In plants, the shoot apical meristem (SAM) serves as a reservoir of pluripotent stem cells from which all above ground organs originate. To sustain proper growth, the SAM must maintain homeostasis between the self-renewal of pluripotent stem cells and cell recruitment for lateral organ formation. At the core of the network that regulates this homeostasis in Arabidopsis are the WUSCHEL (WUS) transcription factor specifying stem cell fate and the CLAVATA (CLV) ligand-receptor system limiting WUS expression. In this study, we identified the ERECTA (ER) pathway as a second receptor kinase signaling pathway that regulates WUS expression, and therefore shoot apical and floral meristem size, independently of the CLV pathway. We demonstrate that reduction in class III HD-ZIP and ER function together leads to a significant increase in WUS expression, resulting in extremely enlarged shoot meristems and a switch from spiral to whorled vegetative phyllotaxy. We further show that strong upregulation of WUS in the inflorescence meristem leads to ectopic expression of the AGAMOUS homeotic gene to a level that switches cell fate from floral meristem founder cell to carpel founder cell, suggesting an indirect role for ER in regulating floral meristem identity. This work illustrates the delicate balance between stem cell specification and differentiation in the meristem and shows that a shift in this balance leads to abnormal phyllotaxy and to altered reproductive cell fate.

Environmental stress often leads to an increased production of reactive oxygen species that are involved in plastid-to-nucleus retrograde signaling. Soon after the release of singlet oxygen ( 1 O 2 ) in chloroplasts of the flu mutant of Arabidopsis , reprogramming of nuclear gene expression reveals a rapid transfer of signals from the plastid to the nucleus. We have identified extraplastidic signaling constituents involved in 1 O 2 -initiated plastid-to-nucleus signaling and nuclear gene activation after mutagenizing a flu line expressing the luciferase reporter gene under the control of the promoter of a 1 O 2 -responsive AAA-ATPase gene ( At3g28580 ) and isolating second-site mutations that lead to a constitutive up-regulation of the reporter gene or abrogate its 1 O 2 -dependent up-regulation. One of these mutants, caa39 , turned out to be a weak mutant allele of the Topoisomerase VI (Topo VI) A-subunit gene with a single amino acid substitution. Transcript profile analysis of flu and flu caa39 mutants revealed that Topo VI is necessary for the full activation of AAA-ATPase and a set of 1 O 2 -responsive transcripts in response to 1 O 2 . Topo VI binds to the promoter of the AAA-ATPase and other 1 O 2 -responsive genes, and hence could directly regulate their expression. Under photoinhibitory stress conditions, which enhance the production of 1 O 2 and H 2 O 2 , Topo VI regulates 1 O 2 -responsive and H 2 O 2 -responsive genes in a distinct manner. These results suggest that Topo VI acts as an integrator of multiple signals generated by reactive oxygen species formed in plants under adverse environmental conditions.

Higher plants continuously and iteratively produce new above-ground organs in the form of leaves, stems and flowers. These organs arise from shoot apical meristems whose homeostasis depends on coordination between self-renewal of stem cells and their differentiation into organ founder cells. This coordination is stringently controlled by the central transcription factor WUSCHEL (WUS), which is both necessary and sufficient for stem cell specification in Arabidopsis thaliana. ULTRAPETALA1 (ULT1) was previously identified as a plant-specific, negative regulator of WUS expression. However, molecular mechanisms underlying this regulation remain unknown. ULT1 protein contains a SAND putative DNA-binding domain and a B-box, previously proposed as a protein interaction domain in eukaryotes. Here, we characterise a novel partner of ULT1, named ULT1 INTERACTING FACTOR 1 (UIF1), which contains a Myb domain and an EAR motif. UIF1 and ULT1 function in the same pathway for regulation of organ number in the flower. Moreover, UIF1 displays DNA-binding activity and specifically binds to WUS regulatory elements. We thus provide genetic and molecular evidence that UIF1 and ULT1 work together in floral meristem homeostasis, probably by direct repression of WUS expression.

The results indicate that the ULT1 and LFY pathways act separately in regulating identity and determinacy at the floral meristem. In particular, they independently induce AG expression in the centre of the flower to terminate meristem activity. A model is proposed whereby these independent contributions bring about a switch at the AG locus from an inactive to an active transcriptional state at the correct time and place during flower development.