STS-95 space experiment for plant growth and development, and auxin polar transport

Comprehensive analyses of plant hormones in etiolated pea and maize seedlings grown under microgravity conditions in space: Relevance to the International Space Station experiment "Auxin Transport"

Functional relationships between endogenous levels of plant hormones in the growth and development of shoots in etiolated Alaska pea and etiolated Golden Cross Bantam maize seedlings under different gravities were investigated in the "Auxin Transport" experiment aboard the International Space Station (ISS). Comprehensive analyses of 31 species of plant hormones of pea and maize seedlings grown under microgravity (μg) in space and 1 g conditions were conducted. Principal component analysis (PCA) and a multiple regression analysis with the dataset from the plant hormone analysis of the etiolated pea seedlings grown under μg and 1 g conditions in the presence and absence of 2,3,5-triiodobenzoic acid (TIBA) revealed endogenous levels of auxin correlated positively with bending and length of epicotyls. Endogenous cytokinins correlated negatively with them. These results suggest an interaction of auxin and cytokinins in automorphogenesis and growth inhibition of etiolated Alaska pea epicotyls grown under μg conditions in space. Less polar auxin transport with reduced endogenous levels of auxin increased endogenous levels of cytokinins, resulting in changing the growth direction of epicotyls and inhibiting growth. On the other hand, almost no close relationship between endogenous plant hormone levels and growth and development in etiolated maize seedlings grown was observed under μg conditions in space, as per Schulze et al. (1992). However, endogenous levels of IAA in the seedlings grown under μg conditions in space were significantly higher than those grown on Earth, similar to the cases of polar auxin transport already reported.

Transcriptomic Analysis of the Interaction Between FLOWERING LOCUS T Induction and Photoperiodic Signaling in Response to Spaceflight

Spaceflight has an impact on the growth and development of higher plants at both the vegetative stage and reproductive stage. A great deal of information has been available on the vegetative stage in space, but relatively little is known about the influence of spaceflight on plants at the reproductive stage. In this study, we constructed transgenic Arabidopsis thaliana plants expressing the flowering control gene, FLOWERING LOCUS T (FT), together with the green fluorescent protein gene (GFP) under control of a heat shock-inducible promoter (HSP17.4), by which we induced FT expression inflight through remote controlling heat shock (HS) treatment. Inflight photography data showed that induction of FT expression in transgenic plants in space under non-inductive short-day conditions could promote flowering and reduce the length of the inflorescence stem in comparison with that of wild-type plants under the same conditions. Whole-genome microarray analysis of gene expression changes in leaves of wild-type and these transgenic plants grown under the long-day and short-day photoperiod conditions in space indicated that the function of the photoperiod-related spaceflight responsive genes is mainly involved in protein synthesis and post-translation protein modulation, notably protein phosphorylation. In addition, changes of the circadian component of gene expression in response to spaceflight under different photoperiods indicated that roles of the circadian oscillator could act as integrators of spaceflight response and photoperiodic signals in Arabidopsis plants grown in space.

Microarray profile of gene expression in etiolated Pisum sativum seedlings grown under microgravity conditions in space: Relevance to the International Space Station experiment "Auxin Transport"

This paper introduces the use of microarray data technology with Medicago (Medicago truncatula) microarrays to characterize global changes in the transcript abundance of etiolated Alaska pea (Pisum sativum L.) seedlings grown under microgravity (µg) conditions in comparison with those under artificial 1 g conditions on the International Space Station. Of the 44,000 genes of the Medicago microarray platform, more than 25,000 transcripts of pea seedlings were hybridized, suggesting that the microarray platform for Medicago could be useful in the study of gene expression of etiolated pea seedlings grown under µg conditions in space. Gene array data were analyzed according to stringent criteria that restricted the scored genes for specific hybridization values at least twofold. Expression of 1362 and 1558 genes in proximal side (the proximal side) and distal side of the epicotyl to the cotyledons (the distal side), respectively, were highly affected by µg conditions in space. Of the genes analyzed, 407 of 1362 transcripts in the proximal side and 740 of 1558 transcripts in the distal side were expressed at ratios at least twofold. However, in the presence of the auxin transport inhibitor TIBA, 212 of 399 transcripts and 255 of 477 transcripts were expressed at ratios at least twofold as high in the proximal and the distal sides of epicotyls in the seedlings grown under µg conditions, respectively. Based on Venn diagram analysis, 31 transcripts and 24 transcripts were found to commonly increase and decrease, respectively, under µg conditions in space. Venn analysis revealed six auxin-related genes and three water channel AQUAPORIN genes that were responsive to gravity. Among 6 auxin-related genes, the accumulation of transcripts of Auxin-induced protein 5NG4 and Indole-3-acetic acid-amido synthetase GH3.3 tended to increase, and that of Auxin-induced protein, Auxin response factor, SAUR-like auxin-responsive family protein and Auxin response factor tended to decrease under µg conditions, whereas there were no statistic differences between under µg and artificial 1 g conditions. Similarly there were no statistic differences between under µg conditions and artificial 1 g, but the accumulation of NIP3-1 and Plasma membrane intrinsic protein11, and AQUAPORIN1/Tonoplast intrinsic protein tended to increase and decrease, respectively. A possible role of auxin-related genes and AQUAPORIN genes in regulating growth of etiolated pea seedlings grown under µg conditions in space is discussed.

