Effects of brassinosteroid on Arabidopsis root system architecture in response to nitrogen and phosphorus deficiency.

Mahamud Al-Mamun, Shyama Chakma, and Priti Krishna

School of Science and health, Western Sydney University, Hawkesbury campus, Richmond, NSW 2753

Abstract

Nutrient stress is a major impediment to crop production worldwide. Plants respond to nutrient stress by inducing numerous physiological, molecular and structural changes in their roots. Brassinosteroids (BRs) are a group of plant steroidal hormones which control several aspects of root development, such as root elongation, lateral root initiation, gravitropic response and nodule formation, but the role of BRs in modulating root system architecture (RSA) in response to nutrition stress has not been studied. In this study, six root characters (total root length, main root length, lateral root number, lateral root length, branching zone and lateral root density) were measured in BR-treated and untreated Arabidopsis seedlings exposed to nitrogen (N) and phosphorus (P) stresses. BR affected RSA in both treated and untreated seedlings with major changes occurring in lateral root numbers. BR-treated seedlings produced 50% and 32% more lateral roots as compared to untreated seedlings under N and P stress, respectively. However, this effect of BR was absent in loss of function mutants of NRT2.1 (high-affinity nitrate transporter) and ANR1 (positive regulator of lateral root growth) under N-deficient conditions, indicating that BR-mediated lateral root growth depends on NRT2.1 and ANR1. BR increased root hair number and length under P stress by 32% and 15%, respectively. Reactive oxygen species (ROS) are signalling molecules and modulate root growth in plants. BR significantly increased ROS production in roots under both N and P -sufficient and -deficient conditions. BR significantly decreased the expression of NRT2.1 (high-affinity nitrate transporter) and increased the expression of NIA1 and NIA2 (nitrate reductase enzymes that catalyse the first step in nitrate assimilation) genes under N-deficient conditions, indicating BR positively affects N uptake and N assimilation in Arabidopsis. These results together suggest that BR modulates RSA under both N and P stresses, as well as gene expression that contribute positively to stress tolerance.