Light-Regulated Hypocotyl Elongation Involves Proteasome-Dependent Degradation of the Microtubule Regulatory Protein WDL3 in Arabidopsis

Light significantly inhibits hypocotyl cell elongation, and dark-grown seedlings exhibit elongated, etiolated hypocotyls. Microtubule regulatory proteins function as positive or negative regulators that mediate hypocotyl cell elongation by altering microtubule organization. However, it remains unclear how plants coordinate these regulators to promote hypocotyl growth in darkness and inhibit growth in the light. Here, we demonstrate that WAVE-DAMPENED 2–LIKE3 (WDL3), a microtubule regulatory protein of the WVD2/WDL family from Arabidopsis thaliana, functions in hypocotyl cell elongation and is regulated by a ubiquitin-26S proteasome–dependent pathway in response to light. WDL3 RNA interference Arabidopsis seedlings grown in the light had much longer hypocotyls than controls. Moreover, WDL3 overexpression resulted in overall shortening of hypocotyl cells and stabilization of cortical microtubules in the light. Cortical microtubule reorganization occurred slowly in cells from WDL3 RNA interference transgenic lines but was accelerated in cells from WDL3-overexpressing seedlings subjected to light treatment. More importantly, WDL3 protein was abundant in the light but was degraded through the 26S proteasome pathway in the dark. Overexpression of WDL3 inhibited etiolated hypocotyl growth in regulatory particle non-ATPase subunit-1a mutant (rpn1a-4) plants but not in wild-type seedlings. Therefore, a ubiquitin-26S proteasome–dependent mechanism regulates the levels of WDL3 in response to light to modulate hypocotyl cell elongation.     

Digital Single-Cell Analysis of Plant Organ Development Using 3DCellAtlas

Diverse molecular networks underlying plant growth and development are rapidly being uncovered. Integrating these data into the spatial and temporal context of dynamic organ growth remains a technical challenge. We developed 3DCellAtlas, an integrative computational pipeline that semiautomatically identifies cell types and quantifies both 3D cellular anisotropy and reporter abundance at single-cell resolution across whole plant organs. Cell identification is no less than 97.8% accurate and does not require transgenic lineage markers or reference atlases. Cell positions within organs are defined using an internal indexing system generating cellular level organ atlases where data from multiple samples can be integrated. Using this approach, we quantified the organ-wide cell-type-specific 3D cellular anisotropy driving Arabidopsis thaliana hypocotyl elongation. The impact ethylene has on hypocotyl 3D cell anisotropy identified the preferential growth of endodermis in response to this hormone. The spatiotemporal dynamics of the endogenous DELLA protein RGA, expansin gene EXPA3, and cell expansion was quantified within distinct cell types of Arabidopsis roots. A significant regulatory relationship between RGA, EXPA3, and growth was present in the epidermis and endodermis. The use of single-cell analyses of plant development enables the dynamics of diverse regulatory networks to be integrated with 3D organ growth.     

Biological implications of the occurrence of 32 members of the XTH (xyloglucan endotransglucosylase/hydrolase) family of proteins in the bryophyte Physcomitrella patens

This comprehensive overview of the xyloglucan endotransglucosylase/hydrolase (XTH) family of genes and proteins in bryophytes, based on research using genomic resources that are newly available for the moss Physcomitrella patens, provides new insights into plant evolution. In angiosperms, the XTH genes are found in large multi‐gene families, probably reflecting the diverse roles of individual XTHs in various cell types. As there are fewer cell types in P. patens than in angiosperms such as Arabidopsis and rice, it is tempting to deduce that there are fewer XTH family genes in bryophytes. However, the present study unexpectedly identified as many as 32 genes that potentially encode XTH family proteins in the genome of P. patens, constituting a fairly large multi‐gene family that is comparable in size with those of Arabidopsis and rice. In situ localization of xyloglucan endotransglucosylase activity in this moss indicates that some P. patens XTH proteins exhibit biochemical functions similar to those found in angiosperms, and that their expression profiles are tissue‐dependent. However, comparison of structural features of families of XTH genes between P. patens and angiosperms demonstrated the existence of several bryophyte‐specific XTH genes with distinct structural and functional features that are not found in angiosperms. These bryophyte‐specific XTH genes might have evolved to meet morphological and functional needs specific to the bryophyte. These findings raise interesting questions about the biological implications of the XTH family of proteins in non‐seed plants.     

