Biosynthesis and Application of 2-Amino-3-Methylhexanoic Acid as a Novel Biostimulant for Tea Plants

2-Amino-3-methylhexanoic acid (AMHA), as a naturally occurring α-amino acid, exhibits excellent bioactivity in inducing plant resistance to biotic and abiotic stresses, promoting plant growth and alleviating pesticide damage. However, its low yield in nature has hindered its industrialization. This study aims to address the current challenges in AMHA production, including synthetic difficulties, low yield, and poor production safety. We investigated the production capacity of Serratia marcescens NAU002-4 using high-performance liquid chromatography (HPLC), orthogonal experiments, and ion exchange chromatography (IEC), supplemented by precursor feeding assays. Optimization of the fermentation medium revealed that 10 g/L corn steep liquor as the nitrogen source and 10 g/L threonine as the precursor resulted in maximal yield. Further optimization of fermentation parameters—initial pH 7.2, inoculation volume 6%, stirring speed 180 rpm, and aeration rate 350 L/h—led to an AMHA titer of 504 mg/L. In parallel, field trials were conducted to evaluate the effects of pure, biosynthetic AMHA on tea plant growth and photosynthetic performance, using plant physiological data and chlorophyll fluorescence rise kinetics. The results demonstrated that biosynthetic AMHA significantly enhances tea plant growth and photosynthetic efficiency, providing a scientific basis for its development and application as a novel biostimulant.