Gravity-induced modification of auxin transport and distribution for peg formation in cucumber seedlings: possible roles for CS-AUX1 and CS-PIN1

Gravistimulation effects on Oryza sativa amino acid profile, growth pattern and expression of OsPIN genes

Gravity is an important ecological factor regulating plant growth and developmental processes. Here we used various molecular and biochemical approaches to investigate artificial and normal gravistimulation's effect on the early growth stages of rice (Oryza sativa L.) by changing the orientations of Petri dishes. Rate of amino acid formation, root and shoot growth, and OsPIN expression was significantly higher under gravistimulation compared with the control. Clinostat rotation positively affected plant growth and amino acid profile. However, under normal gravity, vertical-oriented seedlings showed high amino acid levels compared with clinostat, 90°-rotated, and control seedlings. Similarly, seedling growth significantly increased with 90°-rotated and vertical orientations. Artificial gravity and exogenous indole-3-acetic acid induced OsPIN1 expression in the roots, root shoot junction, and shoots of clinorotated seedlings. Phenyl acetic acid induced OsPIN1 expression in the roots and root shoot junction of clinorotated seedlings but not in the shoot. The current study suggests that OsPIN1 is differentially regulated and that it might be involved in the regulation of plant growth. Conversely, OsPIN2 and OsPIN3a are gravity sensors and highly induced in the roots and root shoot junctions of vertical and 90°-rotated seedlings and play an important role in stress conditions. Thus, on exposure to gravity, hormones, and UV-C radiation, these genes are highly regulated by jasmonic acid, 6-benzylaminopurine and gibberellic acid.

Lateral root initiation and formation within the parental root meristem of Cucurbita pepo: is auxin a key player?

Background and Aims

In some plant families, including Cucurbitaceae, initiation and development of lateral roots (LRs) occur in the parental root apical meristem. The objective of this study was to identify the general mechanisms underlying LR initiation (LRI). Therefore, the first cellular events leading to LRI as well as the role of auxin in this process were studied in the Cucurbita pepo root apical meristem.

Methods

Transgenic hairy roots harbouring the auxin-responsive promoter DR5 fused to different reporter genes were used for visualizing of cellular auxin response maxima (ARMs) via confocal laser scanning microscopy and 3-D imaging. The effects of exogenous auxin and auxin transport inhibitors on root branching were analysed.

Key Results

The earliest LRI event involved a group of symmetric anticlinal divisions in pericycle cell files at a distance of 250-350 µm from the initial cells. The visualization of the ARMs enabled the precise detection of cells involved in determining the site of LR primordium formation. A local ARM appeared in sister cells of the pericycle and endodermis files before the first division. Cortical cells contributed to LR development after the anticlinal divisions in the pericycle via the formation of an ARM. Exogenous auxins did not increase the total number of LRs and did not affect the LRI index. Although exogenous auxin transport inhibitors acted in different ways, they all reduced the number of LRs formed.

Conclusions

Literature data, as well as results obtained in this study, suggest that the formation of a local ARM before the first anticlinal formative divisions is the common mechanism underlying LRI in flowering plants. We propose that the mechanisms of the regulation of root branching are independent of the position of the LRI site relative to the parental root tip.

Procedures for chemical fixation in immunohistochemical analyses of PIN proteins regulating polar auxin transport: Relevance to spaceflight experiments