Root gravity response module guides differential growth determining both root bending and apical hook formation in Arabidopsis

The apical hook is a transiently formed structure that plays a protective role when the germinating seedling penetrates through the soil towards the surface. Crucial for proper bending is the local auxin maxima, which defines the concave (inner) side of the hook curvature. As no sign of asymmetric auxin distribution has been reported in embryonic hypocotyls prior to hook formation, the question of how auxin asymmetry is established in the early phases of seedling germination remains largely unanswered. Here, we analyzed the auxin distribution and expression of PIN auxin efflux carriers from early phases of germination, and show that bending of the root in response to gravity is the crucial initial cue that governs the hypocotyl bending required for apical hook formation. Importantly, polar auxin transport machinery is established gradually after germination starts as a result of tight root-hypocotyl interaction and a proper balance between abscisic acid and gibberellins.This article has an associated 'The people behind the papers' interview.

Gravity-regulated localization of PsPIN1 is important for polar auxin transport in etiolated pea seedlings: Relevance to the International Space Station experiment

To clarify the mechanism of gravity-controlled polar auxin transport, we conducted the International Space Station (ISS) experiment "Auxin Transport" (identified by NASA's operation nomenclature) in 2016 and 2017, focusing on the expression of genes related to auxin efflux carrier protein PsPIN1 and its localization in the hook and epicotyl cells of etiolated Alaska pea seedlings grown for three days in the dark under microgravity (μg) and artificial 1 g conditions on a centrifuge in the Cell Biology Experiment Facility (CBEF) in the ISS, and under 1 g conditions on Earth. Regardless of gravity conditions, the accumulation of PsPIN1 mRNA in the proximal side of epicotyls of the seedlings was not different, but tended to be slightly higher as compared with that in the distal side. 2,3,5-Triiodobenzoic acid (TIBA) also did not affect the accumulation of PsPIN1 mRNA in the proximal and distal sides of epicotyls. However, in the apical hook region, TIBA increased the accumulation of PsPIN1 mRNA under μg conditions as compared with that under artificial 1 g conditions in the ISS. The accumulation of PsPIN1 proteins in epicotyls determined by western blotting was almost parallel to that of mRNA of PsPIN1. Immunohistochemical analysis with a specific polyclonal antibody of PsPIN1 revealed that a majority of PsPIN1 in the apical hook and subapical regions of the seedlings grown under artificial 1 g conditions in the ISS localized in the basal side (rootward) of the plasma membrane of the endodermal tissues. Conversely, in the seedlings grown under μg conditions, localization of PsPIN1 was greatly disarrayed. TIBA substantially altered the cellular localization pattern of PsPIN1, especially under μg conditions. These results strongly suggest that the mechanisms by which gravity controls polar auxin transport are more likely to be due to the membrane localization of PsPIN1. This physiologically valuable report describes a close relationship between gravity-controlled polar auxin transport and the localization of auxin efflux carrier PsPIN1 in etiolated pea seedlings based on the μg experiment conducted in space.

Polar auxin transport is essential to maintain growth and development of etiolated pea and maize seedlings grown under 1 g conditions: Relevance to the international space station experiment

