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How and why do root apices sense light under the soil surface?

Mo M, Yokawa K, Wan Y, Baluška F - Front Plant Sci (2015)

Bottom Line: Recent biological and microscopic advances have improved our understanding of the processes involved in the sensing and transduction of light signals, resulting in subsequent physiological and behavioral responses in growing root apices.Here, we review current knowledge of cellular distributions of photoreceptors and their signal transduction pathways in diverse root tissues and root apex zones.We are discussing also the roles of auxin transporters in roots exposed to light, as well as interactions of light signal perceptions with sensing of other environmental factors relevant to plant roots.

View Article: PubMed Central - PubMed

Affiliation: College of Biological Sciences and Biotechnology, Beijing Forestry University , Beijing, China.

ABSTRACT
Light can penetrate several centimeters below the soil surface. Growth, development and behavior of plant roots are markedly affected by light despite their underground lifestyle. Early studies provided contrasting information on the spatial and temporal distribution of light-sensing cells in the apical region of root apex and discussed the physiological roles of plant hormones in root responses to light. Recent biological and microscopic advances have improved our understanding of the processes involved in the sensing and transduction of light signals, resulting in subsequent physiological and behavioral responses in growing root apices. Here, we review current knowledge of cellular distributions of photoreceptors and their signal transduction pathways in diverse root tissues and root apex zones. We are discussing also the roles of auxin transporters in roots exposed to light, as well as interactions of light signal perceptions with sensing of other environmental factors relevant to plant roots.

No MeSH data available.


Related in: MedlinePlus

(A) Plant organs and their light environment. Shoot part of plants is fully exposed to light during a day. Root part is exposed only to limited amounts of light which penetrates into the soil during a day. Actual light mosaics in the soil depend on numerous factors and it changes with the depth (Woolley and Stoller, 1978; Tester and Morris, 1987; Kasperbauer and Hunt, 1988). (B) Root apex zonation with respect of light-sensitivity. Root cap, meristem and transition zone are expressing phytochromes (Adam et al., 1994; Somers and Quail, 1995a,b; Goosey et al., 1997) whereas only the transition zone is abundantly expressing phototropin phot1 (Wan et al., 2008, 2012). UVR8 is expressed, similarly as phytochromes, in all zones of Arabidopsis root apex (Rizzini et al., 2011; Yokawa et al., 2014). RUS1 and RUS2 are also expressed preferentially in cells of the transition zone (Leasure et al., 2009; Yu et al., 2013). (C) Tissue-specific and polar distribution of phot1 in cells of the transition zone. While epidermis cells do not express phot1, this blue light photoreceptor essential for negative phototropism of roots is abundant and polarly distributed (shown in blue) in underlying cortex cells (Wan et al., 2008) and controls PIN2 distribution and recycling (Wan et al., 2012). This tissue-specific expression and polarity of phot1 fits nicely to the plant “ocelli” concept (the epidermis act as lens-like tissue and the sub-epidermis as retina-like tissue) as proposed by Haberland for shoots (Haberlandt, 1904; Darwin, 1907; von Guttenberg, 1955).
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Figure 1: (A) Plant organs and their light environment. Shoot part of plants is fully exposed to light during a day. Root part is exposed only to limited amounts of light which penetrates into the soil during a day. Actual light mosaics in the soil depend on numerous factors and it changes with the depth (Woolley and Stoller, 1978; Tester and Morris, 1987; Kasperbauer and Hunt, 1988). (B) Root apex zonation with respect of light-sensitivity. Root cap, meristem and transition zone are expressing phytochromes (Adam et al., 1994; Somers and Quail, 1995a,b; Goosey et al., 1997) whereas only the transition zone is abundantly expressing phototropin phot1 (Wan et al., 2008, 2012). UVR8 is expressed, similarly as phytochromes, in all zones of Arabidopsis root apex (Rizzini et al., 2011; Yokawa et al., 2014). RUS1 and RUS2 are also expressed preferentially in cells of the transition zone (Leasure et al., 2009; Yu et al., 2013). (C) Tissue-specific and polar distribution of phot1 in cells of the transition zone. While epidermis cells do not express phot1, this blue light photoreceptor essential for negative phototropism of roots is abundant and polarly distributed (shown in blue) in underlying cortex cells (Wan et al., 2008) and controls PIN2 distribution and recycling (Wan et al., 2012). This tissue-specific expression and polarity of phot1 fits nicely to the plant “ocelli” concept (the epidermis act as lens-like tissue and the sub-epidermis as retina-like tissue) as proposed by Haberland for shoots (Haberlandt, 1904; Darwin, 1907; von Guttenberg, 1955).

Mentions: Roots, the underground organ of all terrestrial plants, do not grow in a completely dark environment. Actually, sunlight can penetrate several millimeters beneath the soil surface, affecting the development of root architecture and guiding the growth direction of roots (Woolley and Stoller, 1978; Tester and Morris, 1987). When sunlight strikes the ground, the spectral characters of light are altered with depth under the soil surface (Figure 1A; Kasperbauer and Hunt, 1988; Mandoli et al., 1990). Photons in the red and far-red part of the spectrum can penetrate deeper than blue light photons. Furthermore, vascular tissue can conduct light to the roots over several centimeters and, again, red to far-red light reaches deeper than blue light (Briggs and Mandoli, 1984; Sun et al., 2003, 2005). Plants have evolved complex and extremely sensitive light sensing systems to react properly to light of different spectra. Plants have several classes of sensory photoreceptors, including the UV-B photoreceptor, UV-A/blue (B) light receptors and red (R)/far-red (FR) receptors (Briggs and Lin, 2012). Most members of these photoreceptors can be expressed in plant roots, giving roots the ability to sense light at wavelengths from the spectral UV-B to FR regions. For laboratory maintained Arabidopsis seedlings, when shoots and cotyledons are exposed to light and roots are grown in shadowed conditions, the root growth and the root-shoot ratio change prominently (Xu et al., 2013; Yokawa et al., 2013, 2014; Novák et al., 2015). Young seedlings with illuminated roots have shorter hypocotyls and longer roots (Novák et al., 2015). The shading roots condition was applied via new method “GLO-Roots” to analyze the root system architecture, showing that light changes the root architecture (Rellán-Álvarez et al., 2015). Importantly, the phot1 mutant is not affected by light exposure (Rellán-Álvarez et al., 2015). Arabidopsis roots exposed to continuous light generate immediate burst of reactive oxygen species (ROS) and show significantly altered responses to salt stresses (Yokawa et al., 2011, 2014). Moreover, Gundel et al. (2014) have hypothesized that the light perceived by the shoots and canopy cover could affect the root architecture. Glucose, synthesized by the photosynthetic process, influences root growth direction and root architecture by adjusting transport and response of phytohormones, for instance, brassinosteroids, auxin, cytokinin, and ethylene (Kircher and Schopfer, 2012; Roycewicz and Malamy, 2012; Singh et al., 2014a,b). Light, directly or/and indirectly, affects root growth, lateral root initiation, root hair formation and root gravitropic and phototropic bending (Lake and Slack, 1961; Klemmer and Schneider, 1979; Burbach et al., 2012; Hopkins and Kiss, 2012; Wan et al., 2012). Another newly proposed system for cultivating young Arabidopsis seedlings with shaded roots is a D–root system (Silva-Navas et al., 2015). In the D-Root system, the light comes from the top and shoots perceive the same amount and intensity of light whereas roots do not get any light. Only in the modified D-Root system, used to analyze specific wavelengths, the light is provided frontally (Silva-Navas et al., 2015).


How and why do root apices sense light under the soil surface?

Mo M, Yokawa K, Wan Y, Baluška F - Front Plant Sci (2015)

(A) Plant organs and their light environment. Shoot part of plants is fully exposed to light during a day. Root part is exposed only to limited amounts of light which penetrates into the soil during a day. Actual light mosaics in the soil depend on numerous factors and it changes with the depth (Woolley and Stoller, 1978; Tester and Morris, 1987; Kasperbauer and Hunt, 1988). (B) Root apex zonation with respect of light-sensitivity. Root cap, meristem and transition zone are expressing phytochromes (Adam et al., 1994; Somers and Quail, 1995a,b; Goosey et al., 1997) whereas only the transition zone is abundantly expressing phototropin phot1 (Wan et al., 2008, 2012). UVR8 is expressed, similarly as phytochromes, in all zones of Arabidopsis root apex (Rizzini et al., 2011; Yokawa et al., 2014). RUS1 and RUS2 are also expressed preferentially in cells of the transition zone (Leasure et al., 2009; Yu et al., 2013). (C) Tissue-specific and polar distribution of phot1 in cells of the transition zone. While epidermis cells do not express phot1, this blue light photoreceptor essential for negative phototropism of roots is abundant and polarly distributed (shown in blue) in underlying cortex cells (Wan et al., 2008) and controls PIN2 distribution and recycling (Wan et al., 2012). This tissue-specific expression and polarity of phot1 fits nicely to the plant “ocelli” concept (the epidermis act as lens-like tissue and the sub-epidermis as retina-like tissue) as proposed by Haberland for shoots (Haberlandt, 1904; Darwin, 1907; von Guttenberg, 1955).
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Related In: Results  -  Collection

