Cytokinins and auxin communicate nitrogen availability as long-distance signal molecules in pineapple (Ananas comosus)

This work aimed at identifying a possible role of phytohormones in long-distance (root-shoot) signaling under nitrogen deficiency. Three-months old pineapple plants were transferred from Murashige and Skoog (MS) medium to nitrogen-free MS (-N). During the first 24 h on -N, 20 plants were harvested every 4 h. After 30 days in -N, the remaining plants were transferred back to regular MS (+N) and 20 plants harvested every 4 h for the first 24 h. Following the harvests, endogenous levels of nitrate (NO3-), indole-3-acetic acid (IAA), isopentenyladenine (iP), isopentenyladenine riboside (iPR), zeatin (Z) and zeatin riboside (ZR) were analyzed in roots and leaves. In N-starved plants, the NO3- level dropped by 20% in roots between the first (4 h) and the second harvest (8 h). In leaves a reduction of 20% was found 4 h later. Accumulation of IAA peaked in leaves at 16 h. In roots, the accumulation of IAA only started at 16 h while the leaf content was already in decline, which suggests that the hormone might have traveled from the leaves to the roots, communicating N-shortage. The contents of the four cytokinins were generally low in both, shoot and roots, and remained almost unchanged during the 24 h of analysis. After N re-supply, roots showed a NO3- peak at 8 h whereas the foliar concentration increased 4 h later. Hormone levels in roots climaxed at 8 h, this coinciding with the highest NO3- concentration. In leaf tissue, a dramatic accumulation was only observed for Z and ZR, and the peak was seen 4 h later than in roots, suggesting that Z-type cytokinins might have traveled from the roots to the leaves. These findings provide evidence that there is a signaling pathway for N availability in pineapple plants, communicated upwards through cytokinins (N-supplemented plants) and downwards through auxin (N-starved plants).