Sweet Enough to Flower: In making the developmental switch from vegetative growth to flowering, plants integrate diverse information, including photoperiod, hormone signals, and carbohydrate status. Wahl et al. (p. 704 ; see the Perspective by Danielson and Frommer ) analyzed the physiology of the signaling sugar trehalose-6-phosphate (T6P) in Arabidopsis . Quantities of T6P cycle in daily rhythms that peak toward the end of the day. T6P levels in the shoot apical meristem mirrored sucrose levels. Disruption of T6P production also disrupted expression of the FLOWERING LOCUS T gene, which responds in leaves to day length and generates signals that direct the meristem to initiate flowering programs. T6P production also affected the signaling pathway that links the age of the plant to flowering. By incorporating requirements for T6P signaling in the flowering induction pathways, the plant ensures that adequate carbohydrate reserves have been accumulated. Thus, T6P regulates the shift to flowering by linking carbohydrate status to day length in the leaves and to developmental age in the shoot apical meristem.

Summary: Optimal timing of flowering, a major determinant for crop productivity, is controlled by environmental and endogenous cues. Nutrients are known to modify flowering time; however, our understanding of how nutrients interact with the known pathways, especially at the shoot apical meristem (SAM), is still incomplete. Given the negative side‐effects of nitrogen fertilization, it is essential to understand its mode of action for sustainable crop production. We investigated how a moderate restriction by nitrate is integrated into the flowering network at the SAM, to which plants can adapt without stress symptoms. This condition delays flowering by decreasing expression of SUPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) at the SAM. Measurements of nitrate and the responses of nitrate‐responsive genes suggest that nitrate functions as a signal at the SAM. The transcription factors NIN‐LIKE PROTEIN 7 (NLP7) and NLP6, which act as master regulators of nitrate signaling by binding to nitrate‐responsive elements (NREs), are expressed at the SAM and flowering is delayed in single and double mutants. Two upstream regulators of SOC1 (SQUAMOSA PROMOTER BINDING PROTEIN‐LIKE3 (SPL3) and SPL5) contain functional NREs in their promoters. Our results point at a tissue‐specific, nitrate‐mediated flowering time control in Arabidopsis thaliana.

Background: Throughout their lives plants produce new organs from groups of pluripotent cells called meristems, located at the tips of the shoot and the root. The size of the shoot meristem is tightly controlled by a feedback loop, which involves the homeodomain transcription factor WUSCHEL (WUS) and the CLAVATA (CLV) proteins. This regulatory circuit is further fine-tuned by morphogenic signals such as hormones and sugars. Results: Here we show that a family of four plant-specific proteins, encoded by the FANTASTIC FOUR (FAF) genes, has the potential to regulate shoot meristem size in Arabidopsis thaliana. FAF2 and FAF4 are expressed in the centre of the shoot meristem, overlapping with the site of WUS expression. Consistent with a regulatory interaction between the FAF gene family and WUS, our experiments indicate that the FAFs can repress WUS, which ultimately leads to an arrest of meristem activity in FAF overexpressing lines. The finding that meristematic expression of FAF2 and FAF4 is under negative control by CLV3 further supports the hypothesis that the FAFs are modulators of the genetic circuit that regulates the meristem. Conclusion: This study reports the initial characterization of the Arabidopsis thaliana FAF gene family. Our data indicate that the FAF genes form a plant specific gene family, the members of which have the potential to regulate the size of the shoot meristem by modulating the CLV3-WUS feedback loop.

Summary: The vegetative phase change marks the beginning of the adult phase in the life cycle of plants and is associated with a gradual decline in the microRNA miR156, in response to sucrose status. Trehalose 6‐phosphate (T6P) is a sugar molecule with signaling function reporting the current sucrose state. To elucidate the role of T6P signaling in vegetative phase change, molecular, genetic, and metabolic analyses were performed using Arabidopsis thaliana loss‐of‐function lines in TREHALOSE PHOSPHATE SYNTHASE1 (TPS1), a gene coding for an enzyme that catalyzes the production of T6P. These lines show a significant delay in vegetative phase change, under both short and long day conditions. Induced expression of TPS1 complements this delay in the TPS1 knockout mutant (tps1‐2 GVG::TPS1). Further analyses indicate that the T6P pathway promotes vegetative phase transition by suppressing miR156 expression and thereby modulating the levels of its target transcripts, the SQUAMOSA PROMOTER BINDING PROTEIN‐LIKE genes. TPS1 knockdown plants, with a delayed vegetative phase change phenotype, accumulate significantly more sucrose than wild‐type plants as a result of a feedback mechanism. In summary, we conclude that the T6P pathway forms an integral part of an endogenous mechanism that influences phase transitions dependent on the metabolic state.