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A novel 3D re-entrant chiral auxetic (RCA) structure has been proposed for the first time in this study. The unit cell has been developed with solid cubes and struts based on the topological features of both re-entrant honeycomb and chiral honeycomb. A recently developed powerful 3D printing process, Multi Jet Fusion (MJF), has been employed to print six samples from polyamide 12 (PA 12). Uniaxial quasi-static compressive tests have been performed in the X, Y and Z directions to investigate the deformation, Poisson's ratio and energy absorption of the proposed structure. Finite element (FE) models have been developed using ANSYS/LS-DYNA and verified by the respective experimental data. The experimental microscopic measurements reveal high accuracy of MJF process to produce robust parts with smooth internal morphology. The designed cubes are found to demonstrate a similar rotation function to that of cylinders of the corresponding 2D RCA structure. The plastic bending and buckling are found to be the dominated deformation mechanisms for the samples compressed in the Y direction, while plastic bending is the dominated deformation mechanism for the samples compressed in the X and Z directions. The proposed structure displays auxetic feature under uniaxial compression in all three directions, which facilitates a more severe compaction of the structure and results in an improved load carrying capacity. It is also found that the compressed RCA structure when loaded in the X direction outperforms in terms of auxeticity and energy absorption compared with the other (Y and Z) directions. The proposed 3D RCA structure has anisotropic properties.

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Flavonols have been shown to respond to a variety of abiotic stresses in plants, including cold stress. Higher total flavonoid content was found in non-heading Chinese cabbage (NHCC, Brassica campestris (syn. Brassica rapa) ssp. chinensis) after cold stress. A non-targeted metabolome analysis showed a significant increase in flavonol content, including that of quercetin and kaempferol. Here, we found that an R2R3–MYB transcription factor, BcMYB111, may play a role in this process. BcMYB111 was up-regulated in response to cold treatment, with an accompanying accumulation of flavonols. Then, it was found that BcMYB111 could regulate the synthesis of flavonols by directly binding to the promoters of BcF3H and BcFLS1. In the transgenic hairy roots of NHCC or stable transgenic Arabidopsis, overexpression of BcMYB111 increased flavonol synthesis and accumulation, while these were reduced in virus-induced gene silencing lines in NHCC. After cold stress, the higher proline content and lower malondialdehyde (MDA) content showed that there was less damage in transgenic Arabidopsis than in the wild-type (WT). The BcMYB111 transgenic lines performed better in terms of antioxidant capacity because of their lower H2O2 content and higher superoxide dismutase (SOD) and peroxidase (POD) enzyme activities. In addition, a key cold signaling gene, BcCBF2, could specifically bind to the DRE element and activate the expression of BcMYB111 in vitro and in vivo. The results suggested that BcMYB111 played a positive role in enhancing the flavonol synthesis and cold tolerance of NHCC. Taken together, these findings reveal that cold stress induces the accumulation of flavonols to increase tolerance via the pathway of BcCBF2–BcMYB111–BcF3H/BcFLS1 in NHCC.

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