Extracellular ATP and nitric oxide signaling pathways regulate redox-dependent responses associated to root hair growth in etiolated Arabidopsis seedlings

Extracellular ATP (eATP) and nitric oxide (NO) have emerged as crucial players in plant development, stress responses and cell viability. Glutathione (GSH) is an abundant reducing agent with proposed roles in plant growth, development and stress physiology. In a recent publication, we demonstrated that eATP and NO restore hypocotyl elongation of etiolated Arabidopsis seedlings treated with GSH. Here it is reported that exogenous ATP also restores root hair growth suggesting a role for ATP and NO in the regulation of redox balance associated to specific processes of plant morphogenesis. A tentative model integrating redox-, eATP- and NO-signaling pathways during root hair growth in Arabidopsis seedlings is presented.Key words: Arabidopsis thaliana, extracellular ATP signaling, nitric oxide, redox system, root hair     

Ride to cell wall: Arabidopsis XTH11, XTH29 and XTH33 exhibit different secretion pathways and responses to heat and drought stress

The xyloglucan endotransglucosylase/hydrolases (XTHs) are enzymes involved in cell wall assembly and growth regulation, cleaving and re-joining hemicellulose chains in the xyloglucan–cellulose network. Here, in a homologous system, we compare the secretion patterns of XTH11, XTH33 and XTH29, three members of the Arabidopsis thaliana XTH family, selected for the presence (XTH11 and XTH33) or absence (XTH29) of a signal peptide, and the presence of a transmembrane domain (XTH33). We show that XTH11 and XTH33 reached, respectively, the cell wall and plasma membrane through a conventional protein secretion (CPS) pathway, whereas XTH29 moves towards the apoplast following an unconventional protein secretion (UPS) mediated by exocyst-positive organelles (EXPOs). All XTHs share a common C-terminal functional domain (XET-C) that, for XTH29 and a restricted number of other XTHs (27, 28 and 30), continues with an extraterminal region (ETR) of 45 amino acids. We suggest that this region is necessary for the correct cell wall targeting of XTH29, as the ETR-truncated protein never reaches its final destination and is not recruited by EXPOs. Furthermore, quantitative real-time polymerase chain reaction analyses performed on 4-week-old Arabidopsis seedlings exposed to drought and heat stress suggest a different involvement of the three XTHs in cell wall remodeling under abiotic stress, evidencing stress-, organ- and time-dependent variations in the expression levels. Significantly, XTH29, codifying the only XTH that follows a UPS pathway, is highly upregulated with respect to XTH11 and XTH33, which code for CPS-secreted proteins.     

MDP25, a novel calcium regulatory protein, mediates hypocotyl cell elongation by destabilizing cortical microtubules in Arabidopsis

The regulation of hypocotyl elongation is important for plant growth. Microtubules play a crucial role during hypocotyl cell elongation. However, the molecular mechanism underlying this process is not well understood. In this study, we describe a novel Arabidopsis thaliana microtubule-destabilizing protein 25 (MDP25) as a negative regulator of hypocotyl cell elongation. We found that MDP25 directly bound to and destabilized microtubules to enhance microtubule depolymerization in vitro. The seedlings of mdp25 mutant Arabidopsis lines had longer etiolated hypocotyls. In addition, MDP25 overexpression resulted in significant overall shortening of hypocotyl cells, which exhibited destabilized cortical microtubules and abnormal cortical microtubule orientation, suggesting that MDP25 plays a crucial role in the negative regulation of hypocotyl cell elongation. Although MDP25 localized to the plasma membrane under normal conditions, increased calcium levels in cells caused MDP25 to partially dissociate from the plasma membrane and move into the cytosol. Cellular MDP25 bound to and destabilized cortical microtubules, resulting in their reorientation, and subsequently inhibited hypocotyl cell elongation. Our results suggest that MDP25 exerts its function on cortical microtubules by responding to cytoplasmic calcium levels to mediate hypocotyl cell elongation.     

