Brassinosteroids (BRs) are growth-promoting natural products found at low levels in pollen, seeds, and young vegetative tissues throughout the plant kingdom. Detailed studies of BR biosynthesis and metabolism, coupled with the recent identification of BR-insensitive and BR-deficient mutants, has greatly expanded our view of steroids as signals controlling plant growth and development. This review examines the microchemical and molecular genetic analyses that have provided convincing evidence for an essential role of BRs in diverse developmental programs, including cell expansion, vascular differentiation, etiolation, and reproductive development. Recent advances relevant to the molecular mechanisms of BR-regulated gene expression and BR signal transduction are also discussed.

BRASSINOSTEROIDS: Essential Regulators of Plant Growth and Development

Research in recent years on the biology of guard cells has shown that these specialized cells integrate both extra- and intra-cellular signals in the control of stomatal apertures. Among the phytohormones, abscisic acid (ABA) is one of the key players regulating stomatal function. In addition, auxin, cytokinin, ethylene, brassinosteroids, jasmonates, and salicylic acid also contribute to stomatal aperture regulation. The interaction of multiple hormones can serve to determine the size of stomatal apertures in a condition-specific manner. Here, we discuss the roles of different phytohormones and the effects of their interactions on guard cell physiology and function.

Hormone interactions in stomatal function.

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 Mechanisms of Brassinosteroids' Action: From Signal Transduction to Plant Development

The development of anther and pollen is important for male reproduction, and this process is coordinately regulated by many external and internal cues. In this study, we systematically examined the male reproductive phenotypes of a series of brassinosteroid biosynthetic and signaling mutants and found that, besides the expected cell-expansion defects, these mutants also showed reduced pollen number, viability, and release efficiency. These defects were related with abnormal tapetum and microspore development. Using both real-time quantitative RT-PCR and microarray experiments, we found that the expression of many key genes required for anther and pollen development was suppressed in these mutants. ChIP analysis demonstrated that BES1, an important transcription factor for brassinosteroid signaling, could directly bind to the promoter regions of genes encoding transcription factors essential for anther and pollen development, SPL/NZZ, TDF1, AMS, MS1, and MS2. Taken together, these data lead us to propose that brassinosteroids control male fertility at least in part via directly regulating key genes for anther and pollen development in Arabidopsis. Our work provides a unique mechanism to explain how a phytohormone regulates an essential genetic program for plant development.

https://www.pnas.org/content/pnas/107/13/6100.full.pdf

Brassinosteroids control male fertility by regulating the expression of key genes involved in Arabidopsis anther and pollen development

A new multiple hanging-drop assay has been tested with a growth inhibitor, cycloheximide, and a growth-promoter, fusicoccin. This assay has been used to examine the growth-response of pollen-tubes of Prunus avium to brassinolide, a naturally occurring promoter of plant growth. This response is compared with that of indol-3-ylacetic acid, gibberellic acid, and kinetin. Pollen tubes responded to brassinolide and fusicoccin at 1 nM and above, a concentration one order of magnitude lower than that for indol-3-ylacetic acid and gibberellic acid. Kinetin did not stimulate growth of the pollen tubes.

P O'Neill

Papers

Effect of Brassinolide and other Growth Regulators on the Germination and Growth of Pollen Tubes of Prunus avium using a Multiple Hanging-drop Assay