The identification of brassinosteroid (BR) deficient and BR insensitive mutants provided conclusive evidence that BR is a potent growth-promoting phytohormone. Arabidopsis mutants are characterized by a compact rosette structure, decreased plant height and reduced root system, delayed development, and reduced fertility.
Schröder et al BMC Plant Biology 2014, 14:309 http://www.biomedcentral.com/1471-2229/14/309 RESEARCH ARTICLE Open Access Consequences of induced brassinosteroid deficiency in Arabidopsis leaves Florian Schröder1*, Janina Lisso1, Toshihiro Obata2, Alexander Erban2, Eugenia Maximova2, Patrick Giavalisco2, Joachim Kopka2, Alisdair R Fernie2, Lothar Willmitzer2 and Carsten Müssig1 Abstract Background: The identification of brassinosteroid (BR) deficient and BR insensitive mutants provided conclusive evidence that BR is a potent growth-promoting phytohormone Arabidopsis mutants are characterized by a compact rosette structure, decreased plant height and reduced root system, delayed development, and reduced fertility Cell expansion, cell division, and multiple developmental processes depend on BR The molecular and physiological basis of BR action is diverse The BR signalling pathway controls the activity of transcription factors, and numerous BR responsive genes have been identified The analysis of dwarf mutants, however, may to some extent reveal phenotypic changes that are an effect of the altered morphology and physiology This restriction holds particularly true for the analysis of established organs such as rosette leaves Results: In this study, the mode of BR action was analysed in established leaves by means of two approaches First, an inhibitor of BR biosynthesis (brassinazole) was applied to 21-day-old wild-type plants Secondly, BR complementation of BR deficient plants, namely CPD (constitutive photomorphogenic dwarf)-antisense and cbb1 (cabbage1) mutant plants was stopped after 21 days BR action in established leaves is associated with stimulated cell expansion, an increase in leaf index, starch accumulation, enhanced CO2 release by the tricarboxylic acid cycle, and increased biomass production Cell number and protein content were barely affected Conclusion: Previous analysis of BR promoted growth focused on genomic effects However, the link between growth and changes in gene expression patterns barely provided clues to the physiological and metabolic basis of growth Our study analysed comprehensive metabolic data sets of leaves with altered BR levels The data suggest that BR promoted growth may depend on the increased provision and use of carbohydrates and energy BR may stimulate both anabolic and catabolic pathways Keywords: Brassinosteroids, Arabidopsis, Tricarboxylic acid cycle, Biomass, Cell expansion, Growth Background Brassinolide (BL) was identified in 1979 through its ability to promote internode growth [1] Since then, the growth-promoting effect has been studied in hundreds of articles Excised tissues (e.g hypocotyl, epicotyl, cotyledons, internodes, leaves, and roots), protoplasts, cell suspension cultures, intact seedlings, and whole plants were subjected to brassinosteroid (BR) treatments, and in all cases BR had the potential to stimulate growth [2] A large number of BR deficient and BR insensitive mutants in Arabidopsis thaliana and in crops such as rice, * Correspondence: schroeder@mpimp-golm.mpg.de University of Potsdam, c/o Max Planck Institute of Molecular Plant Physiology, Am Mühlenberg 1, 14476 Potsdam-Golm, Germany Full list of author information is available at the end of the article tomato, and pea were identified [3] These mutants are generally dwarfed and exhibit rounded, dark green leaves, delayed flowering, reduced male fertility and seed set, and delayed senescence Further proven roles of BR include the control of xylem formation [4,5], stomata development [6-8], and further developmental processes [9-12] Numerous studies analysed gene expression patterns upon BR treatment and BR deficiency However, these studies barely clarified the metabolic and physiological basis of BR dependent growth because the precise functions of isoenzymes, cell wall proteins, and other factors often remains obscure BR plays non redundant roles since it is not possible to complement BR mutants with other phytohormones or their antagonists [13] Overexpression