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Figure 1: (A) Plant organs and their light environment. Shoot part of plants is fully exposed to light during a day. Root part is exposed only to limited amounts of light which penetrates into the soil during a day. Actual light mosaics in the soil depend on numerous factors and it changes with the depth (Woolley and Stoller, 1978; Tester and Morris, 1987; Kasperbauer and Hunt, 1988). (B) Root apex zonation with respect of light-sensitivity. Root cap, meristem and transition zone are expressing phytochromes (Adam et al., 1994; Somers and Quail, 1995a,b; Goosey et al., 1997) whereas only the transition zone is abundantly expressing phototropin phot1 (Wan et al., 2008, 2012). UVR8 is expressed, similarly as phytochromes, in all zones of Arabidopsis root apex (Rizzini et al., 2011; Yokawa et al., 2014). RUS1 and RUS2 are also expressed preferentially in cells of the transition zone (Leasure et al., 2009; Yu et al., 2013). (C) Tissue-specific and polar distribution of phot1 in cells of the transition zone. While epidermis cells do not express phot1, this blue light photoreceptor essential for negative phototropism of roots is abundant and polarly distributed (shown in blue) in underlying cortex cells (Wan et al., 2008) and controls PIN2 distribution and recycling (Wan et al., 2012). This tissue-specific expression and polarity of phot1 fits nicely to the plant “ocelli” concept (the epidermis act as lens-like tissue and the sub-epidermis as retina-like tissue) as proposed by Haberland for shoots (Haberlandt, 1904; Darwin, 1907; von Guttenberg, 1955).
Mentions: Roots, the underground organ of all terrestrial plants, do not grow in a completely dark environment. Actually, sunlight can penetrate several millimeters beneath the soil surface, affecting the development of root architecture and guiding the growth direction of roots (Woolley and Stoller, 1978; Tester and Morris, 1987). When sunlight strikes the ground, the spectral characters of light are altered with depth under the soil surface (Figure 1A; Kasperbauer and Hunt, 1988; Mandoli et al., 1990). Photons in the red and far-red part of the spectrum can penetrate deeper than blue light photons. Furthermore, vascular tissue can conduct light to the roots over several centimeters and, again, red to far-red light reaches deeper than blue light (Briggs and Mandoli, 1984; Sun et al., 2003, 2005). Plants have evolved complex and extremely sensitive light sensing systems to react properly to light of different spectra. Plants have several classes of sensory photoreceptors, including the UV-B photoreceptor, UV-A/blue (B) light receptors and red (R)/far-red (FR) receptors (Briggs and Lin, 2012). Most members of these photoreceptors can be expressed in plant roots, giving roots the ability to sense light at wavelengths from the spectral UV-B to FR regions. For laboratory maintained Arabidopsis seedlings, when shoots and cotyledons are exposed to light and roots are grown in shadowed conditions, the root growth and the root-shoot ratio change prominently (Xu et al., 2013; Yokawa et al., 2013, 2014; Novák et al., 2015). Young seedlings with illuminated roots have shorter hypocotyls and longer roots (Novák et al., 2015). The shading roots condition was applied via new method “GLO-Roots” to analyze the root system architecture, showing that light changes the root architecture (Rellán-Álvarez et al., 2015). Importantly, the phot1 mutant is not affected by light exposure (Rellán-Álvarez et al., 2015). Arabidopsis roots exposed to continuous light generate immediate burst of reactive oxygen species (ROS) and show significantly altered responses to salt stresses (Yokawa et al., 2011, 2014). Moreover, Gundel et al. (2014) have hypothesized that the light perceived by the shoots and canopy cover could affect the root architecture. Glucose, synthesized by the photosynthetic process, influences root growth direction and root architecture by adjusting transport and response of phytohormones, for instance, brassinosteroids, auxin, cytokinin, and ethylene (Kircher and Schopfer, 2012; Roycewicz and Malamy, 2012; Singh et al., 2014a,b). Light, directly or/and indirectly, affects root growth, lateral root initiation, root hair formation and root gravitropic and phototropic bending (Lake and Slack, 1961; Klemmer and Schneider, 1979; Burbach et al., 2012; Hopkins and Kiss, 2012; Wan et al., 2012). Another newly proposed system for cultivating young Arabidopsis seedlings with shaded roots is a D–root system (Silva-Navas et al., 2015). In the D-Root system, the light comes from the top and shoots perceive the same amount and intensity of light whereas roots do not get any light. Only in the modified D-Root system, used to analyze specific wavelengths, the light is provided frontally (Silva-Navas et al., 2015).

Bottom Line: Recent biological and microscopic advances have improved our understanding of the processes involved in the sensing and transduction of light signals, resulting in subsequent physiological and behavioral responses in growing root apices.Here, we review current knowledge of cellular distributions of photoreceptors and their signal transduction pathways in diverse root tissues and root apex zones.We are discussing also the roles of auxin transporters in roots exposed to light, as well as interactions of light signal perceptions with sensing of other environmental factors relevant to plant roots.

View Article: PubMed Central - PubMed

Affiliation: College of Biological Sciences and Biotechnology, Beijing Forestry University , Beijing, China.

ABSTRACT
Light can penetrate several centimeters below the soil surface. Growth, development and behavior of plant roots are markedly affected by light despite their underground lifestyle. Early studies provided contrasting information on the spatial and temporal distribution of light-sensing cells in the apical region of root apex and discussed the physiological roles of plant hormones in root responses to light. Recent biological and microscopic advances have improved our understanding of the processes involved in the sensing and transduction of light signals, resulting in subsequent physiological and behavioral responses in growing root apices. Here, we review current knowledge of cellular distributions of photoreceptors and their signal transduction pathways in diverse root tissues and root apex zones. We are discussing also the roles of auxin transporters in roots exposed to light, as well as interactions of light signal perceptions with sensing of other environmental factors relevant to plant roots.

No MeSH data available.


Related in: MedlinePlus