Glucose Inhibits Root Growth By Regulating Auxin Via ABA INSENSITIVE5in Arabidopsis

All living organisms rely on metabolic pathways to generate cellular energy. Glucose, a universal nutrient preferred by most organisms, not only fuels metabolism, but also functions as a hormone-like signaling molecule during plant growth and development. For example, high concentrations of glucose repress hypocotyl elongation, cotyledon greening and expansion, and shoot development during early seedling development in Arabidopsis. However, the role of glucose in root elongation remains elusive. Our study demonstrates that high concentrations of glucose inhibit root growth, especially root meristem length. Plant glucose signaling is reported to interact with classical hormone signaling networks. Using physiological, biochemical, cellular and genetic methods, we demonstrate that glucose inhibits root growth by regulating auxin via ABA INSENSITIVE5(ABI5) in Arabidopsis. Our research results are summarized as follows:1. Glucose represses primary root growth by shortening the meristematic zoneIn our study, high concentrations of glucose were shown to shorten the root meristematic zone in a dose-dependent manner. We found that glucose reduces the size of the primary root meristem by increasing the rate of meristematic cell differentiation and positively regulating the exit of cells from the primary root meristem into the elongation and differentiation zones.2. Glucose affects auxin activities in the root meristem cellsThe DII-VENUS signal was stronger in the root meristems of the DII-VENUS plants treated with higher concentrations of glucose, indicating that high concentrations of glucose down-regulate auxin activity. This was further supported by measurements of IAA content using GC-SIM-MS. 3. High glucose concentrations regulate PIN1accumulationWe found that pin1plants were less sensitive to high concentrations of glucose compared with wild-type plants in terms of root meristem inhibition. Further studies found that the levels of PIN1protein, rather than PIN1transcript, were reduced in roots treated with high concentrations of glucose, indicating the importance of the post-transcriptional regulation of PIN1in this processes.4. ABI5is involved in the glucose-mediated shortening of the root meristem zoneabi5-1plants were shown to be less sensitive to high concentrations of glucose compared with wild-type plants in terms of root meristem inhibition. Furthermore, ABI5expression in the root meristem of ABI5::GUS line was dramatically induced when subjected to either3%or5%glucose, implying that glucose-induced ABI5expression is accompanied by glucose-mediated inhibition of the root meristem zone. Consistent with this, our transgenic lines over-expressing ABI5had short root meristem zones.5. PIN1protein is affected by ABI5Our results also indicated that PIN1protein levels were markedly reduced in the roots of wild-type plants treated with high concentrations of glucose, but were only slightly reduced in those of abi5-1plants. These observations were further supported by the analyses of PIN1-GFP fluorescences in the ABI5over-expressing lines. The GFP fluorescence was reduced and confined to the stele in35S::ABI5-GR PIN1::PIN1-GFP lines.In summary, our data indicate that glucose induces ABI5expression in the root, which, in turn, reduces PIN1protein accumulation, leading reduced auxin activity and resulting in a short root meristem zone.