The mechanism by which gravity controls the polar transport of auxin, a plant hormone regulating multiple physiological processes in higher plants, remains unclear, although an important role of PIN proteins as efflux carriers/facilitators in polar auxin transport is suggested. We are going to study the effect of microgravity on the polar transport of auxin, focusing on the cellular localization of its efflux carrier, PsPIN1 in etiolated pea seedlings and ZmPIN1a in etiolated maize seedlings grown under microgravity conditions on the International Space Station (ISS) using immunohistochemical analyses according to space experimental plans (Ueda, 2016). To obtain adequate results regarding the cellular localization of functional proteins, prolonged chemical fixation processes as well as chemical fixatives should be well-matched to the properties of functional proteins as antigens since experimental analyses will be performed on the ground after keeping samples for a long duration on the ISS. As a result of ground verification, clear detection of the cellular localization of PsPIN1 and ZmPIN1a immunohistochemically was successful based on the results of several kinds of chemical fixation tested, even when etiolated pea and maize seedlings were fixed by immersion in chemical fixative for a long duration. The addition of 0.1% (w/v) Nonidet P-40 to chemical fixative composed of 50% (v/v) ethanol and 5% (v/v) acetic acid or that of 50% (v/v) methanol and 5% (v/v) acetic acid has led to a significant improvement in the immunohistochemical detection of PsPIN1 or ZmPIN1a. These chemical fixatives were also shown to be storage-stable for a long time before use. In this study, adequate chemical fixatives and fixation protocols were developed, which can be used to detect localization of PsPIN1 and ZmPIN1a proteins in young etiolated pea and maize seedlings, respectively, using anti PsPIN1 and ZmPIN1a antibodies. These protocols can be used in spaceflight experiments to investigate the effects of the microgravity environment on the ISS on PIN protein localization in pea and maize seedlings.

Two Paralogous Genes Encoding Auxin Efflux Carrier Differentially Expressed in Bitter Gourd (Momordica charantia)

The phytohormone auxin regulates various developmental programs in plants, including cell growth, cell division and cell differentiation. The auxin efflux carriers are essential for the auxin transport. To show an involvement of auxin transporters in the coordination of fruit development in bitter gourd, a juicy fruit, we isolated novel cDNAs (referred as McPIN) encoding putative auxin efflux carriers, including McPIN1, McPIN2 (allele of McPIN1) and McPIN3, from developing fruits of bitter gourd. Both McPIN1 and McPIN3 genes possess six exons and five introns. Hydropathy analysis revealed that both polypeptides have two hydrophobic regions with five transmembrane segments and a predominantly hydrophilic core. Phylogenetic analyses revealed that McPIN1 shared the highest homology to the group of Arabidopsis, cucumber and tomato PIN1, while McPIN3 belonged to another group, including Arabidopsis and tomato PIN3 as well as PIN4. This suggests different roles for McPIN1 and McPIN3 in auxin transport involved in the fruit development of bitter gourd. Maximum mRNA levels for both genes were detected in staminate and pistillate flowers. McPIN1 is expressed in a particular period of early fruit development but McPIN3 continues to be expressed until the last stage of fruit ripening. Moreover, these two genes are auxin-inducible and qualified as early auxin-response genes. Their expression patterns suggest that these two auxin transporter genes play a pivotal role in fruit setting and development.

Gravitropism interferes with hydrotropism via counteracting auxin dynamics in cucumber roots: clinorotation and spaceflight experiments

Roots of land plants show gravitropism and hydrotropism in response to gravity and moisture gradients, respectively, for controlling their growth orientation. Gravitropism interferes with hydrotropism, although the mechanistic aspects are poorly understood. Here, we differentiated hydrotropism from gravitropism in cucumber roots by conducting clinorotation and spaceflight experiments. We also compared mechanisms regulating hydrotropism and auxin-regulated gravitropism. Clinorotated or microgravity (μG)-grown cucumber seedling roots hydrotropically bent toward wet substrate in the presence of moisture gradients, but they grew straight in the direction of normal gravitational force at the Earth's surface (1G) on the ground or centrifuge-generated 1G in space. The roots appeared to become hydrotropically more sensitive to moisture gradients under μG conditions in space. Auxin transport inhibitors significantly reduced the hydrotropic response of clinorotated seedling roots. The auxin efflux protein CsPIN5 was differentially expressed in roots of both clinorotated and μG-grown seedlings; with higher expression in the high-humidity (concave) side than the low-humidity (convex) side of hydrotropically responding roots. Our results suggest that roots become hydrotropically sensitive in μG, and CsPIN5-mediated auxin transport has an important role in inducing root hydrotropism. Thus, hydrotropic and gravitropic responses in cucumber roots may compete via differential auxin dynamics established in response to moisture gradients and gravity.

