Our previous papers have shown that genome size limits minimum cell size and that small cells (and high cell packing densities) in leaf veins and stomata facilitate high rates of metabolism. However, as we laid out recently in a paper in the International Journal of Plant Sciences, for the benefits of high densities of leaf veins and stomata to be realized, the interior mesophyll tissue of the leaf that is responsible for photosynthesis likely also needs to be re-engineered.
Two papers currently in review discuss this idea further. First, we show that genome size-cell size allometry extends to the mesophyll cells and, critically, that genome size limits the amount of 3D surface are per leaf volume in the mesophyll. Thus, genome size limits not only cell size but also the tissue-level trait that fundamentally limits CO2 into the mesophyll cells. Further, we show that differentiation of the palisade and spongy mesophyll layers in the leaf results in optimization for opposing gradients of light and CO2.
A second paper focuses on the lower, spongy mesophyll tissue, which has been poorly described, predominantly becaus of the difficulties of visualizing this tissue. Using microCT imaging, we show that in the paradermal plane (i.e. parallel to the flat surface of the leaf) the spongy mesophyll of most species shows sonvergence towards a tissue-level honeycomb topology, which conforms to some simple physical laws. Interestingly, this honeycomb topology only occurs among species with genomes and cells above a certain size threshold, suggesting that one way of overcoming potential constraints of having large cells is to change cell shape in a way that increases the surface area-to-volume ratio.
Together, these two papers show how genome size fundamentally limits cell size and tissue structure in the leaf mesophyll and how leaves with large genomes are able to compensate for the potential costs of having large cells. These papers extend ideas as part of our Rules of Life grant from NSF.