GmAP1 delays flowering time and confers sensitivity to salt stress in soybean

Aspartic proteases represent a class of proteolytic enzymes widely implicated in plant growth regulation and abiotic stress responses. Our previous work identified an aspartic protease, GmAP1, whose expression is strongly correlated with flowering time in wild soybean populations. In this study, we further demonstrated that GmAP1 overexpression (OE) significantly delays flowering, whereas CRISPR/Cas9-mediated knockout (CR) accelerates this process. Specifically, OE lines exhibited suppressed expression of CONSTANS (CO) and FLOWERING LOCUS T (FT), along with elevated transcript levels of FLOWERING LOCUS C (FLC). Subcellular localization assays confirmed that GmAP1 is targeted to chloroplasts, where it downregulates the PHD-type transcription factor gene PTM without affecting GOLDEN2-LIKE 1/2 (GLK1/2). Notably, GmAP1 expression was strongly induced by salt stress. GmAP1-OE lines exposed to salt stress showed chlorophyll degradation and reduced net photosynthetic rate (Pn). Both in vivo and in vitro biochemical assays verified that GmAP1 interacts with and degrades the Rubisco large subunit (rbcL), a process correlating with decreased Pn and delayed flowering in OE lines under salt stress. Collectively, our findings reveal that GmAP1 coordinates photosynthetic energy supply and reproductive transition in soybean. Precise modulation of this gene holds substantial practical value for developing early-maturing, salt-tolerant soybean varieties well adapted to saline-alkali environments.