Gravitropism ensures the growth of plant organs along a specific
vector relative to gravity, and dictates the upward growth
of shoots and that of roots down into the soil. Root gravitropism
plays an important role in determining the distribution of
the root system in the soil, and is therefore critical to the physical
anchorage of the plant, as well as to water and nutrient
acquisition (Forde & Lorenzo, 2001; Perrin et al., 2005).
Changes in orientation relative to gravity (gravistimulation)
induce root bending towards the original growth direction, a
process that can be conceptually divided into four successive
steps: gravity perception, signal transduction, signal transmission
and curvature response (Perrin et al., 2005). Columella
cells containing sedimentable amyloplasts in the root cap are
the principal gravity-perceptive sites in roots (Caspar &
Pickard, 1989; Kiss et al., 1989; Blancaflor et al., 1998). The
sedimentation of amyloplasts in the root cap has long been
thought to trigger a signal transduction pathway that promotes
the development of a lateral gradient of auxin, which is then
transported to the elongation zone (EZ), where it leads to differential
cellular elongation on opposite flanks of the corresponding
EZ (Chen et al., 2002).
The formation and maintenance of auxin gradients are
dependent on polar auxin transport, mediated by special transporters,
including auxin influx carriers (in particular AUX1) and
efflux facilitators (PIN-FORMED3 (PIN3) and homologous
protein PIN2/AGR1 (AGRAVITROPIC1)/WAV6 (WAVY
ROOTS 6)/EIR1 (ETHYLENE INSENSITIVE ROOT 1); Ge
et al., 2010). For example, AUX1 encodes an auxin influx-mediating
transmembrane protein, localized in the stele, the apical
side of protophloem cells, columella, epidermis and lateral root
cap tissues (Swarup et al., 2001). By contrast, PINs encode
auxin efflux-mediating proteins, and polar PIN localization
directs auxin flow (Wisniewska et al., 2006). A relocation of
PIN3 within statocytes from a symmetrical distribution at the
plasma membrane is believed to represent the initial step in the
establishment of the lateral auxin gradient on gravistimulation
(Friml et al., 2002;
gradient is shifted basipetally through the combined actions of
AUX1 and PIN2 via lateral root cap and epidermal cells towards
EZ (Friml, 2003; Swarup et al., 2005). AUX1 is present in the
same cells as PIN3 and PIN2, and probably facilitates the
uptake of auxin into the lateral root cap and the epidermal
region, and PIN2 is believed to mediate its directional translocation
towards EZ (Friml, 2003). However, differential cellular
elongation on opposite flanks of the central EZ induced by the
lateral auxin gradient may be responsible for only part of the
gravitropic curvature (Chen et al., 2002). Recent findings have
suggested the involvement of the transition zone (TZ), also
known as the distal elongation zone (DEZ), as a secondary site/
mechanism of gravity sensing for root gravitropism, which may
be independent of the auxin gradient (Wolverton et al., 2002;
Chavarrıa-Krauser et al., 2008; Baluska et al., 2010). Roots of
pin3 mutants lack the bending response in the elongation
region, but bending is not affected in TZ (Chavarrıa-Krauser
et al., 2008; Baluska et al., 2010).
Given that the appropriate distribution of roots within the
soil greatly affects plant survival, roots have evolved to sense
adverse environmental cues and to modulate their growth direction
through a variety of pathways. For example, moisture gradients
and water stress can cause the desensitization of
gravitropism in Arabidopsis by the degradation of amyloplasts in
root columella cells, allowing roots to exhibit positive hydrotropism
(Takahashi et al., 2003). Similarly, roots exposed to salt
stress show inhibited gravitropism, which permits the active
avoidance of stressed regions (Li & Zhang, 2008; Sun et al.,
2008). In addition, the availability of phosphorus has been
shown to regulate the root configuration of legumes by altering
the growth angle of basal roots, so as to facilitate improved
acquisition of phosphorus from the soil (Bonser et al., 1996;
Liao et al., 2001). Similarly, reductions in external potassium
have been observed to trigger agravitropic growth in Arabidopsis,
so that roots grow away from potassium-impoverished regions,
which may well represent a mechanism by which plants respond
to mineral deficiencies in general (Vicente-Agullo et al., 2004).
Furthermore, ammonium (NH4
+), both a major nitrogen source
and common toxicant (Kronzucker et al., 1997), frequently produces
the inhibition of root growth and lateral root formation
(Gerendas et al., 1997; Britto & Kronzucker, 2002; Qin et al.,
2008; Li et al., 2010, 2011a,b, 2012, 2013; Kempinski et al.,
2011), but has also been shown to affect the root gravitropism
response (Zou et al., 2012).
