The regulation of flowering in plants is a highly complex process that is dependent on many internal and external factors. Flowering at the wrong time point leads to a loss of seed production and endangers the plant’s reproduction and survival of the species. To combat this problem, plants use a complex network of proteins to continuously monitor environmental factors, such as light and temperature, to determine the best time point for flowering. The work group of Dr. Vanessa Wahl from the Max Planck Institute of Molecular Plant Physiology, in cooperation with Dr. Anne Krapp from the National Institute for Agricultural Research (INRA) in Paris, have now revealed that nitrogen is involved in the regulation of the flowering time. In March, Dr. Wahl’s group published their exciting results in the scientific journal New Phytologist.
Nitrogen is one of the main nutrients required for plant growth, and is known to influence various developmental processes. Although our atmosphere is made up of 78% molecular nitrogen, this gaseous form cannot be used by plants. Instead, plants use their root systems to absorb nitrogen from the soil in the form of mineral compounds, such as ammonia or nitrate. As plants grow, they gradually deplete nitrogen from the soil; therefore, nitrogen fertilizers have to be added to the soil to guarantee optimal plant growth and yield. If the amount of nitrogen in the soil exceeds the demand of the plant, or if nitrogen cannot be absorbed by roots due to environmental factors, such as drought, significant amounts of nitrogen are lost from the soil as run-off or leaching to groundwater, leading to environmental pollution. To avoid this scenario and promote sustainable agricultural practices, it is crucial that we gain a detailed knowledge of how nitrogen influences the plant’s life cycle, and use this information to direct and optimize fertilizer application practices.
After nitrogen is absorbed by roots, it is transported through all plant organs by the vascular system. Nitrogen is used as a component in many important biological processes, such as amino acid and protein synthesis, and as part of the green pigment chlorophyll, which has an important role in photosynthesis. Plants grown in nitrogen-depleted soil experience chlorosis and a delay of growth, which directly results in a massive yield loss. In addition to being used as a nutrient, nitrate as the major form of nitrogen is also important as a signaling molecule. Dr. Vanessa Wahl explains her findings:
“In the model plant Arabidopsis thaliana, we showed that nitrogen is transported as nitrate into the shoot apical meristem (SAM) to influence flowering time. There, the protein SUPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) plays a crucial role as a central regulator of flowering at the SAM. SOC1 gene expression is activated by environmental signals like temperature and light, or by phytohormones. It is a so-called transcriptional activator, which can initiate the expression of other genes. As a result, SOC1 expression causes the plant to stop producing leaves and induces flowering.”
(Upper panel) Arabidopsis plants grown with optimal nitrogen supply (ON) are flowering after few weeks with the help of SOC1, a central regulator of flowering, that is produced in the shoot apical meristem (yellow asterisk). (Lower panel) If the soil contains low amounts of nitrogen (LN), the flowering of the plants is delayed and the SOC1 production is reduced.
Her group’s experiments showed that the production of SOC1 is nitrate-dependent. As proof, plants were grown on nitrogen-limited soil and compared to plants grown on full-nutrition soil. Dr. Vanessa Wahl explains: “The higher the nitrate level of the soil, the more SOC1 was produced, causing plants to flower earlier. Conversely, under nitrogen-limitation, flowering was delayed”. In addition, nitrate responsive elements (NREs) were identified in many genes already known to be involved in the regulation of SOC1 and flowering time. Nitrate can activate these genes, which subsequently explains the elevated level of SOC1. Finally, it was shown for the first time that nitrate is transported into the SAM where it can be metabolized to ammonia. This conversion is mediated by nitrate reductase, found to be present in the SAM. The tissue-specific expression of nitrate reductases was published in the journal Signaling & Behavior in August.
Because flowering is an energy-consuming process, the time point of flowering also depends on the availability of sugars, which act as the plant’s energy source. In this respect, Dr. Vanessa Wahl showed that the sugar trehalose 6-phosphate (T6P) plays an important role in flowering, as published by her group in the journal Science (2013). Interestingly, flowering initiation is signaled by T6P and nitrate independent from each other. If both signals are inhibited flowering is not induced.
In summary, plants are able to adapt their life cycles depending on the availability of nitrogen by delaying their flowering time, due to suboptimal growth conditions. Deeper knowledge about the interplay of nitrogen and sugar sensing in terms of flowering time will help to develop new strategies to increase the yield of crops when grown on nitrogen-limited soil. By understanding these biological processes, we stand to gain an important approach to optimizing the use and application of nitrogen fertilizer.