Analysis of the Metabolic and Structural Changes in Ulmus pumila ‘Zhonghua Jinye’ Leaf Under Shade Stress

<i>Ulmus pumila</i> L. 'Zhonghua Jinye' is a plant variety with colourful leaves that is widely used in landscaping. In our study, the leaves of <i>U. pumila</i> 'Zhonghua Jinye' fade and turn green under light (LT, 45%), moderate (MD, 70%) and high (HG, 95%) shading treatment, reducing its ornamental value. However, the mechanism underlying this adaptation to shade is poorly understood. The objective of this study was to elucidate the mechanism of physiological, cellular microstructural and transcriptional changes involved in leaf regreening in <i>U. pumila</i> 'Zhonghua Jinye'. Our results showed that the pigment content of <i>U. pumila</i> 'Zhonghua Jinye' leaves increased under shade stress, with a corresponding colour change from yellow to dark green. Thus, <i>U. pumila</i> 'Zhonghua Jinye' adapted to shade stress by increasing leaf pigment and chlorophyll content. Transmission electron microscopy showed that thylakoid stacking in the grana lamellae changed significantly from a loose state to a closely packed structure under shaded conditions. Because plant pigments were located mainly in thylakoids, this closer stacking increased photosynthetic efficiency and pigment accumulation. RNA sequencing analysis showed that <i>Lhcb1</i>, a key thylakoid membrane gene, was upregulated under shade, which promoted thylakoid stacking and light absorption. In the chlorophyll synthesis pathway, haeme metabolism was inhibited, increasing protoporphyrin IX flow to the chlorophyll pathway and promoting the synthesis of chlorophyll a/b. The simultaneous upregulation of plant hormone-related genes promoted an increase in plant leaf area, improving the light energy utilisation ratio. This study is the first to report the self-regulatory mechanism that leads to colour change in <i>U. pumila</i> 'Zhonghua Jinye' under shade stress and provides a theoretical basis for the cultivation of tree species with colourful leaves.