Growing in darkness: The etiolated lupin hypocotyls

Epigeal germination of a dicot, like lupin (Lupinus albus L.), produces a seedling with a characteristic hypocotyl, which grows in darkness showing a steep growth gradient with an elongation zone just below the apex. The role of phytohormones, such as auxin and ethylene, in etiolated hypocotyl growth has been the object of our research for some time. The recent cloning and expression of three genes of influx and efflux carriers for polar auxin transport (LaAUX1, LaPIN1 and LaPIN3) reinforces a previous model proposed to explain the accumulation of auxin in the upper growth zone of the hypocotyl.Key words: auxin carriers, auxin transport gradient, etiolated hypocotyl growth, Lupinus albusMost plants show a typical axial polar and branched (dendritic) morphology to compensate for their immobility by optimally exploiting the resources available in a limited environment.From Julius von Sachs¹ to Tsvi Sachs² many plant physiologists sought to explain how the axis is maintained and what type of signals are interchanged between poles. It was demonstrated that auxins were the determining factors in maintaining the polarity in shoots and roots and a reductionistic approach leads to conclude that such polarity had to be established at the cellular level. A chemiosmotic theory was then proposed, which implied an asymmetric distribution of efflux carriers at the bottom of a cell, linked to pH gradients to maintain different undissociated/dissociated forms of auxin separated between apoplast and symplast spaces.³In recent years, the use of Arabidopsis thaliana as a plant model has given additional support to the hypothesis that polar auxin transport is restricted to certain cells and mediated by influx (AUX1 and LAX1–4 proteins) and efflux carriers (PIN1–8 proteins).^4–6 Currently, we have a good idea of the topology of Arabidopsis carrier distribution, especially in roots.^4,5 Additional (MDR/PGP)⁷ or parallel (TRH1)⁸ components of the transport system are now emerging.However, while accepting the enormous advances and contributions to plant science provided by the use of Arabidopsis thaliana, we remain true (loyal) to the particular model adopted by the Department of Plant Biology, University of Murcia (Spain) in the 1970''s: the hypocotyl of lupin seedlings cultivated in darkness. In such conditions, the organ grows heterotrophically and longer than in light.The cotyledons and meristem at the top supply nutrients and hormones in a basipetal direction.The hypocotyl is a cylindrical column, with a radial symmetry that clearly shows differentiated tissues: epidermis, cortex, vascular cylinder and pith. Its size allows surgical separation of the tissues using suitable glass capillaries.At the beginning lupin was chosen because it had higher IAA-oxidase activity than pea, bean, oat or barley seedlings. At the time, it was thought that growth was mainly controlled through auxin catabolism (a fruitful line involving peroxidases was developed later). However, the etiolated hypocotyl was soon adopted preferentially by our group because of its qualities as a model for studying the relationship between hormone levels (auxin and ethylene) and growth. Our Portuguese colleagues have also used lupin as a model with successful results.⁹Bellow, we detail the landmarks of our research to date. Hypocotyl growth shows a characteristic pattern. Unlike plants grown in the light, in which all the cells along the hypocotyl elongate continuously throughout the growth period,^10,11 there is a steep growth gradient in the dark with an elongation zone just below the apex¹² (see Fig. 1 for details). This cell growth pattern in etiolated hypocotyls was described in lupin and then in Arabidopsis.¹¹ In this pattern, it is important to note that there is compensation along the organ between the cell diameter and the cell wall thickness. Once the cell growth pattern was known, we investigated its relation with the level of two phytohormones, auxin and ethylene, which might participate in the growth regulation. Special attention was paid to the distribution of endogenous IAA and its relation with growth. The results showed good correlation between the auxin levels and the cell size.^13,14 Auxin from the apex appears to be responsible for hypocotyl growth, since decapitation of seedlings strongly reduced growth, which was restored after the application of exogenous IAA to the cut surface.¹⁵ In light of the fact that growth depended on auxin from the apex, we investigated the nature of the auxin transport and demonstrated that this transport is polarized and sensitive to inhibition by specific inhibitors of polar auxin transport (PAT) such as 2,3,5-triiodobenzoic acid and 1-N-naphthylphtalamic acid (NPA).^16,17 Basipetal PAT mainly occurred in the stele,¹⁵ while cells in the epidermis and outer cortex are the limiting factor in auxin-induced shoot growth.^18–20 The finding that during PAT auxin can move laterally from transporting cells in the stele to the outer tissues of the elongation zone¹⁵ could explain the apparent conflict between the localization of PAT and the auxin target cells for elongation. In fact, epidermal cells acted as a sink for lateral auxin movement (LAM).