The transcription factors BEL1 and SPL are required for cytokinin and auxin signaling during ovule development in Arabidopsis

Hormones, such as auxin and cytokinin, are involved in the complex molecular network that regulates the coordinated development of plant organs. Genes controlling ovule patterning have been identified and studied in detail; however, the roles of auxin and cytokinin in ovule development are largely unknown. Here we show that key cytokinin pathway genes, such as isopentenyltransferase and cytokinin receptors, are expressed during ovule development. Also, in a cre1-12 ahk2-2 ahk3-3 triple mutant with severely reduced cytokinin perception, expression of the auxin efflux facilitator PIN-FORMED 1 (PIN1) was severely reduced. In sporocyteless/nozzle (spl/nzz) mutants, which show a similar phenotype to the cre1-12 ahk2-2 ahk3-3 triple mutant, PIN1 expression is also reduced. Treatment with the exogenous cytokinin N-6-benzylaminopurine also altered both auxin distribution and patterning of the ovule; this process required the homeodomain transcription factor BELL1 (BEL1). Thus, this article shows that cytokinin regulates ovule development through the regulation of PIN1. Furthermore, the transcription factors BEL1 and SPL/NZZ, previously described as key regulators of ovule development, are needed for the auxin and cytokinin signaling pathways for the correct patterning of the ovule

A paternal signal induces endosperm proliferation upon fertilization in Arabidopsis

In multicellular organisms, sexual reproduction relies on the formation of highly differentiated cells, the gametes, which await fertilization in a quiescent state. Upon fertilization, the cell cycle resumes. Successful development requires that male and female gametes are in the same phase of the cell cycle. The molecular mechanisms that reinstate cell division in a fertilization-dependent manner are poorly understood in both animals and plants. Using Arabidopsis, we show that a sperm-derived signal induces the proliferation of a female gamete, the central cell, precisely upon fertilization. The central cell is arrested in S phase by the activity of the RETINOBLASTOMA RELATED1 (RBR1) protein. Upon fertilization, delivery of the core cell cycle component CYCD7;1 causes RBR1 degradation and thus S phase progression, ensuring the formation of functional endosperm and, consequently, viable seeds

Auxin-Induced Modulation of ETTIN Activity Orchestrates Gene Expression in Arabidopsis

The phytohormone auxin governs crucial developmental decisions throughout the plant life cycle. Auxin signaling is effectuated by auxin response factors (ARFs) whose activity is repressed by Aux/IAA proteins under low auxin levels, but relieved from repression when cellular auxin concentrations increase. ARF3/ETTIN (ETT) is a conserved noncanonical Arabidopsis thaliana ARF that adopts an alternative auxin-sensing mode of translating auxin levels into multiple transcriptional outcomes. However, a mechanistic model for how this auxin-dependent modulation of ETT activity regulates gene expression has not yet been elucidated. Here, we take a genome-wide approach to show how ETT controls developmental processes in the Arabidopsis shoot through its auxin-sensing property. Moreover, analysis of direct ETT targets suggests that ETT functions as a central node in coordinating auxin dynamics and plant development and reveals tight feedback regulation at both the transcriptional and protein-interaction levels. Finally, we present an example to demonstrate how auxin sensitivity of ETT-protein interactions can shape the composition of downstream transcriptomes to ensure specific developmental outcomes. These results show that direct effects of auxin on protein factors, such as ETT-TF complexes, comprise an important part of auxin biology and likely contribute to the vast number of biological processes affected by this simple molecule