showbizvn.com June 24, 2014 111 (25) 9319-9324; first published June 9, 2014; https://doi.org/10.1073/showbizvn.com.1400966111
fInstituto de Biologia Molecular y Celular de Plantas, Consejo Superior de Investigaciones Científicas–Universitat Politécnica de Valéncia, ES-46022 Valencia, Spain;
gLaboratoire de Reproduction et Développement des Plantes, Centre National de la Recherche Scientifique, Institut National de la Recherche Agronomique, École Normale Supérieure de Lyon, Université Claude Bernard de Lyon, Université de Lyon, F-69364 Lyon Cedex 07, France; and
Edited by Maarten J. Chrispeels, University of California, San Diego, La Jolla, CA, and approved May 14, 2014 (received for review January 17, 2014)
Fig. 1.
Hydropatterning of root development in Arabidopsis, maize and rice. (A) Diagram showing asymmetries in the local environment generated when seedlings grow on the surface of an agar-based media or the symmetric environment generated when roots are grown through agar. (B and C) LR primordia emerging from the contact (B) or air side (C) of the primary root. (D) Quantification of LR emergence patterns from the primary root under different conditions (n > 10). Various phenotypic categories are indicated with different colors and are marked in A. (E) Cross-section of a rice primary root grown on agar, stained with calcofluor. Image shows the development of aerenchyma (AE) and root hairs (RH) on the air side and an LR emerging from the contact side. (F) Cross-section of a maize root grown on agar and stained with propidium iodide. (G) Diagram showing the construction of “agar sandwiches” used to test the effects of local differences in media composition on LR development in maize. (H) LR outgrowth is induced on two sides by contact with agar (Mock/Control); this effect is diminished on the “Treatment” side when the water potential of the media is reduced using PEG infusion (PEG/Control). Contact of the root with a glass surface does not induce LR outgrowth (Glass/Control). Growth of roots along a single agar surface results in the suppression of LR development on the air side (Air/Control) (n > 10). (I–K) MicroCT-generated images of maize seedlings grown through a macropore of air (I and K) or a continuous volume of soil (J). The root in K is growing in air, whereas in I the root is contacting the soil surface. Root tissue is false-colored in white, and soil is false-colored in brown. Average number of LRs per seedling (D) and per centimeter of primary root (H) is shown at base of columns in bar charts. Error bars indicate SEM. Significant differences based on Fisher’s exact test (P w) and the amount of expressed water on the surface of the media, which reduced the circumferential area of the root contacting liquid water (SI Appendix, Fig. S1) (7). This change in media composition also significantly affected the bias in the distribution of LRs (Fig. 1D). Use of different growth substrates indicated that no specific component of the media besides water was necessary to elicit biased LR development, although these media differed significantly in water potential (range, −0.22 MPa) (SI Appendix, Fig. S2). These data suggest that contact with water, in or on the surface of the media, had a greater influence on the local induction of LR development than small differences in water potential.
Bạn đang xem: Plant roots use a patterning mechanism to position lateral root branches toward available water
Growth of Oryza sativa (rice) and Zea mays (maize) seedlings on agar also resulted in the development of LRs predominantly on the contact side of the root (Fig. 1 E, F, and H and SI Appendix, Fig. S3). In maize, LR development was also locally induced when seedlings were grown on wet germination paper, again indicating that no specific component of the medium besides water was necessary to elicit biased LR development (SI Appendix, Fig. S3). Using a similar experimental system as Karahara et al. (8) (Fig. 1G), we tested the effects of placing the maize primary root between two slabs of control media. Interestingly, LRs developed along both sides contacting the media, demonstrating that multiple distinct domains along the circumferential axis of the root can simultaneously form LRs with intervening areas lacking LR development (Fig. 1H).