The gravity-induced re-localization of auxin efflux carrier CsPIN1 in cucumber seedlings: spaceflight experiments for immunohistochemical microscopy

Reorientation of cucumber seedlings induces re-localization of CsPIN1 auxin efflux carriers in endodermal cells of the transition zone between hypocotyl and roots. This study examined whether the re-localization of CsPIN1 was due to the graviresponse. Immunohistochemical analysis indicated that, when cucumber seedlings were grown entirely under microgravity conditions in space, CsPIN1 in endodermal cells was mainly localized to the cell side parallel to the minor axis of the elliptic cross-section of the transition zone. However, when cucumber seeds were germinated in microgravity for 24 h and then exposed to 1g centrifugation in a direction crosswise to the seedling axis for 2 h in space, CsPIN1 was re-localized to the bottom of endodermal cells of the transition zone. These results reveal that the localization of CsPIN1 in endodermal cells changes in response to gravity. Furthermore, our results suggest that the endodermal cell layer becomes a canal by which auxin is laterally transported from the upper to the lower flank in response to gravity. The graviresponse-regulated re-localization of CsPIN1 could be responsible for the decrease in auxin level, and thus for the suppression of peg formation, on the upper side of the transition zone in horizontally placed seedlings of cucumber.

Gravistimulation changes the accumulation pattern of the CsPIN1 auxin efflux facilitator in the endodermis of the transition zone in cucumber seedlings

Cucumber (Cucumis sativus) seedlings grown in a horizontal position develop a specialized protuberance (or peg) on the lower side of the transition zone between the hypocotyl and the root. This occurs by suppressing peg formation on the upper side via a decrease in auxin resulting from a gravitational response. However, the gravity-stimulated mechanism of inducing asymmetric auxin distribution in the transition zone is poorly understood. The gravity-sensing tissue responsible for regulating auxin distribution in the transition zone is thought to be the endodermal cell. To characterize the gravity-stimulated mechanism, the auxin efflux facilitator PIN-FORMED1 (CsPIN1) in the endodermis was identified and the localization of CsPIN1 proteins during the gravimorphogenesis of cucumber seedlings was examined. Immunohistochemical analysis revealed that the accumulation pattern of CsPIN1 protein in the endodermal cells of the transition zone of cucumber seedlings grown horizontally differed from that of plants grown vertically. Gravistimulation for 30 min prompted changes in the accumulation pattern of CsPIN1 protein in the endodermis as well as the asymmetric distribution of auxin in the transition zone. Furthermore, 2,3,5-triiodobenzoic acid inhibited the differential distribution of auxin as well as changes in the accumulation pattern of CsPIN1 in the endodermis of the transition zone during gravistimulation. These results suggest that the altered pattern of CsPIN1 accumulation in the endodermis in response to gravistimulation influences lateral auxin transport through the endodermis, resulting in asymmetric auxin distribution in the transition zone.

AUX/LAX family of auxin influx carriers-an overview

Auxin regulates several aspects of plant growth and development. Auxin is unique among plant hormones for exhibiting polar transport. Indole-3-acetic acid (IAA), the major form of auxin in higher plants, is a weak acid and its intercellular movement is facilitated by auxin influx and efflux carriers. Polarity of auxin movement is provided by asymmetric localization of auxin carriers (mainly PIN efflux carriers). PIN-FORMED (PIN) and P-GLYCOPROTEIN (PGP) family of proteins are major auxin efflux carriers whereas AUXIN1/LIKE-AUX1 (AUX/LAX) are major auxin influx carriers. Genetic and biochemical evidence show that each member of the AUX/LAX family is a functional auxin influx carrier and mediate auxin related developmental programmes in different organs and tissues. Of the four AUX/LAX genes, AUX1 regulates root gravitropism, root hair development and leaf phyllotaxy whereas LAX2 regulates vascular development in cotyledons. Both AUX1 and LAX3 have been implicated in lateral root (LR) development as well as apical hook formation whereas both AUX1 and LAX1 and possibly LAX2 are required for leaf phyllotactic patterning.

Auxin transport routes in plant development

The differential distribution of the plant signaling molecule auxin is required for many aspects of plant development. Local auxin maxima and gradients arise as a result of local auxin metabolism and, predominantly, from directional cell-to-cell transport. In this primer, we discuss how the coordinated activity of several auxin influx and efflux systems, which transport auxin across the plasma membrane, mediates directional auxin flow. This activity crucially contributes to the correct setting of developmental cues in embryogenesis, organogenesis, vascular tissue formation and directional growth in response to environmental stimuli.

