BRANCHING PLASTICITY · Plasticity in the Shoot Branching Regulatory Network

Plasticity in the shoot branching regulatory network

As sessile organisms, plants are unable to forage for a suitable environment, but simply depend on the prevailing environmental conditions in which they are growing. However, plant development is remarkably plastic, and information from the external environment can be integrated with the endogenous developmental programmes so that a single plant genotype is able to give rise to a wide range of phenotypes. During domestication, there has been strong selection for extreme phenotypes, low phenotypic variability, and reduced competition with neighbours. This is likely to have compromised developmental plasticity, resulting in restriction of cultivation of breeding lines to certain climatic zones. Limited plasticity is also likely to reduce yield stability in the increasingly unpredictable climate. Therefore, it would be of great interest to obtain plants with extreme yield phenotypes, while maintaining high plasticity in order to cope with the rapid climate changes. To achieve this, it is necessary to understand how plasticity is encoded in the networks that regulate plant development. Shoot system architecture is an excellent model for this endeavour. Both developmental and environmental inputs, such as the position of the bud along the primary axis, phase of growth of the plant, levels and quality of light and nutrient availability have profound effects on the outgrowth of axillary buds. The knowledge on how shoot branching is controlled has advanced significantly over the last few years and has provided the necessary intellectual framework to investigate complex properties, such as branching plasticity. Moreover, shoot branching traits are of great importance across agriculture, horticulture and forestry, what makes the understanding this system particularly valuable. Aiming to understand the basis for dynamic variation in branch number and thus the molecular basis for plasticity in shoot branching, the host laboratory collected branching data from Arabidopsis recombinant inbred lines that were grown under high- or low-N conditions. These inbred lines are derived from a multiparent advanced generation inter-cross (MAGIC) strategy lines (Kover et al., 2009) and have been single nucleotide polymorphism (SNP)-genotyped to high density, allowing quantitative trait loci (QTL) mapping for branch number and branching plasticity. The primary aim of this project is to identify genes that underlie these QTL and characterise them to determine their role in the shoot branching regulatory network. Using the software HAPPY v1.3 (Mott and Flint, 2002), four QTL for shoot branching plasticity were found, in which plasticity is defined as the number of branches formed on high nitrate (N) minus the number of branches formed on low N. At the start of the project, the QTL that accounted for a relatively high proportion of the variation in branching plasticity was selected as the focus of the project. However, this QTL was delineated to a region of 2.5 Mb, containing over 650 genes, and none of these genes are known to be involved in bud activation. To reduce the list of possible candidate genes, the traditional linkage mapping method was combined with an association mapping approach. Therefore, an experiment was done to collect branching data from 99 SNP-genotyped natural accessions of Arabidopsis grown under high or low N conditions. At the end of this experiment, data were obtained from only 53 lines. The remaining 46 lines never flowered or had a very long flowering time, causing them to grow in environmental conditions different from the conditions in which the early flowering lines were growing. The data of the 53 lines were associated with the polymorphisms in the QTL-delimited region and resulted in the selection of two candidate genes: AT1G69880, encoding a thioredoxin H-type 8 (ATH8), and AT1G70000, encoding a myb-like transcription factor family protein, but their function in the regulation of shoot branching plasticity could not be verified. The reliability of the association mapping approach improves when more lines are included in the analysis. Therefore, a new experiment was performed in summer 2011 to collect the branching data of 187 natural accessions grown on high and low N. At the start of the experiment the lines were vernalised for 8 weeks to reduce flowering time. However, the vernalisation treatment did not only reduce flowering time, but also reduced the variation in branching plasticity as most lines adopted a moderate branching plasticity. This made it very difficult to map the trait, but nevertheless provided new insights in the seasonal control of shoot branching plasticity. The analysis of the QTL mapping population showed that there is a tight correlation between branch number and branching plasticity, and that plants with relatively low plasticity tend to have a moderate number of branches on low N and high N while plants with high plasticity tend to have high branch numbers on high N and low numbers on low N. The correlation between flowering time and plasticity on overall data was found to be low, but especially in the early flowering lines there is a tight positive correlation between these traits, suggesting that there is a gradient from lines that flower very early and branch regardless of the N status to lines that flower later and branch depending on the N availability. The MAGIC lines provide information about the phenotype space that can potentially be occupied given the existing allelic diversity, but analysis of natural accessions provides more information on the parameter space that is actually occupied. Most of the correlations that were found between traits of the MAGIC lines were also observed between traits of the unvernalised natural accessions, except for the correlation between number of branches formed on low N and plasticity. In the natural accessions almost all lines adopted a low branching phenotype on low N suggesting that natural selection possibly favoured a conservative use of resources to ensure the production of a limited number of well-supplied seed, rather than risk resources on branches for which insufficient nutrients may be available later at the time of seed filling. More detailed phenotypic analysis of selected genotypes showed that there is a positive correlation between branch number, branching plasticity and seed yield. These correlations demonstrate the agronomic importance of studying plasticity. Shoot branching plasticity is a complex trait, and it is this complexity that makes it difficult to map. Although no candidate genes have been identified, the work described in this report greatly contributed to our understanding of this trait, which will help to unravel the molecular mechanisms that control shoot branching plasticity.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз