Brassinosteroids improve the redox state of wheat florets under low-nitrogen stress and alleviate degeneration

Reducing nitrogen application rates can mitigate issues such as environmental degradation and resource wastage. However, it can also exacerbate problems such as wheat floret degeneration, leading to reduced yields. Therefore, investigating wheat floret degeneration mechanisms under low nitrogen stress and identifying mitigation measures are conducive to achieving high yields and sustainable development. To investigate the physiological mechanism of low nitrogen stress affecting wheat floret degradation and whether exogenous brassinosteroids can alleviate this stress, three nitrogen application rates (N0, no nitrogen application; N1, 120 kg ha-1 pure nitrogen; and N2, 240 kg ha-1 pure nitrogen) and exogenous spraying experiments (N0CK, no nitrogen with water spraying; N0BR, no nitrogen with 24-epibrassinolide (an active brassinosteroids) spraying; and N1, 120 kg ha-1 pure nitrogen with water spraying) were designed. The results indicated that low nitrogen stress induced a large amount of reactive oxygen species generation. Although wheat spikes synthesized flavonoids to combat oxidative stress, their energy metabolism (glycolysis and tricarboxylic acid cycle) and ascorbate-glutathione cycle were inhibited, keeping reactive oxygen levels elevated within the spike, inducing cell death and exacerbating floret degeneration. Furthermore, brassinosteroids played a role in regulating wheat floret degeneration under low-nitrogen stress. Exogenous foliar spraying of 24-epibrassinolide promoted energy metabolism and the ascorbate-glutathione cycle within the spike, enhancing energy charge and effectively mitigating a portion of reactive oxygen induced by low nitrogen stress, thereby alleviating floret degeneration caused by low nitrogen stress. In summary, low-nitrogen stress disrupts the redox homeostasis of wheat spikes, leading to floret degeneration. Brassinosteroids alleviate floret degeneration by improving the redox state of wheat spikes. This research provides theoretical support for balancing the contradiction between high yields and sustainable development and is beneficial for the application of low nitrogen in production.