Résumé
Foliar fertilization, a key agronomic practice, enhances crop growth and yield by facilitating nutrient uptake through leaf surfaces. This practice involves that plants absorb fertilizers from the leaf surfaces before assimilation. Nitrogen (N) is an important nutrient supplied to crops via the foliar route, mainly as nitrate (NO3-, mineral) and urea (CO(NH2)2, organic). Despite the well-established understanding of root nitrate perception and absorption, the molecular mechanisms underlying nitrate uptake by plant leaves remain elusive. Adressing this gap, our study focuses on characterizing nitrate and urea absorption by Arabidopsis thaliana leaves, aiming to identify the molecular actors involved in transport, sensing and signalling of these small N-based molecules in leaves. Our research objectives encompass critical aspects: (i) the dynamics of foliar nitrate absorption over short periods (0-240 min); (ii) the genome-wide transcriptional response to shoot nitrate supply; (iii) the role of foliar structures - cuticles, stomata and trichomes - in mediating absorption. Through meticulous experimentations using wild genotypes and mutants with altered leaf structures, we aim to untangle the intricate molecular pathways governing nitrate uptake through leaves.Our findings confirmed nitrate is uptaken in leaves, in consistence with the rapid activation of known nitrate-responsive genes. Employing RNA-sequencing techniques, we identified novel molecular candidates potentially involved in both leaf nitrate uptake and the leaf-specific nitrate response pathway. Furthermore, when using a combination of the two N forms, this revealed a synergistic effect on leaf N accumulation. However, looking at the physiological effect of these distinct forms when sprayed on leaves, plants demonstrated different responses.Our investigation extended beyond absorption kinetics to investigate whether plants responded differently to N from foliar versus root sources, addressing long-standing uncertainties surrounding nutrient acquisition mechanisms. By elucidating the molecular underpinnings of foliar nitrate absorption, our study contributes to the broader understanding of plant physiology and agriculture.