of the major BR © 2014 Schröder et al.; licensee BioMed Central Ltd This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated Schröder et al BMC Plant Biology 2014, 14:309 http://www.biomedcentral.com/1471-2229/14/309 receptor, BRI1 (BRASSINOSTEROID-INSENSITIVE 1), stimulated growth However, the underlying changes at transcript and metabolite level are largely different from other growth-stimulating pathways [14] The most prominent direct BR effect is the modification of gene expression patterns [15] Transcription factors such as BES1 (bri1-EMS-suppressor 1) and BZR1 (Brassinazole-resistant 1) regulate BR responsive genes [16-19] The physiological mechanisms underlying BR promoted growth appear to be manifold, and depend on the tissue and developmental stage [2] They include the control of aquaporin activity and water movement across membranes [20], cytoskeleton organisation [21-23], and the modification of mechanical cell wall properties [24,25] A few studies in Arabidopsis and crops addressed the influence of BR on primary metabolism The focus of these studies was on photosynthesis and sink strength, and specific enzyme activities or metabolites were measured An early article showed that BR stimulates CO2 assimilation in wheat [26] This finding was confirmed in other plants such as cucumber [27] and rice [28] In Arabidopsis, expression of a mutated BRI1 receptor (Y831F) enhanced shoot growth and conferred elevated photosynthetic rates and starch levels at the end of the light period [29] Rubisco activity and regeneration of Ribulose-1,5-bisphosphate are the most important limiting factors of photosynthesis under natural conditions Positive effects of BR on Rubisco activity have been shown [26,27] Yu et al [27] also postulated a positive effect on Ribulose-1,5-bisphosphate-regeneration Enhanced photosynthesis correlated with an increase of soluble sugar and starch content and parallel enhancement of fresh and dry weights In line with these data, BR deficient Arabidopsis mutants showed drastically reduced CO2 assimilation rates, reduced starch levels, a tendency to reduced sucrose levels, and reduced biomass accumulation [30] In addition to source efficiency, BR also increases sink strength The tomato dx mutant produces bioactive BR in fruits but not in the shoot, and provides an option to dissect BR dependent processes in fruits and shoots Dry weight and starch levels of dx fruits were significantly reduced [31] BR application to leaves partly normalized metabolic changes in dx fruits suggesting that shootderived BR dependent factors are required for proper fruit metabolism Previous research emphasized the relevance of BR for invertase activity in the growing zone of tomato hypocotyls [32] Thus, several reports demonstrated the requirement of BR for source efficiency and sink strength In this study, induced BR deficiency was analysed in Arabidopsis rosette leaves by means of complementary time-series experiments First, BR deficient plants were complemented by exogenous BR Subsequent omission of BR treatment caused BR deficiency Secondly, brassinazole (BRZ) was applied to wild-type plants BRZ binds Page of 14 to the DWF4 enzyme and specifically blocks BR biosynthesis at the C-22 hydroxylation step [33] BR dependent growth of established leaves is associated with elevated starch levels, higher metabolic flux through the tricarboxylic acid (TCA) cycle, and increased cell expansion and biomass production Results Time course experiments for the analysis of morphological and biochemical consequences of BR deficiency The analysis of BR deficient or BR insensitive mutants is complicated by the severe dwarfism and major morphological changes [3] The use of mutants with mild phenotypic changes such as cbb1/dwf1 (cabbage1/dwarf1) [13,34] can alleviate that difficulty The known alleles result in milder phenotypic changes in comparison to det2 (de-etiolated2), cpd (constitutive photomorphogenic dwarf), dwf4 (dwarf4), and other biosynthetic mutants The cbb1/dwf1 mutants are presumably able to produce unusual bioactive BR as a consequence of the accumulation of precursors and display altered BR responses [34,35] In order to start the analysis of BR deficiency symptoms with morphologically intact plants, two complementary sets of time course experiments were performed (Figure 