Stem node has been found to be a hub for controlling
mineral nutrient distribution in gramineous plants. However, the characteristics of stem nodes associated with whole-plant
carbon partitioning in maize (Zea mays L.) and their responses to
nitrogen (N) availability remains elusive. Maize plants were grown in greenhouse under low to high N supply. Plant growth,
sugar accumulation, and
sugar transporters in nodes and leaves, as well as the anatomical structure of nodes, were investigated at vegetative phase. When compared to N-sufficient plants, low-N availability
stunted growth and resulted in 49-64% less
sugars in leaves, which was attributed to low photosynthesis or the accelerated
carbon export, as evidenced by more 13C detected further below leaf
tips. Invariably higher
sugar concentrations were found in the stem nodes, rather than in the leaves across N treatments, indicating a crucial role of nodes in facilitating whole-plant
carbon partitioning. More and smaller vascular bundles and phloem were observed in stem nodes of N-deficient plants, while higher
sugar levels were found in the bottom nodes than in the upper ones. Low-N availability upregulated the gene expressions of
sugar transporters, which putatively function in nodes such as ZmSWEETs and ZmSUTs at the bottom stem, but suppressed them in the upper ones, showing a developmental impact on node function. Further, greater activity of
sugar transporters in the bottom nodes was associated with less
sugars in leaves. Overall, these results highlighted that stem nodes may play an important role in facilitating long-distance
sugar transport in maize.