristem by enhancing stem cell proliferation in the root apical meristem and inducing differentiation of root transition zone cells. Ethylene affects root growth mainly by inhibiting root cell elongation. The two hormones interact in biosynthesis and signaling, and cytokinins have been found to affect root growth by regulating ethylene synthesis and signaling. However, the role of cytokinins and ethylene in spatial specificity in root growth regulation is not well understood.
On July 3, 2024, Jan Hejatko's team from Masaryk University published a research paper titled "Cytokinins regulate spatially-specific ethylene production to control root growth in Arabidopsis" in Plant Communications, revealing how cytokinins regulate spatially-specific ethylene production in plant roots, thereby affecting plant root growth and development. These findings reveal the complex regulatory network of plant hormone signaling pathways in root growth, providing important clues for our understanding of the molecular mechanisms of plant growth and development.
To evaluate potential cell types for cytokinin-induced root elongation, this study first specifically expressed the cytokinin biosynthetic gene ISOPENTENYLTRANSFERASE (IPT) in different cell layers of the root through the GAL4>>UAS activator–reporter system. The results showed that upregulation of cytokinins in root external tissues (epidermis and cortex) significantly inhibited root growth and promoted root hair formation, and these effects could be inhibited by the ethylene biosynthesis inhibitor AVG. In contrast, upregulation of IPT in root internal tissues (primordial/phloem) had no significant effect on root length and induced a weak induction of root hair formation. Upregulation of IPT both outside and inside the root resulted in a reduction in RAM size and an increase in endogenous ACC (ethylene precursor) levels, but the effect was more pronounced outside the root.
Further analysis showed that plants overexpressing the negative regulator of ethylene signaling, EIN3-BINDING F BOX PROTEIN 2 (EBF2), in the outer root tissues exhibited insensitivity to cytokinin, whereas plants overexpressing EBF2 in internal roots showed sensitivity similar to that of the wild type. All plants overexpressing EBF2 externally and internally in roots showed significant reduction in the cytokinin-induced reduction of cell elongation and RAM size. These findings indicate that cytokinin-induced, ethylene-mediated inhibition of root growth occurs primarily through the inhibition of cell elongation, a process that requires the activation of endogenous cytokinin production in root external cells.
Figure 1. Research model. (Yamoune, et al., 2024)
Finally, through experiments such as reporter genes and mutants, the study found that cytokinins can stimulate the production of 1-aminocyclopropane-1-carboxylate (ACC) and ethylene in a cytokinin- and ethylene-specific manner by inducing the transcription of multiple ACC SYNTHASEs (ACSs) and ACC OXIDASEs (ACOs) genes, making the synthesis of ethylene show spatial specificity in different cell types in the root. The specific regulation of ethylene mainly occurs in peripheral and proximal tissues, while the specific regulation of cytokinins mainly occurs in internal and distal tissues. This spatial specific regulation is crucial for cytokinin-induced ethylene-mediated root shortening and RAM size control.
