IMMUTANS Does Not Act as a Stress-Induced Safety Valve in the Protection of the Photosynthetic Apparatus of Arabidopsis during Steady-State Photosynthesis

Abstract

IMMUTANS (IM) encodes a thylakoid membrane protein that has been hypothesized to act as a terminal oxidase that couples the reduction of O 2 to the oxidation of the plastoquinone (PQ) pool of the photosynthetic electron transport chain. Because IM shares sequence similarity to the stress-induced mitochondrial alternative oxidase (AOX), it has been suggested that the protein encoded by IM acts as a safety valve during the generation of excess photosynthetically generated electrons. We combined in vivo chlorophyll fluorescence quenching analyses with measurements of the redox state of P 700 to assess the capacity of IM to compete with photosystem I for intersystem electrons during steady-state photosynthesis in Arabidopsis (Arabidopsis thaliana).Comparisons were made between wild-type plants, im mutant plants, as well as transgenics in which IM protein levels had been overexpressed six (OE-6 3) and 16 (OE-16 3) times. Immunoblots indicated that IM abundance was the only major variant that we could detect between these genotypes. Overexpression of IM did not result in increased capacity to keep the PQ pool oxidized compared to either the wild type or im grown under control conditions (25°C and photosynthetic photon flux density of 150 mmol photons m 22 s 21 ). Similar results were observed either after 3-d cold stress at 5°C or after full-leaf expansion at 5°C and photosynthetic photon flux density of 150 mmol photons m 22 s 21 . Furthermore, IM abundance did not enhance protection of either photosystem II or photosystem I from photoinhibition at either 25°C or 5°C. Our in vivo data indicate that modulation of IM expression and polypeptide accumulation does not alter the flux of intersystem electrons to P 700 1 during steady-state photosynthesis and does not provide any significant photoprotection. In contrast to AOX1a, meta-analyses of published Arabidopsis microarray data indicated that IM expression exhibited minimal modulation in response to myriad abiotic stresses, which is consistent with our functional data. However, IM exhibited significant modulation in response to development in concert with changes in AOX1a expression. Thus, neither our functional analyses of the IM knockout and overexpression lines nor meta-analyses of gene expression support the model that IM acts as a safety valve to regulate the redox state of the PQ pool during stress and acclimation. Rather, IM appears to be strongly regulated by developmental stage of Arabidopsis.

Photosynthetic Redox Imbalance Governs Leaf Sectoring in theArabidopsis thalianaVariegation Mutantsimmutans,spotty,var1, andvar2

“…Taken together, these results do not support the model of IM as a safety valve to regulate the redox state of the PQ pool during stress and acclimation in fully developed Arabidopsis leaves. By contrast, the meta-analysis revealed that IM did appear to be strongly regulated by development in Arabidopsis (Rosso et al, 2006). This is consistent with the observations of Aluru et al (2001Aluru et al ( , 2009 who suggested that IM is required to protect against the potential for photooxidation during the development of chloroplasts, amyloplasts, and etioplasts.…”

“…If the PQ pool remains reduced due to the lack of IM, carotenoid biosynthesis is blocked at the phytoene desaturase step, phytoene accumulates, and photobleaching results from an increase in chloroplastic ROS (Wetzel et al, 1994;Wu et al, 1999). Consistent with the notion that white sectoring is triggered by photooxidation (Aluru et al, 2009) is the fact that the variegated phenotype can be suppressed in im plants when grown under low-light conditions (Ré dei, 1963;Wetzel et al, 1994;Aluru and Rodermel, 2004;Rosso et al, 2006).…”

“…Moreover, through metaanalyses of published Arabidopsis microarray data, Rosso et al (2006) reported that IM expression was insensitive to a range of abiotic stresses. Taken together, these results do not support the model of IM as a safety valve to regulate the redox state of the PQ pool during stress and acclimation in fully developed Arabidopsis leaves.…”

“…However, recent research comparing the function of IM in a knockout mutant as well as in overexpressing lines of Arabidopsis has shown that modulation of IM expression and polypeptide accumulation does not alter the flux of intersystem electrons reaching PSI during steady state photosynthesis nor does the presence or absence of IM affect sensitivity to photoinhibition in Arabidopsis (Rosso et al, 2006). Moreover, through metaanalyses of published Arabidopsis microarray data, Rosso et al (2006) reported that IM expression was insensitive to a range of abiotic stresses.…”

Phylogenetic viewpoints on regulation of light harvesting and electron transport in eukaryotic photosynthetic organisms

“…During de-etiolation, the NDH-like complex rapidly starts to interact with PSI, and after just 48 h in light, the PSI-NDH supercomplex is assembled (Peng et al 2008). This result may support the speculated co-operation between NDH and the plastid terminal oxidase PTOX, which is known to be crucial for the early leaf development and for the proper chloroplast biogenesis (Rosso et al 2006(Rosso et al , 2009. Alternatively, the etioplastic NDH-like complex might be physiologically nonfunctional until the appearance of its assembly partner, PSI.…”