All above ground plant organs initiate or derive from stem cells at the shoot apical meristem. The activity of the shoot apical meristem determines the rate of leaf initiation, which is repressed by the ACTIN RELATED PROTEIN2/3 (ARP2/3) complex in the dark. The ARP2/3 complex is an ancient nucleator of actin filament branches, with roles in a variety of subcellular processes. However, the mechanism by which the ARP2/3 complex regulates shoot apical meristem activity is unknown.
In this thesis I show that the increased shoot apical meristem activity of arp3 in prolonged darkness required the polar auxin efflux carrier PIN-FORMED1 (PIN1). Wild-type shoot apical meristem activity was largely unaffected by inhibitors of polar auxin transport in the dark, and a pin1 mutant had similar shoot apical meristem activity to wild-type. By stark contrast, the increased shoot apical meristem activity of arp3 was hypersensitive to inhibitors of polar auxin transport, and abolished in an arp3pin1 double mutant. Furthermore, multiple phenotypes of a brassinosteroid biosynthesis mutant det2, reported to have reduced PIN1 expression and polar auxin transport, were rescued in an arp3det2 double mutant grown in the light. These results indicate that the ARP2/3 complex regulates the activity of PIN1, possibly by facilitating PIN1 endocytosis, and suggest that the ARP2/3 complex is a repressor of brassinosteroid responses. The auxin response factors ARF4 and ARF5 were found to be repressors of shoot apical meristem activity in the same pathway as the ARP2/3 complex. This result led to the proposal of a model where auxin minima, rather than auxin maxima (where ARF4 and ARF5 are active) are required to initiate new leaves at the shoot apical meristem.
The increased shoot apical meristem activity of arp3 required sugar, the glucose sensor TOR kinase, and the initial steps of glycolysis which generate precursors for cell wall biosynthesis.
In a candidate approach to identify novel transcriptional regulators of dark development, the IND transcription factor was found to repress shoot apical meristem activity redundantly with its homologue HEC2. IND was also found to interact genetically with the phytochrome interacting factors PIF3 and PIF4 to differentially regulate shoot apical meristem activity. Microarray analysis revealed that the primary target of IND is the sugar transporter SWEET15 (upregulated), which promoted shoot apical meristem activity.
This research identifies potential avenues for generating crop varieties with increased shoot apical meristem activity in the dark, which might be advantageous in a mulched system, and for generating semi-dwarf crop varieties, by activating a subset of brassinosteroid responses in brassinosteroid deficient crops.
