Brassinosteroid: a biotechnological target for enhancing crop yield and stress tolerance

Annette Kauschmann 1, Alison Jessop 1,t, Csaba Koncz 2,3, Miklos Szekeres 3, Lothar Willmitzer 1.t and Thomas Altmann 1,t.* llnstitut fQr Genbiologische Forschung Berlin GmbH, Ihnestra~e 63, D-14195 Berlin, Germany, 2Max-Planck-lnstitut fQr Z0chtungsforschung, Carl-yon Linn6-Weg 10, D-50829 K61n, Germany, and 3Institute of Plant Biology, Biological Research Center of Hungarian Academy of Sciences, H-6701 Szeged, P.O. Box 521, Temesv ri krt. 62, Hungary

Genetic evidence for an essential role of brassinosteroids in plant development

Plant hormones regulate plant growth and development by affecting an array of cellular, physiological, and developmental processes, including, but not limited to, cell division and elongation, stomatal regulation, photosynthesis, transpiration, ion uptake and transport, initiation of leaf, flower and fruit development, and senescence. Environmental factors such as salinity, drought, and extreme temperatures may cause a reduction in plant growth and productivity by altering the endogenous levels of plant hormones, sensitivity to plant hormones, and/or signaling pathways. Molecular and physiological studies have determined that plant hormones and abiotic stresses have interactive effects on a number of basic biochemical and physiological processes, leading to reduced plant growth and development. Various strategies have been considered or employed to maximize plant growth and productivity under environmental stresses such as salt-stress. A fundamental approach is to develop salt-tolerant plants through gene...

The Physiological, Biochemical and Molecular Roles of Brassinosteroids and Salicylic Acid in Plant Processes and Salt Tolerance

The dumpy (dpy) mutant of tomato (Lycopersicon esculentum Mill.) exhibits short stature, reduced axillary branching, and altered leaf morphology. Application of brassinolide and castasterone rescued the dpy phenotype, as did C-23-hydroxylated, 6-deoxo intermediates of brassinolide biosynthesis. The brassinolide precursors campesterol, campestanol, and 6-deoxocathasterone failed to rescue, suggesting that dpy may be affected in the conversion of 6-deoxocathasterone to 6-deoxoteasterone, similar to the Arabidopsis constitutive photomorphogenesis and dwarfism (cpd) mutant. Measurements of endogenous brassinosteroid levels by gas chromatography-mass spectrometry were consistent with this hypothesis. To examine brassinosteroid-regulated gene expression in dpy, we performed cDNA subtractive hybridization and isolated a novel xyloglucan endotransglycosylase that is regulated by brassinosteroid treatment. The curl-3 (cu-3) mutant (Lycopersicon pimpinellifolium [Jusl.] Mill.) shows extreme dwarfism, altered leaf morphology, de-etiolation, and reduced fertility, all strikingly similar to the Arabidopsis mutant brassinosteroid insensitive 1 (bri1). Primary root elongation of wild-type L. pimpinellifolium seedlings was strongly inhibited by brassinosteroid application, while cu-3 mutant roots were able to elongate at the same brassinosteroid concentration. Moreover, cu-3 mutants retained sensitivity to indole-3-acetic acid, cytokinins, gibberellin, and abscisic acid while showing hypersensitivity to 2,4-dichlorophenoxyacetic acid in the root elongation assay. The cu-3 root response to hormones, coupled with its bri1-like phenotype, suggests that cu-3 may also be brassinosteroid insensitive.

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A Putative Role for the Tomato Genes DUMPY and CURL-3 in Brassinosteroid Biosynthesis and Response

Brassinosteroids promote stem elongation in a variety of plants but little is known about the mechanism of action of these plant growth regulators. We investigated a number of physiological and molecular parameters associated with brassinosteroid-enhanced elongation. Continuous growth recordings of soybean (Glycine max L. cv Williams 82) epicotyls showed that there was a 45-min lag before 0.1 [mu]M brassinolide (BR) exerted a detectable effect on elongation. BR caused a marked increase in Instron-measured plastic extensibility, suggesting that BR may promote elongation in part by altering mechanical properties of the cell wall (wall loosening). Structure-function studies suggested that the dimensions of the brassinosteroid side chain were critical for promotion of elongation and expression of BRU1, a gene regulated specifically by active brassinosteroids. Auxin-BR interactions were examined by using small auxin up RNA (SAUR) gene probes and the auxin-insensitive diageotropica (dgt) mutant of tomato (Lycopersicon esculentum Mill.). We have shown that in wild-type tomato, which elongates in response to exogenous auxin, a transcript of identical size to the soybean SAUR 15A is strongly induced within 1 h by 50 [mu]M 2,4-dichlorophenoxyacetic acid or indoleacetic acid, whereas in the dgt mutant, which does not elongate in response to auxin, no transcript is expressed. Furthermore, BR promotes equal elongation of hypocotyls in both wild-type and dgt tomatoes but does not rapidly induce the SAUR 15A homolog in either genotype. BR does not cause rapid induction of SAUR 6B in elongating soybean epicotyls but does lead to increased expression after 18 h. This late BR activation of SAUR 6B is controlled, at least in part, at the transcriptional level and is not accompanied by an increase of free indoleacetic acid in the tissue. We conclude that although both BR and auxin affect wall relaxation processes, BR-promoted elongation in soybean and tomato stems acts via a mechanism that most likely does not proceed through the auxin signal transduction pathway.

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Masami Koguchi

Papers

Structure-activity relationship of brassinosteroids