Consequences of induced brassinosteroid deficiency in Arabidopsis leaves.
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.


CONCLUSIONS
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.
Florian
F
Schröder
Janina
J
Lisso
Toshihiro
T
Obata
Alexander
A
Erban
Eugenia
E
Maximova
Patrick
P
Giavalisco
Joachim
J
Kopka
Alisdair R
AR
Fernie
Lothar
L
Willmitzer
Carsten
C
Müssig
D017360:Arabidopsis / Q000187:drug effects* / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D060406:Brassinosteroids / Q000494:pharmacology*; D018506:Gene Expression Regulation, Plant / Q000187:drug effects*; D010937:Plant Growth Regulators / Q000494:pharmacology*; D018515:Plant Leaves / Q000187:drug effects / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D011897:Random Allocation; D013250:Steroid Hydroxylases / Q000235:genetics / Q000378:metabolism; D014230:Triazoles / Q000494:pharmacology*
D029681:Arabidopsis Proteins; D060406:Brassinosteroids; C091574:DIM1 protein, Arabidopsis; D010937:Plant Growth Regulators; D014230:Triazoles; C113844:CPD protein, Arabidopsis; D013250:Steroid Hydroxylases; C412342:brassinazole