Abstract
Our phenotyping and hormonal study has characterized the role of cytokinins (CK) in the drought and recovery responses of Arabidopsis thaliana. CK down-regulation was achieved by overexpression of the gene for CK deactivating enzyme cytokinin oxidase/dehydrogenase (CKX): constitutive (35S:CKX) or at the stress onset using a dexamethasone-inducible pOp/LhGR promoter (DEX:CKX). The 35S:CKX plants exhibited slow ontogenesis and higher expression levels of stress-associated genes, e.g., AtP5CS1, already at well-watered conditions. CK down-regulation resulted during drought in higher stress tolerance (indicated by relatively low up-regulation of the expression of drought stress marker gene AtRD29B) accompanied with lower leaf water loss. Nevertheless, these plants exhibited slow and delayed recovery after re-watering. CK levels were increased at the stress onset by stimulation of the expression of CK biosynthetic gene isopentenyl transferase (ipt) (DEX:IPT) or by application of exogenous CK meta-topolin. After water withdrawal, long-term CK elevation resulted in higher water loss in comparison with CKX transformants as well as with plants overexpressing ipt driven by senescence-inducible SAG12 promoter (SAG:IPT), which gradually enhanced CKs during the stress progression. In all cases, CK up-regulation resulted in fast and more vigorous recovery. All drought-stressed plants exhibited growth suppression associated with elevation of abscisic acid and decrease of auxins and active CKs (with the exception of SAG:IPT plants). Apart from the ipt overexpressers, also increase of jasmonic and salicylic acid was found.
Highlights
Drought belongs to the most frequent abiotic stresses which worldwide reduce crop yields (Daryanto et al, 2016)
The first noticeable reduction of growth was detected after 4 days of drought stress (29 days after sowing (DAS)), the growth rate gradually decreased
The growth suppression was accompanied by the decrease of active CKs, predominantly of trans-zeatin
Summary
Drought belongs to the most frequent abiotic stresses which worldwide reduce crop yields (Daryanto et al, 2016). Plants had to evolve different mechanisms for sensing and responding to drought (Zwack and Rashotte, 2015). Their interactions with the environment as well as their growth and development are regulated by plant hormones (Ha et al, 2012). ABA exhibits a complex cross-talk with other plant hormones. Participation of salicylic acid (SA) in drought responses is indicated by elevation of this hormone after water withdrawal as well as by positive effects of exogenous SA application on plant tolerance (Miura and Tada, 2014). A complex cross-talk among different phytohormones is underlying drought stress responses
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