Biological Activity of Brassinosteroids – Direct Comparison of Known and New Analogs in planta

Abstract

A systematic investigation of structural modifications of brassinosteroids is presented. We describe in detail their synthetic preparation, which includes significant improvements of previously reported protocols as well as access to new analogs with functional modifications of the steroid skeleton and of the C(17)‐attached side chain. We report the biological potency of the prepared brassinosteroid analogs as plant hormones, which were carefully established in the French bean second internode elongation assay and discuss our observations in light of the recently reported structural data detailing the molecular interactions between brassinolide in the trimeric complex with the protein receptors kinases BRASSINOSTEROID INSENSITIVE 1 (BRI1) and SOMATIC EMBRYOGENESIS RECEPTOR KINASE 1 (SERK1). In a further part of this work we describe the preparation of H2O‐soluble pro‐forms of 24‐epicastasterone and we discuss their physical properties, hydrolytic stabilities and biological activity.

Synthesis and Biological Activity of Brassinosteroid Analogues with a Nitrogen-Containing Side Chain

“…The structure–activity relationship of brassinosteroids has been studied in detail in recent years and it has been postulated that the (2α,3α)- and (22R,23R)-vicinal diol moieties are required for optimum bioactivity and 7-oxalactone BRs have stronger biological activity than 6-oxo types, whereas B-ring non-oxidized BRs reveal no activity in biological tests [ 25 , 40 , 41 ]. Recently, several research groups prepared BR synthetic analogues with various nitrogen modifications in the side chain.…”

“…Such modifications include shortening or prolonging the side chain, oxidation or reduction of oxygen functional groups, incorporation of heteroatoms other than oxygen (mostly nitrogen and halogens) and modification of newly synthesized functional groups (e.g., acetylation, methylation, etc.) [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ]. Preparation of derivatives with a nitrogen-containing side chain has not been at the forefront of the interest of steroidal chemists.…”

“…Unfortunately, the rice lamina inclination biotest showed no biological activity [ 29 ]. Several BR analogues with nitrogen-containing heterocycles (e.g., isoxazoline, oxazole, thiazole and pyridine) were synthesized by Wendeborn’s group [ 25 ]. Other analogues with nitrogen groups in the side chain were prepared mostly for other chemical modifications, e.g., azide derivatives for photoaffinity labeling [ 30 ] or activated esters for conjugation reactions [ 31 , 32 ].…”

Comprehensive Overview of the Brassinosteroid Biosynthesis Pathways: Substrates, Products, Inhibitors, and Connections

“…Based on the cholesterol (CR) side chain, BRs are divided by different substituents into C-23, C-24, C-25, 23-oxo, 24 S -methyl, 24 R -methyl, 24-methylene, 24 S -ethyl, 24-ethylidene, 24-methylene-25-methyl, 24-methyl-25-methyl; without substituent at C-23, without substituent at C-24, and without substituents at C-23, C-24. BRs can also conjugate with glucose and fatty acids ( Fujioka and Yokota, 2003 ; Bajguz, 2007 ; Piotrowska and Bajguz, 2011 ; Wendeborn et al., 2017 ; Zullo and Bajguz, 2019 ).…”

“…BRs play an essential role in the development and growth of plants. They elicit a broad spectrum of morphological and physiological responses as well as a tolerance against abiotic and biotic stress (Bajguz and Hayat, 2009;Bajguz and Piotrowska-Niczyporuk, 2014;Wei and Li, 2016;Wendeborn et al, 2017;Ahanger et al, 2018;Siddiqui et al, 2018;Nolan et al, 2020).…”