No abstract

Extensive communication occurs between plants and microorganisms during different stages of plant development in which signaling molecules from the two partners play an important role. Volatile organic compounds (VOCs) emission by certain plant-growth promoting rhizobacteria (PGPR) has been found to be involved in plant growth. However, little is known about the role of bacterial VOCs in plant developmental processes. In this work, we investigated the effects of inoculation with twelve bacterial strains isolated from the rhizosphere of lemon plants (Citrus aurantifolia) on growth and development of Arabidopsis thaliana seedlings. Several bacterial strains showed a plant growth promoting effect stimulating biomass production, which was related to differential modulation of root-system architecture. The isolates L263, L266, and L272a stimulated primary root growth and lateral root development, while L254, L265a and L265b did not significantly alter primary root growth but strongly promoted lateral root formation. VOC emission analysis by SPME-GC-MS identified aldehydes, ketones and alcohols as the most abundant compounds common to most rhizobacteria. Other VOCs, including 1-octen-3-ol and butyrolactone were strain specific. Characterization of L254, L266 and L272a bacterial isolates by 16S rDNA analysis revealed the identity of these strains as Bacillus cereus, Bacillus simplex and Bacillus sp, respectively. Taken together, our data suggest that rhizospheric bacterial strains can modulate both plant growth promotion and root-system architecture by differential VOC emission.

Ribosomal protein S6 (S6rp) is phosphorylated by the p70S6K enzyme in mammals, under mitogen/IGF regulation. This event has been correlated with an increase in 5'TOP mRNA translation. In this research, a maize S6 kinase (ZmS6K) was isolated from maize (Zea mays L.) embryonic axes by human p70S6K antibody immunoprecipitation. This enzyme, a 62 kDa peptide, proved to be specific for S6rp phosphorylation, as revealed by in vivo and in vitro kinase activity using either the 40S ribosomal subunit or the RSK synthetic peptide as the substrates. ZmS6K activation was achieved by phosphorylation on serine/threonine residues. Specific phospho-Threo recognition by the p70S6K antibody directed to target phospho-Threo residue 389 correlated with ZmS6K activation. The ZmS6K protein content remained almost steady during maize seed germination, whereas the ZmS6K activity increased during this process, consistent with Zm6SK phosphorylation. Addition of insulin to germinating maize axes proved to increase ZmS6K activity and the extent of S6rp phosphorylation. These events were blocked by rapamycin, an inhibitor of the insulin signal transduction pathway in mammals, at the TOR (target of rapamycin) enzyme level. We conclude that ZmS6K is a kinase, structurally and functionally ortholog of the mammalian p70S6K, responsible for in vivo S6rp phosphorylation in maize. Its activation is induced by insulin in a TOR-dependent manner by phosphorylation on conserved serine/threonine residues.

Insulin and ‘insulin‐like’ growth factors (IGFs) are known to regulate cell growth in eukaryotes by stimulating a signal transduction pathway that exerts translational control. Intermediate kinases of this pathway, target of rapamycin (TOR) and ribosomal protein S6 kinase (S6K), have been reported in Arabidospsis thaliana and Zea mays. However, upstream signal inducers and downstream targets of the pathway are not well known in plants. The objective of this work is to inquire whether plant growth is regulated by a signal transduction pathway similar to the insulin/IGF‐stimulated pathway in other metazoans. Insulin as well as Zea mays insulin‐related peptide (ZmIGF), which is a maize, 20‐kDa peptide fraction recognized by insulin antibody, were used as effectors to stimulate maize axes growth from germinating seeds. ZmIGF expression was identified in axes from germinating maize seeds and immunolocalized in the meristems of these tissues. Significant enhancement of specific de novo protein synthesis of the translational apparatus components was found in the stimulated axes. Reverse‐transcription‐polymerase chain reaction analysis of total and polysomal RNA pools in ZmIGF‐ or insulin‐stimulated axes confirmed these data by revealing specific mRNA recruitment into polysomes. In addition, the same stimuli induced activation of S6 ribosomal protein kinase (ZmS6K) in germinating maize axes. All the above effects were inhibited by rapamycin, indicating that they depend on TOR activity. We conclude that a TOR–S6K signal transduction pathway is functional in maize germination, as that found for non‐photosynthetic eukaryotes. The evolutionary implications of these findings are discussed.

Insulin and insulin-like growth factors (IGFs) are well-characterized regulators in higher eukaryotic cells that control biological processes such as cell growth and survival, and selective translation of mRNAs. This research presents the purification of a 20kDa protein, isolated from maize tissue, with IGF activity. The protein was purified from 48h-germinated maize embryonic axes by G-50 Sephadex fractionation followed by affinity chromatography through a bovine insulin antibody–Sepharose column. This protein proved to significantly speed up maize germination and seedling growth. At the molecular level, Zea mays IGF (ZmIGF) enhanced phosphorylation of S6 ribosomal protein (rp) on the 40S ribosomal subunit, in a similar way as observed when bovine insulin is applied to maize axes during germination. Rapamycin, a specific inhibitor of the insulin-stimulated signal transduction pathway, prevented S6 rp phosphorylation in maize axes. Moreover, ZmIGF stimulated [35S]methionine incorporation into rps, above the level of overall cytoplasmic proteins. Either incubation with anti-insulin antibody, heat treatment (60°C) or trypsin digestion abolished this ZmIGF effect. It is proposed that ZmIGF is an endogenous maize growth factor that regulates the synthesis of specific proteins through a pathway similar to that of insulin or IGFs in animal tissues.