¹⁷Open in a separate windowFigure 1Distribution of growth and cell size along the hypocotyl in etiolated lupin seedlings. At 3 d, hypocotyls were marked with ink, delimiting four 5-mm long zones including the apical, middle and basal zones. The hypocotyl growth ceased at day 12 and almost no growth was observed in the basal zone after day 3. From 3 to 6 d the growth was localized between the apical and basal zones, while most growth occurring from 6 to 12 d was localized in apical and middle zones. The cell size represents the cell length and cell diameter (the cell wall excluded) and corresponds to the second cell layer of cortex near the vascular cylinder. Similar results were obtained in cells from epidermis and pith. In each zone the cell length increased and the cell diameter showed little change during hypocotyl ageing. The final size at the end of the growth period varied along the hypocotyl, the cells becoming shorter and broader from the apical to the basal zones. In spite of the fact that cell diameter increased basipetally, no significant variation in hypocotyl diameter was found along the organ during the growth period. A morphometric study revealed that cell wall thickness in the apical cells was twice that in the basal cells at the end of the growth period i.e., the thinner apical cells had thicker cell walls, which may help explain the consistency of hypocotyl diameter along the organ.If PAT provides the auxin for growth and elongating growth is restricted to the apical region in etiolated hypocotyls, the question is: how does auxin accumulate in the elongation region?In a former study, we proposed that variations in auxin transport along actively growing lupin hypocotyl could produce such accumulation.²¹ Recently we extensively studied the variation of PAT along the lupin hypocotyls in seedlings of different ages, finding that certain parameters of PAT, such as transport intensity, polarity (basipetal vs acropetal) and sensitivity to NPA inhibition, showed a good correlation with the distribution of growth along the hypocotyl and its variation with ageing.²² These results suggest that a basipetally decreasing gradient in PAT along the hypocotyl may be responsible for the auxin distribution pattern controlling growth, since the existence of such a PAT gradient might generate the so-called barrier effect, which could produce an auxin gradient along the hypocotyl, the auxin content being higher in the apical elongation zone. To investigate whether these PAT variations can be explained in terms of auxin carrier distribution, we isolated three genes coding for auxin influx (LaAUX1) and efflux (LaPIN1 and LaPIN3) carriers, and studied their expression in different tissues along the hypocotyl at different ages.²³ The expression of LaAUX1 and LaPIN3 occurred both in the stele and in the outer tissues, while the expression of LaPIN1 was restricted to the stele and showed a basipetally decreasing gradient along the hypocotyl. The decisive role ascribed to PIN1 in polar auxin transport due to its localization in the basal end of transporting cells,²⁴ 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 as being responsible for the auxin gradient which controls the growth pattern in etiolated lupin hypocotyls.The acid-growth theory of auxin action was also tested, observing that the elongation growth of etiolated hypocotyl segments of lupin was stimulated by acid pH and IAA. Both factors stimulated growth in a more than additive way, suggesting a synergistic action between them.²⁵ The recent finding of a soluble auxin receptor (intracellular) reinforces the interest of the above study (which has remained a “sleeping beauty”) because pH affects IAA uptake.There are still several questions that must be answered before we can fully understand the growth pattern exhibited by etiolated lupin hypocotyls. Thus, as regards the cause of the PAT gradient, other factors besides the LaPIN1 gradient must be considered. For example, auxin carriers such as some phosphoglycoproteins (PGP), are also expressed differentially along the Arabidopsis hypocotyl and specific PIN-PGP pairings influence PAT by modulating the rates of cellular auxin movement.⁷ The pathway (symplast or apoplast) and mechanism of LAM remains unknown. Although alternative mechanisms have been proposed,²⁶ a previous study in lupin¹⁵ suggested that LAM is a diffusive process and that the IAA metabolism observed in the outer tissues might generate the radial gradient of auxin necessary for the maintenance of its lateral flow. It is thought that this metabolism of IAA occurs once the hormonal action is completed.^25,27 Although NPA does not inhibit LAM, the involvement of auxin efflux carriers cannot be discarded. In fact, the role of PIN carriers in lateral auxin transport towards and from the stele has been described in the root.²⁸ Other phytohormones besides auxin can modulate hypocotyl growth. Thus, the ethylene production rate, the 1-aminocyclopropane-1-carboxylic acid (ACC) content and the ACC oxidase activity decreased along the hypocotyl during the hypocotyl growth period.²⁹ Sensitivity to exogenous ethylene varied during growth, the young apical region being less sensitive than the older basal region.³⁰ Ethylene modified the cell growth pattern in the different tissues.³¹ The ethylene-induced lupin hypocotyl thickening was irreversible and mainly due to an increase in cell diameter. However, the inhibition of hypocotyl elongation produced by ethylene was reversible and involved irreversible inhibition of cell division and, paradoxically, stimulation of cell elongation to produce cells longer than those of the control.³²Studies in Arabidopsis showed that the hypocotyl growth in both light- and dark-grown plants is a process driven by cross-talk between multiple hormones. Interactions between auxins, ethylene, gibberellins and brassinosteroids have been described.^33,34 We think that the etiolated lupin hypocotyl remains a suitable model for confirming some of these results and for opening up new approaches in phytohormone research.     