Isolation and characterization of a cDNA clone encoding an auxin influx carrier in carnation cuttings. Expression in different organs and cultivars and its relationship with cold storage

Polar auxin transport (PAT) is necessary for the formation of adventitious roots in the base of leafy stem cuttings, as has been demonstrated in several studies in which the application of PAT inhibitors strongly inhibited the rooting of cuttings. However, unlike in the case of lateral roots, there is almost no information on the molecular mechanism that controls PAT in the formation of adventitious roots. A novel cDNA encoding an auxin influx carrier has been isolated and characterized from carnation (Dianthus caryophyllus) cuttings. The full length of DcAUX1 was obtained and the deduced aminoacid sequence revealed a high degree of identity with the corresponding auxin carrier proteins from several species. The expression of this gene depended on the organ, the carnation cultivar and the length of time cuttings had been stored in a cold chamber. As a rule, expression was higher in stem than in leaves, in the basal than in the first internode and in mature than in young leaves irrespective of the cultivar and the duration of the storage. This pattern of expression agrees with the results of a previous study showing that auxin from mature leaves was essential for rooting, while exogenous auxin applied to mature leaves was polarly transported in the stem and accumulated in the basal internode (the rooting zone). Variations in the expression observed during storage (depending of the cultivar) might be related to the variation in PAT and rooting reported in previous studies.

Role of xyloglucan in gravitropic bending of azuki bean epicotyl

The mechanism of the gravitropic bending was studied in azuki bean epicotyls. The cell wall extensibility of the lower side became higher than that of the upper side in the epicotyl bending upward. The contents of matrix polysaccharides of the cell wall (pectin and xyloglucan in hemicellulose-II) in the lower side became smaller than those in the upper side. The molecular mass of xyloglucans in the lower side decreased. After an epicotyl was fixed to a metal rod to prevent the bending, gravistimulation was applied. Fundamentally the same results were obtained with respect to rheological and chemical characteristics of the cell wall as those of epicotyls showing gravitropic bending. The present results suggested that the initial gravitropic bending was caused by the increase in extensibility of the lower side and the decrease in extensibility of the upper side via the change of the cell wall matrix, especially xyloglucans.

Functional characterization of PaLAX1, a putative auxin permease, in heterologous plant systems

We have isolated the cDNA of the gene PaLAX1 from a wild cherry tree (Prunus avium). The gene and its product are highly similar in sequences to both the cDNAs and the corresponding protein products of AUX/LAX-type genes, coding for putative auxin influx carriers. We have prepared and characterized transformed Nicotiana tabacum and Arabidopsis thaliana plants carrying the gene PaLAX1. We have proved that constitutive overexpression of PaLAX1 is accompanied by changes in the content and distribution of free indole-3-acetic acid, the major endogenous auxin. The increase in free indole-3-acetic acid content in transgenic plants resulted in various phenotype changes, typical for the auxin-overproducing plants. The uptake of synthetic auxin, 2,4-dichlorophenoxyacetic acid, was 3 times higher in transgenic lines compared to the wild-type lines and the treatment with the auxin uptake inhibitor 1-naphthoxyacetic acid reverted the changes caused by the expression of PaLAX1. Moreover, the agravitropic response could be restored by expression of PaLAX1 in the mutant aux1 plants, which are deficient in auxin influx carrier activity. Based on our data, we have concluded that the product of the gene PaLAX1 promotes the uptake of auxin into cells, and, as a putative auxin influx carrier, it affects the content and distribution of free endogenous auxin in transgenic plants.

