Simple SummarySucrose phosphate synthases catalyze the rate-limiting step in the sucrose biosynthetic pathway. Considering the essential role of sucrose in plant metabolism and molecular signaling, sucrose phosphate synthases are finely tuned both at transcriptional and post-translational levels. In plants, sucrose phosphate synthases are organized in protein families. Arabidopsis contains four sucrose phosphate synthase isoforms, each with a specific but overlapping expression profile, although the literature shows some discrepancies. In the present study, the role of isoform A2 was investigated under control conditions and in response to drought stress. The lack of sucrose phosphate synthase A2 affects (i) carbon partitioning through the activation of the oxidative pentose phosphate pathway and (ii) influences plant response to drought.AbstractSucrose is essential for plants for several reasons: It is a source of energy, a signaling molecule, and a source of carbon skeletons. Sucrose phosphate synthase (SPS) catalyzes the conversion of uridine diphosphate glucose and fructose-6-phosphate to sucrose-6-phosphate, which is rapidly dephosphorylated by sucrose phosphatase. SPS is critical in the accumulation of sucrose because it catalyzes an irreversible reaction. InArabidopsis thaliana, SPSs form a gene family of four members, whose specific functions are not clear yet. In the present work, the role of SPSA2 was investigated in Arabidopsis under both control and drought stress conditions. In seeds and seedlings, major phenotypic traits were not different in wild-type compared withspsa2knockout plants. By contrast, 35-day-old plants showed some differences in metabolites and enzyme activities even under control conditions. In response to drought,SPSA2was transcriptionally activated, and the divergences between the two genotypes were higher, withspsa2showing reduced proline accumulation and increased lipid peroxidation. Total soluble sugars and fructose concentrations were about halved compared with wild-type plants, and the plastid component of the oxidative pentose phosphate pathway was activated. Unlike previous reports, our results support the involvement of SPSA2 in both carbon partitioning and drought response.