We conducted "Auxin Transport" space experiments in 2016 and 2017 in the Japanese Experiment Module (JEM) on the International Space Station (ISS), with the principal objective being integrated analyses of the growth and development of etiolated pea (Pisum sativum L. cv Alaska) and maize (Zea mays L. cv Golden Cross Bantam) seedlings under true microgravity conditions in space relative to auxin dynamics. Etiolated pea seedlings grown under microgravity conditions in space for 3 days showed automorphogenesis. Epicotyls and roots bent ca. 45° and 20° toward the direction away from the cotyledons, respectively, whereas those grown under artificial 1 g conditions produced by a centrifuge in the Cell Biology Experimental Facility (CBEF) in space showed negative and positive gravitropic response in epicotyls and in roots, respectively. On the other hand, the coleoptiles of 4-day-old etiolated maize seedlings grew almost straight, but the mesocotyls curved and grew toward a random direction under microgravity conditions in space. In contrast, the coleoptiles and mesocotyls of etiolated maize seedlings grown under 1 g conditions on Earth were almost straight and grew upward or toward the direction against the gravity vector. The polar auxin transport activity in etiolated pea epicotyls and in maize shoots was significantly inhibited and enhanced, respectively, under microgravity conditions in space as compared with artificial 1 g conditions in space or 1 g conditions on Earth. An inhibitor of polar auxin transport, 2,3,5-triiodobenzoic acid (TIBA) substantially affected the growth direction and polar auxin transport activity in etiolated pea seedlings grown under both artificial 1 g and microgravity conditions in space. These results strongly suggest that adequate polar auxin transport is essential for gravitropic response in plants. Possible mechanisms enhancing polar auxin transport in etiolated maize seedlings grown under microgravity conditions in space are also proposed.

Seed-to-seed-to-seed growth and development of Arabidopsis in microgravity

Arabidopsis thaliana was grown from seed to seed wholly in microgravity on the International Space Station. Arabidopsis plants were germinated, grown, and maintained inside a growth chamber prior to returning to Earth. Some of these seeds were used in a subsequent experiment to successfully produce a second (back-to-back) generation of microgravity-grown Arabidopsis. In general, plant growth and development in microgravity proceeded similarly to those of the ground controls, which were grown in an identical chamber. Morphologically, the most striking feature of space-grown Arabidopsis was that the secondary inflorescence branches and siliques formed nearly perpendicular angles to the inflorescence stems. The branches grew out perpendicularly to the main inflorescence stem, indicating that gravity was the key determinant of branch and silique angle and that light had either no role or a secondary role in Arabidopsis branch and silique orientation. Seed protein bodies were 55% smaller in space seed than in controls, but protein assays showed only a 9% reduction in seed protein content. Germination rates for space-produced seed were 92%, indicating that the seeds developed in microgravity were healthy and viable. Gravity is not necessary for seed-to-seed growth of plants, though it plays a direct role in plant form and may influence seed reserves.

Analysis of apical hook formation in Alaska pea with a 3-D clinostat and agravitropic mutant ageotropum

The formation of the apical hook in dicotyledonous seedlings is believed to be effected by gravity in the dark. However, this notion is mostly based on experiments with the hook formed on the hypocotyl, and no detailed studies are available with the developmental manners of the hook, particularly of the epicotyl hook. The present study aims at clarifying the dynamics of hook formation including the possible involvement of gravity. Time-course studies with normal Alaska pea (Pisum sativum L., cv. Alaska) and an agravitropic pea mutant, ageotropum, under the 1-g conditions and on a 3-D clinostat revealed that (1) the apical hook of the epicotyl forms by the development of the arc-shaped plumule of the embryo existing in the non-germinated seed. The process of formation consists of two stages: development and partial opening, which are controlled by some intrinsic property of the plumule, but not gravity. Approximately when the epicotyl emerges from the seed coat, the hook is established in both pea varieties. In Alaska the established hook is sustained or enhanced by gravity, resulting in a delay of hook opening compared with on a clinostat, which might give an incorrect idea that gravity causes hook formation. (2) During the hook development and opening processes the original plumular arc holds its orientation unchanged to be an established hook, which, therefore, is at the same side of the epicotyl axis as the cotyledons. This is true for both Alaska and ageotropum under 1-g conditions as well as on the clinostat, supporting finding (1). (3) Application of auxin polar transport inhibitors, hydroxyfluorenecarboxylic acid, naphthylphthalamic acid, and triiodobenzoic acid, suppressed the curvature of hook by equal extents in Alaska as well as ageotropum, suggesting that the hook development involves auxin polar transport probably asymmetrically distributed across the plumular axis by some intrinsic property of the plumule not directly related with gravity action.

Close relationships between polar auxin transport and graviresponse in plants

Gravitational force on Earth is one of the major environmental factors affecting plant growth and development. Spacecraft and the International Space Station (ISS), and a three-dimensional (3-D) clinostat have been available to clarify the effects of gravistimulation on plant growth and development in space and on ground conditions, respectively. Under a stimulus-free environment such as space conditions, plants show a growth and developmental habit designated as 'automorphosis' or 'automorphogenesis'. Recent studies in hormonal physiology, together with space and molecular biology, have demonstrated the close relationships between automorphosis and polar auxin transport. Reduced polar auxin transport in space conditions, or induced by the application of polar auxin transport inhibitors, substantially induced automorphosis or automorphosis-like growth and development, indicating that polar auxin transport is responsible for graviresponse in plants. This concise review covers graviresponse in plants and automorphosis observed in space conditions, and polar auxin transport related to graviresponse in etiolated Alaska and ageotropum pea seedlings. Molecular aspects of polar auxin transport clarified in recent studies are also described.

