Responses to drought, heat, and combined stress were compared in tobacco (Nicotiana tabacum L.) plants ectopically expressing the cytokinin oxidase/dehydrogenase CKX1 gene of Arabidopsis thaliana L. under the control of either the predominantly root-expressed WRKY6 promoter or the constitutive 35S promoter, and in the wild type. WRKY6:CKX1 plants exhibited high CKX activity in the roots under control conditions. Under stress, the activity of the WRKY6 promoter was down-regulated and the concomitantly reduced cytokinin degradation coincided with raised bioactive cytokinin levels during the early phase of the stress response, which might contribute to enhanced stress tolerance of this genotype. Constitutive expression of CKX1 resulted in an enlarged root system, a stunted, dwarf shoot phenotype, and a low basal level of expression of the dehydration marker gene ERD10B. The high drought tolerance of this genotype was associated with a relatively moderate drop in leaf water potential and a significant decrease in leaf osmotic potential. Basal expression of the proline biosynthetic gene P5CSA was raised. Both wild-type and WRKY6:CKX1 plants responded to heat stress by transient elevation of stomatal conductance, which correlated with an enhanced abscisic acid catabolism. 35S:CKX1 transgenic plants exhibited a small and delayed stomatal response. Nevertheless, they maintained a lower leaf temperature than the other genotypes. Heat shock applied to drought-stressed plants exaggerated the negative stress effects, probably due to the additional water loss caused by a transient stimulation of transpiration. The results indicate that modulation of cytokinin levels may positively affect plant responses to abiotic stress through a variety of physiological mechanisms.

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.

Mitochondrial gene loss and functional transfer to the nucleus is an ongoing process in many lineages of plants, resulting in substantial variation across species in mitochondrial gene content. The Caryophyllaceae represents one lineage that has experienced a particularly high rate of mitochondrial gene loss relative to other angiosperms.

Summary: In angiosperms, mitochondrial‐encoded genes can cause cytoplasmic male sterility (CMS), resulting in the coexistence of female and hermaphroditic individuals (gynodioecy). We compared four complete mitochondrial genomes from the gynodioecious species Silene vulgaris and found unprecedented amounts of intraspecific diversity for plant mitochondrial DNA (mtDNA). Remarkably, only about half of overall sequence content is shared between any pair of genomes. The four mtDNAs range in size from 361 to 429 kb and differ in gene complement, with rpl5 and rps13 being intact in some genomes but absent or pseudogenized in others. The genomes exhibit essentially no conservation of synteny and are highly repetitive, with evidence of reciprocal recombination occurring even across short repeats (< 250 bp). Some mitochondrial genes exhibit atypically high degrees of nucleotide polymorphism, while others are invariant. The genomes also contain a variable number of small autonomously mapping chromosomes, which have only recently been identified in angiosperm mtDNA. Southern blot analysis of one of these chromosomes indicated a complex in vivo structure consisting of both monomeric circles and multimeric forms. We conclude that S. vulgaris harbors an unusually large degree of variation in mtDNA sequence and structure and discuss the extent to which this variation might be related to CMS.