Nitrogen Deficiency-Induced Decrease in Cytokinins Content Promotes Rice Seminal Root Growth by Promoting Root Meristem Cell Proliferation and Cell Elongation

Abstract

Rice (Oryza sativa L.) seedlings grown under nitrogen (N) deficiency conditions show a foraging response characterized by increased root length. However, the mechanism underlying this developmental plasticity is still poorly understood. In this study, the mechanism by which N deficiency influences rice seminal root growth was investigated. The results demonstrated that compared with the control (1 mM N) treatment, N deficiency treatments strongly promoted seminal root growth. However, the N deficiency-induced growth was negated by the application of zeatin, which is a type of cytokinin (CK). Moreover, the promotion of rice seminal root growth was correlated with a decrease in CK content, which was due to the N deficiency-mediated inhibition of CK biosynthesis through the down-regulation of CK biosynthesis genes and an enhancement of CK degradation through the up-regulation of CK degradation genes. In addition, the N deficiency-induced decrease in CK content not only enhanced the root meristem cell proliferation rate by increasing the meristem cell number via the down-regulation of OsIAA3 and up-regulation of root-expressed OsPLTs, but also promoted root cell elongation by up-regulating cell elongation-related genes, including root-specific OsXTHs and OsEXPs. Taken together, our data suggest that an N deficiency-induced decrease in CK content promotes the seminal root growth of rice seedlings by promoting root meristem cell proliferation and cell elongation.

The Exudation of Surplus Products Links Plant Functional Traits and Plant-Microbial Stoichiometry

“…The flow goes belowground, where transported sucrose serves as a signal for root growth. The N required for building root tissues [69] can be obtained either from remobilised shoot reserves, which alters PFT (e.g., SLA, plant N content) or from the soil solution, which affects the concentration of N-forms in the soil solution. Graminoids growing under low N availability increase their phloem load with C and amino acids to support root growth [69][70][71].…”

“…The N required for building root tissues [69] can be obtained either from remobilised shoot reserves, which alters PFT (e.g., SLA, plant N content) or from the soil solution, which affects the concentration of N-forms in the soil solution. Graminoids growing under low N availability increase their phloem load with C and amino acids to support root growth [69][70][71]. The active remobilisation of leaf reserves may reduce leaf thickness and result in an increase in SLA.…”

Faster leaf senescence after flowering in wet direct‐seeded rice was mainly regulated by decrease in cytokinin content as compared with transplanted‐flooded rice

“…Previous studies have shown that CTK content in plants was downregulated by nitrogen deficiency, thus affecting the distribution and transport of dry matters (Wang et al., 2020; Xu et al., 2015). Higher leaf senescence rate after flowering in WDSR could reduce the photosynthesis rate of leaves and thus decrease the dry matter synthesis after flowering and the distribution of dry matter to grain (Cassman, Whitney, & Stockinger, 1980).…”

Limitation of Cytokinin Export to the Shoots by Nucleoside Transporter ENT3 and Its Linkage with Root Elongation in Arabidopsis

“…Regulation of cytokinin concentration and distribution in plants is important for the control of their adaptation to environmental changes. A decline in the content of these hormones in shoots has been frequently detected under water deficiency in tobacco [ 1 ] and tomato [ 2 ], as well as under deficiency of mineral nutrients in Arabidopsis [ 3 ], wheat [ 4 ], barley [ 5 ], and rice [ 6 ]. Cytokinins are known to exert opposite effects on shoots and roots, promoting the shoot growth, while inhibiting the root growth [ 7 ].…”

“…Consequently, a reduction in cytokinin levels detected under deficiency of mineral nutrients or water [ 8 ] should inhibit shoot growth, while at the same time promoting root growth. Cytokinins have been shown to inhibit root elongation [ 9 ] and accumulation of root biomass [ 10 ] and consequently a decline in root cytokinin concentration is expected to activate root growth [ 6 ]. There are various mechanisms regulating cytokinin concentration under the changing environment.…”