Improvement of drought resistance through manipulation of the gibberellic acid pathway

Authors: ZHANG, YAPING - University Of California, Davis; Norris, Ayla; REID, MICHAEL - University Of California, Davis; Jiang, Cai-Zhong

Submitted to: Ornamental Plant Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/28/2021
Publication Date: 11/17/2021
Citation: Zhang, Y., Norris, A.M., Reid, M., Jiang, C. 2021. Improvement of drought resistance through manipulation of the gibberellic acid pathway. Ornamental Plant Research. 1. Article 11. https://doi.org/10.48130/OPR-2021-0011.
DOI: https://doi.org/10.48130/OPR-2021-0011

Interpretive Summary: Over their life cycle, plants often experience undesirable abiotic stresses, such as salinization, heat, waterlogging, and drought. It has been reported that approximately one-third of the cultivated area around the world suffers from constantly inadequate supply of water. Continuous drought can cause economic and financial problems for both agricultural producers and consumers. In 2012, the drought in America destroyed or damaged many of the major field crops in the Midwest, particularly field corn and soybeans, causing price increases of corn, soybeans, and other field crops. Western United States including California experienced extremely drought stress in recent years. For example, California statewide, in 2013, the 12-month accumulated precipitation was less than one-third of average precipitation from the year 1895 to 2013, which greatly impacted agricultural irrigation because of the low rainfall and water deficiency. Therefore, increasing plants’ ability to survive under drought stress conditions is of great importance to the horticultural industry. Recent research demonstrated that the absence of the phytohormone gibberellic acid (GA) or a lack of GA sensitivity increased plants’ drought resistance. GA is a plant hormone that plays important roles in the regulation of plant growth and development including seed development and germination, diverse organ (e.g. leaves, stem, flower and fruit) elongation and expansion. Studies on number of the GA-deficient mutants in various plants, such as Arabidopsis, maize, rice, pea, and tomato, suggested that decreased levels of GA in the mutants consequently caused typical GA-deficiency dwarf phenotypes and the enhancement of stress tolerance. Research has found that DELLA deficient mutants, such as rice slr1 and Arabidopsis gai-t6 and rga-24, are taller and flower earlier than wild type plants. In contrast, DELLA sufficient mutants or transgenic plants are dwarf and flower late. Such a mutant, the DELLA protein mutant gai-1 (gibberellin acid insensitive) from Arabidopsis has been shown to reduce plant height and alter GA response. It has been suggested that when GID and gai form a complex together, they cannot be degraded by the 26S proteasome, which inhibits GA signaling, stops cell division and expansion, and results in an undesirable dwarf phenotype with improved drought resistance. However, overexpression of gai using the constitutively active 35S promoter would result in completely dwarf plants that cannot ensure product quality in terms of flower or fruit size. In order to generate normal plants that can have similar products as wild type plants and can survive from drought stress as well, we proposed to use stress-inducible promoters to drive the expression of the gai mutant gene. The RD29A promoter from Arabidopsis has been demonstrated that its activity was quickly and strongly induced by abiotic stresses such as drought, heat and salinity. Studies indicated that within the RD29A promoter region, there are number of cis-acting elements involved in the dehydration-induced activation, including the drought response element (DRE), and ABA response element (ABRE). Therefore, in this study, we propose to use a stress-inducible promoter to drive the expression of the gai-1 gene, which will allow us to initiate the expression of gai-1 at specific times or under drought stress conditions. We hypothesize that the temporary inhibition of cell growth caused by inducible expression of the gai-1 gene would lead to enhancement of drought stress tolerance. We generated transgenic plants in which the gai-1 gene was over-expressed in petunia under a stress-inducible RD29A promoter from Arabidopsis. When these plants were subjected to limited irrigation and drought treatments, transgenic plants showed phenotypes of darker green leaves and compact flowers compared to the wild type plants.

Technical Abstract: Improving plants' ability to survive under drought is of great importance to horticultural industry. The plant hormone gibberellic acid (GA) mediates diverse aspects of plant growth and development. The Arabidopsis gibberellin acid insensitive mutant gai-1 displays reduced plant height, altered GA response, and enhanced drought resistance. However, overexpression of gai-1 using the constitutive 35S promoter would result in dwarf plants with drought resistance. Here, we tested the hypothesis that the temporary inhibition of cell growth caused by inducible expression of the gai-1 gene would lead to better drought resistance and improve crop productivity without an undesirable dwarf phenotype. We generated transgenic plants in which the gai-1 gene was over-expressed in petunia under a stress-inducible RD29A promoter from Arabidopsis. When these plants were subjected to limited irrigation and drought treatments, transgenic plants showed phenotypes of darker green leaves and compact flowers compared to the wild type plants. Importantly, these transgenic plants recovered sooner than wild type and the empty vector-transformed control plants. This study provides evidences that temporary inhibition of cell growth caused by over-expression of the gai-1 mutant gene with a drought stress-inducible promoter leads to better drought resistance when the plants experience the drought.