The Targeted Metabolomic Signatures of Phytohormones in Leaves of Mulberry (<i>Morus alba</i> L.) Are Crucial for Regrowth and Specifically Modulated by the Differential Stubble Lengths

Vegetative propagation of mulberry (<i>Morus alba</i> L.) via sapling methods, due to the ability to exponentially multiply lateral buds on stem cuttings to enhance rapid shoot formation, is crucial for sericulture industries. The sprouting of mulberry using stubbles is an emerging method for rapid and mass production of mulberry leaves, but the growth mechanisms associated with its use remain obscure. This study is the first to report how the differential stubble lengths from mulberry plants alter and modulate phytohormones and the associated mechanisms. This study seeks to evaluate the growth mechanisms by elucidating the phytohormone signature modulation in response to differential stubble lengths of 0 cm, 5 cm, 10 cm, 20 cm, and a control via targeted metabolomics analysis in mulberry leaves. The results consistently show that the use of differential stubble lengths of mulberry promoted growth, the number of buds, aboveground biomass, and branch and leaf weights by improving the net photosynthesis, transpiration rate, stomatal conductance, and intercellular CO₂ relative to the control. The differential stubble lengths not only caused contrasting responses in the contents of plant hormones, including salicylic acid (SA), abscisic acid (ABA), indole-3-acetic acid (IAA), jasmonic acid (JA), and gibberellin (GA), but also modulated higher elemental contents relative to the control. The results further reveal significant and positive correlations between the phytohormones and all growth, biomass, and photosynthetic parameters, highlighting the role of phytohormones in the sprouting and rejuvenation of mulberry stubbles. Meanwhile, the targeted metabolomics analysis identified a total of 11 differentially accumulated phytohormones in response to the differential stubble lengths, which were significantly implicated and enriched in three major pathways, including the biosynthesis of plant hormones (ko01070), metabolic pathways (ko01100), and the plant hormone signal transduction pathway (ko04575). The use of stubbles for rapid leaf production in mulberry plants is of great importance to improve early sprouting and cutting survival, as well as shortening growth and rooting time, and is highly recommended for the sericulture industries.