Fundamental plant biology enabled by the space shuttle

The relationship between fundamental plant biology and space biology was especially synergistic in the era of the Space Shuttle. While all terrestrial organisms are influenced by gravity, the impact of gravity as a tropic stimulus in plants has been a topic of formal study for more than a century. And while plants were parts of early space biology payloads, it was not until the advent of the Space Shuttle that the science of plant space biology enjoyed expansion that truly enabled controlled, fundamental experiments that removed gravity from the equation. The Space Shuttle presented a science platform that provided regular science flights with dedicated plant growth hardware and crew trained in inflight plant manipulations. Part of the impetus for plant biology experiments in space was the realization that plants could be important parts of bioregenerative life support on long missions, recycling water, air, and nutrients for the human crew. However, a large part of the impetus was that the Space Shuttle enabled fundamental plant science essentially in a microgravity environment. Experiments during the Space Shuttle era produced key science insights on biological adaptation to spaceflight and especially plant growth and tropisms. In this review, we present an overview of plant science in the Space Shuttle era with an emphasis on experiments dealing with fundamental plant growth in microgravity. This review discusses general conclusions from the study of plant spaceflight biology enabled by the Space Shuttle by providing historical context and reviews of select experiments that exemplify plant space biology science.

Parabolic flight induces changes in gene expression patterns in Arabidopsis thaliana

Our primary objective was to evaluate gene expression changes in Arabidopsis thaliana in response to parabolic flight as part of a comprehensive approach to the molecular biology of spaceflight-related adaptations. In addition, we wished to establish parabolic flight as a tractable operations platform for molecular biology studies. In a succession of experiments on NASA's KC-135 and C-9 parabolic aircraft, Arabidopsis plants were presented with replicated exposure to parabolic flight. Transcriptome profiling revealed that parabolic flight caused changes in gene expression patterns that stood the statistical tests of replication on three different flight days. The earliest response, after 20 parabolas, was characterized by a prominence of genes associated with signal transduction. After 40 parabolas, this prominence was largely replaced by genes associated with biotic and abiotic stimuli and stress. Among these responses, three metabolic processes stand out in particular: the induction of auxin metabolism and signaling, the differential expression of genes associated with calcium-mediated signaling, and the repression of genes associated with disease resistance and cell wall biochemistry. Many, but not all, of these responses are known to be involved in gravity sensing in plants. Changes in auxin-related gene expression were also recorded by reporter genes tuned to auxin signal pathways. These data demonstrate that the parabolic flight environment is appropriate for molecular biology research involving the transition to microgravity, in that with replication, proper controls, and analyses, gene expression changes can be observed in the time frames of typical parabolic flight experiments.

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.

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.

Automorphosis of etiolated pea seedlings in space is simulated by a three-dimensional clinostat and the application of inhibitors of auxin polar transport

Etiolated pea (Pisum sativum L. cv. Alaska) seedlings grown under microgravity conditions in space show automorphosis: bending of epicotyls, inhibition of hook formation and changes in root growth direction. In order to determine the mechanisms of microgravity conditions that induce automorphosis, we used a three-dimensional clinostat and obtained the successful induction of automorphosis-like growth of etiolated pea seedlings. Kinetic studies revealed that epicotyls bent at their basal region towards the clockwise direction far from the cotyledons from the vertical line (0 degrees) at approximately 40 degrees in seedlings grown both at 1 g and in the clinostat within 48 h after watering. Thereafter, epicotyls retained this orientation during growth in the clinostat, whereas those at 1 g changed their growth direction against the gravity vector and exhibited a negative gravitropic response. On the other hand, the plumular hook that had already formed in the embryo axis tended to open continuously by growth at the inner basal portion of the elbow; thus, the plumular hook angle initially increased; this was followed by equal growth on the convex and concave sides at 1 g, resulting in normal hook formation; in contrast, hook formation was inhibited on the clinostat. The automorphosis-like growth and development of etiolated pea seedlings was induced by auxin polar transport inhibitors (9-hydroxyfluorene-9-carboxylic acid, N-(1-naphthyl)phthalamic acid and 2,3,5-triiodobenzoic acid), but not by anti-auxin (p-chlorophenoxyisobutyric acid) at 1 g. An ethylene biosynthesis inhibitor, 1-aminooxyacetic acid, inhibited hook formation at 1 g, and ethylene production of etiolated seedlings was suppressed on the clinostat. Clinorotation on the clinostat strongly reduced the activity of auxin polar transport of epicotyls in etiolated pea seedlings, similar to that observed in space experiments (Ueda J, Miyamoto K, Yuda T, Hoshino T, Fujii S, Mukai C, Kamigaichi S, Aizawa S, Yoshizaki I, Shimazu T, Fukui K (1999) Growth and development, and auxin polar transport in higher plants under microgravity conditions in space: BRIC-AUX on STS-95 space experiment. J Plant Res 112: 487492). These results suggest that clinorotation on a three-dimensional clinostat is a valuable tool for simulating microgravity conditions, and that automorphosis of etiolated pea seedlings is induced by the inhibition of auxin polar transport and ethylene biosynthesis.

