Jasmonic acid negatively regulates wheat grain filling by affecting assimilate transport and starch biosynthesis

Grain filling is a pivotal determinant of wheat (Triticum aestivum L.) yield, relying on the efficient coordination of source–sink dynamics and assimilate partitioning. Jasmonic acid (JA) is widely involved in plant growth and development, and also plays an important role in resource allocation. However, it remains unclear if JA is involved in regulation of grain filling and assimilate transport in wheat. The objective of this study was to identify the roles of JA in regulating wheat grain filling and weight, by comparing cultivars, comparing superior and inferior grains of the same spikes, manipulating source–sink relationship by removing superior grains, and applying exogenous JA. The results showed the negative association between endogenous JA level and grain filling rate and final grain weight, irrespective of cultivars and source-sink treatments. JA increased assimilate retention in stems by reducing fructan mobilization and diminishing sucrose availability for grain development. In addition, JA suppressed sucrose unloading by reducing invertase (CWI and SAI) activities, and inhibited starch biosynthesis through reducing starch synthase (AGPase, GBSS and SBE) activities. These coordinated changes limited substrate sugar supply and starch accumulation, ultimately slowing grain filling rate. Overall, our findings reveal that JA changes the source–sink relationship, favoring carbon retention in vegetative tissues and reducing assimilate partitioning into grain.