P-chlorophenoxyisobutyric acid impairs auxin response for gravity-regulated peg formation in cucumber (Cucumis sativus) seedlings

Cucumber (Cucumis sativus L.) seedlings form a specialized protuberance, the peg, on the transition zone between the hypocotyl and the root. When cucumber seeds germinate in a horizontal position, the seedlings develop a peg on the lower side of the transition zone. To verify the role of auxin action in peg formation, we examined the effect of the anti-auxin, p-chlorophenoxyisobutyric acid (PCIB), on peg formation and mRNA accumulation of auxin-regulated genes. Application of PCIB to cucumber seedlings inhibited peg formation. The application of indole-3-acetic acid (IAA) competed with PCIB and induced peg formation. Furthermore, application of PCIB decreased auxin-inducible CsIAA1 mRNA and increased auxin-repressible CsGRP1 mRNA in the lower side of the transition zone. The differential accumulation of CsIAA1 and CsGRP1 mRNAs in the transition zone of cucumber seedlings grown in a horizontal position was smaller in the PCIB-treated seedlings. These results demonstrate that endogenous auxin redistributes and induces the differential expression of auxin-regulated genes, and ultimately results in the suppression or induction of peg formation in the gravistimulated transition zone of cucumber seedlings.

The expression of genes coding for auxin carriers in different tissues and along the organ can explain variations in auxin transport and the growth pattern in etiolated lupin hypocotyls

Novel cDNA clones encoding putative auxin influx and efflux carriers have been isolated and characterized from etiolated lupin (Lupinus albus L) hypocotyls. The full length of LaAUX1 and LaPIN1 and the partial length of LaPIN3 were obtained and the deduced amino acid sequence revealed a high degree of identity with the corresponding auxin carrier proteins from several species. The expression of these genes depended on the tissue, the hypocotyl zone and seedling age. LaAUX1 and LaPIN3 were expressed in stele and outer tissues, while LaPIN1 was restricted to the stele. From the above-mentioned results and taking into account the role proposed for the efflux carrier PIN1, it is suggested that LaPIN1 could mediate the basipetal auxin transport already described in this organ. LaAUX1 might facilitate auxin influx in the transport cells. The expression of the three genes decreased down the hypocotyl. The basipetally decreasing gradient in the expression of LaPIN1 coincides with previous results showing a similar gradient in the intensity and polarity of auxin transport. The decisive role ascribed to PIN1 in polar auxin transport due to its localization in the basal end of transporting cells and the existence of such a gradient in the expression of LaPIN1 support the hypothesis of a barrier effect (generated by decreasing auxin transport) previously proposed by our research group as being responsible for the auxin gradient, which controls the growth pattern in etiolated lupin hypocotyls.

Gravity-controlled asymmetrical transport of auxin regulates a gravitropic response in the early growth stage of etiolated pea (Pisum sativum) epicotyls: studies using simulated microgravity conditions on a three-dimensional clinostat and using an agravitropic mutant, ageotropum

Increased expression of the auxin-inducible gene PsIAA4/5 was observed in the elongated side of epicotyls in early growth stages of etiolated pea (Pisum sativum L. cv. Alaska) seedlings grown in a horizontal or an inclined position under 1 g conditions. Under simulated microgravity conditions on a 3D clinostat, accumulation of PsIAA4/5 mRNA was found throughout epicotyls showing automorphosis. Polar auxin transport in the proximal side of epicotyls changed when the seedlings were grown in a horizontal or an inclined position under 1 g conditions, but that under clinorotation did not, regardless of the direction of seed setting. Accumulation of PsPIN1 and PsPIN2 mRNAs in epicotyls was affected by gravistimulation, but not by clinorotation. Under 1 g conditions, auxin-transport inhibitors made epicotyls of seedlings grown in a horizontal or inclined position grow toward the proximal direction to cotyledons. These inhibitors led to epicotyl bending toward the cotyledons in seedlings grown in an inclined position under clinorotation. Polar auxin transport, as well as growth direction, of epicotyls of the agravitropic mutant ageotropum did not respond to various gravistimulation. These results suggest that alteration of polar auxin transport in the proximal side of epicotyls regulates the graviresponse of pea epicotyls.

