‘happy on norflurazon’ (hon) mutations implicate perturbance of plastid homeostasis with activating stress acclimatization and changing nuclear gene expression in norflurazon‐treated seedlings

Abstract

). † These authors contributed equally to this work. SUMMARYVarious mutant screens have been undertaken to identify constituents involved in the transmission of signals from the plastid to the nucleus. Many of these screens have been performed using carotenoid-deficient plants grown in the presence of norflurazon (NF), an inhibitor of phytoene desaturase. NF-treated plants are bleached and suppress the expression of nuclear genes encoding chloroplast proteins. Several genomes uncoupled (gun) mutants have been isolated that de-repress the expression of these nuclear genes. In the present study, a genetic screen has been established that circumvents severe photo-oxidative stress in NF-treated plants. Under these modified screening conditions, happy on norflurazon (hon) mutants have been identified that, like gun mutants, de-repress expression of the Lhcb gene, encoding a light-harvesting chlorophyll protein, but, in contrast to wild-type and gun mutants, are green in the presence of NF. hon mutations disturb plastid protein homeostasis, thereby activating plastid signaling and inducing stress acclimatization. Rather than defining constituents of a retrograde signaling pathway specifically associated with the NF-induced suppression of nuclear gene expression, as proposed for gun, hon mutations affect Lhcb expression more indirectly prior to initiation of plastid signaling in NF-treated seedlings. They pre-condition seedlings by inducing stress acclimatization, thereby attenuating the impact of a subsequent NF treatment.

Influence of plastids on light signalling and development

“…Additionally, mutant alleles that were obtained from gun mutant screens were not obtained from screens for norflurazon-resistant mutants, and in contrast to norflurazon-resistant mutants, gun mutants were not resistant to low concentrations of norflurazon [45]. Other attempts to show that gun mutants are resistant to norflurazon were not successful [26,44,45]. Indeed, ROS and blocking chloroplast biogenesis with norflurazon appeared to activate distinct plastid-to-nucleus signalling mechanisms [44,46].…”

“…in the dark and without norflurazon) [19] also indicates that gun5 regulated PhANG expression without increasing the levels of 1 O 2 , regardless of whether this increase in ROS is transient, localized or sustained. Additionally, mutant alleles that were obtained from gun mutant screens were not obtained from screens for norflurazon-resistant mutants, and in contrast to norflurazon-resistant mutants, gun mutants were not resistant to low concentrations of norflurazon [45]. Other attempts to show that gun mutants are resistant to norflurazon were not successful [26,44,45].…”

Plastids Are Major Regulators of Light Signaling in Arabidopsis

“…We also tested whether the fluence-rate shift affects the expression of five genes whose expression is induced by chloroplastic ROS, namely AAA, BAP1, NodL, FER1, and ZAT12 (Saini et al, 2011). The expression of AAA, BAP1, and NodL is induced by singlet oxygen (Baruah et al, 2009a(Baruah et al, , 2009b.…”

“…The reduced expression of photosynthesis-related genes in these lincomycin-treated end mutants and the variable effects of these end alleles on Lhcb1.4 and CHS expression indicate that these mutants are not resistant to lincomycin. We also tested whether the end mutants accumulate more chlorophyll than the wild type when they are treated with low concentrations of lincomycin, an expected phenotype for mutants that are resistant to inhibitors of chloroplast biogenesis (Saini et al, 2011). We found that the end mutants appear indistinguishable from the wild type regardless of whether they are grown on medium that lacks lincomycin or medium that contains 0.005, 0.015, 0.05, 0.15, or 0.5 mM lincomycin (Supplemental Figs.…”

A Shoot-Specific Hypoxic Response of Arabidopsis Sheds Light on the Role of the Phosphate-Responsive Transcription Factor PHOSPHATE STARVATION RESPONSE1

“…To this end, the herbicide norflurazon, an inhibitor of phytoene desaturase (Susek et al, 1993;Gray et al, 2003;Saini et al, 2011), was applied to seedlings, and 4 d later, the response to hypoxia was tested. Norflurazon inhibits the synthesis of carotenoids and causes a disintegration of the photosynthetic apparatus.…”