Boron-regulated hypocotyl elongation is affected in Arabidopsis mutants with defects in light signalling pathways

We studied the effect of elevated boron (B) concentrations on the growth and development of Arabidopsis thaliana in vitro with respect to different light conditions. Two basic responses were observed. At high concentrations (above 5 mM) a clear toxicity effect of B on plant growth was apparent. Seedlings were short, stunted and pale. However at concentrations between 1 and 3 mM H₃BO₃, hypocotyl elongation was stimulated in all Arabidopsis ecotypes tested relative to plants grown at 0.1 mM H₃BO₃. The stimulation of hypocotyl elongation by elevated B was proportionally greater with increasing irradiance. We also showed that blue light (BL) and red light (RL) did not alter the sensitivity of Arabidopsis hypocotyls to boron, but, dependent on genotype, BL and RL increased or reduced capacity of boron-induced hypocotyl elongation. Analysis of photomorphogenic mutants indicated the existence of an interaction between boron and light signalling pathways during plant growth and development. This interaction was supported by the observation that the expression of the BOR1 gene in Arabidopsis hypocotyls was stimulated by BL and RL. Our results suggest that in etiolated or light-grown seedlings the stimulation of hypocotyl growth by boron can be mediated by cryptochromes and phytochromes.     

Endogenous Abscisic Acid Promotes Hypocotyl Growth and Affects Endoreduplication during Dark-Induced Growth in Tomato (Solanum lycopersicum L.)

