Drought-induced Abscisic Acid Accumulation Reduces the Resistance of Next Generation Dryland Rice to Rice Brown Spot

Purpose Next generation dryland rice (NGDR) is a new rice type bred for rain-fed dryland cultivation, combining high yield and quality with drought tolerance. The pathogenic fungus of rice brown spot disease (Bipolaris oryzae) is a necrotrophic fungus. Field observations have shown that the dryland cultivation environments readily promote the occurrence of this disease, but the underlying mechanisms remain unclear. This study aimed to elucidate the molecular mechanisms underlying the reduced resistance of rice to brown spot under dryland conditions, and to provide theoretical guidance for field control of this desease.

Methods Using the NGDR variety Dianheyou 615 (DHY615) and its paternal parent Nan 615 (N615) as materials, flooded and dryland cultivation models were compared. The plants were spray-inoculated with B. oryzae, and relative lesion area was measured to assess the effects of the two cultivation methods on rice resistance. Meanwhile, flooded and drought treatments were applied to quantify the endogenous abscisic acid (ABA) content and the expression of genes involved in ABA metabolism and signal transduction in rice. Furthermore, rice plants were treated with an ABA biosynthesis inhibitor to evaluate its inhibitory effect on rice brown spot development.

Results Both drought stress and dryland cultivation significantly reduced rice resistance to B. oryzae and strongly elevated endogenous ABA levels. Under normal flooded conditions, the endogenous ABA content was 25.32 ng/g, which increased to 993.24 ng/g after drought stress, representing an approximately 38-fold increase. RT-qPCR results showed significant up-regulation of the ABA biosynthesis gene OsNCED4 and the transcription factor OsbZIP23 involved in ABA signaling, and significant down-regulation of the ABA catabolic gene OsABA8ox1. Exogenous application of ABA biosynthesis inhibitor significantly enhanced rice resistance to B. oryzae. Compared with the control, relative lesion area decreased from 16.32% to 7.81%, representing a 52.14% reduction.

Conclusion ABA is a pivotal phytohormone that orchestrates plant responses to diverse abiotic stresses. Dryland farming conditions often lead to drought stress, triggering substantial synthesis and accumulation of ABA, which represses rice growth and results in increased susceptibility to rice brown spot.