A Central Role of Abscisic Acid in Drought Stress Protection of Agrobacterium-Induced Tumors on Arabidopsis

Abstract

Crown gall tumors induced by Agrobacterium tumefaciens represent a sink that has to be provided with nutrients and water by the host plant. The lack of an intact epidermis or cuticle results in uncontrolled loss of water. However, neither the tumor nor the host plant displays wilting. This phenomenon points to drought adaptation in both tumors and the crown gall host plant. To understand the underlying molecular mechanisms of protection against desiccation the gene expression pattern of Arabidopsis (Arabidopsis thaliana) tumors was integrated with the profile of stress metabolites: Arabidopsis tumors accumulated high amounts of abscisic acid (ABA), the ethylene precursor aminocyclopropyl carboxylic acid, osmoprotectants, and form a suberized periderm-like protective layer. Suberization of the outer tumor cell layers most likely is mediated by ABA since external application of ABA induced suberization of Arabidopsis roots. However, the expression level of the classical marker genes, known to respond to drought stress and/or ABA, was lower in tumors. Instead another set of drought and/or ABAinducible genes was more highly transcribed. Elevated transcription of several ABA-dependent aquaporin genes might indicate that ABA controls the water balance of the tumor. The retarded tumor growth on abi and aba mutant plants underlined the importance of a tumor-specific ABA signaling pathway. Taken together, we propose that ABA is an important signal for protection of tumors against desiccation and thus supports tumor development.

The Nonspecific Lipid Transfer Protein AtLtpI-4 Is Involved in Suberin Formation of Arabidopsis thaliana Crown Galls

“…For example, two desaturases, the stearoyl-acyl carrier protein desaturase SAD6 and the fatty acid desaturase FAD3, are highly expressed and enrich the unsaturated fatty acid pool, which helps in maintaining membrane integrity (Klinkenberg et al, 2014). Second, a suberin-containing periderm-like layer covers the surface of crown galls to minimize water loss and avoid pathogen invasion (Efetova et al, 2007).…”

Agrobacterium tumefaciensPromotes Tumor Induction by Modulating Pathogen Defense inArabidopsis thaliana

“…These differences revealed that the host responded to both agrobacterial strains as early as 3 h postinoculation and that the oncogenic strain C58 incites a much stronger pathogen defense response than strain GV3101 at both early time points. Furthermore, our studies documented that the development of Agrobacterium-induced tumors on Arabidopsis was accompanied by elevated production of abscisic acid (ABA) (Efetova et al, 2007), IAA, SA, and ET, but not JA. This was in agreement with previous findings that the development of Agrobacterium-induced tumors on Ricinus communis involves the production of the phytohormones JA, auxin, cytokinin, ET, and ABA (Veselov et al, 2003).…”

“…For successful tumor development, morphological adaptations are essential: these include neovascularization to supply the growing tumor with nutrients (Ullrich and Aloni, 2000) and suberization of the outer cell layers to protect the disrupted tumor surface against drought stress (Efetova et al, 2007). Hydrogen peroxide production in fully developed tumors ( Figure 1H) and the expression of 34 genes involved in oxidative stress ( Figure 4B) are at least partly associated with suberization of outer tumor cell layers.…”

“…The polymerization of suberin monomers involves peroxidases for which H 2 O 2 is the electron donor. Suberization together with ET-triggered ABA production (Veselov et al, 2003) induces drought protective mechanisms in tumors (Efetova et al, 2007).…”

Euglena gracilis(Euglenophyceae) produces abscisic acid and cytokinins and responds to their exogenous application singly and in combination with other growth regulators

“…ABA may enable Euglena gracilis cells in culture to mitigate abiotic and/or biotic stress conditions developing in vitro, thereby producing high cell concentrations, similar to beneficial effects on Arabidopsis plants of inoculation with ABA-producing bacteria under stress conditions (Cohen et al, 2008). In Euglena, ABA may play a promotive role and enhance growth, as in vascular plant systems where ABA is not only a hormone that regulates stress-signalling and controls stress responses, but often is critical for seed filling and tumour development (Efetova et al, 2007;Tuteja, 2007;Santner et al, 2009). Euglena gracilis cultures showed a significant increase in cell numbers in response to exogenous CKs, tZ, iP and BAP at their optimal concentrations.…”