Download Molecular and physiological significance of leaf size and shape for

Molecular and physiological significance of leaf size and shape for plant water use
efficiency.
Supervisors Maurizio Mencuccini (Edinburgh GeoSciences), A. Hudson (Edinburgh Biology)
Email: [email protected]; webpage: http://www.geos.ed.ac.uk/homes/maurizio
Introduction. Darwinian evolution occurs because organisms that are better adapted to their
environment leave proportionately more offspring - i.e. are fitter. Plants from drier habitats, for
example, tend to have smaller leaves, leading to the belief that small leaves are an adaptation to
drought. However, fitness is difficult to measure experimentally. In this project, we will adapt a
recently-created genetic resource to overcome these problems and test the fitness effects of leaf
size and shape. The genus Antirrhinum (Plantaginaceae) consists of ~20 species from the
Mediterranean region of variable morphology and physiology. Antirrhinum charidemi, which has
very small leaves and is endemic to the driest place in mainland Europe, has been crossed with
the garden snapdragon, A. majus, which has much larger leaves and is less tolerant of drought.
About 200 hybrids have been self pollinated for eight generations to produce recombinant inbred
lines (RILs), each of which comprises genetically identical plants. These RILs allow the adaptive
value of any character to be measured as the average fitness of the line that has inherited it - the
effects of other characters effectively cancel each other out. Leaf shape and size genes from A.
charidemi have also been introduced into the genetic background of A. majus. Each of the
resulting near-isogenic lines (NILs) produces offspring that differ only for a single character,
providing further material to test the effects of characters in isolation.
Figure 1 (A) Parental species and (B) node 5 leaves from a selection of RILs
produced from their hybrids. (C) Effects of a single leaf size/shape gene,
segregating in an isogenic background. The images show the mean leaf size and
shape for each of the three genotypes produced when a NIL heterozygous for a leaf
size gene was self-pollinated. Not that the gene also affects leaf shape. Leaves in
A-C are shown to the same scale.
Objectives. RILs and NILs will be used to (1) measure the
fitness effects of leaf shape and size in response to water
availability and interacting environmental factors, including
temperature, light and humidity; (2) map the genes
responsible for drought tolerance of A. charidemi and test
whether they correspond to genes that we already know
control plant morphology and (3) measure the effects of leaf
shape and size on plant water and carbon relations to
quantify the costs and benefits that contribute to fitness in dry
environments.
Methods. 1) Measuring fitness. The relative fitness of ~100 selected RILs will be estimated as the
proportion of the total number of viable seeds that each produces after artificial self-pollination.
Characters such as leaf number and area, branch number and length, flower number and fruit size
will also be measured, as well as the density of stomata, the distribution of veins in leaves and leaf
hydraulic conductance, photosynthetic rates and transpiration. 2) Mapping adaptive genes. The
regions of the A. charidemi chromosomes that have been inherited by each of the RILs and NILs
are already known. This information will be used to map the genes responsible for drought
adaptation as QTL. 3) Dissecting the physiological basis for fitness. The underlying
physiological basis for the fitness effects of leaf shape and size will be examined by combining gas
exchange analysis with chlorophyll fluorescence and δ13C/12C ratios for selected RILs and NIL.
Research Training. Collaboration between the Schools of GeoSciences and Biological Sciences
at the University of Edinburgh will provide the PhD student with training in a wide range of
research techniques from both physiological ecology and molecular biology. A group of postdoctoral fellows and a number of PhD students working on related issues will be available for
advice and practical assistance. We will encourage the student’s participation in a number of
training courses both outside the UK and in the MSc courses active within the two Schools.
Summary. Are small and narrow leaves advantageous under drought conditions? Can we map
genes responsible for drought adaptation? Which physiological features make small narrow
leaves advantageous?
References. Mencuccini M, Comstock J (1999). Aust J Pl Physiol. 26:115-24. Langlade NB, Feng X, Dransfield T,
Copsey L, Hanna AI, Thébaud C, Bangham A, Hudson A Coen ES (2005). PNAS USA 102(29): 10221-10226.