Exogenous auxin mediates the NnARF9-NnSUS2 module to regulate sucrose synthesis in lotus leaves (Nelumbo nucifera).

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conclusionNnARF9 directly represses NnSUS2 transcription via TGTCTC motif binding, inhibiting sucrose cleavage; NAA activates this pathway to boost sucrose storage in photosynthetic tissues. Sucrose is the most common type of sugar produced in photosynthetic organs and plays a crucial role in the growth and development of plants. Auxin can mediate various physiological effects, but the regulatory mechanism of auxin on sucrose synthesis remains unclear. In this study, 1-naphthaleneacetic acid (NAA) treatment promoted photosynthesis in lotus leaves, thereby facilitating the accumulation of sucrose. Through in vitro enzymatic activity determination experiments, the results showed that NnSUS2 could decompose sucrose using sucrose and UDP as substrates in vitro, demonstrating its decomposition function. Transient overexpression of NnSUS2 in the leaves significantly reduces the sucrose content in the leaves, while silencing NnSUS2 leads to a significant increase in the sucrose content. NnARF9 activates and inhibits the gene expression of sucrose synthase (NnSUS2) by directly binding to the TGTCTC element in the promoter of NnSUS2. Moreover, overexpression of NnAFR9 would decrease the expression level of NnSUS2 in the resting tissue, thereby inhibiting the decomposition of sucrose by NnSUS2, and ultimately leading to an increase in the sucrose content. This study revealed the molecular mechanism by which NnARF9 regulates NnSUS2 and thereby affects the sucrose metabolism of the source organs of lotus plants. NAA promotes the expression of NnARF9 to inhibit the expression of NnSUS2, thereby increasing the sucrose accumulation in the source tissues. This discovery provides a theoretical basis for the breeding of new high-quality and high-yield lotus varieties.