Differential accumulation of the mRNA of the auxin-repressed gene CsGRP1 and the auxin-induced peg formation during gravimorphogenesis of cucumber seedlings

When cucumber seeds are germinated horizontally, an outgrowth (peg) develops on the lower side of the transition zone between the hypocotyl and the root for pulling the cotyledons and plumule out of the seed coat. We previously suggested that gravistimulation suppresses peg formation on the upper side of the transition zone when placed in a horizontal position. In the gravistimulated transition zone, auxin and the mRNAs of auxin-inducible genes are more abundant in the lower side than in the upper side. Here, using fluorescent differential display, we identified Cucumis sativus glycine-rich protein1(CsGRP1) as a gene whose mRNA accumulated more abundantly on the upper side than on the lower side of the transition zone in response to gravistimulation. Auxin starvation increased CsGRP1 mRNA in segments of the transition zone, and inhibition of polar auxin transport with 2,3,5-triiodobenzoic acid (TIBA) prevented the asymmetric accumulation of CsGRP1 mRNA. These results suggest that gravistimulation increases not only the expression of auxin-inducible genes on the lower side of the transition zone, but also the expression of auxin-repressed genes, such as CsGRP1, on the upper side of cucumber seedlings. In the hypocotyls of 3-day-old seedlings, neither gravistimulation nor changes in auxin level influenced the accumulation of CsGRP1 mRNA. These results suggest that the transition zone responds to gravistimulation in a specific manner by an asymmetric expression of CsGRP1 gene during regulation of peg formation.

Requirement for the gravity-controlled transport of auxin for a negative gravitropic response of epicotyls in the early growth stage of etiolated pea seedlings

Gravity-controlled transport of auxin was studied for a negative gravitropic response in the early growth stage of etiolated pea (Pisum sativum L. cv. Alaska) seedlings, in which epicotyl bending was observed near the cotyledon nodes of the seedlings grown continuously from seeds germinated in a horizontal or an inclined position. Increased expression of an auxin-inducible gene, PsIAA4/5, was observed in the elongated side of epicotyls grown in a horizontal or an inclined position. Regardless of the conditions of seed germination, polar auxin transport in the proximal side of the first internodes of the seedlings was significantly higher than in the distal side. Polar auxin transport in the proximal side of epicotyls grown in an inclined position was significantly lower than in those grown in a horizontal position. In contrast, lateral auxin distribution from the proximal to distal sides in epicotyls grown in an inclined position was significantly higher than in epicotyls grown in a horizontal position. Accumulation of PsPIN1 mRNA encoding a putative auxin efflux facilitator, which was observed in vascular tissue, cortex and epidermis in the proximal and distal sides of epicotyls, was markedly influenced by gravistimulation. These results strongly suggest that gravistimulation induces changeable polar auxin transport and one-way lateral auxin distribution in epicotyls as well as asymmetric auxin accumulation in the proximal and distal sides of epicotyls, resulting in a negative gravitropic response of epicotyls in the early growth stage of pea seedlings.

Shoot circumnutation and winding movements require gravisensing cells

Circumnutation and winding in plants are universal growth movements that allow plants to survive despite their sessile nature. However, the detailed molecular mechanisms controlling these phenomena remain unclear. We previously found that a gravitropic mutant of Japanese morning glory (Pharbitis nil or Ipomoea nil), Shidare-asagao (weeping), is defective not only in circumnutation but also in the winding response. This phenotype is similar to that of the Arabidopsis SCARECROW (SCR) mutant. We therefore investigated whether morning glory SCR (PnSCR) is involved in the weeping phenotype. We found that one amino acid was inserted into the highly conserved VHIID motif in weeping-type PnSCR; this mutation caused abnormal endodermal differentiation. We introduced either the mutant or WT PnSCR into Arabidopsis scr mutants for complementation tests. PnSCR of the WT, but not of weeping, rescued the shoot gravitropism and circumnutation of scr. These results show that both the abnormal gravitropism and the circumnutation defect in weeping are attributable to a loss of PnSCR function. Thus, our data show that gravisensing endodermal cells are indispensable for shoot circumnutation and the winding response and that PnSCR is responsible for the abnormal phenotypes of weeping.

Possible involvement of CS-ACS1 and ethylene in auxin-induced peg formation of cucumber seedlings

BACKGROUND AND AIMS

Cucumber (Cucumis sativus) seedlings develop a peg on the concave side of the gravitropically bending transition zone between the hypocotyl and the root after seed germination. Peg initiation occurs in response to auxin when its levels in the concave side of the transition zone exceed a particular threshold through the graviresponse. Ethylene also plays an important role in peg formation, but its relationship to auxin in this event is not understood. Here, the role ethylene plays in auxin-induced peg formation is studied.

