Transgenic tobacco plants co‐expressing Agrobacterium iaa and ipt genes have wild‐type hormone levels but display both auxin‐ and cytokinin‐overproducing phenotypes

Abstract

). ² These authors contributed equally to this work. SummaryTransgenic tobacco lines simultaneously expressing the Agrobacterium iaaM, iaaH and ipt genes, obtained by crossing lines expressing ipt with lines expressing iaaM and iaaH, were used to study in planta interactions between auxin and cytokinins. All phenotypic traits of the respective parental lines characteristic of cytokinin and auxin overproduction were present in the cross. Indole-3-acetic acid (IAA) and combined zeatin riboside (ZR) and zeatin riboside-5¢-monophosphate (ZRMP) contents were analysed by mass spectrometry in young, developing leaves from the cross, the parental lines and the wild type. Unexpectedly, hormone levels in the cross were very similar to wild-type levels. Thus IAA levels in the cross were much lower throughout vegetative development than in the parental IAA overproducing line, although expression of the bacterial IAA biosynthesis genes was not reduced. The results suggest that effects on apical dominance, adventitious root formation, leaf morphology and other traits commonly Kassociated with IAA and cytokinin overproduction, and observed in the iaa ¥ ipt cross, cannot be explained solely by analysis of auxin and cytokinin contents in individual organs. As traits associated with both hormones are expressed in close spatial and temporal proximity, it is likely that cellular resolution of hormone contents is essential to explain physiological responses to auxins and cytokinins.

Heterologous expression of the BABY BOOM AP2/ERF transcription factor enhances the regeneration capacity of tobacco (Nicotiana tabacum L.)

“…Classical tissue culture experiments have demonstrated that exogenous application of a low ratio of auxin to cytokinin promotes shoot regeneration from tobacco callus, while a high auxin to cytokinin ratio stimulates root formation (Skoog and Miller 1957). These observations are supported and extended by numerous studies in which heterologous expression of bacterial cytokinin-(isopentenyltransferase; ipt) and auxin biosynthesis enzymes (tryptophan monooxygenease, iaaM; indoleacetamide hydrolase; iaaH) has been used to alter endogenous levels of these hormones (Smigocki and Owens 1988;Sitbon et al 1992;Hewelt et al 1994;Eklöf et al 2000). BBM over-expression transgenics show elements of both cytokinin and auxin over-production phenotypes including ectopic shoot production and leaf wrinkling (cytokinin) and ectopic root production (auxin).…”

“…BBM over-expression transgenics show elements of both cytokinin and auxin over-production phenotypes including ectopic shoot production and leaf wrinkling (cytokinin) and ectopic root production (auxin). The simultaneous production of ectopic roots and shoots in a single plant has to our knowledge only been reported in the oVspring of crosses between cytokinin-and auxin-overproducing transgenic lines (Eklöf et al 2000). Endogenous cytokinin and auxin levels in these plants were similar to wild-type hormone levels, suggesting that the observed phenotypes are due to local physiological responses to auxin and cytokinin levels/ratios at the cellular rather than at the organ level.…”

Cytokinin-Deficient Transgenic Arabidopsis Plants Show Multiple Developmental Alterations Indicating Opposite Functions of Cytokinins in the Regulation of Shoot and Root Meristem Activity

“…Interactions between cytokinin and auxin have been described as occurring at multiple levels, including mutual regulation at the level of active hormone (Coenen and Lomax, 1997). Cytokinin-overproducing tobacco plants have been shown to contain lower levels of free indole-3-acetic acid (IAA) and reduced rates of IAA synthesis and turnover (Eklöf et al, 1997(Eklöf et al, , 2000. It was suggested that cytokinins act as downregulators of IAA and vice versa.…”

Auxin regulation of cytokinin biosynthesis in Arabidopsis thaliana : A factor of potential importance for auxin–cytokinin-regulated development

“…It has, for example, been observed that cytokininoverproducing tobacco had lower levels of indole-3-acetic acid (IAA), and that overproduction of IAA in tobacco leads to a reduced pool size of cytokinins (8,9). The problem with these observations, as well as many other studies on auxin-cytokinin cross talk, is the difficulty of resolving cause and effect.…”