The hollow truth: ethylene-triggered ROS, PCD, senescence, and autophagy drive hollow stem formation

Hollow stems have independently evolved multiple times across the plant kingdom, and play crucial roles in plant development and various environmental adaptations. However, the mechanisms underlying stem hollowness remain poorly understood. Water spinach (Ipomoea aquatica) is one of the few hollow-stemmed plants in the Convolvulaceae family, and its hollow stems are essential for thriving in aquatic environments. Using histochemical staining and transcriptome analysis, we identified programmed cell death (PCD) is involved in cavity formation of water spinach shoot tips. Single-cell transcriptome analysis further revealed that ethylene and reactive oxygen species (ROS) drive and regulate this process by activating transcription factors IaNAC074, IaNAC087, IaNAC029, IaNTL9, and IaTGA9, which initiate PCD, senescence, and autophagy, collectively controlling cell death. These findings were validated through treatments with ethylene and ROS reagents in water spinach and transient expression in tobacco. Additionally, transcriptomic data suggest that these mechanisms may also play a role in hollow stem formation in bamboo, highlighting the conservation of PCD regulatory mechanisms in plant development. This work not only fills a major knowledge gap in the adaptive mechanisms of hollow stem formation but also opens new avenues for agricultural and ecological applications, offering strategies to enhance crop tolerance to flooding and accelerate crop growth.