Boosting crop yields with plant steroids

Abstract

Plant sterols and steroid hormones, the brassinosteroids (BRs), are compounds that exert a wide range of biological activities. They are essential for plant growth, reproduction, and responses to various abiotic and biotic stresses. Given the importance of sterols and BRs in these processes, engineering their biosynthetic and signaling pathways offers exciting potentials for enhancing crop yield. In this review, we focus on how alterations in components of sterol and BR metabolism and signaling or application of exogenous steroids and steroid inhibitors affect traits of agronomic importance. We also discuss areas for future research and identify the fine-tuning modulation of endogenous BR content as a promising strategy for crop improvement.

Figure 1.

BR Homeostasis and Signaling Mechanisms and Their Effects on Plant Growth and Development in Arabidopsis (Dicot) and Rice (Monocot). Low levels of BRs promote BR biosynthesis. Conversely, under high BR levels, the signaling of BRs is active and the synthesis of BRs is repressed through BR signaling–mediated transcriptional negative feedback regulation. Downregulation of BR biosynthetic enzymes or overexpression of BR inactivation enzymes leads to decreased levels of the end product of BR synthesis, whereas downregulation of the positive regulators or overexpression of the negative regulators of the BR signaling pathway also decreases BR levels as well as increasing the sensitivity to BRs. On the contrary, BR levels are increased by overexpression of a BR biosynthetic enzyme (rate-limiting step)/BR signaling positive regulator or by downregulation of a BR catabolic enzyme/BR signaling negative regulator. A decrease in plant endogenous BR levels generally results in dwarfism, shorter petiole (in Arabidopsis)/more erect leaves (in rice), and reduced fertility. By contrast, Arabidopsis or rice plants with increased endogenous BR levels generally display improved plant growth and reproduction capacity. To date, the impact of altering endogenous BR content on the plant tolerance to abiotic and biotic stresses remains unclear. DR, downregulation; GOF, gain of function; LOF, loss of function; OX, overexpression.

Figure 2.

Actions of BRs in Regulating Plant Development and Traits of Agronomic Importance in Model Plant Species and Crops. Positive effects of increased BR content or signaling are boxed in red. Mixed effects are boxed in orange. Dose-dependent effects are observed on root development (positive effects at low concentrations and negative effects at high concentrations). Overall effects of BRs on the plant biotic and abiotic stress tolerance remain unclear to date.