Cytokinin biosynthesis and transport for systemic nitrogen signaling

Abstract

Summary The plasticity of growth and development in response to environmental changes is one of the essential aspects of plant behavior. Cytokinins play an important role as signaling molecules in the long‐distance communication between organs in systemic growth regulation in response to nitrogen. The spatial distribution of the expression sites of cytokinin biosynthesis genes leads to structural differences in the molecular species transported through the xylem and phloem, giving root‐borne trans‐hydroxylated cytokinins, namely trans‐zeatin (tZ) type, a specialized efficacy in regulating shoot growth. Furthermore, root‐to‐shoot translocation via the xylem, tZ, and its precursor, the tZ riboside, controls different sets of shoot growth traits to fine‐tune shoot growth in response to nitrogen availability. In addition to nitrogen, photosynthetically generated sugars positively regulate de novo cytokinin biosynthesis in the roots, and contribute to plant growth under elevated CO2 conditions. In shoot‐to‐root signaling, cytokinins also play a role in the regulation of nutrient acquisition and root system growth in cooperation with other types of signaling molecules, such as C‐TERMINALLY ENCODED PEPTIDE DOWNSTREAMs. As cytokinin is a key regulator for the maintenance of shoot apical meristem, deepening our understanding of the regulatory mechanisms of cytokinin biosynthesis and transport in response to nitrogen is important not only for basic comprehension of plant growth, but also to ensure the stability of agricultural production.

TaCKX2.2 Genes Coordinate Expression of Other TaCKX Family Members, Regulate Phytohormone Content and Yield-Related Traits of Wheat

“…Other differences concerning CKs are a larger increase of cZ and glucosides of tZ and DZ in TaCKX2.2.2 silenced compared to TaCKX1 silenced plants, which in contrast were very rich in tZ7G and tZ9GOG. Both riboside forms and free base cZ are transporter metabolites, which when transported from roots to shoots control shoot traits [7,20]. Moreover, tZ riboside and cZ are degradation substrates of CKX [23,24].…”

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

“…As ENT3 is highly expressed in the root vasculature of seedlings carrying AtENT3-promoter–GUS construct, a similar function in roots is highly likely [ 22 ]. Roots are a site for the synthesis of cytokinin ribosides which are transport form of cytokinins and precursors of active cytokinin bases [ 39 ]). Accordingly, ENT3 might function in cellular uptake of cytokinin ribosides, their conversion or signaling purposes in roots, thereby prohibiting export to the shoot via the xylem.…”

“…In accordance, we can interpret the present results as suggesting that knocking out the gene coding for the transporter of ribosides decreases capacity of root cells to retain cytokinins in the root cells contributing to their apoplast loading into xylem and transport to the shoots through xylem. The choice of trans-zeatin for supplying roots with exogenous cytokinin in our experiments is supported by information that trans-zeatin-type species are the major form in xylem sap [ 39 ], while we used antibodies raised against trans-zeatin riboside having low cross-reactivity to cis-zeatin forms.…”

Nitrate inhibits nodule organogenesis through inhibition of cytokinin biosynthesis inLotus japonicus

“…In addition to local responses to nitrate, plants possess systemic regulatory circuits allowing response to nitrate to be coordinated between roots and shoots. Cytokinin signalling is one of the pathways underlying this systemic signalling of nitrate availability (Sakakibara, 2020). Outside of legumes, induction of cytokinin biosynthesis by nitrate has been described in several species including maize (Takei et al, 2001), rice (Kamada-Nobusada et al, 2013) and Arabidopsis (Miyawaki et al, 2004).…”