Melatonin-Induced Leaf Growth in Lithocarpus litseifolius: A Synergistic Interplay Among Hormone Homeostasis, Photosynthetic Enhancement, and Transcriptional Regulation

Lithocarpus litseifolius is a medicinal tea plant recognized for its sweet flavor and anti-diabetic properties, but its limited leaf yield under cultivation restricts its economic sustainability. Melatonin (MLT) is a multifunctional plant growth regulator, but its roles in leaf growth under normal conditions remain not fully understood. Herein, we investigated the effects and mechanisms of foliar-applied MLT on L. litseifolius seedlings, including growth indices, phytohormone profiles, photosynthetic characteristics, and transcriptome alterations. The 100 μM MLT treatment significantly enhanced leaf dry weight by 33.8% and leaf dry matter content by 22.2% compared to the control group. MLT decreased both free and bound abscisic acid (ABA), while increasing gibberellins (GAs), 5-deoxystrigol, auxins (e.g., IAM), and cytokinins (e.g., cZ9G). Additionally, exogenous MLT improved photosynthetic rate, stomatal conductance, chlorophyll content, and soluble sugars in leaves. RNA-seq revealed that MLT up-regulated DEGs involved in hormone biosynthesis and signaling (CYP707A, BAK1, D14, CCD1, and IAA6), photosynthesis (PsbC/B, PetH, PsaB, and ATPase β), and sugar metabolism (WAXY, glgC, and otsB). Our results demonstrate that MLT promotes leaf dry matter accumulation through coordinated phytohormone homeostasis, photosynthetic enhancement, and transcriptional regulation, offering a cost-effective strategy to improve leaf yield in L. litseifolius.