Investigation of Gene Expression, Growth Kinetics, and Wall Extensibility during Brassinosteroid-Regulated Stem Elongation

Abstract

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 mar 1. cv Williams 82) epicotyls showed that there was a 45-min lag before 0.1 f i~ 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 auxininsensitive 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 f i~ 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 leve1 and i 5 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.Brassinosteroids are growth-promoting natural products with structural similarities to animal steroid hormones (Mandava, 1988). The wide distribution of brassinosteroids in the plant kingdom, their marked effect on cell proliferation and elongation at nanomolar levels, and their interactions with

Brassinosteroid perception in the epidermis controls root meristem size

“…The size of cells in the meristem is a result of increasing cytoplasmic volume (Beemster et al, 2003). Although we do not exclude the involvement of BRs in the increase of cell-biomass, the role of BRs in regulating expansion processes through cell-wall modification is evident (Zurek et al, 1994). In addition, short BR treatment is known to positively regulate the expression of cell-wall organization enzymes required for cell expansion and division (Nemhauser et al, 2004).…”

The Auxin-induced Maize Gene ZmSAUR2 Encodes a Short-lived Nuclear Protein Expressed in Elongating Tissues

“…Similarly, expansin LeExp2, which is thought to be a target gene for auxin signaling, is also not induced by brassinolide (51). On the other hand in the auxin-insensitive tomato mutant dgt (diageotropica) brassinolide is able to stimulate elongation growth, suggesting independent signal pathways for auxin-and brassinolide-regulated processes (52).…”

Constitutive Expression of Arabidopsis SMALL AUXIN UP RNA19 (SAUR19) in Tomato Confers Auxin-Independent Hypocotyl Elongation

“…Thus, the auxin induction of SAUR expression correlates well with the onset of auxin-mediated elongation growth. Likewise, auxininduced expression of SAUR gene expression in tomato has also been found to occur with very rapid kinetics (Wu et al, 2012), including in assays employing auxin-depleted hypocotyl segments (Zurek et al, 1994).…”