Effects of Elevated Root-Zone CO2 on Root Morphology and Nitrogen Metabolism Revealed by Physiological and Transcriptome Analysis in Oriental Melon Seedling Roots

Abstract

Rhizosphere CO2 is vital for crop growth, development, and productivity. However, the mechanisms of plants’ responses to root-zone CO2 are unclear. Oriental melons are sensitive to root-zone gas, often encountering high root-zone CO2 during cultivation. We investigated root growth and nitrogen metabolism in oriental melons under T1 (0.5%) and T2 (1.0%) root-zone CO2 concentrations using physiology and comparative transcriptome analysis. T1 and T2 increased root vigor and the nitrogen content in the short term. With increased treatment time and CO2 concentration, root inhibition increased, characterized by decreased root absorption, incomplete root cell structure, accelerated starch accumulation and hydrolysis, and cell aging. We identified 1280 and 1042 differentially expressed genes from T1 and T2, respectively, compared with 0.037% CO2-grown plants. Among them, 683 co-expressed genes are involved in stress resistance and nitrogen metabolism (enhanced phenylpropanoid biosynthesis, hormone signal transduction, glutathione metabolism, and starch and sucrose metabolism). Nitrogen metabolism gene expression, enzyme activity, and nitrogen content analyses showed that short-term elevated root-zone CO2 mainly regulated plant nitrogen metabolism post-transcriptionally, and directly inhibited it transcriptionally in the long term. These findings provided a basis for further investigation of nitrogen regulation by candidate genes in oriental melons under elevated root-zone CO2.

Root-Zone CO2 Concentration Affects Partitioning and Assimilation of Carbon in Oriental Melon Seedlings

“…During the labeling period of the 0.2% root-zone 13 CO 2 treatment, the size of δ 13 C was in the order of root > stem > leaf, which was consistent with the results of previous studies [38]. The δ 13 C value under the 0.5% and 1% 13 CO 2 treatments is in the order of stem > root > leaf, which may be due to the fact that the increase in root-zone 13 CO 2 concentration specifically promotes root growth in the short term [39]. Oriental melon seedlings have a higher transpiration rate, which facilitates the upward transport of carbon dioxide, enhances the photosynthesis of stems and intercepts the carbon dioxide diffused into the atmosphere [20].…”

Physiological and Transcriptomic Analyses Reveal the Effects of Elevated Root-Zone CO2 on the Metabolism of Sugars and Starch in the Roots of Oriental Melon Seedlings

“…The seedlings were treated with an automatic root-zone CO 2 concentration treatment system based on the root-zone CO 2 treatment method by Chen et al, 2020. ( Figure 9 ) [ 24 ]. The root-zone CO 2 cultivation and treatment system mainly comprised a CO 2 -air mixing system, a CO 2 concentration monitoring system, and an aeroponic cultivation system.…”

“…Therefore, this study aimed to investigate the alterations in the accumulation and metabolism of starch and sugar in the root system of oriental melon seedlings under elevated root-zone CO 2 . Three different root-zone concentrations of CO 2 were selected for treatment, and the previously designed automatic root-zone gas control device was used [ 24 ]. The physiological indices of root growth, root cell microstructure, sugar and starch contents, and the activities of enzymes related to sugar and starch metabolism in the roots were determined.…”