Here, we report a novel Arabidopsis thaliana mutant, gsa-1
(gravitropism sensitive to ammonium-1), which displays
reduced root gravitropism in response to NH4
+ stress. Gene
cloning shows gsa-1 to be allelic to ARG1 (ALTERED
RESPONSE TO GRAVITY1), which is required for the establishment
of the lateral auxin gradient across the root cap following
gravistimulation (Boonsirichai et al., 2003; Harrison &
Masson, 2008).Our results further demonstrate that the disruption
of GSA-1/ARG1 can reduce basipetal auxin transport and
the expression of AUX1 protein in root apices. Moreover, we
demonstrate the involvement of PIN2 in the NH4
+ regulation
Gravitropism มั่นใจการเติบโตของอวัยวะพืชตามการเวกเตอร์ที่สัมพันธ์กับแรงโน้มถ่วง และบอกการเจริญเติบโตขึ้นถ่ายภาพและที่รากลงในดิน ราก gravitropismมีบทบาทสำคัญในการกำหนดชั้นระบบรากในดิน และดังนั้นจึงมีความสำคัญจริงแองเคอเรจพืช เช่นเป็นน้ำและธาตุอาหารซื้อ (Forde และ Lorenzo, 2001 Perrin et al., 2005)เปลี่ยนแปลงในสัมพันธ์กับแรงโน้มถ่วง (gravistimulation)ก่อให้เกิดรากดัดต่อทิศทางการเจริญเติบโตต้นฉบับ การกระบวนการที่สามารถทางแนวคิดแบ่งออกเป็นสี่ต่อเนื่องขั้นตอน: รับรู้แรงโน้มถ่วง transduction สัญญาณ สัญญาณส่งและตอบสนองในโค้ง (Perrin et al., 2005) Columellaamyloplasts sedimentable ในหมวกรากที่ประกอบด้วยเซลล์อยู่ไซต์หลักแรงโน้มถ่วงตาต่ำในราก (Caspar &Pickard, 1989 จูบ et al., 1989 Blancaflor และ al., 1998) ที่ตกตะกอนของ amyloplasts ในหมวกรากได้ความคิดที่ทำให้เกิดทางเดิน transduction สัญญาณที่ส่งเสริมการพัฒนาของการไล่ระดับด้านข้างของออกซิน ซึ่งเป็นแล้วขนส่งไปยังโซน elongation (EZ), ซึ่งจะนำไปสู่การแตกต่างกันelongation โทรศัพท์มือถือบน flanks ตรงข้ามของให้สอดคล้องกับEZ (Chen et al., 2002)ผู้แต่งและบำรุงรักษาของการไล่ระดับสีออกซินขึ้นอยู่กับขนส่งออกซินและโพลาร์ mediated โดยผู้พิเศษรวมทั้งออกซินอีกสายการบิน (ในเฉพาะ AUX1) และefflux facilitators (PIN-FORMED3 (PIN3) and homologousprotein PIN2/AGR1 (AGRAVITROPIC1)/WAV6 (WAVYROOTS 6)/EIR1 (ETHYLENE INSENSITIVE ROOT 1); Geet al., 2010). For example, AUX1 encodes an auxin influx-mediatingtransmembrane protein, localized in the stele, the apicalside of protophloem cells, columella, epidermis and lateral rootcap tissues (Swarup et al., 2001). By contrast, PINs encodeauxin efflux-mediating proteins, and polar PIN localizationdirects auxin flow (Wisniewska et al., 2006). A relocation ofPIN3 within statocytes from a symmetrical distribution at theplasma membrane is believed to represent the initial step in theestablishment of the lateral auxin gradient on gravistimulation(Friml et al., 2002;gradient is shifted basipetally through the combined actions ofAUX1 and PIN2 via lateral root cap and epidermal cells towardsEZ (Friml, 2003; Swarup et al., 2005). AUX1 is present in thesame cells as PIN3 and PIN2, and probably facilitates theuptake of auxin into the lateral root cap and the epidermalregion, and PIN2 is believed to mediate its directional translocationtowards EZ (Friml, 2003). However, differential cellularelongation on opposite flanks of the central EZ induced by thelateral auxin gradient may be responsible for only part of thegravitropic curvature (Chen et al., 2002). Recent findings havesuggested the involvement of the transition zone (TZ), alsoknown as the distal elongation zone (DEZ), as a secondary site/mechanism of gravity sensing for root gravitropism, which maybe independent of the auxin gradient (Wolverton et al., 2002;Chavarrıa-Krauser et al., 2008; Baluska et al., 2010). Roots ofpin3 mutants lack the bending response in the elongationregion, but bending is not affected in TZ (Chavarrıa-Krauseret al., 2008; Baluska et al., 2010).Given that the appropriate distribution of roots within thesoil greatly affects plant survival, roots have evolved to senseadverse environmental cues and to modulate their growth directionthrough a variety of pathways. For example, moisture gradientsand water stress can cause the desensitization ofgravitropism in Arabidopsis by the degradation of amyloplasts inroot columella cells, allowing roots to exhibit positive hydrotropism(Takahashi et al., 2003). Similarly, roots exposed to saltstress show inhibited gravitropism, which permits the activeavoidance of stressed regions (Li & Zhang, 2008; Sun et al.,2008). In addition, the availability of phosphorus has beenshown to regulate the root configuration of legumes by alteringthe growth angle of basal roots, so as to facilitate improvedacquisition of phosphorus from the soil (Bonser et al., 1996;Liao et al., 2001). Similarly, reductions in external potassiumhave been observed to trigger agravitropic growth in Arabidopsis,so that roots grow away from potassium-impoverished regions,which may well represent a mechanism by which plants respondto mineral deficiencies in general (Vicente-Agullo et al., 2004).Furthermore, ammonium (NH4+), both a major nitrogen sourceand common toxicant (Kronzucker et al., 1997), frequently producesthe inhibition of root growth and lateral root formation(Gerendas et al., 1997; Britto & Kronzucker, 2002; Qin et al.,2008; Li et al., 2010, 2011a,b, 2012, 2013; Kempinski et al.,2011), but has also been shown to affect the root gravitropismresponse (Zou et al., 2012).Here, we report a novel Arabidopsis thaliana mutant, gsa-1(gravitropism sensitive to ammonium-1), which displaysreduced root gravitropism in response to NH4+ stress. Genecloning shows gsa-1 to be allelic to ARG1 (ALTEREDRESPONSE TO GRAVITY1), which is required for the establishmentof the lateral auxin gradient across the root cap followinggravistimulation (Boonsirichai et al., 2003; Harrison &Masson, 2008).Our results further demonstrate that the disruptionof GSA-1/ARG1 can reduce basipetal auxin transport andthe expression of AUX1 protein in root apices. Moreover, wedemonstrate the involvement of PIN2 in the NH4+ regulation
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