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Have you ever wondered how plants decide when and where to grow their next branch, especially when the environment fluctuates unpredictably? For plants, such developmental choices are matters of survival. When essential nutrients become scarce, plants cannot migrate in search of more fertile ground. Instead, they respond through remarkable growth plasticity where the plants reprogram their growth patterns, reallocating resources and reshaping their architecture to make the most of what is available.

At the heart of this adaptability lies a sophisticated internal communication network governed by phytohormones, or plant hormones. Much like hormones in the human body that coordinate complex physiological responses, such as insulin regulating blood glucose level or adrenaline preparing the body for fight/flight responses, plant hormones orchestrate growth, development, and stress responses across diverse tissues. Among the five classical groups of plant hormones, auxin stands out as a master regulator, directing where growth occurs and ensuring that the plant’s form survives the environment in which it lives.

Environmental cues continually modulate this intricate system. Light, gravity, and nutrient availability all influence the localisation and activity of PIN proteins, thereby shaping the auxin gradients in response to changing conditions. One of the earliest and most striking demonstrations of auxin function can be observed in the bending of a plant towards a light source known as phototropism. When light illumination comes from one side of a plant, researchers have shown that PIN proteins relocate to the shaded side of the stem, causing auxin to accumulate there. This asymmetric distribution of auxin stimulates cell elongation on the shaded side, resulting in the characteristic bending of the shoot towards light[2]. It provides a vivid illustration of how a single hormone can translate environmental information into coordinated growth.

Nitrogen: The Essential but Limited Nutrient

Nitrogen is one of the most essential nutrients for plant growth, serving as a fundamental component of amino acids, proteins, nucleic acids, and chlorophyll. Some plant groups, particularly legumes, have evolved mutualistic relationships with soil bacteria such as Rhizobium. These bacteria are capable of converting atmospheric nitrogen into forms that plants can use, thereby meeting their nitrogen requirements. Most non-leguminous plants, however, depend on nitrogen available in the soil, and this often presents a challenge since natural soils are frequently nitrogen-deficient. Under nitrogen-deficient conditions, plants exhibit slower growth, reduced chlorophyll content, and significant changes in their overall structure and development[3].

In the early twentieth century, Fritz Haber and Carl Bosch developed a method to capture atmospheric nitrogen and convert it into ammonia through what is now known as the Haber–Bosch process. This breakthrough made large-scale fertiliser production possible and revolutionised global agriculture. The ammonia produced through this process forms the nitrogen (N) component in the widely used NPK (Nitrogen, Phosphate, Potassium) fertiliser. Yet, the process remains energy-intensive and heavily dependent on fossil fuels, raising concerns about its sustainability and long-term impact on food security. 

Nitrogen does much more than provide the basic building blocks for plant growth. Studies have shown that a plant’s nutrient status, especially the amount of nitrogen available, can influence plants’ hormonal activities. This includes changes in the auxin production, movement, and signalling, which together help determine how a plant shapes its shoots, roots, and overall structure.

Auxin and Branching: A Delicate Tug-of-Growth

The relationship between auxin and nitrogen becomes particularly evident when examining how plants control shoot branching. Auxin produced at the main shoot tip moves downward through the stem and suppresses the growth of lateral buds, a process known as apical dominance. This continuous movement of auxin, facilitated by the PIN transport proteins, maintains apical dominance from the early stages of seedling growth throughout the vegetative phase.

Nitrogen availability strongly influences this process by affecting both hormone signalling and lateral bud growth. When nitrogen levels are sufficient, plants generally exhibit enhanced branching as they transition to reproductive growth. This effect is partly mediated through changes in hormonal balance, including hormones other than auxin, such as strigolactones and cytokinins. These hormonal shifts allow lateral buds to activate and develop, producing new branches and a fuller shoot structure. However, when nitrogen levels decline, this delicate balance can be disrupted. Limited nitrogen availability may alter auxin production or interfere with its transport within the plant. As a result, branching remains suppressed and apical dominance is reinforced[4]. This interaction illustrates how nitrogen availability can directly influence plant architecture through hormonal signalling.

Beyond the shoot system, plants demonstrate remarkable flexibility in how their roots respond to different nitrogen conditions. This adaptability is largely guided by auxin activity, which helps shape the overall root growth. Several recent studies have shown that when nitrogen levels in the soil are low, plants adjust the expression of specific genes, leading to higher auxin accumulation in their roots. This increase in auxin encourages the formation of new root branches and improves the plant’s ability to absorb nitrogen-usable form from the soil[5].