1) During the time course experiments, plants were grown in parallel in a randomized manner in a controlled growth chamber (see Methods for details) The time course experiments were performed three times each, resulting in a total of six independent experiments The first approach used BR deficient mutants CPDantisense (aCPD) plants and the BR-deficient cbb1/dwf1-6 mutant [13,30] were treated with 200 nM brassinolide (BL) for three weeks Wild-type plants were grown in parallel and were simultaneously treated with a control solution The control solution was identical to the BR solution apart from the addition of BR (for details see Methods) BR supplementation fully normalized the morphology and biomass production of CPD-antisense plants The CPDantisense plants were nearly indistinguishable from the Figure Design of time series experiments Plants were grown on soil for weeks in controlled growth chambers under the specified conditions Black bars indicate a treatment of the plants with brassinolide (BL) or brassinazole (BRZ) Samples were taken at day (21 days after sowing), 1, 3, 6, and 14 LD, long day; SD, short day; CN, cold night Schröder et al BMC Plant Biology 2014, 14:309 http://www.biomedcentral.com/1471-2229/14/309 wild type The growth defect of cbb1 plants was partly complemented by exogenous BR (Figure 2, day 0) Fresh weight of 21-day-old cbb1 shoots was identical to the wild type However, leaf length and width were diminished in comparison to the wild type, leaves were more erect and had a slightly crinkled surface, and rosettes appeared compact Thus, exogenous BR could not fully substitute for endogenous BR After three weeks, BR treatment was stopped (day 0) The CPD-antisense and cbb1 plants started to run into BR deficiency or pronounced BR deficiency, respectively At this point the sampling began Samples for biochemical analysis were taken at day 0, 1, 3, 6, and 14 after the respective treatment was stopped Under the applied conditions, plants were in the vegetative phase during the complete experiments and did not start bolting The second approach used wild-type plants (C24) that were grown for three weeks without any treatment (Figure 3, day 0) Subsequently, plants were treated with 10 μM BRZ, 20 μM BRZ, or control solution Lower concentrations such as to μM BRZ have previously been applied in synthetic growth medium (e.g [36,37]) but were inapplicable in our time course experiments since they induced only minor growth effects in soilgrown plants The necessity for higher BRZ concentrations may reflect weaker uptake by leaves through a functional epidermis After the onset of BRZ application Page of 14 (day 0), samples for biochemical analysis were taken at the same points in time as described above (i.e day 1, 3, 6, 9, and 14) The parallel analysis of CPD-antisense, cbb1, and BRZ treated wild-type plants allows avoiding genotype- or treatment-specific limitations during the evaluation of BR deficiency Elevated CPD and DWF4 transcript levels indicate emerging BR deficiency The CPD [38] and DWF4 [39] genes encode enzymes involved in BR biosynthesis The expression of these genes is negatively associated with endogenous BR levels High transcript levels indicate low BR levels and vice versa [40] CPD and DWF4 transcript levels were analysed by means of quantitative RT-PCR (Figure 4) Unchanged transcript levels one day after the BR supplementation was stopped or after the BRZ treatment was started may indicate the presence of remaining BR or a time lag in the induction of BR biosynthetic genes Stronger differences were observed from day onwards CPD transcript levels in CPD-antisense plants were previously described [30] Due to incomplete CPD gene repression, the phenotypic changes of CPD-antisense plants are considerably milder in comparison to the cpd/cbb3 knockout mutant and other BR deficient mutants such as cbb1/ Figure Growth parameters of CPD-antisense and cbb1 plants in comparison to the wild type Wild-type (C24), CPD-antisense, and cbb1 plants were grown as described in Figure A, Shoot fresh weight B, Representative plants at day 0, 1, 3, 6, 9, and 14 C, Length of rosette leaves three and four D, Width of rosette leaves three and four Data are given as mean ± SE (n =10 plants) Values denoted with an asterisk are significantly different from the wild type (t test, P