Search

Inadequate availability of inorganic phosphate (Pi) in the rhizosphere is a common challenge to plants, which activate metabolic and developmental responses to maximize Pi acquisition. The sensory mechanisms that monitor environmental Pi status and regulate root growth via altered meristem activity are unknown. Here, we show that PHOSPHATE DEFICIENCY RESPONSE 2 (PDR2) encodes the single P(5)-type ATPase of Arabidopsis thaliana. PDR2 functions in the endoplasmic reticulum (ER) and is required for proper expression of SCARECROW (SCR), a key regulator of root patterning, and for stem-cell maintenance in Pi-deprived roots. We further show that the multicopper oxidase encoded by LOW PHOSPHATE ROOT 1 (LPR1) is targeted to the ER and that LPR1 and PDR2 interact genetically. Because the expression domains of both genes overlap in the stem-cell niche and distal root meristem, we propose that PDR2 and LPR1 function together in an ER-resident pathway that adjusts root meristem activity to external Pi. Our data indicate that the Pi-conditional root phenotype of pdr2 is not caused by increased Fe availability in low Pi; however, Fe homeostasis modifies the developmental response of root meristems to Pi availability.

Phosphorus, one of the essential elements for plants, is often a limiting nutrient because of its low availability and mobility in soils. Significant changes in plant morphology and biochemical processes are associated with phosphate (Pi) deficiency. However, the molecular bases of these responses to Pi deficiency are not thoroughly elucidated. Therefore, a comprehensive survey of global gene expression in response to Pi deprivation was done by using Arabidopsis thaliana whole genome Affymetrix gene chip (ATH1) to quantify the spatio-temporal variations in transcript abundance of 22,810 genes. The analysis revealed a coordinated induction and suppression of 612 and 254 Pi-responsive genes, respectively. The functional classification of some of these genes indicated their involvement in various metabolic pathways, ion transport, signal transduction, transcriptional regulation, and other processes related to growth and development. This study is a detailed analysis of Pi starvation-induced changes in gene expression of the entire genome of Arabidopsis correlated with biochemical processes. The results not only enhance our knowledge about molecular processes associated with Pi deficiency, but also facilitate the identification of key molecular determinants for improving Pi use by crop species.

The early light-induced proteins (ELIPs) belong to the multigenic family of light-harvesting complexes, which bind chlorophyll and absorb solar energy in green plants. ELIPs accumulate transiently in plants exposed to high light intensities. By using an Arabidopsis thaliana mutant (chaos) affected in the posttranslational targeting of light-harvesting complex-type proteins to the thylakoids, we succeeded in suppressing the rapid accumulation of ELIPs during high-light stress, resulting in leaf bleaching and extensive photooxidative damage. Constitutive expression of ELIP genes in chaos before light stress resulted in ELIP accumulation and restored the phototolerance of the plants to the wild-type level. Free chlorophyll, a generator of singlet oxygen in the light, was detected by chlorophyll fluorescence lifetime measurements in chaos leaves before the symptoms of oxidative stress appeared. Our findings indicate that ELIPs fulfill a photoprotective function that could involve either the binding of chlorophylls released during turnover of pigment-binding proteins or the stabilization of the proper assembly of those proteins during high-light stress.

TOR (target of rapamycin) protein kinases were identified in yeasts, mammals, and Drosophila as central controllers of cell growth in response to nutrient and growth factors. Here we show that Arabidopsis thaliana possesses a single TOR gene encoding a protein able to complex with yeast 12-kDa FK506-binding protein and rapamycin despite the insensitivity of Arabidopsis vegetative growth to rapamycin. Analysis of two T-DNA insertion mutants shows that disruption of AtTOR leads to the premature arrest of endosperm and embryo development. A T-DNA-mediated translational fusion of AtTOR with the GUS reporter gene allows us to show that AtTOR is expressed in primary meristem, embryo, and endosperm, but not in differentiated cells. The implications of these features for the plant TOR pathway are discussed.

The mechanisms involved in the posttranslational targeting of membrane proteins are not well understood. The light-harvesting chlorophyll proteins (LHCP) of the thylakoid membrane are a large family of hydrophobic proteins that are targeted in this manner. They are synthesized in the cytoplasm, translocated across the chloroplast envelope membranes into the stroma, bound by a stromal factor to form a soluble intermediate, "transit complex", and then integrated into the thylakoid membrane by a GTP dependent reaction. Signal recognition particle (SRP), a cytoplasmic ribonucleoprotein, is known to mediate the GTP dependent cotranslational targeting of proteins to the endoplasmic reticulum. We show that chloroplasts contain an SRP consisting of, cpSRP54, a homologue of SRP54 and a previously undescribed 43-kDa polypeptide (cpSRP43) instead of an RNA. We demonstrate that both subunits of cpSRP are required for the formation of the transit complex with LHCP. Furthermore, cpSRP54, cpSRP43, and LHCP are sufficient to form a complex that appears to be identical to authentic transit complex. We also show that the complex formed between LHCP and cpSRP, together with an additional soluble factor(s) are required for the proper integration of LHCP into the thylakoid membrane. It appears that the expanded role of cpSRP in posttranslational targeting of LHCP has arisen through the evolution of the 43-kDa protein.