The mechanisms by which polyamines regulate wheat grain filling under drought stress conditions

Drought stress (DS) is a major factor limiting wheat grain filling. Polyamines (PAs) play crucial roles in plant responses to DS; however, the mechanisms underlying their effects on grain filling are not fully understood. This study aimed to clarify the regulatory role of PAs in grain filling through source-sink dynamics in two wheat cultivars representing distinct drought tolerance under non-stress and DS conditions, with or without exogenous spermine and putrescine. Data indicated that DS significantly disrupted the grain-filling process (P < 0.01), accompanied by severe limitations in source-sink capacity. However, PA application improved chlorophyll content (9.45-23.39 %), Fv/Fm values (1.86-5.56 %), assimilate partitioning to non-structural carbohydrates (4.24-7.17 %), and stem reserves (6.42-24.44 %), thereby enhancing source capacity. PAs also reduced abscisic acid (ABA) levels during early grain-filling stages and increased auxin and cytokinin levels, which were associated with enhanced endosperm cell division and number (P < 0.05), thereby improving sink capacity. In later grain-filling stages, PAs caused a controlled increase in ABA levels, serving as physiological signals for reserve mobilization and significantly inducing the expression of 1-FEH-w3 and SPSI genes (P < 0.01). These changes were accompanied by improved stem reserve remobilization (RM) (6-16.70 %), grain-filling rate, and grain yield (P < 0.05). The cultivars' responses to spermine application were more evident than to putrescine, particularly in the sensitive cultivar. Overall, PAs could significantly enhance grain filling and sustain wheat yield under DS conditions, likely through a multifaceted mechanism involving hormonal regulation, maintaining source-sink capacity, and facilitating RM.