[Graviresponse in higher plants and its regulation in molecular bases: relevance to growth and development, and auxin polar transport in etiolated pea seedlings]

We review the graviresponse under true and simulated microgravity conditions on a clinostat in higher plants, and its regulation in molecular bases, especially on the aspect of auxin polar transport in etiolated pea (Pisum sativum L. cv. Alaska) seedlings which were the plant materials subjected to STS-95 space experiments. True and simulated microgravity conditions substantially affected growth and development in etiolated pea seedlings, especially the direction of growth of stems and roots, resulting in automorphosis. In etiolated pea seedlings grown in space, epicotyls were the most oriented toward the direction far from the cotyledons, and roots grew toward the aerial space of Plant Growth Chamber. Automorphosis observed in space were well simulated by a clinorotation on a 3-dimensional clinostat and also phenocopied by the application of auxin polar transport inhibitors of 2,3,5-triiodobenzoic acid, N-(1-naphtyl)phthalamic acid and 9-hydroxyfluorene-9-carboxylic acid. Judging from the results described above together with the fact that activities of auxin polar transport in epicotyls of etiolated pea seedlings grown in space substantially were reduced, auxin polar transport seems to be closely related to automorphosis. Strenuous efforts to learn in molecular levels how gravity contributes to the auxin polar transport in etiolated pea epicotyls resulted in successful identification of PsPIN2 and PsAUX1 genes located in plasma membrane which products are considered to be putative efflux and influx carriers of auxin, respectively. Based on the results of expression of PsPIN2 and PsAUX1 genes under various gravistimulations, a possible role of PsPIN2 and PsAUX1 genes for auxin polar transport in etiolated pea seedlings will be discussed.

Reduced allelopathic inhibition of lettuce (Lactuca sativa) growth caused by velvet bean (Mucuna pruriens) under 3D-clinorotation

Allelopathy between Mucuna pruriens (velvet bean) and Lactuca sativa (lettuce) was studied under 3D-clinorotation. Growth of both roots and shoots of lettuce seedlings was suppressed by the presence of velvet bean. The degree of suppression was less on the clinostat compared to the normal static earth gravity. L-DOPA (L-3, 4-dihydroxyphenylalanine) is known to be a major substance in allelopathy of velvet bean. Amount of L-DOPA diffused out from a sintered filter paper into agar medium was compared between clinorotation and control group, and found no significant difference. It was concluded that some factors related to release, transport, and sensing phenomena of allelopathic substances may be responsible to the new findings in this study.

The molecular characterization and in situ expression pattern of pea SCARECROW gene

Certain mutants of shoot gravitropism were reported to be ascribed to the SCR and SHR loci in Arabidopsis thaliana. The SCR gene was known to regulate the development of endodermis cells that are responsible for sensing gravity in a shoot. With the aim of elucidating the molecular mechanism for gravitropic responses in pea seedlings, we have isolated a putative pea SCR ortholog from a shoot cDNA library. Analyses of the cDNA clones revealed the structure of a full-length ORF coding for 819 amino acid residues. A remarkable feature of pea SCR protein was the presence of asparagine stretches at the N-terminal transcriptional activation domain, which was distinct from the occurrence of glutamine or alanine stretches in the Arabidopsis or maize SCR. A Northern blot analysis revealed a single 3.2-kb pea SCR transcript in addition to a closely related 2.5-kb transcript. Our in situ hybridization data indicated that pea SCR mRNA accumulated in the shoot apical meristem, leaf primordia and a root single cell layer corresponding to the endodermis. The expression patterns were similar to those reported for A. thaliana and Zea mays, suggesting that SCR may be functionally conserved among plants and involved in the differentiation of the endodermis.