19Expansins facilitate cell expansion via mediating pH-dependent cell wall (CW) loosening. 20However, the role of expansins in the control of biomechanical CW properties in the tissue and 21 organ context remains elusive. We determined hormonal responsiveness and specificity of 22 expression and localization of expansins predicted to be direct targets of cytokinin signalling. We 23 found EXPA1 homogenously distributed throughout the CW of columella/ lateral root cap, while 24 EXPA10 and EXPA14 localized predominantly at the three-cell boundaries of epidermis/cortex in 25 various root differentiation zones. Expression of EXPA15, revealing cell type-specific localization 26 pattern, overlaps with higher CW stiffness measured via Brillouin light scattering microscopy. As 27 indicated by both higher Brillouin frequency shift and AFM-measured Youngs' modulus, EXPA1 28 overexpression upregulated CW stiffness, associated with shortening of the root apical meristem 29 and root growth arrest. We propose that root growth in Arabidopsis requires delicate orchestration 30 of biomechanical CW properties via tight regulation of various expansins' localization to specific 31 cell types and extracellular domains. 32

Biomechanical properties of the cell wall (CW) are important for many developmental and adaptive responses in plants. Expansins were shown to mediate pH-dependent CW enlargement via a process called CW loosening. Here, we provide a brief overview of expansin occurrence in plant and non-plant species, their structure and mode of action including the role of hormone-regulated CW acidification in the control of expansin activity. We depict the historical as well as recent CW models, discuss the role of expansins in the CW biomechanics and address the developmental importance of expansin-regulated CW loosening in cell elongation and new primordia formation. We summarise the data published so far on the role of expansins in the abiotic stress response as well as the rather scarce evidence and hypotheses on the possible mechanisms underlying expansin-mediated abiotic stress resistance. Finally, we wrap it up by highlighting possible future directions in expansin research.