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PROLIFERA - Proliferative Arrest and its Ecological Relevance as a Resource Allocation Strategy in Arabidopsis

Final Report Summary - PROLIFERA (PROLIFERA - Proliferative Arrest and its Ecological Relevance as a Resource Allocation Strategy in Arabidopsis.)

The study of genetic variation in ecologically relevant plant traits can lead to an understanding of evolutionary processes and result in an increased agricultural output. Recent technological advances in molecular biology and genetics, together with the adoption of model organisms for research, have opened new possibilities to dissect genetic networks. However, to understand the evolutionary forces that shape these networks, an ecological context is necessary. This project aimed at bridging disciplines to study genetic networks underlying an ecologically relevant plant trait. More specifically, we examined the mechanisms underlying seed number control in plants. Such studies on the mechanisms were planned to be complemented by studies of the ecological relevance of the examined traits. The project was implemented through the work of the research fellow Samuel Wüst at the Institute of Evolutionary Biology and Environmental Studies as well as the Institute of Plant Biology of the University of Zurich, Switzerland.

The control over seed numbers in plants often occurs through control over the continuous production of flowers from an inflorescence meristem. In flowering plants, embryogenesis depends on maternal resources, and mothers provision the developing seeds over an extended period of time. Many plants globally arrest meristem growth (and thus flower production) at a specific time during development, depending both on resources and the number of seeds already in production. In this project, we aimed to study this phenomenon termed proliferative arrest by genomic technologies, and isolate genotypes that exhibit variation in the timing of arrest. Furthermore, the project aimed to identify possible ecological or physiological trade-offs of different reproductive allocation strategies under different environmental conditions.

Implementation and Results:

1) The molecular bases of the proliferative arrest was studied by combining laser-assisted microdissection for isolation of inflorescence meristems with high-throughput sequencing technologies for transcript profiling. We isolated meristems of a) infertile plants, b) fruit-bearing growing plants, c) fruit-bearing plants that have stopped growing, and d) plants that had stopped growth but from which the fruits were removed prior to tissue collection (which should result in a reactivation of growth). From the analysis of this material we gained detailed insights into changing molecular processes in maternal tissue when fruits are produced or meristems are arrested. We found that the growth arrest is associated with many changes in gene expression. Interestingly, a full reversal to a growing transcriptional state can be seen within 48 hours after fruit removal. This suggests, that arrested meristems enter a state of cellular quiescence maintaining their identity as meristems. A comparison with genes expressed during the release of axillary shoot dormancy further suggests that the suppression of growth through apical dominance (as described in the literature) and the suppression of growth through fruit production exhibit commonalities but also differences. From our dataset it is clear that the proliferative arrest is associated with a complex regulatory gene network affecting most known plant hormones. Building up on such a dataset, we are planning to infer, which genes and genetic regulatory elements are involved in the cross-talk between mother and offspring.

2) Forward genetics have traditionally played an important role in the understanding of genetic factors underlying developmental processes. We performed a forward mutant screen to isolate genotypes affected in the timing of the proliferative arrest. After screening 5500 individual plants for altered reproductive allocation patterns and confirming the observed phenotypes over at least three generation, we isolated five genotypes with increased seed output (estimated increase between 22 and 47%), one genotype that exhibits an early growth arrest upon fruit production, as well as five genotypes with significantly increased seed size. The genetic characterization of several of these genotypes is in progress, and it is expected that they will provide deeper insights into the genetics of maternal resource provisioning towards offspring. In addition, they represent excellent material for testing fundamental questions about the ecological relevance of different reproductive allocation strategies.

3) It was originally planned to complement the study of the mechanisms underlying proliferative arrest by studies of its ecological relevance through examining trade-offs and constraints to increasing or decreasing seed numbers and sizes. However, because the isolation and description of genetic variants was more time-consuming than previously anticipated, this part of the project was postponed and replaced by a pilot study on a related ecological question: trait optimization in individuals (e.g. maximizing seed number) is only one promising approach to increasing agricultural output. An alternative, yet novel and underexploited approach is the optimization of community- (i.e. field-) level performance. In recent ecological research, a positive relationship between biological diversity (including genotypic diversity) and productivity at the community level has often been found. We established a simple model system to study the genetic basis for such productivity increases in communities with higher genotypic diversity. In further work, we are planning to utilize this for a proof-of-principle test, using simple quantitative genetic methods. Community-level approaches offer the potential to optimize productivity independently of trait-level optimization at the individual level.

In summary, this project established
i) functional insights into the molecular basis of proliferative arrest upon fruit production
ii) important genetic resources to study mechanisms and ecological aspects of reproductive allocation in plants
iii) a new experimental system for a proof-of-principle about the benefits of optimizing productivity at community-level via genotypic diversity.

Overall, our research addresses several fundamental questions in plant development and evolution and generates much-needed new ideas for crop improvement. The latter is effected by proof-of-principles for the use of new ways to optimize traits at both the individual as well as community (i.e. field-) level. The continuation and dissemination of this research is guaranteed by a follow-up project funded by the Swiss National Foundation.