Root-to-shoot iron partitioning in Arabidopsis requires IRON-REGULATED TRANSPORTER1 (IRT1) protein but not its iron(II) transport function.

IRON-REGULATED TRANSPORTER1 (IRT1) is the root high-affinity ferrous iron (Fe) uptake system and indispensable for the completion of the life cycle of Arabidopsis thaliana without vigorous Fe supplementation. Here we provide evidence supporting a second role of IRT1 in root-to-shoot partitioning of Fe. We show that irt1 mutants overaccumulate Fe in roots, most prominently in the cortex of the differentiation zone in irt1-2, compared to the wild type. Shoots of irt1-2 are severely Fe-deficient according to Fe content and marker transcripts, as expected. We generated irt1-2 lines producing IRT1 mutant variants carrying single amino-acid substitutions of key residues in transmembrane helices IV and V, Ser206 and His232, which are required for transport activity in yeast. Root short-term 55 Fe uptake rates were uninformative concerning IRT1-mediated transport. Overall irt1-like concentrations of the secondary substrate Mn suggested that the transgenic Arabidopsis lines also remain incapable of IRT1-mediated root Fe uptake. Yet, IRT1S206A partially complements rosette dwarfing and leaf chlorosis of irt1-2, as well as root-to-shoot Fe partitioning and gene expression defects of irt1-2, all of which are fully complemented by wild-type IRT1. Taken together, these results suggest a regulatory function for IRT1 in root-to-shoot Fe partitioning that does not require Fe transport activity of IRT1. Among the genes of which transcript levels are partially dependent on IRT1, we identify MYB DOMAIN PROTEIN10, MYB DOMAIN PROTEIN72 and NICOTIANAMINE SYNTHASE4 as candidates for effecting IRT1-dependent Fe mobilization in roots. Understanding the biological functions of IRT1 will help to improve Fe nutrition and the nutritional quality of agricultural crops.

Tocopherol as singlet oxygen scavenger in photosystem II.

Singlet oxygen is formed in the photosystem II reaction center in the quench of P680 triplets, and the yield is dependent on light intensity and the reduction level of plastoquinone. Singlet oxygen in PS II triggers the degradation of the D1 protein. We investigated the participation of tocopherol as a singlet oxygen scavenger in this system. For this purpose, we inhibited tocopherol biosynthesis at the level of the HPP-dioxygenase in the alga Chlamydomonas reinhardtii under conditions in which plastoquinone did not limit the photosynthesis rate. In the presence of the inhibitor and in high light for 2 h, photosynthesis in vivo and photosystem II was inactivated, the D1 protein was degraded, and the tocopherol pool was depleted and fell below its turnover rate/h. The inhibited system could be fully resuscitated upon the addition of a chemical singlet oxygen quencher (diphenylamine), and partly by synthetic cell wall permeable short chain alpha- and gamma-tocopherol derivatives. We conclude that under conditions of photoinhibition and extensive D1 protein turnover tocopherol has a protective function as a singlet oxygen scavenger.

Tocopherol content and activities of tyrosine aminotransferase and cystine lyase in Arabidopsis under stress conditions.

Tocopherols are presumed to be important antioxidants and scavengers of lipid radicals and reactive oxygen species in plants. Age is known to be a condition under which oxidative stress increases. In leaves of aging Arabidopsis thaliana plants, the content of alpha-tocopherol as well as of gamma-tocopherol increased significantly. The activity of tyrosine aminotransferase, which supplies the biosynthetic pathway with 4-hydroxyphenylpyruvate, was increased as well. On the other hand, coronatine, a phytotoxin mimicking octadecanoids and leading to symptoms of senescence, caused a moderate increase in alpha-tocopherol as well as some enhancement of gamma-tocopherol.

Jasmonate is involved in the induction of tyrosine aminotransferase and tocopherol biosynthesis in Arabidopsis thaliana.

Coronatine-inducible tyrosine aminotransferase (TAT), which catalyses the transamination from tyrosine to p-hydroxyphenylpyruvate, is the first enzyme of a pathway leading via homogentisic acid to plastoquinone and tocopherols, the latter of which are known to be radical scavengers in plants. TAT can be also induced by the octadecanoids methyl jasmonate (MeJA) and methyl-12-oxophytodienoic acid (MeOPDA), as well as by wounding, high light, UV light and the herbicide oxyfluorfen. In order to elucidate the role of octadecanoids in the process of TAT induction in Arabidopsis thaliana (L.) Heynh., the jasmonate-deficient mutant delayed dehiscence (dde1) was used, in which the gene for 12-oxophytodienoic acid reductase 3 is disrupted. The amount of immunodetectable TAT was low. The enzyme was still fully induced by coronatine as well as by MeJA although induction by the latter was to a lesser extent and later than in the wild type. Treatment with MeOPDA, wounding and UV light, however, had hardly any effects. Tocopherol levels that showed considerable increases in the wild type after some treatments were much less affected in the mutant. However, starting levels of tocopherol were higher in non-induced dde1 than in the wild type. We conclude that jasmonate plays an important role in the signal transduction pathway regulating TAT activity and the biosynthesis of its product tocopherol.

A specific role for tocopherol and of chemical singlet oxygen quenchers in the maintenance of photosystem II structure and function in Chlamydomonas reinhardtii.

alpha-Tocopherol concentrations were determined at low and high light intensities and compared with the rate of photosynthesis, photosystem II (PS II) and its reaction center D1 protein. Blocking of tocopherol biosynthesis at the 4-hydroxyphenylpyruvate dioxygenase by the herbicide pyrazolynate led to a quick disappearance of alpha-tocopherol in high light, as well as of PS II activity and the D1 protein. Homogentisic acid rescued all activities. It is concluded that alpha-tocopherol has a continuous turnover as a scavenger of the singlet oxygen that arises from the quenching by oxygen of the triplet of the PS II reaction center and triggers the degradation of the D1 protein. Thus tocopherols are essential to keep photosynthesis active. We suggest that this is why plants make and need tocopherols. Chemical quenchers of singlet oxygen, notably diphenylamines, completely protect PS II, prevent D1 protein degradation and keep tocopherol levels even at very high light intensities. This supports the notion that 1O2 is the intermediate in light triggered D1 protein turnover.