Endogenous SPD content decreases with leaf aging. Abundance of PUT (A) and SPD (B) in IDLs of WT, pif5-621, and ore1-2 across a 6-day time course. n = 3 ± sd. ^*P < 0.05, Student’s t test versus WT. C, Scheme illustrating biosynthesis of ethylene and SPD. Ethylene biosynthesis starts with the conversion of S-adenosyl-l-methione (SAM), which is converted from methionine by SAM synthase (SAMS), into 1-aminocyclopropane-1-carboxylic acid (ACC) by the enzyme ACC synthase (ACS). ACC can then be converted to the end product ethylene by ACC oxidase (ACO). SAM decarboxylase (SAMDC1/4) converts SAM into decarboxylated SAM (DcSAM). SPD is synthesized from PUT and DcSAM by SPDS (SPDS1/2). SPD is further metabolized to spermine by spermine synthase (SPMS). D, SPD content in the fourth leaf of WT (Col-0) at different developmental stages: 7-, 14-, 21-, 28-, 35-, and 42-day-old. Student’s t test, *P < 0.05, **P < 0.01; n = 3 ± sd. E, Transcript abundance of SPDS1 and SPDS2 in the fourth leaf across the same time course. n = 3 ± sd, and two technical replicates were performed. F, Senescence phenotype of spds1-2, spds2-2, VIGS-SPDS2/spds1-2, and WT plants. VIGS-SPDS2/spds1-2 plants were treated with 10 μM SPD. Rosette leaves detached from 45-day-old plants were arranged according to their age. G, PA contents in SPDS1/2 loss-of-function mutants. PAs were isolated from the fourth leaves of each line (30-day-old) and measured by HPLC. Bars represent mean ± sd (n = 3) (Student’s t test, *P < 0.05, **P < 0.01). H, RT-qPCR analysis of SAG12 expression. Bars represent mean ± sd (n = 3) (Student’s t test, *P < 0.05, **P < 0.01, ***P < 0.001). I, The senescence phenotype of SPDS1ox. Rosette leaves of 6.5-week-old Col-0 and SPDS1ox were detached and arranged according to their age. J, RT-qPCR analysis of SPDS1 expression in SPDS1ox lines. K, Determination of SPD contents in SPDS1ox plants as described in G. J and K: mean ± sd; n = 3; Student’s t test, *P < 0.05, **P < 0.01, ***P < 0.001.