Balancing Water Uptake and Loss through the Coordinated Regulation of Stomatal and Root Development

Root development is influenced by nutrient and water availabilities. Plants are able to adjust many attributes of their root in response to environmental signals including the size and shape of the primary root, lateral roots and root hairs. Here we investigated the response of roots to changes in the levels of leaf transpiration associated with altered stomatal frequency. We found that plants with high stomatal density and conductance produce a larger rooting area and as a result have enhanced phosphate uptake capacity whereas plants with low stomatal conductance produce a smaller root. Manipulating the growth environment of plants indicated that enhanced root growth is most likely a result of an increased demand for water rather than phosphate. Plants manipulated to have an increase or reduction in root hair growth show a reduction or increase respectively, in stomatal conductance and density. Our results demonstrate that plants can balance their water uptake and loss through coordinated regulation of both stomatal and root development.

Citation: Hepworth C, Turner C, Landim MG, Cameron D, Gray JE (2016) Balancing Water Uptake and Loss through the Coordinated Regulation of Stomatal and Root Development. PLoS ONE 11(6): e0156930. https://doi.org/10.1371/journal.pone.0156930

Editor: Keqiang Wu, National Taiwan University, TAIWAN

Received: March 18, 2016; Accepted: May 20, 2016; Published: June 8, 2016

Copyright: © 2016 Hepworth et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the paper and its Supporting Information files.

Funding: CH received funding from the Engineering and Physical Sciences Research Council, Grant code: X/003926-12-45-4. CT received funding from the Biotechnology and Biological Research Council, Grant code: X/004216-14-16. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Recent advances in molecular genetics provide the resources to dissect the effect of water loss on plant growth and performance. Work in the genetic model species Arabidopsis thaliana, to unravel the molecular basis of stomatal development, has resulted in a series of mutants which range in stomatal density (D), stomatal conductance, transpiration (E), water use efficiency and carbon assimilation in the same genetic background [1–4]. Arabidopsis plant lines with either low or high D have been produced by manipulating the level of expression of EPIDERMAL PATTERNING FACTORS (EPF). In the current study we focused on genotypes with manipulated levels of EPF1 and EPF2; peptides which normally act to suppress inappropriate stomatal development. Plants manipulated to ectopically overexpress EPF2 have substantially reduced D on leaves, and in our previous study, had approximately 50% of the E of wild-type mature leaves [5]. In contrast, epf1epf2 double mutant plants, lacking both EPF1 and EPF2 expression, have a high D and approximately 170% of the E of wild-type leaves.

Here we investigated direct uptake of phosphate (P) by the roots of plants with manipulated stomatal densities. P is an essential plant nutrient which is actively taken up by transporters predominately located on the surface of root hairs. A relationship between D and nutrient accumulation via mass flow has been previously reported [5]. We sought to address whether this relationship extended to direct nutrient uptake via the roots by examining the accumulation of a radioactive phosphate isotope, supplied to the roots as 33P-orthophosphate. Furthermore, It is generally accepted that the presence of root hairs is important for both water and nutrient uptake by increasing the extent of the rhizosphere and maximising the root to soil interface. Root hairs may also act as environmental sensors in particular of water stress [6]. The molecular genetic basis of root development has been well studied [7] and in particular a family of transcription factors including ROOT HAIR DEFECTIVE 6-LIKE 4 (RSL4), ROOT HAIR DEFECTIVE 6 (RHD6) and RHD SIX LIKE1 (RSL1) have been shown to regulate root hair patterning/development [8, 9]. Plants lacking the expression of these genes display shorter root hairs whilst those overexpressing RSL4 develop longer root hairs.

To our knowledge no study has previously compared the root structure or phosphate uptake between plants of the same species with significantly differing levels of D or E grown under the same conditions. Likewise it is currently unknown what effect altering root hair development may have on D or E. Via manipulation of both the stomatal and root hair development pathways, we sought to address these questions.

Five hours after transfer of Arabidopsis thaliana seedling roots to an aqueous nutrient solution, plants with high D (epf1epf2) had taken up over twice as much radioactive phosphate as wild-type. Low D plants, (the EPF2OE genotype) showed only a slight reduction in phosphate (P) accumulation, and this was not significantly different to the level of P taken up by wild-type plants under the same conditions (Fig 1). This suggested that, whilst an increase in D and E may promote P accumulation the opposite did not hold true; a large decrease in E had no significant effect on P uptake.

Mean total [33P] concentrations (ng g−1) taken up directly by whole plants over 5 hours (n = 5). Bars with no letters in common are significantly different, P