In rice, plants with modifications in these regulatory genes show stronger root growth and better adaptation to varying nitrogen supplies. They also tend to grow slightly taller, produce fewer tillers, and yield more grain compared to normal plants[5]. A similar trend has been observed in wheat, where overexpression of the gene responsible for auxin production results in more lateral root branching, greater plant height, more spikes, higher grain yield, and increased nitrogen accumulation in the shoots under different nitrogen conditions[6]. These examples demonstrate how fine-tuning auxin activity can improve root growth in ways that enhance nitrogen uptake and efficiency, providing useful insights for developing crop varieties that perform well even under nitrogen-limited conditions.

Recent studies have uncovered crosstalk between auxin and other hormones, especially cytokinins and strigolactones, which adds further complexity. These hormones often work together, sometimes in opposition, to fine-tune both shoot and root development according to nutrient conditions. Scientists are still attempting to understand how this hormonal communication works, and every discovery adds another layer to our understanding of how plants make growth decisions.

Implications for Agriculture

Understanding how nitrogen availability influences hormonal networks has profound implications for modern agriculture. Plant architecture, including the number and angle of branches, directly affects crop yield and resource use efficiency. By modulating auxin signalling or its interaction with nitrogen metabolism, researchers hope to produce crops that exhibit optimal branching and enhanced Nitrogen Use Efficiency (NUE). Such advances could lead to varieties that thrive on less fertiliser, lowering environmental impact while maintaining productivity, an essential step toward sustainable agriculture.

At first glance, plants may appear passive, but beneath the surface, they are remarkably strategic organisms, constantly assessing their environment and reshaping themselves to survive and thrive. Decoding these intricate internal signals lays the foundation for a new generation of more efficient crops, making the most of limited resources to feed a growing world population.

References:

1. Luschnig, C., Friml, J. 2024. Over 25 years of decrypting PIN-mediated plant development. Nat Commun 15, 9904. https://doi.org/10.1038/s41467-024-54240-y

2. Ding, Z., Galván-Ampudia, C., Demarsy, E., Łangowski, L., Jürgen, K.-V., Yuanwei, F., Miyo T., M., Masao, T., Christian, F., Remko, O. & Jiří, F. 2011. Light-mediated polarization of the PIN3 auxin transporter for the phototropic response in Arabidopsis. Nat Cell Biol 13, 447–452. https://doi.org/10.1038/ncb2208

3. Mus, F., Crook, M. B., Garcia, K., Garcia Costas, A., Geddes, B. A., Kouri, E. D., Paramasivan, P., Ryu, M. H., Oldroyd, G. E. D., Poole, P. S., Udvardi, M. K., Voigt, C. A., Ané, J. M. & Peters, J. W. 2016. Symbiotic Nitrogen Fixation and the Challenges to Its Extension to Nonlegumes. Appl Environ Microbiol 13, 82(13), 3698-3710. doi: 10.1128/AEM.01055-16.

4. De Jong, M., George, G., Ongaro, V., Williamson, L., Willetts, B., Ljung, K., McCulloch, H. & Leyser, O. 2014. Auxin and strigolactone signaling are required for modulation of Arabidopsis shoot branching by nitrogen supply. Plant Physiol 166(1), 384-95. doi: 10.1104/pp.114.242388.

5. Huang, Y., Ji, Z., Tao, Y., Wei, S., Jiao, W., Fang, Y., Jian, P., Shen, C., Qin, Y., Zhang, S., Li, S., Liu, X., Kang, S., Tian, Y., Song, Q., Harberd, N. P., Wang, S. & Li. S. 2023. Improving rice nitrogen-use efficiency by modulating a novel monouniquitination machinery for optimal root plasticity response to nitrogen. Nat Plants 9(11), 1902-1914. doi: 10.1038/s41477-023-01533-7. 

6. Shao, A., Ma, W., Zhao, X., Hu, M., He, X., Teng, W., Li, H. & Tong, Y. 2017. The Auxin Biosynthetic TRYPTOPHAN AMINOTRANSFERASE RELATED TaTAR2.1-3A Increases Grain Yield of Wheat. Plant Physiol 174(4),2274-2288. doi: 10.1104/pp.17.00094.