Nitrate modulates stem cell dynamics in Arabidopsis shoot meristems through cytokinins

Abstract

The shoot apical meristem (SAM) is responsible for the generation of all the aerial parts of plants. Given its critical role, dynamical changes in SAM activity should play a central role in the adaptation of plant architecture to the environment. Using quantitative microscopy, grafting experiments, and genetic perturbations, we connect the plant environment to the SAM by describing the molecular mechanism by which cytokinins signal the level of nutrient availability to the SAM. We show that a systemic signal of cytokinin precursors mediates the adaptation of SAM size and organogenesis rate to the availability of mineral nutrients by modulating the expression of WUSCHEL, a key regulator of stem cell homeostasis. In time-lapse experiments, we further show that this mechanism allows meristems to adapt to rapid changes in nitrate concentration, and thereby modulate their rate of organ production to the availability of mineral nutrients within a few days. Our work sheds light on the role of the stem cell regulatory network by showing that it not only maintains meristem homeostasis but also allows plants to adapt to rapid changes in the environment.Arabidopsis | shoot apical meristem | plant nutrition | plant development | cytokinin hormones P lants have evolved specific mechanisms to adapt their growth and physiology to the availability of mineral nutrients in their environment (1). Various hormones such as auxin, abscisic acid, gibberellin, and cytokinin have been shown to act in this process either locally or systemically (1). Cytokinins in particular play an essential role in plant response to nitrate, where they act as second messengers (2). For example, cytokinins promote lateral root development in areas rich in NO 3 if the overall NO 3 availability for the plant is low (3). In the shoot, cytokinins have been shown to modulate key traits such as leaf size (4, 5) and branch number (6) in response to nitrate.Cytokinins have also been shown to be critical for the maintenance of stem cell homeostasis in the shoot apical meristem (SAM). By modulating the expression of WUSCHEL (WUS), encoding for a homeodomain transcription factor expressed in the center of the meristem, cytokinins promote stem cell proliferation, thus controlling the size of the meristem and the rate of shoot organogenesis (7-10).Using grafting experiments, a recent study showed that a systemic signal of a cytokinin precursor [trans-zeatin riboside (tZR)], traveling from root to shoot through xylem, could influence the size of the vegetative meristem (11). However, it remains unclear whether cytokinin signaling can allow the SAM to respond to changes in nutrient levels in the environment. Here, we examined how a core stem cell regulator in the SAM dynamically responds to changes in mineral nutrient levels in the soil and whether systemic cytokinin signals can account for the dynamic adaptation of meristem function to nutrient levels. We used the inflorescence meristem of Arabidopsis as a model, as this structure produces all the flowers of the plant...

Nitrate acts at the Arabidopsis thaliana shoot apical meristem to regulate flowering time

“…However, evidence that nitrate signaling can directly interact with the flowering network at the SAM has not yet been reported. A recent publication describes a root‐borne cytokinin signal that transduces nitrate availability to the SAM within a matter of days and controls the stem cell population and hence meristem size and growth (Landrein et al ., ). It remains an open question whether this systemic signal also contributes to the regulation of flowering as previously suggested (D'Aloia et al ., ).…”

Connecting the pieces: uncovering the molecular basis for long‐distance communication through plant grafting

“…Coordinated energy allocation between root and shoot growth is essential for optimizing whole‐plant performance. New studies using grafting have identified signaling molecules that coordinate root–shoot responses to drought stress (Takahashi et al ., ), light (Chen et al ., ), and nitrogen (N) availability (Tabata et al ., ; Chen et al ., ; Ohkubo et al ., ; Landrein et al ., ).…”

“…Low‐N conditions induce the expression of C‐ TERMINALLY ENCODED PEPTIDE ( CEP ) peptides in roots (a), which were shown to travel into the shoot where they bind CEP receptors (c; highlighted in blue) and trigger the expression of CEP ‐ DOWNSTREAM ( CEPD ) response peptides (highlighted in pink) that move back down into the root to induce NRT 2.1 expression in a nitrate‐dependent manner (d) (Tabata et al ., ; Ohkubo et al ., ). High‐nitrate conditions in roots activate the production of trans ‐Zeatin cytokinin precursors (highlighted in purple), which were shown, through genetic grafting experiments (e), to move through the xylem into shoots where they participate in the regulation of shoot apical meristem (SAM) size (Osugi et al ., ; Landrein et al ., ). Note that (b–e) illustrate key reciprocal grafting experiments that were used to demonstrate the movement of long‐distance N signals, but are not meant to illustrate all grafting experiments used in the discoveries described.…”

“…Recognizing that well‐nourished Arabidopsis grown with supplemental nitrate exhibits a significantly larger SAM size and higher growth rate, Landrein et al . () set out to identify the molecular signals that connect nutrient sensing in the root with SAM regulation. The researchers identified mutants that are defective in early steps of cytokinin biosynthesis (isopentyl transferase3.5.7 and cyp735a1.2) that specifically desensitize the SAM to increased nitrate availability when they are grafted as rootstocks, but not as scions, indicating that cytokinin precursors relay nutrient information regarding nitrate status from roots to shoots (Landrein et al ., ) (Fig.…”

“…Although grafting plays a central role in the successful production of many crops, the underlying mechanisms that make particular graft combinations productive remain largely unknown. Independent of its application in agriculture, grafting experiments have been used to discover mobile proteins, mRNAs, small RNAs, and small molecules that impact plant morphology (Kim, ; Haywood et al ., ), reproductive transitions (Corbesier et al ., ; Jaeger & Wigge, ), host–microbe and plant–parasite interactions (Shahid et al ., ; Tsikou et al ., ), root–shoot balance (Lin et al ., ; Spiegelman et al ., ; Chen et al ., ; Landrein et al ., ), and other fundamental organismal processes (Martin et al ., ; Navarro et al ., ; Takahashi et al ., ; Tylewicz et al ., ). Advances in genomic resources and molecular techniques that support grafted crops make it possible for the distinct fields of agricultural and experimental grafting to be merged.…”