Functional Replacement of Ferredoxin by a Cyanobacterial Flavodoxin in Tobacco Confers Broad-Range Stress Tolerance

Abstract

Chloroplast ferredoxin (Fd) plays a pivotal role in plant cell metabolism by delivering reducing equivalents to various essential oxidoreductive pathways. Fd levels decrease under adverse environmental conditions in many microorganisms, including cyanobacteria, which share a common ancestor with chloroplasts. Conversely, stress situations induce the synthesis of flavodoxin (Fld), an electron carrier flavoprotein not found in plants, which can efficiently replace Fd in most electron transfer processes. We report here that chloroplast Fd also declined in plants exposed to oxidants or stress conditions. A purified cyanobacterial Fld was able to mediate plant Fd-dependent reactions in vitro, including NADP 1 and thioredoxin reduction. Tobacco (Nicotiana tabacum) plants expressing Fld in chloroplasts displayed increased tolerance to multiple sources of stress, including redox-cycling herbicides, extreme temperatures, high irradiation, water deficit, and UV radiation. Oxidant buildup and oxidative inactivation of thioredoxin-dependent plastidic enzymes were decreased in stressed plants expressing plastid-targeted Fld, suggesting that development of the tolerant phenotype relied on productive interaction of this flavoprotein with Fd-dependent oxidoreductive pathways of the host, most remarkably, thioredoxin reduction. The use of Fld provides new tools to investigate the requirements of photosynthesis in planta and to increase plant stress tolerance based on the introduction of a cyanobacterial product that is free from endogenous regulation in higher plants.

Enhanced plant tolerance to iron starvation by functional substitution of chloroplast ferredoxin with a bacterial flavodoxin

“…We have introduced the Fld gene from the cyanobacterium Anabaena PCC7119 into the genome of tobacco and demonstrated that transgenic plants expressing this flavoprotein in chloroplasts developed generalized tolerance to various sources of oxidative and environmental stress (26). Here we show that these transformants were also able to grow on Fe-limited substrates.…”

“…We therefore analyzed the response to Fe limitation of transgenic tobacco plants in which Fld accumulated in plastids (pfld lines, for plastidic Fld) using WT siblings and transformants expressing the flavoprotein in the cytosol (cfld lines, for cytosolic Fld) as controls. Preparation of stable homozygous pfld and cfld tobacco lines expressing various levels of Fld has been described elsewhere (26). Most experiments were carried out with plants from lines pfld5-8, pfld4-2, and cfld1-4, containing Fld at 70, 57, and 108 pmol⅐g Ϫ1 fresh weight, quantities comparable to 98 pmol⅐g Ϫ1 fresh weight for endogenous Fd (26).…”

“…Preparation of stable homozygous pfld and cfld tobacco lines expressing various levels of Fld has been described elsewhere (26). Most experiments were carried out with plants from lines pfld5-8, pfld4-2, and cfld1-4, containing Fld at 70, 57, and 108 pmol⅐g Ϫ1 fresh weight, quantities comparable to 98 pmol⅐g Ϫ1 fresh weight for endogenous Fd (26). When grown in Fe-replete media, transformed plants displayed WT phenotypes with respect to biomass accumulation (Table 1), flower development, and seed production (26).…”

“…Up-regulation of Fld levels is also considered as one of the factors determining colonization of Fe-poor waters by phytoplankton (23), and photosynthetic microorganisms lacking the Fld gene are usually restricted to Fe-rich coastal environments (24). The Fld gene is absent from the plant genome (25), indicating that this adaptive resource was lost somewhere in the transition between algae and plants, although at least some of the plant enzymes whose bacterial ancestors used Fld as a normal or occasional substrate have retained the ability to productively interact with this electron carrier (26).…”

Drought effects and water use efficiency: improving crop production in dry environments

“…These crops and others under development show overexpression of bacterial RNA chaperones (Castiglioni et al 2008), and NF-Y class transcriptional regulators (Nelson et al 2007) in which drought tolerance is reported. Tolerance to a whole range of stresses in plants may also be induced by RNA silencing to downregulate poly ADP ribose polymerase (Vanderauwera et al 2007), and overexpression of a cyanobacterial flavodoxin (Tognetti et al 2006). Until comprehensive field trials are conducted with this material, some caution must be exercised.…”