A conserved ethylene-triggered cell death mechanism may underlie hollow stem formation across plant species

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 (<i>Ipomoea aquatica</i>) is one of the few hollow-stemmed plants in the <i>Convolvulaceae</i> family (eudicot: asterid), and its hollow stems are essential for thriving in aquatic environments. Using histochemical staining and transcriptome analysis, we found that programmed cell death (PCD) is involved in cavity formation at water spinach shoot tips. Single-cell and spatial transcriptome analyses further revealed that ethylene and reactive oxygen species (ROS) likely drive and regulate this process by activating transcription factors <i>IaNAC074</i>, <i>IaNAC087</i>, <i>IaNAC029</i>, <i>IaNTL9</i>, and <i>IaTGA9</i>, which likely initiate PCD, senescence, and autophagy, collectively leading to pith cell death. These findings were validated through treatments with ethylene and ROS reagents in water spinach, as well as transient expression assays in tobacco. Additionally, transcriptomic data suggest that these mechanisms may also play a role in hollow stem formation in horsetail (fern), moso bamboo (monocot), and broad bean (eudicot: rosid), highlighting the conservation of PCD regulatory mechanisms in hollow stem formation. This study not only fills a major knowledge gap in the adaptive mechanisms of hollow stem formation but also opens broad avenues for agricultural and ecological applications, offering strategies to enhance crop tolerance to flooding and accelerate crop growth.