The Effects of Fruit Load, Cytokinin, WUSCHEL and CLAVATA3 on Meristem Maintenance

The ability of plants to produce new leaves and flowers depends largely on a small population of stem cells found at the apex of the stem, which is known as the Shoot Apical Meristem (SAM). The activity of the SAM can be influenced by the development of immature fruit, often causing a biennial pattern in yield called Alternate Bearing (AB). In order to understand how the SAM responds to the fruit at a genetic level, the expression profile of inflorescence meristems from Arabidopsis thaliana were studied under high fruit load conditions. The pattern of responses was found to resemble carbohydrate starvation, supporting the competition hypothesis. Several additional discussions are provided to revise old concepts, offer suggestions for improvement, and describe how genetic tools could be best applied to alternate bearing research. On a more basic level, the SAM is maintained by the WUSCHEL-CLAVATA feedback loop, where WUS is thought to directly regulate CLV3. The nature of the CLV3 regulome was studied by combining phylogenetic footprinting with promoter deletions. This found that the 5’ CLV3 promoter is less than 70bp long, and is likely to be regulated in part by an auxin response element. In addition, three large cis-regulatory modules were found in the 3’ enhancer, two of which were found in a naturally occurring transposon. These modules are proposed to be nucleation sites for chromatin silencing, while the role of transposition in the evolution of the WUS-CLV3 feedback loop is discussed. Finally, the role of cytokinin hormones on WUS regulation was investigated by adjusting native cytokinin levels with receptor mutants and genetic constructs, in order to observe changes in WUS, CLV3 and cytokinin fluorescent reporters. The results found that cytokinin had little or no effect on the transcription, nuclear transport, or protein degradation of either gene, while auxin responses rapidly reduced WUS protein levels. Surprisingly, the complete absence of cytokinin responses in the central zone was found to be critical for meristem maintenance, and the response-free zone existed independently of WUS-CLV3 feedback loop