Abstract
The molecular mechanisms of abscisic acid (ABA) signalling have been studied for many years; however, how mitochondria-localized proteins play roles in ABA signalling remains unclear. Here an Arabidopsis mitochondria-localized protein RRL (RETARDED ROOT GROWTH-LIKE) was shown to function in ABA signalling. A previous study had revealed that the Arabidopsis mitochondria-localized protein RRG (RETARDED ROOT GROWTH) is required for cell division in the root meristem. RRL shares 54% and 57% identity at the nucleotide and amino acid sequences, respectively, with RRG; nevertheless, RRL shows a different function in Arabidopsis. In this study, disruption of RRL decreased ABA sensitivity whereas overexpression of RRL increased ABA sensitivity during seed germination and seedling growth. High expression levels of RRL were found in germinating seeds and developing seedlings, as revealed by β-glucuronidase (GUS) staining of ProRRL-GUS transgenic lines. The analyses of the structure and function of mitochondria in the knockout rrl mutant showed that the disruption of RRL causes extensively internally vacuolated mitochondria and reduced ABA-stimulated reactive oxygen species (ROS) production. Previous studies have revealed that the expression of alternative oxidase (AOX) in the alternative respiratory pathway is increased by mitochondrial retrograde regulation to regain ROS levels when the mitochondrial electron transport chain is impaired. The APETALA2 (AP2)-type transcription factor ABI4 is a regulator of ALTERNATIVE OXIDASE1a (AOX1a) in mitochondrial retrograde signalling. This study showed that ABA-induced AOX1a and ABI4 expression was inhibited in the rrl mutant, suggesting that RRL is probably involved in ABI4-mediated mitochondrial retrograde signalling. Furthermore, the results revealed that ABI4 is a downstream regulatory factor in RRL-mediated ABA signalling in seed germination and seedling growth.
Highlights
The phytohormone abscisic acid (ABA) regulates many important aspects of plant development, such as promotion of seed maturation and dormancy and inhibition of the transitions from the embryonic to the germinative phase (Rock, 2000; Finkelstein et al, 2002; Hirayama and Shinozaki, 2007)
This study showed that ABA-induced ALTERNATIVE OXIDASE1a (AOX1a) and ABI4 expression was inhibited in the rrl mutant, suggesting that ROOT GROWTH-LIKE (RRL) is probably involved in ABI4-mediated mitochondrial retrograde signalling
The mitochondrial localization of RRL–green fluorescent protein (GFP) was determined by tobacco leaf infiltration experiments with a well-known mitochondria marker, mt-rk CD3-991 (Nelson et al, 2007), which was generated with a red fluorescent protein, mCherry (Shaner et al, 2004)
Summary
The phytohormone abscisic acid (ABA) regulates many important aspects of plant development, such as promotion of seed maturation and dormancy and inhibition of the transitions from the embryonic to the germinative phase (Rock, 2000; Finkelstein et al, 2002; Hirayama and Shinozaki, 2007). ABA binds to the receptor PYR1/PYLs/RCARs, which together inactivate type 2C protein phosphatases (PP2Cs) such as ABI1 and ABI2. Inactivation of the protein phosphatases enables the release of active OST1 and related SNF1-type kinases (SnRKs) via PP2C inactivation to initiate ABA signal transduction (Fujii et al, 2009; Miyazono et al, 2009; Nishimura et al, 2009). In ABA signalling to the nucleus, OST1 and related SnRKs target ABA-responsive elementbinding factor (ABFs/AREBs) such as the basic leucine zipper (bZIP) transcriptional factor ABI5 (Finkelstein et al, 2005) to regulate ABA-responsive gene expression. ABI4, an APETALA2 (AP2)-type transcription factors binds the CE1 cis-element in the promoter of target genes in ABA signalling (Woodson and Chory, 2008)
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