Dark-induced growth (skotomorphogenesis) is primarily characterized by rapid elongation of the hypocotyl. We have studied the role of abscisic acid (ABA) during the development of young tomato (Solanum lycopersicum L.) seedlings. We observed that ABA deficiency caused a reduction in hypocotyl growth at the level of cell elongation and that the growth in ABA-deficient plants could be improved by treatment with exogenous ABA, through which the plants show a concentration dependent response. In addition, ABA accumulated in dark-grown tomato seedlings that grew rapidly, whereas seedlings grown under blue light exhibited low growth rates and accumulated less ABA. We demonstrated that ABA promotes DNA endoreduplication by enhancing the expression of the genes encoding inhibitors of cyclin-dependent kinases SlKRP1 and SlKRP3 and by reducing cytokinin levels. These data were supported by the expression analysis of the genes which encode enzymes involved in ABA and CK metabolism. Our results show that ABA is essential for the process of hypocotyl elongation and that appropriate control of the endogenous level of ABA is required in order to drive the growth of etiolated seedlings.     

Expression of xyloglucan endotransglucosylase/hydrolase (XTH) genes and XET activity in ethylene treated apple and tomato fruits

Xyloglucan endotransglucosylase/hydrolase (XTHs: EC 2.4.1.207 and/or EC 3.2.1.151), a xyloglucan modifying enzyme, has been proposed to have a role during tomato and apple fruit ripening by loosening the cell wall. Since the ripening of climacteric fruits is controlled by endogenous ethylene biosynthesis, we wanted to study whether XET activity was ethylene-regulated, and if so, which specific genes encoding ripening-regulated XTH genes were indeed ethylene-regulated. XET specific activity in tomato and apple fruits was significantly increased by the ethylene treatment, as compared with the control fruits, suggesting an increase in the XTH gene expression induced by ethylene. The 25 SlXTH protein sequences of tomato and the 11 sequences MdXTH of apple were phylogenetically analyzed and grouped into three major clades. The SlXTHs genes with highest expression during ripening were SlXTH5 and SlXTH8 from Group III-B, and in apple MdXTH2, from Group II, and MdXTH10, and MdXTH11 from Group III-B. Ethylene was involved in the regulation of the expression of different SlXTH and MdXTH genes during ripening. In tomato fruit fifteen different SlXTH genes showed an increase in expression after ethylene treatment, and the SlXTHs that were ripening associated were also ethylene dependent, and belong to Group III-B (SlXTH5 and SlXTH8). In apple fruit, three MdXTH showed an increase in expression after the ethylene treatment and the only MdXTH that was ripening associated and ethylene dependent was MdXTH10 from Group III-B. The results indicate that XTH may play an important role in fruit ripening and a possible relationship between XTHs from Group III-B and fruit ripening, and ethylene regulation is suggested.     

POLYGALACTURONASE INVOLVED IN EXPANSION1 Functions in Cell Elongation and Flower Development in Arabidopsis

Pectins are acidic carbohydrates that comprise a significant fraction of the primary walls of eudicotyledonous plant cells. They influence wall porosity and extensibility, thus controlling cell and organ growth during plant development. The regulated degradation of pectins is required for many cell separation events in plants, but the role of pectin degradation in cell expansion is poorly defined. Using an activation tag screen designed to isolate genes involved in wall expansion, we identified a gene encoding a putative polygalacturonase that, when overexpressed, resulted in enhanced hypocotyl elongation in etiolated Arabidopsis thaliana seedlings. We named this gene POLYGALACTURONASE INVOLVED IN EXPANSION1 (PGX1). Plants lacking PGX1 display reduced hypocotyl elongation that is complemented by transgenic PGX1 expression. PGX1 is expressed in expanding tissues throughout development, including seedlings, roots, leaves, and flowers. PGX1-GFP (green fluorescent protein) localizes to the apoplast, and heterologously expressed PGX1 displays in vitro polygalacturonase activity, supporting a function for this protein in apoplastic pectin degradation. Plants either overexpressing or lacking PGX1 display alterations in total polygalacturonase activity, pectin molecular mass, and wall composition and also display higher proportions of flowers with extra petals, suggesting PGX1’s involvement in floral organ patterning. These results reveal new roles for polygalacturonases in plant development.     

