Brassinosteroids alleviate wheat floret degeneration under low nitrogen stress by promoting the redistribution of sucrose from stems to spikes

The trade-off between yield and environmental effects caused by nitrogen fertilizer application is an important issue in wheat production. A reduction in fertile florets is one of the main reasons for the lower yields under low nitrogen application rates. Brassinosteroids (BRs) have been found to play a role in nitrogen-induced rice spikelet degeneration. However, whether BRs play a role in wheat floret development and the mechanisms involved are not clear. Therefore, a nitrogen gradient experiment and exogenous spraying experiment were conducted to investigate the role and mechanism of BRs in wheat floret development under low nitrogen stress. The results showed that as the nitrogen application decreased, the endogenous BRs content of the spikes decreased, photosynthesis weakened, and total carbon, soluble sugar and starch in the spikes decreased, leading to a reduction in the number of fertile florets. Under low nitrogen stress, exogenous spraying of 24-epibrassinolide promoted photosynthesis, and stimulated stem fructan hydrolysis and the utilization and storage of sucrose in spikes, which directed more carbohydrates to the spikes and increased the number of fertile florets. In conclusion, BRs mediate the effects of nitrogen fertilizer on wheat floret development, and under low nitrogen stress, foliar spraying of 24-epibrassinolide promotes the flow of carbohydrates from the stem to the spikes, alleviating wheat floret degeneration.