Functional characterization of <scp>NLG14</scp> provides novel insights into the synergistic improvement of grain length, grain quality and salt tolerance in rice

Rice, as a vital food crop, faces persistent challenges in breeding programs aimed at achieving stable high yield under environmental stresses due to intrinsic trade-off mechanisms. This study functionally characterizes NARROW AND LONGER GRAIN 14 (NLG14), which encodes a spermine synthase. Loss-of-function nlg14 mutants exhibit slender grains due to enhanced cell expansion and proliferation, alongside significantly improved grain quality-manifested as reduced chalkiness, lower amylose/protein content, higher gel consistency, and superior taste value. These improvements correlate with decreased reactive oxygen species (ROS) accumulation and programmed cell death (PCD) in developing endosperm. Crucially, nlg14 confers enhanced salt tolerance by elevating the K⁺/Na⁺ ratio and antioxidant enzyme activities. Mechanistically, disrupted spermine biosynthesis in nlg14 redirects metabolic flux toward ethylene synthesis, activating ethylene signaling to enhance ROS scavenging and ion homeostasis. Furthermore, the transcription factor OsMYB2 directly binds to the promoter of NLG14 and represses its expression via the abscisic acid (ABA) pathway. Haplotype analysis identifies natural NLG14 variants (Class A) associated with longer grains, improved quality, and higher salt tolerance, demonstrating breeding potential. Collectively, NLG14 integrates grain morphology, quality, and stress adaptation through polyamine-ethylene-ABA crosstalk. Our results provide useful gene and germplasm resources for rice molecular breeding and shed insights for understanding yield and salt tolerance trade-off mechanisms.