The to provide a range of trichome densities.

The
results of our study found a negative correlation between trichome density and
stomatal index within Arabidopsis
thaliana (Fig. 2A), and a reduced stomatal density in trichome-producing
plants in a wild population of Arabidopsis
halleri (Fig. 3A). This suggests a cost associated with the production of
trichomes, specifically a trade-off between trichome and stomata formation.

However,
within accessions of Arabidopsis
thaliana, we found no significant correlation between stomatal index and
trichome number, likely due to the small sample size. Exploring the benefit of
trichomes in Arabidopsis, we
conducted a drought experiment, however the results were of little significance.

4.1 Negative correlation between trichome
number and stomatal index

Our findings
suggest that across a range of accessions of Arabidopsis thaliana there is a significant negative correlation
between trichome density and stomatal density (-0.242, p = 0.02). This suggests
that during development, the plants are undergoing a trade-off between the
formation of stomata and trichomes. Previous studies have indicated a cost of
trichomes in relation to growth, with trichome producing plants facing a growth
penalty in control treatments compared to glabrous plants (Sletvold and Ågren 2012). Our observations regarding increased
trichome number correlating with reduced stomatal density could explain this
detrimental effect of trichomes under herbivore and drought free conditions. A
reduction in stomatal density may lead to reduced stomatal conductance, therefore
reducing carbon fixation (Tanaka et al. 2013), and negatively impacting upon
plant growth.

However,
the number of trichomes is not consistent throughout a plants life. In Arabidopsis, leaves produced early in
rosette development lack abaxial trichomes, but in older plants, newly
developing leaves possess trichomes on both abaxial and adaxial surfaces (Telfer et al. 1997). Additionally, trichome production on stem internodes and floral organs has been
shown to be gradually repressed following bolting, via temporal control of SPL
genes (Yu et al. 2010). For further research it would be
of interest to measure the correlation between trichomes and stomata at multiple
stages throughout development, to see if plant age has any effect.

4.2 Positive correlation between trichome
number and plant mass and negative correlation for leaf size

In
this study we used four different accessions of Arabidopsis thaliana to provide a range of trichome densities.
However, this introduced a number of confounding variables affecting growth,
most notably in the accession WS. WS grew to a much larger size, and possibly
skewed the correlation between mass and trichome number, which was shown to be
strongly positive across accessions (0.401, p = 0.000). Future research should
be conducted within one accession, polymorphic for trichome production, in
order to control for these variables.

In
our study, we found a significant negative correlation between leaf size and
trichome density (-0.342, p = 0.001). Trichome formation occurs early in
development and during leaf maturation the distance between trichomes increases
due to mitotic divisions in the epidermal cells, separating trichomes (Hülskamp and Schnittger 1998) and reducing density, explaining
this negative trend.

 

4.3 Tolerance to drought

The
drought experiment, although showing significant differences between accessions
in above ground mass and area, was severely limited. In the control treatment
(Fig. 4A, Fig. 4B), Col0 had a significantly greater mean mass and mean area
than C24. This indicates that other factors influencing growth are present, and
that the significant difference in mass between the accessions is due to other
effects. If we were to repeat the study, a single accession would be used,
polymorphic for trichome production to control for these other factors.
Additionally, the plants were grown in a growth chamber lacking in humidity
control and under time constraint, meaning that the severity of drought was
hard to control. Finally, due to time constraints we were unable to record the
trichome density of the individual plants, which would have provided information
about the relationship between drought tolerance and trichome density.

Despite
our findings, previous field experiments indicate that tolerance of plants to
drought conditions are affected by trichome density (Sletvold and Ågren 2012). Leading on from our findings that
increased trichome production is correlated with reduced stomatal density (Fig.
2A, 3C), this could provide a possible mechanism for the higher tolerance to
drought found by Sletvold. To explore whether these differences in tolerance to
drought are due to the presence of trichomes or other confounding effects,
further investigation into a range of physiological traits, such as stomatal
conductance, photosynthesis and evapotranspiration, could expose the mechanisms
affecting drought tolerance.

 

 

4.4 Wild population of Arabidopsis halleri showed reduced
stomatal density in trichome producing morphs

The
wild population of Arabidopsis halleri studied
was formed from a single subspecies, polymorphic for trichome production, and
explores whether the trend we observed in laboratory conditions is present in a
wild population. We found a significantly reduced stomatal index among
trichome-producing plants when compared to their glabrous counterparts (Fig. 3C).
This suggests that a correlation between trichomes and stomata exists within
wild populations. However, we have no indication of the density of trichomes,
and due to other unaccounted environmental effects (Fig. S1) this could vary
significantly between samples. Additionally, Arabidopsis halleri is self-incompatible, maintaining high genetic
diversity which could cause differential responses to the environment (Clauss and Koch 2006).

Previous
studies have shown that interactions between stomatal complexes and trichomes during
development are usually competitive, acting in a complex web of positive and
negative interactions. In Arabidopsis
thaliana, mutants in the gene STOMATAL DENSITY AND DISTRIBUTION 1 (SDD1)
controlling stomatal patterning and density have up to  a
fourfold increase in stomatal density (Von Groll et al. 2002), and demonstrate a strong
reduction in trichome number (Berger and Altmann 2000). Additionally, in tobacco plants,
ectopic expression of the MIXTA gene from Antirrginum
majus results in a significant increase in trichomes, coupled with a
reduction in stomatal density (Glover et al. 1998). Possible linkage of stomatal
density and trichome number through gene expression (e.g. SDD1) could explain
the negative correlation that we have observed in Arabidopsis (Fig. 2A, Fig. 3C).

Over-expression
of SDD1 shows reduced stomatal density, and subsequently reduced photosynthesis
due to limited CO2 at high light (Büssis et al. 2006). In agriculture, a major field of
research is the production of drought resistant crops, with maximisation of
photosynthesis and reduction of unnecessary transpiration being the main aim. Previous
research into Arabidopsis thaliana
has shown increased tolerance to drought through a reduction in stomatal
density (Hepworth et al. 2015). However, without considering the
complex set of biochemical interactions occurring on the leaf surface, a change
in stomatal density may have unforeseen impacts on the plant, possibly even
reducing crop yield. This explains trends observed in previous studies, where
glabrous plants grew larger than trichome-producing plants in the control
treatment (Sletvold and Ågren 2012). More research into the effects of
trichome density on rate of transpiration is required, as it is possible that
the presence of trichomes is having a significant effect on plants ability to
accumulate biomass.

Trichomes
provide many benefits in agriculture through resistance to a range of arthropod
herbivores, and also increasing drought tolerance. However, the possible cost
of trichomes in relation to stomatal conductance should be considered when
breeding for insect resistance. Selection for increased herbivore resistance in
agronomically important crops, such as Solanum
berthaultii (Kalazich and Plaisted 1991),  has been shown to come with
many unwelcome traits, such later maturing plants, and even reductions in yield.

4.5 Conclusions

In
summary, a negative correlation between stomata and trichomes was found to be
expressed in Arabidopsis thaliana, and
also in a wild population of Arabidopsis
halleri. These results indicate that plants may be undergoing a trade-off
in development of trichomes and stomata, which may have an impact of transpiration,
and ultimately stress tolerance in relation to drought.

 

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