The Role Of Gibberellin Metabolism In Response To Abiotic Stress

Throughout their life cycle, plants regularly face various environmental conditions beyond their growth and productivity optimum range. To survive, plants must respond effectively to adverse conditions such as drought, cold, and salinity, referred to as abiotic stressors. Defense against these stressors is costly, often occurring at the expense of growth and development. However, studies have shown that the cost of growth inhibition is worthwhile to facilitate survival. Evidence has emerged that gibberellin (GA) plays a major role in these growth regulatory mechanisms. However, the role and benefit of reducing the active GA level in abiotic stress responses are largely unknown. Using transgenitc technology, the GA over-accumulation and deficiency, CBF1constitutive expression transgenic tobacco plants (Nicotiana tabacum) were generated.The expression of GA metabolism-related genes in wild type and transgentic tobacco under the stress was analyzed by Real time PCR. The transcriptomes changes of transgentic plant were compared based on microarry analysis.These findings provide more comprehensive insight into the roles of GA in CBF1-mediated abiotic stress response.The main results and conclusions obtained in this study are shown as follows:1. Transgenic tobacco plants were generated expressing AtCBF1, PtGA20ox, or PtGA2oxl under the control of the promoter of the cauliflower mosaic virus35S using the following constructs35S::CBF1,35S::PtGA20ox, and35S::PtGA2ox1(as35:CBF,35:G20, and35:G2). The level of major active GA in tobacco was significantly altered in35:G20and35:G2plants typically resulting in GA overproduction and GA deficiency syndrome, respectively. Like35:G2,35:CBF plants showed a reduction in height and in GA1content albeit to a lesser extent. The35:G20plants exhibited increases in internode length, whereas35:G2and35:CBF plants exhibited dwarfism.The content of inactive GA8, converted from GA1, was substantial increasesec both35:G20and35:G2plants,however, the GA8content showed no obvious difference in35:CBF plants, indicting that the precursor input of the GA metabolism pathway was limited in the35:CBF plants.2. The Under the cold, osmotic and salt stress conditions,the synthetic genes for GA metabolic pathway upstream precursor GGPP biosynthetic were widely suppressed in Arabidopsis thaliana. Similarly, the expression of genes coding GGPP directly synthase GGPPS was also inhibited. These result suggesting that the shortage of GGPP caused by CBF-mediated GGPPS1repression was responsible for the decrease in active GA and consequent growth retardation. However, under the cold stress, the expression of GA2oxs was also regulated that is different with35:CBF1plants, indicating there is at least have another regulatory mechanism for GA metabolism regulation in addition to the CBF1pathway.3. The transcriptome of35:CBF,35:G20, and35:G2plants compared with wild-type tobacco plants were remodeling.When a1.5-fold change (FC) and with respect to WT plants in gene expression was used to filter gene sets,293genes were obtained between35:G2vs. WT, and88genes were upregulated, the other205genes were downregulated.536genes were obtained between35:G20vs. WT, and224 genes were upregulated, the other312genes were downregulated.354genes were obtained between35:CBF vs. WT, and95genes were upregulated, the other259genes were downregulated. The ratio of upreguled genes and down reguled genes of35:CBF plants is sililar to the35:G2plants, the numerous genes expressional repression probably associated with the growth retardation. The genes differentially expressed in neither transgentic plants were enrichment in the "stimulus response" and "metabolic activity" function, the former function directly relatd with sress response and the latter probably involved in the control of growth rate.4. In order to further understand the role of GA in the CBF1-mediated abiotic stress response, The subgroup of GA-regulated genes in the CBF1-mediated abiotic stress response were identified. When a1.5-fold change (FC) with respect to WT plants in gene expression was used to filter our gene list,43genes were obtained. Fourteen genes were upregulated, and the other29genes were downregulated in35:CBF plants that associated with the low GA level. This result indicates that the low GA level in35:CBF plants primarily represses gene expression. High GA level was inhibited the accumulation of peroxidase, and low levels of GA was promoted the osmotic adjustment related to substance accumulation and gene activation.The high GA level has a negative regulatory role in leaf water retention.GA was associated with SA/JA balance in the CBF1-mediated stress response, but inhibition of SA signaling is the principal regulator in this pathway. These results demonstrate that JA may be another important player in the CBF1-mediated stress-response pathway.This study analyzed the regulatory sites in the GA metabolic pathway under the biological stress. The roles of GA metabolic regulation in the abiotic stress response were revealed.These findings provide new insight into the theory and strategy development about the stress-resistance breeding. These results also provide a potentially invaluable scientifi resource for future studies of the balance regulation between growth and tolence in plants under abiotic stress.