Brassinosteroids facilitate controlled soil drying to mitigate heat stress on pistil fertilization in photo-thermosensitive genetic male-sterile rice

Globally recurrent extreme high temperature (HT) events severely limit rice production. This study investigated whether a controlled moderate soil drying (MD) could replace the conventional well-watered (WW) regime to more effectively mitigate HT stress on pistil fertilization in photo-thermosensitive genetic male-sterile (PTGMS) rice, and examined the role of brassinosteroids (BRs). Two PTGMS rice varieties were cultivated under normal temperature (NT) and HT conditions, paired WW and MD strategies during anthesis. In conventional WW regime, waterlogging reduces BRs levels in roots and pistils due to excessive decomposition, weakening active water uptake driven by root activity and failing to alleviate transpiration-pulled passive water extraction hampered by restricted stomatal openings. Thereby, it causes water imbalance in plants and weakened pistil function due to a suppressed ascorbate-glutathione (AsA-GSH) cycle and hyperactive nicotinamide adenine dinucleotide phosphate oxidase (NOX) activity. This exacerbates pistil fertilization impairment and hybrid seed yield loss under HT stress. Conversely, by promoting BR synthesis and inhibiting its decomposition in roots and pistils, the MD strategy enhanced root activity and transpiration-driven water uptake. It maintained plant water balance and supported pistil function through suppressed NOX activity and an enhanced AsA-GSH cycle-driven redox homeostasis. Thus, it mitigated HT-induced pistil fertilization impairment and hybrid seed yield loss. The precise function of BRs in moderating the protective effects of MD against the detrimental impacts of HT stress on pistil fertilization in PTGMS rice was confirmed through genetic and chemical approaches. Consequently, a controlled MD method proved to be more effective than the conventional WW regime in alleviating HT stress on pistil fertilization in PTGMS rice by promoting BR enhancement.