X-ray microscale computed tomography (microCT) visualization of maize roots growing through a macropore (large air space) in the soil matrix revealed a similar positioning of LRs biased toward the root face in direct contact with the soil (Fig. 1I, Movie S1, and SI Appendix, Table S1). When roots were grown in pots without a macropore, LRs developed around the entire circumference of the primary root (Fig. 1J and Movie S2). Interestingly, when roots did not contact the soil surface in the macropore (Fig. 1K and Movie S3), LRs emerged sporadically in all directions, suggesting that a nonuniform environment is required for the bias in LR development but that contact is not required for LR development per se in this condition. These data support the physiological relevance of the patterning phenomenon observed in vitro.
In addition to LR emergence, rice and maize seedlings showed preferential accumulation of aerenchyma (air pockets forming in the cortex cell layers that may aid in gas exchange) on the air side of the root (Fig. 1 E and F and SI Appendix, Fig. S3). In maize, the pigment anthocyanin accumulated on the air side of the root whereas it was depleted from the contact side, especially in those regions where preemergent LRs were developing (SI Appendix, Fig. S3). This provided a useful visual marker to distinguish contact and air sides in root cross-sections. Anthocyanin biosynthesis is light-dependent; however, hydropatterning of LRs was not disrupted by growth of plants in the dark (SI Appendix, Fig. S3).
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Rice, maize, and Arabidopsis also showed a clear bias in root hair development on the air side (Fig. 1 E and F and SI Appendix, Fig. S4). In Arabidopsis, suppression of root hair development often occurred before the initiation stage but was not associated with obvious changes in the expression of genes involved in root hair patterning (SI Appendix, Fig. S4). The presence of root hairs was used as a visual marker to distinguish air and contact sides of roots removed from the media for imaging. Root hair initiation on the contact side could be rescued by treatment with abscisic acid (ABA) or the ethylene precursor 1–aminocyclopropane-1–carboxylic acid (ACC), suggesting that the lack of root hair development was not simply a consequence of physical impedance of the growth medium (SI Appendix, Fig. S4). Together these data demonstrated that plant roots are adept at sensing and developmentally responding to local differences in the environment in ways that we hypothesize take advantage of microscopic variations in the distribution of liquid water and air in soil.
The rate at which water is absorbed by the root (Jv) is the product of the driving force for water flow (ΔΨw, the difference in water potential between the root and the growth medium) and the resistance to water flow (inversely proportional to the hydraulic conductivities of the medium and the root, Lp) (9). In our in vitro growth systems, the air is likely at water-potential equilibrium with the culture medium; thus water potential does not distinguish these environments. Hydraulic conductivity, however, differs dramatically; the conductivity of agar (1 × 10−5 m2 s−1 MPa−1) is orders of magnitude higher than that of air (4.18 × 10−12 m2 s−1 MPa−1) (9, 10).
To specifically test the effects that media water potential and hydraulic conductivity have on the local regulation of LR development, we again used the “agar sandwich” approach to vary the media contacting the maize root (treatment agar slab) while a second agar slab contacting the root served as a control. In rice, Karahara et al. (8) previously showed that growth of roots between two slabs of agar results in asymmetries in aerenchyma development if one of the slabs contains mannitol, which reduces water potential of the medium. We performed similar experiments using polyethylene glycol (PEG; infused agar Ψw was −0.63 ± 0.02 MPa, and control agar was −0.10 ± 0.01 MPa) and observed a significant reduction in LR emergence on the treatment side, which partially mimicked the effect of air (Fig. 1H) (11). We severely reduced hydraulic conductivity by placing various non-water-conducting materials between the root and the treatment agar slab. This eliminated the inductive effect of this media on LR development, indicating that hydraulic conductivity of the contacted surface, rather than contact alone, was important for hydropatterning (SI Appendix, Fig. S3). Similar results were obtained when a sheet of glass or silicone rubber was used to contact the root, suggesting that the pliancy of the material was inconsequential (Fig. 1H and SI Appendix, Fig. S3). Together these data suggest that the rate with which water is absorbed by a root from the media determines whether a contacted surface will induce LR development.
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To describe the environmental response phenomena shown here, we have designated the term hydropatterning: a nonuniform distribution of available water causes asymmetries in root development. This term is primarily used to simplify discussion of the process, and we do not intend to imply any specific physiological or molecular mechanisms used by the plant to detect differences in water availability.