METHODS

Peg formation of cucumber seedlings exposed to ethylene at different stages of growth or during exogenous auxin treatment was observed. In addition, ethylene evolution from the concave and convex sides of the transition zone was compared and their transcription of CS-ACS (1-aminocyclopropane-1-carboxylic acid synthase) genes was analysed by RT-PCR and in situ hybridization.

KEY RESULTS

Seedlings treated with ethylene after peg initiation produced an enlarged peg, whereas ethylene treatment before peg initiation inhibited peg formation. Ethylene also promoted the development of the peg in the auxin-treated seedlings. Furthermore, the concave side of the transition zone at peg initiation produced more ethylene and CS-ACS1 mRNA than the convex side.

CONCLUSIONS

Since CS-ACS1 is an auxin-inducible gene, the greater abundance of auxin in the concave side of the transition zone causes peg initiation and increases CS-ACS1-mediated ethylene biosynthesis, which then facilitates peg development.

Spatial and Temporal Tracing of Vessel Differentiation in Young Arabidopsis Seedlings by the Expression of an Immature Tracheary Element-specific Promoter

The vascular system is a complex tissue composed of several vascular cell types. However, little is known about the differentiation process of each vascular cell in situ. In this study, we found that the expression of the Zinnia cysteine protease 4 (ZCP4) promoter is restricted to only immature tracheary elements (TEs) in situ. Therefore, we monitored the early TE differentiation process in young Arabidopsis seedlings using a fusion gene of the ZCP4 promoter and the beta-glucuronidase gene as a molecular marker. This approach revealed unique processes of vessel differentiation during early seedling development, in which discontinuous initiation of vessel element differentiation occurs at distinct regions, followed by the simultaneous differentiation of protoxylem vessels and bidirectional differentiation of metaxylem vessels to form a vessel in the plant body.

Isolation of cucumber CsARF cDNAs and expression of the corresponding mRNAs during gravity-regulated morphogenesis of cucumber seedlings

Cucumber seedlings show positive gravitropism and bend in the transition zone between the hypocotyl and the root. The peg, a specialized protuberance, develops on the concave side of the bending transition zone. Auxin and the mRNA of an auxin-inducible gene (CsIAA1) isolated from cucumber are differentially accumulated across the transition zone during the gravity-regulated peg formation. In this study, five cDNAs of Auxin Response Factors (ARFs) from cucumber were isolated and their mRNA accumulation was compared with that of CsIAA1. The tissue specificity of CsARF2 mRNA accumulation was similar to that of CsIAA1. Because the structural character of CsARF2 predicts that it is a transcriptional activator, CsARF2 may be involved in the activation of CsIAA1 transcription, which plays a role in gravity-regulated peg formation. Neither gravity nor auxin affected mRNA accumulation of five CsARFs including CsARF2, suggesting that CsARF2 may be regulated at a post-transcriptional level to induce the asymmetric expression of the CsIAA1 gene in response to gravistimulation and auxin in cucumber seedlings.

Auxin-mediated response of cucumber seedlings to gravity

Gravity regulates peg formation because cucumber seedlings grown in a horizontal position develop a peg on the lower side of the transition zone (TR zone) but not on the upper side. Studies on peg formation have suggested the regulation of peg formation by gravity as follows. Cucumber seedlings potentially develop a peg on both the lower and upper sides of the TR zone. The development of the peg on upper side of the TR zone is suppressed in response to gravity. A phytohormone, auxin, induces peg formation. Upon gravistimulation the auxin concentration on the upper side of the TR zone is reduced to a level below the threshold value necessary for peg formation. The unequally distributed auxin across TR zone is caused by a change in accumulation of auxin influx carrier (CsAUX1) protein and auxin efflux carrier (CsPIN1) protein in response to gravity. In addition, TR zone before peg initiation expresses both CsARF2 (putative activator of auxin response factor) and CsIAA1 (putative repressor of auxin-inducible gene expression), by which TR zone could respond the auxin gradient regulated by gravity.