SHORTROOT-Mediated Increase in Stomatal Density Has No Impact on Photosynthetic Efficiency

Abstract

ORCID IDs: 0000-0003-4287-6927 (M.L.S.); 0000-0002-7609-0378 (O.V.S.); 0000-0001-5932-6468 (B.J.W.); 0000-0002-4621-1490 (P.W.); 0000-0001-7648-3924 (J.A.L.).The coordinated positioning of veins, mesophyll cells, and stomata across a leaf is crucial for efficient gas exchange and transpiration and, therefore, for overall function. In monocot leaves, stomatal cell files are positioned at the flanks of underlying longitudinal leaf veins, rather than directly above or below. This pattern suggests either that stomatal formation is inhibited in epidermal cells directly in contact with the vein or that specification is induced in cell files beyond the vein. The SHORTROOT pathway specifies distinct cell types around the vasculature in subepidermal layers of both root and shoots, with cell type identity determined by distance from the vein. To test whether the pathway has the potential to similarly pattern epidermal cell types, we expanded the expression domain of the rice (Oryza sativa ssp japonica) OsSHR2 gene, which we show is restricted to developing leaf veins, to include bundle sheath cells encircling the vein. In transgenic lines, which were generated using the orthologous ZmSHR1 gene to avoid potential silencing of OsSHR2, stomatal cell files were observed both in the normal position and in more distant positions from the vein. Contrary to theoretical predictions, and to phenotypes observed in eudicot leaves, the increase in stomatal density did not enhance photosynthetic capacity or increase mesophyll cell density. Collectively, these results suggest that the SHORTROOT pathway may coordinate the positioning of veins and stomata in monocot leaves and that distinct mechanisms may operate in monocot and eudicot leaves to coordinate stomatal patterning with the development of underlying mesophyll cells.The coordinated differentiation of cell types within an organ is a crucial component of morphogenesis, necessary to ensure that the final form is appropriate for function. In this regard, photosynthetic function in plant leaves requires that chloroplast-containing cells in the middle leaf layers are interspersed with veins (to supply water and to redistribute metabolites) and are overlaid with stomatal pores through which carbon dioxide can enter the leaf. In grass leaves, cellular arrangements are defined by parallel longitudinal veins that extend from the base of the leaf sheath to the tip of the leaf blade, with short transverse veins interconnecting the longitudinal network (Sharman, 1942;Esau, 1943;Nelson and Dengler, 1997). This vascular framework underpins linear files of stomata in the epidermis, with each vein being flanked by one to three rows of stomata on both the medial and lateral sides (Stebbins and Shah, 1960). The genetic mechanisms that ensure optimal photosynthetic capacity in grass leaves, by coordinating the development of veins, photosynthetic cell types, and stomata, are not known.Grass leaves develop basipetally such that cellular differentiation proceeds from the tip of the leaf to the base, ...

Stomatal density affects gas diffusion and CO2 assimilation dynamics in Arabidopsis under fluctuating light

“…SD differences had no significant effect on the response of gs to the light intensity change from low to high in the present (Fig. 4A and B) and previous studies (Papanatsiou et al, 2016; Schuler et al, 2018). Contrastingly, ST-OX and ST-RNAi showed gs responses slower or faster, respectively, than that of WT when pre-adapted to the dark condition (Fig.…”

“…Previously, we reported that higher SD resulted in the enhancement of gs and A in Arabidopsis under constant and saturated light conditions (Tanaka et al, 2013). Lawson and Blatt (2014) suggested that with higher SD , it would be instructive to determine biomass productivity under fluctuating light, although only a few studies investigated the relationship between SD and photosynthetic or growth characteristics under that condition (Drake et al, 2013; Papanatsiou et al, 2016; Schuler et al, 2018). Here, we attempted to examine how the change in SD affects gs and A dynamics, biomass production, and water use in Arabidopsis under fluctuating light.…”

“…These facts evidence that rapid stomatal movement can be beneficial for the effective carbon gain and water use under fluctuating light conditions. However, how SD changes affect gs and A dynamics, biomass production, and water use under these conditions has been understudied (Drake et al, 2013; Papanatsiou et al, 2016; Schuler et al, 2018).…”

Conservation and Divergence of YODA MAPKKK Function in Regulation of Grass Epidermal Patterning

“…As disorderly as epidermal identities become within bdyda1 cell files, they still obey tissue-wide fate arrangements, indicating that different factors control lateral (and proximal-distal) positional information in the leaf. While none of the work with grass EPF homologues has yet demonstrated an effect on lateral patterning (Hughes et al, 2017; Yin et al, 2017), expanded expression of SHORTROOT, a factor normally involved in internal cell fates, leads to production of supernumerary stomatal rows in rice (Schuler et al, 2018) suggesting that lateral information may be provided through different pathways.…”

Form, development and function of grass stomata

“…These rows are usually located on both sides of vascular bundles, indicating a developmental signal produced by developing leaf vessels to position stomatal rows (Figure b). In fact, the ectopic expression of mobile ZmSHORTROOT1 (ZmSHR1) in rice bundle sheath cells, which has been implicated with leaf vascular development and Kranz anatomy in maize (Slewinski et al , ), produced additional stomatal rows between veins (Schuler et al , ), which is potentially owed to the higher degree of mobility of grass SHR proteins (Wu et al , ). Consistently, osshr1 osshr2 double‐mutants in rice display strongly reduced stomatal density (Wu et al , ), indicating that this factor might be a vein‐derived morphogenetic signal involved in positioning stomatal rows (Figure b).…”