The role of cytokinins in responses to water deficit in tobacco plants over‐expressing trans‐zeatin O‐glucosyltransferase gene under 35S or SAG12 promoters

Abstract

The impact of water deficit progression on cytokinin (CK), auxin and abscisic acid (ABA) levels was followed in upper, middle and lower leaves and roots of Nicotiana tabacum L. cv. Wisconsin 38 plants [wild type (WT)]. ABA content was strongly increased during drought stress, especially in upper leaves. In plants with a uniformly elevated total CK content, expressing constitutively the trans-zeatin O-glucosyltransferase gene (35S::ZOG1), a delay in the increase of ABA was observed; later on, ABA levels were comparable with those of WT.As drought progressed, the bioactive CK content in leaves gradually decreased, being maintained longer in the upper leaves of all tested genotypes. Under severe stress (11 d dehydration), a large stimulation of cytokinin oxidase/ dehydrogenase (CKX) activity was monitored in lower leaves, which correlated well with the decrease in bioactive CK levels. This suggests that a gradient of bioactive CKs in favour of upper leaves is established during drought stress, which might be beneficial for the preferential protection of these leaves.During drought, significant accumulation of CKs occurred in roots, partially because of decreased CKX activity. Simultaneously, auxin increased in roots and lower leaves. This indicates that both CKs and auxin play a role in root response to severe drought, which involves the stimulation of primary root growth and branching inhibition.

Analysis of Cytokinin Mutants and Regulation of Cytokinin Metabolic Genes Reveals Important Regulatory Roles of Cytokinins in Drought, Salt and Abscisic Acid Responses, and Abscisic Acid Biosynthesis

“…Consistent with our finding, Havlová et al (2008) detected an increase in ABA levels in the CK-overproducing 35S:ZOG1 plants under nonstressed conditions. It is possible that an increase of ABA:CK ratio caused by CK deficiency might result in constitutive stomatal closure under well-watered conditions, leading to reduced transpiration, which inhibits CO 2 uptake and thereby negatively impacts photosynthesis (Cornic and Fresneau, 2002).…”

“…threshold also caused stunted plant growth, abnormal tissue development, and sensitivity but not tolerance to drought (Li et al, 1992;Hewelt et al, 1994;Wang et al, 1997;Synkova et al, 1999;Havlová et al, 2008). Therefore, appropriate manipulation of CK levels is necessary to increase leaf longevity and photosynthetic capacity under drought stress, taking into account the dosage effect of CKs on plant growth and stress responses.…”

“…Increasing evidence suggests that cytokinins (CKs) are involved in stress responses (Tran et al, 2007b;Havlová et al, 2008;Argueso et al, 2009). CKs have been recognized as an important signal that travels from roots to shoots (Letham, 1994).…”

Conditional up-regulation of cytokinin status increases growth and survival of sugarcane in water-limited conditions

“…The current studies show that bioactive CK content was reduced by water stress in sugarcane (Table 5-4). This is in line with reports in various plant species which demonstrated a reduction of biosynthesis, metabolism, perception and signal transduction of CK in response to environmental stress (Argueso et al, 2009, Nishiyama et al, 2011, Schachtman and Goodger, 2008, Havlova et al, 2008. However, during the experiment, simultaneously with the development of stress levels, P RAB17 ::IPT plants produced substantially more bioactive CKs than wild-type plants.…”

“…However, the hormone analysis results showed that levels of free auxin (indole-3-acetic acid, IAA) were elevated in all plants under water stress conditions. The increase in IAA content was higher in severely stressed leaves of P RAB17 ::IPT than wild-type plants, a finding consistent with the increased bioactive CK and IAA in drought-affected tobacco (Havlova et al, 2008). The P RAB17 ::IPT finding in sugarcane is in line with Bertell and Eliasson (1992) who detected increased in IAA content when CK (6-benzyladenine) was applied to pea plants.…”