Compared to many other crops, maize has a significantly higher productivity. The special leaf anatomy and the special form of photosynthesis (referred to as "C4") that were developed during its development allow maize to grow significantly faster than comparable plants. As a result, maize needs more efficient transport strategies to distribute the photoassimilates generated during photosynthesis throughout the plant.
Researchers at the HHU have now discovered a previously unknown phloem loading mechanism – the bundle shell surrounding the vascular system as a location for the actual transport of compounds such as sugars or amino acids. The development of this mechanism could have been the decisive evolutionary step towards a higher transport rate, which has made maize plants particularly successful and useful. This is probably also related to the more effective C4 photosynthesis of maize compared to other plants that only use C3 photosynthesis. The study was conducted by Dr. Ji Yun Kim and Prof. Dr. Wolf B. Frommer from the Institute for Molecular Physiology at HHU.
Plant leaves have different structures on the upper (adaxial) and lower (abaxial) sides, and each side performs different tasks. In maize, for example, sucrose transporters (SWEET) act in the "bundle shell cells" (which frame the vascular bundle like a wreath) on the abaxial side of the leaf. In the model plant Arabidopsis thaliana, sugars released from phloem parenchyma cells are transported directly to the neighboring accompanying cells via SWEETs. In corn, sugar is released in the direction of the phloem by two large bundle-shell cells. The large surface of the bundle envelope cells compared to the phloem parenchyma enables much higher transport rates. Compared to Arabidopsis, corn could transport sugar more effectively.
The doctoral student and first author Margaret Bezrutczyk from HHU emphasizes: "The bundle-coat cells arranged in a wreath look the same at first glance. The single-cell sequencing approach we used made it possible for the first time to differentiate between different types of bundle-coat cells in a maize leaf. With this technology, we expect that more cell types, especially in the vascular bundles, will be discovered in the future. "
Institute director Prof. Frommer emphasizes the importance of the finding and says: "Maize plants are extremely productive due to their C4 photosynthesis. It is conceivable that the productivity of rice or other crops can be increased by transferring the loading mechanism of maize to these crops."