Exogenous putrescine regulates leaf starch overaccumulation in cucumber under salt stress

To understand the mechanism by which exogenous putrescine (Put) alleviated starch overaccumulation in the leaves of cucumber subjected to salt stress, a greenhouse experiment was conducted. Our research findings focused on NaCl stress leading to an increase in the number of starch granules in the chloroplast and starch content in the leaves, resulting in the destruction of photosynthetic organs and reduced photosynthesis. However, exogenous Put reduced the number of starch granules and protected the photosynthetic organs, thereby enhancing photosynthesis. On the other hand, Put-treated seedlings reduced AGPase activity by downregulating AGPase and increased beta-amylase activity by upregulating BAMI, resulting in relief from excess accumulation of leaf starch. The application of D-Arg under salt stress decreased the expression of ADC, inhibited Put synthesis, reduced the endogenous polyamine (PAs) content, and inhibited the growth of seedlings; exogenous Put partly alleviated the inhibitory effect of D-Arg and significantly increased the endogenous PM content, indicating that the ADC pathway is important for Put synthesis in plants suffering from salt stress. Taken together, exogenous application of PM could increase levels of endogenous PAs via important ADC pathways, mainly inducing AGPase and BAM1 expression, leading to changes in AGPase and 0-amylase activities involved in starch metabolism to reduce excessive starch accumulation in leaves, relieve photosynthetic feedback repression and protect photosynthetic organs, thus enhancing cucumber seedling salt stress tolerance.