Effects of SAG12-ipt and HSP18.2-ipt Expression on Cytokinin Production, Root Growth, and Leaf Senescence in Creeping Bentgrass Exposed to Drought Stress

Abstract

<jats:p>Drought stress is a widespread abiotic stress that causes a decline in plant growth. Drought injury symptoms have been associated with an inhibition in cytokinin (CK) synthesis. The objectives of this study were to investigate whether expression of a gene (<jats:italic>ipt</jats:italic>) encoding the enzyme adenine isopentenyl phosphotransferase for CK synthesis ligated to a senescence-activated promoter (<jats:italic>SAG12</jats:italic>) or a heat shock promoter (<jats:italic>HSP18.2</jats:italic>) would improve drought tolerance in creeping bentgrass (<jats:italic>Agrostis stolonifera</jats:italic>) and to examine shoot and root growth responses to drought stress associated with changes in endogenous production of CK, and the proportional change in CK and abscisic acid (ABA) due to <jats:italic>ipt</jats:italic> transformation. Most <jats:italic>SAG12-ipt</jats:italic> and <jats:italic>HSP18.2-ipt</jats:italic> transgenic lines exhibited significantly higher turf quality, photochemical efficiency, chlorophyll content, leaf relative water content, and root:shoot ratio under drought stress than the null transformant or the wild-type ‘Penncross’ plants. Transgenic lines that had better growth and turf performance generally had higher CK content and a higher CK-to-ABA ratio, although the direct correlation of CK and ABA content with individual physiological parameters in individual lines was not clear. Our results demonstrated that expressing <jats:italic>ipt</jats:italic> resulted in the improvement of turf performance under drought stress in creeping bentgrass in some of the transgenic plants with <jats:italic>SAG12-ipt</jats:italic> or <jats:italic>HSP18.2-ipt</jats:italic>, which could be associated with the suppression of leaf senescence and promoting root growth relative to shoot growth due to the maintenance of higher CK level and a higher ratio of CK to ABA.</jats:p>

Exogenously applied spermidine improves drought tolerance in creeping bentgrass associated with changes in antioxidant defense, endogenous polyamines and phytohormones

“…It has been reported that drought stress can lead to an increase in ABA level of creeping bentgrass (DaCosta and Huang 2007a, b). We noticed that drought stress resulted in an ABA increase in both treatments, while exogenous Spd pretreatment almost had no effect on ABA accumulation in creeping bentgrass with significant change of endogenous PA. Merewitz et al (2010) reported that cytokinin regulate drought tolerance of creeping bentgrass through changing the balance of ABA, rather than accumulating more ABA. As a drought resistant bentgrass, velvet bentgrass exhibited significantly less ABA accumulation than creeping bentgrass, which could allow continued gas exchange and sustained plant survival during drought stress (DaCosta and Huang 2007a, b).…”

Differentially Expressed Genes Associated with Improved Drought Tolerance in Creeping Bentgrass Overexpressing a Gene for Cytokinin Biosynthesis

“…The ipt gene was cloned from agrobacterium ( Agrobacterium tumefaciens ) and SAG12-ipt plants were transformed using the Agrobacterium method as described previously [2–4, 19–20]. In order for ease of comparison to previous results, the same plant material used for proteomic and metabolomic evaluation [3–4] was used for SSH analysis.…”

“…Gene sequences were categorized based on their predicted function based on the system used previously in Bevan et al . [23] and [2–3]. Sequences were evaluated to remove redundant base pairs and vector sequences manually and through use of VecScreen BLAST database.…”

“…Increasing CK synthesis through overexpressing a gene encoding isopentyl transferase controlling CK synthesis ( ipt ) has been shown to increase drought tolerance in creeping bentgrass [2–4] and other species [5–7]. SAG12-ipt creeping bentgrass exhibits a greater ability to maintain CK content and improvement in major physiological characteristics governing drought tolerance, as manifested by reduced electrolyte leakage, maintenance of higher relative water content, improvement in photosynthetic rate, and reduction in oxidative damage, which provided strong evidence of improved drought tolerance by increasing endogenous CK [2–4]. Rivero et al .…”

Advances in upstream players of cytokinin phosphorelay: receptors and histidine phosphotransfer proteins

“…Particularly cytokinins are known to delay leaf senescence as shown in experiments that prolonged the life span of leaves when the cytokinin biosynthetic IPT gene was expressed under the control of a senescence-specific promoter SAG12 (Gan and Amasino 1995) or a heat-shock promoter HSP18.2 (Merewitz et al 2010). Identification of an AHK3 gain-of-function mutant, ore12-1, revealed that AHK3, not AHK2 or AHK4, is the primary cytokinin receptor functionally regulating leaf senescence via an ARR2-specific phosphorelay cascade (Kim et al 2006).…”