Brassinosteroid Signaling Pathways Interplaying with Diverse Signaling Cues for Crop Enhancement

Hyeona Hwang,Hojin Ryu,Hyunwoo Cho

doi:10.3390/agronomy11030556

Abstract

The signaling pathways of brassinosteroids (BRs), a unique plant steroid hormone, are critically involved in a diverse range of plant growth and developmental processes as well as many important agronomic traits. Recent advances in the understanding of BR biosynthetic and signaling pathways in model organisms and crops have increased the feasibility of modulating BR responses in crop plants to enhance adaptation to various vulnerable environmental changes. In particular, the identification and functional analysis of BR signaling components in rice (Oryza sativa) present the possibility of their utilization to improve many agricultural traits involved in crop yields. In this review, we summarize recent advances and progress in the understanding of the BR signaling pathway and its interactions with diverse internal and external signaling cues. We also discuss how these physiological modulations of BR and the abundant signaling crosstalk can be applied to enhance rice productivity through the manipulation of plant architecture and fine-tuning of stress responses. Finally, we discuss how the complex regulation of BR signaling pathways could favor application in the molecular design of plant growth and development, precise breeding strategies, and cultivation methods for rice crop improvement.

Highlights

Brassinosteroids (BR) regulate a diverse spectrum of processes in plant growth and development as well as internal modulation in response to environmental fluctuation. Since they are essential in plant adaptation and seed plant evolution, the biosynthesis and signaling pathways have been identified in the model plant Arabidopsis, providing a comprehensive understanding of how BR synthesis is controlled and how the signaling pathways are coordinated during the plant’s life cycle [1,2,3,4,5,6]
The initiation of BR signaling is tightly mediated by a receptor-like kinase, BRASSINOSTEROID INSENSITIVE 1 (BRI1), and co-receptor kinase, BRI1-ASSOCIATED KINASE 1 (BAK1), at the plasma membrane [1,4]
BR perception through these receptor complexes triggers the dissociation of a negative regulator, BRI1 KINASE INHIBITOR 1 (BKI1), and confers a transphosphorylation of BRI1 and BAK1, leading to the activation of BRI1 SUPPRESSOR 1 (BSU1) and consequent inactivation of BRASSINOSTEROID INSENSITIVE 2 (BIN2) kinase, a representative of the plant GLYCOGEN SYNTHASE KINASE 3 (GSK3) [1,2,3,4,10]

Summary

Introduction

Brassinosteroids (BR) regulate a diverse spectrum of processes in plant growth and development as well as internal modulation in response to environmental fluctuation. Rice BR signaling involves a diverse range of transcriptional regulators which modulate the primary BR-responsive genes These include DWARF AND LOW-TILLERING (DLT), LEAF AND TILLER ANGLE INCREASED CONTROLLER (LIC), GRAIN LENGTH 2/GROWTH REGULATING FACTOR 4 (GL2/OsGRF4), OVATE FAMILY PROTEIN (OFP), REDUCED LEAF ANGLE 1/SMALL ORGAN SIZE 1 (RLA1/SMOS1), and U-TYPE CYCLIN (CYC-U), which fine-tune the BR response spatiotemporally via direct and/or indirect interaction with canonical BR signaling components (OsBRI1, OsBAK1, OsGSKs, OsBZR1) in rice (Figure 1). Among a number of genes controlling the seed and embryo/endosperm development, SHORT HYPOCOTYL UNDER BLUE 1 (SHB1), HAIKU 1 (IKU1), HAIKU 2 (IKU2), and MINISEED 3 (MINI3) are known to be the main regulators of seed size and weight through their interactions with canonical BR signaling [46,47,48,49] These proteins act downstream of BZR1 under the control of the BR-BRI1-BIN2 phosphorylation cascade. BR inhibits the protein accumulation and expression level of CYC-U4;1, suppressing cell proliferation in the abaxial side of the lamina joint region through inactivation of OsGSK2 and activation of OsBZR1, respectively [57]

BR Crosstalk with the Environmental Signal in Dicot and Monocot

Findings

BR Application for Increasing Crop Yield in Rice