Hormone cross-talk drives nutrient dependent plant development

For their growth and development plants are vitally dependent on the mineral nutrition acquired in form of inorganic ions from the soil. However, the availability of these ions dramatically fluctuates in both time and space, which makes nutrient-limiting conditions a general rule rather than an exception in almost all natural ecosystems. To face these constraints, plants developed a wide range of adaptive responses triggered by sensing systems that perceive the external nutrient availability. Plants ability to perceive and respond fluctuations in nutrient availability by adequate developmental response is mediated by plant hormones. Auxin and its tightly controlled polar distribution mediated by specific transporters from at least three families (PIN, AUX/LAX and PGP) play one of the prominent roles in these dynamic plant developmental adaptations. Importantly, recent findings demonstrate that plant hormones do not act exclusively downstream of the exogenous signals, but actively feedback on mechanisms involved in nutrient acquisition and thus constitute retro-control of growth on nutrient uptake and assimilation. Hence complex nutrient hormonal regulatory network consisting of feedback forward loops operates in plants to control plant behavior in response to fluctuating nitrogen environments. However molecular mechanisms underlying this nutrient hormones regulated plant development are scarcely understood so far. Within the different nutrient, nitrate is one of the major nitrogen source but also a signaling molecule that modifies root growth and development. This sensing mechanism is, at least partly, mediated by the nitrate dependent auxin transport ability of NRT1.1/NPF6.3, one of the 53 NPF nitrate transporters in Arabidopsis. Hence, NRT1.1 might act as an important convergence point at which nitrate and hormone mediated signaling converge. However, how the activity of NRT1.1 is integrated within the hormonal, in particular auxin transport regulating network that control root growth and development is unclear. To address mechanisms integrating nitrate and hormonal regulation of root growth and development we will follow complementary research strategies focusing on 1. Mechanisms of auxin transport adaptation to nitrate availability. 2. Interplay between nitrate and hormones in regulation of auxin transport directing root development. 3. Posttranscriptional modification as mechanisms of nitrate regulated auxin transport. Results of the project will bring new important insights elucidating the molecular bases of the interaction between nitrate and hormones and their role in control of plant development.

Exploitation and acquisition of nutrients from the soil is one of the most challenging aspects of plant adaptation to a sessile life-style. Plant must cope with varying availabilities of mineral elements and secure their optimal income into plant body. This is achieved through adjustment of mechanisms and pathways mediating soil exploitation, uptake of elements, and distribution within the plant body. This vital function of effective soil exploitation and balanced acquisition of all elements is executed by a root organ. In the soil, the root system perceives and integrates local and systemic signals on the plants nutrient status to regulate activity of pathways mediating nutrient uptake and distribution. An important component of this nutrient management strategy involves rapid modulation of root growth and development. In response to nutrient availability, root meristem activity, root elongation growth as well as branching pattern are adjusted in order to optimize nutrient provision to the plant body. Plant hormone auxin and its tightly controlled polar distribution mediated by specific transporters from at least three families (PIN, AUX/LAX and PGP) play one of the prominent role in this dynamic plant developmental adaptations. Importantly, plant hormones do not act exclusively downstream of the exogenous signals, but actively feedback on mechanisms involved in nutrient acquisition and thus constitute retro-control of growth on nutrient uptake and assimilation. Hence complex nutrient hormonal regulatory network consisting of feedback forward loops operates in plants to control plant behavior in response to fluctuating nitrogen environments. However molecular mechanisms underlying this nutrient hormones regulated plant development are scarcely understood so far. In collaboration with our French partner from CNRS/INRA/SupAgro/ in Montpellier we have explored mechanisms underlying this nutrient hormonal interaction. We have established unique imaging platform that enabled us to detect and dissect earliest adaptive developmental responses of the root system to nitrate provision. We found that root responds to nitrate availability by rapid enhancement of growth and we identified auxin transporter PIN2 as an essential determinant for this nitrate dependent root growth regulation. We showed that nitrate through modulation of PIN2 phosphorylation status impact on the subcellular trafficking and abundance of this protein at plasma membrane. In addition we have recognised novel CRF-PIN regulatory module as essential components of nitrate regulated establishment of root branching pattern and its specific function in shoot to root communication. Results of our project provide important insights into molecular basis of plant interaction with environment and adaptive developmental responses.