Similarity between Cytokinin and Blue Light Inhibition of Cucumber Hypocotyl Elongation

The cytokinin benzyladenine inhibited endogenous hypocotyl elongation in intact etiolated seedlings of cucumber (Cucumis sativus L.). In hypocotyl segments, the inhibitory effect of benzyladenine on growth was clearly detectable in the presence of indoleacetic acid. Fusicoccin-induced elongation was unaffected by the presence of cytokinin. The effect of cytokinin on elongation of the segments was determined by measuring changes in fresh weight, a linear function of extension growth. The effect of benzyladenine on hypocotyl growth was at least as large in segments prepared from red-light-grown seedlings as in those from seedlings grown in total darkness. A comparison was made between the inhibitory effects of cytokinin and blue light. The use of the calcium chelator ethyleneglycol-bis(β-aminoethyl ether)-N, N′-tetraacetic acid indicated that calcium ions are required for manifestation of benzyladenine-induced inhibition.     

Blue-light-mediated shade avoidance requires combined auxin and brassinosteroid action in Arabidopsis seedlings

Plant growth in dense vegetation can be strongly affected by competition for light between neighbours. These neighbours can not only be detected through phytochrome-mediated perception of a reduced red:far-red ratio, but also through altered blue light fluence rates. A reduction in blue light (low blue) induces a set of phenotypic traits, such as shoot elongation, to consolidate light capture; these are called shade avoidance responses. Here we show that both auxin and brassinosteroids (BR) play an important role in the regulation of enhanced hypocotyl elongation of Arabidopsis seedlings in response to blue light depletion. Only when both hormones are experimentally blocked simultaneously, using mutants and chemical inhibitors, will the response be fully inhibited. Upon exposure to low blue several members of the cell wall modifying XYLOGLUCAN ENDOTRANSGLUCOSYLASE/HYDROLASE (XTH) protein family are regulated as well. Interestingly, auxin and BR each regulate a subset of these XTHs, by which they could regulate cell elongation. We hypothesize that auxin and BR regulate specific XTH genes in a non-redundant and non-synergistic manner during low-blue-induced shade avoidance responses of Arabidopsis seedlings, which explains why both hormones are required for an intact low-blue response.     

Photophysiology of the Elongated Internode (ein) Mutant of Brassica rapa: ein Mutant Lacks a Detectable Phytochrome B-Like Polypeptide

Several phytochrome-controlled processes have been examined in etiolated and light-grown seedlings of a normal genotype and the elongated internode (ein/ein) mutant of rapid-cycling Brassica rapa. Although etiolated ein seedlings displayed normal sensitivity to prolonged far-red light with respect to inhibition of hypocotyl elongation, expansion of cotyledons, and synthesis of anthocyanin, they displayed reduced sensitivity to prolonged red light for all three of these deetiolation responses. In contrast to normal seedlings, light-grown ein seedlings did not show a growth promotion in response to end-of-day far-red irradiation. Additionally, whereas the first internode of light-grown normal seedlings showed a marked increase in elongation in response to reduced ratio of red to far-red light, ein seedlings showed only a small elongation response. When blots of protein extracts from etiolated and light-treated ein and normal seedlings were probed with monoclonal antibody to phytochrome A, an immunostaining band at about 120 kD was observed for both extracts. The immunostaining intensity of this band was substantially reduced for extracts of light-treated normal and ein seedlings. A mixture of three monoclonal antibodies directed against phytochrome B from Arabidopsis thaliana immunostained a band at about 120 kD for extracts of etiolated and light-treated normal seedlings. This band was undetectable in extracts of ein seedlings. We propose that ein is a photoreceptor mutant that is deficient in a light-stable phytochrome B-like species.