Multiscale imaging locates thermogenic tissues and reveals the role of Ca2+ in floral thermogenesis

Floral thermogenesis is an ancient feature that facilitates mutualism between flowers and pollinators. Yet, localization of specific thermogenic tissues within floral organs has received little attention. Here, we integrated infrared (IR) thermal imaging and micro X-ray fluorescence (μ-XRF) to localize the thermogenic tissues in the lotus (Nelumbo nucifera Gaertn.) receptacle. IR imaging preliminarily identified the primary thermogenic tissues of the receptacle as the carpels and epidermis. The calcium distribution visualized by μ-XRF complemented the results of IR imaging, indicating that the thermogenic tissues include the epidermis and the upper parts of the carpels. This ensures that heat reaches the chamber formed by the petals and receptacle over the shortest distance, thereby minimizing heat loss. Additionally, we observed a higher rate of Ca2+ transport from the apoplast to the cytosol and upregulation of genes associated with mitochondrial calcium uniporters (MCU) at the thermogenesis initiation stage as compared to the prethermogenic stage. Increasing the cytosolic Ca2+ (cCa2+) concentration reversed the inhibition of alternative respiratory pathways, further illustrating the close relationship between Ca2+ concentration and thermogenesis. Our research not only presents a precise method for identifying thermogenic tissues in plants but also demonstrates the evolutionary efforts of lotus to maximize energy utilization efficiency.