A LEA 4 protein up-regulated by ABA is involved in drought response in maize roots.

Late embryogenesis abundant (LEA) proteins are hydrophilic proteins that accumulate to high concentrations during the late stages of seeds development, which are integral to desiccation tolerance. LEA proteins also play a protective role under other abiotic stresses. We analyzed in silico a maize protein predicted to be highly hydrophilic and intrinsically disordered. This prediction was experimentally corroborated by solubility assays under denaturing conditions. Based on its amino acid sequence, we propose that this protein belongs to group four of the LEA proteins. The accumulation pattern of this protein was similar to that of dehydrins during the desiccation process that takes place during seed development. This protein was induced by exogenous abscisic acid in immature embryos, but during imbibition was down-regulated by gibberellins. It was also induced in maize roots under osmotic stress. So far, this is the first member of the LEA proteins belonging to group four to be characterized in maize, and it plays a role in the response to osmotic stress.

TOR pathway activation in Zea mays L. tissues: conserved function between animal and plant kingdoms.

In most non-photosynthetic eukaryotes it has been demonstrated a conserved signal transduction pathway, namely TOR-S6K, that coordinates growth and cell proliferation. This pathway targets the translational apparatus to induce selective translation of ribosomal mRNAs as well as stimulate the cell cycle transition through the G1/S phase. Thus, by activation of this pathway through environmental signals, nutrients, stress, or specific growth factors, such as insulin or insulin-like growth factors (IGF), this pathway allows organisms to regulate growth and cell division. In plants, evidence has shown that TOR protein has been highly conserved through evolution, being involved in growth and cell proliferation control as well. Particularly in maize, a peptide named ZmIGF has been found in actively growing tissues. It targets the maize TOR pathway at the same extent as insulin and, by doing so it induces growth, as well as ribosomal proteins and DNA synthesis. Thus, higher metazoans and plants seem to conserve similar biochemical paths to regulate cell growth through equivalent targets that conduce to activation of the TOR-S6K pathway. Recent research shows evidence that supports this proposal by uncovering the ZmIGF receptor in maize, providing further means for analyzing the role of the conserved TOR signaling pathway in this plant.

Biochemical characterization of a new maize (Zea mays L.) peptide growth factor.

Coordination of cell growth and cell division is very important for living organisms in order for these to develop harmonically. The present research is concerned with the purification and characterization of a new peptide hormone, namely ZmIGF (Zea mays insulin-like growth factor), which regulates growth and cell division in maize tissues. ZmIGF is a peptide of 5.7 kDa, as determined by mass spectroscopy. It was isolated either from maize embryonic axes of 48-h germinated seeds or from embryogenic callus and purified through several chromatographic procedures to obtain a single peak as shown by Reverse Phase High-Performance Liquid Chromatography (RP-HPLC). This peptide exhibits a well defined α-helix structure by circular dichroism analysis, similar to that reported for Insulin or for Insulin-like growth factor (IGF-1). Further, ZmIGF seems to perform, in maize, a similar function to that reported for insulin or peptides from the IGF family in animals. Indeed, maize tissues stimulated either by ZmIGF or insulin showed to induce selective synthesis of ribosomal proteins as well as of DNA. Taken together, the previously mentioned data strongly suggest that plants contain a peptide hormone of the IGF family, highly conserved through evolution that regulates growth and development.

Cap-independent translation of maize Hsp101.

Maize embryonic axes contain stored mRNAs, some of which are able to undergo cap-independent translation initiation during germination. The Hsp101 mRNA, encoding a heat shock protein, is essential for thermo-tolerance induction and is present among the stored transcripts. This research aimed to investigate whether the Hsp101 transcript is IRES-driven regulated upon heat stress. Hsp101 transcribed either in vitro or in vivo was efficiently translated via a cap-independent mechanism. This was observed either in an animal in vitro translation system containing proteolytically cleaved eukaryotic initiation factor eIF4G or in a plant system lacking both eIF4E and eIFiso4E initiation factors. Deletion of the 5' untranslated region (UTR) from the Hsp101 mRNA abolished its cap-independent translation indicating that this nucleotide sequence is required to confer cap-independent initiation. Bicistronic constructs containing the Hsp101 mRNA 5'UTR in sense and anti-sense directions between two reporter genes were translated in both cap-independent systems. A similar bicistronic construct containing a viral internal ribosome entry site (IRES) element between the reporter genes was used as control. Internal translation of the second reporter gene was observed when the Hsp101 5'UTR was in the sense but not in the anti-sense orientation in the bicistronic construct. Taken together, these data suggest that the 5'UTR of maize Hsp101, a plant cellular mRNA, functions as an IRES-like element accounting for its cap-independent translation during heat stress.

Auxin stimulates S6 ribosomal protein phosphorylation in maize thereby affecting protein synthesis regulation.

Auxin is known to stimulate protein synthesis in many plant tissues, but the mechanisms involved in this process are unknown. The present research inquires whether auxin might regulate selective translation of mRNAs by inducing S6 ribosomal protein phosphorylation on the 40S ribosomal subunit in maize (Zea mays L.). Maize embryonic axes auxin-stimulated by natural (IAA) or synthetic (Dicamba or 1-NAA) auxins, selectively increased ribosomal protein synthesis. This effect was not reproduced by auxin inactive analogue 2-NAA. Enhanced S6 ribosomal protein phosphorylation on the 40S ribosomal subunit was also observed after auxin stimulation, as measured by [32P] incorporation into this protein. This increment did not occur when stimulation was performed with the inactive auxin analogue. Further, increased recruitment into polysomes of two 5'TOP-like mRNAs, encoding for the initiation translation factor eIF-iso4E and the S6 ribosomal protein, was also found after auxin stimulation of maize axes. A positive correlation was established between the levels of S6 ribosomal protein phosphorylation and the S6 ribosomal protein transcript recruitment into polysomes by means of okadaic acid or heat shock application to maize axes. These data indicate that auxin stimulates S6 ribosomal protein phosphorylation on maize ribosomes, concomitant to the recruitment of specific mRNAs (5'TOP-like mRNAs) into polysomes for translation. It is proposed that by this mechanism auxin regulate the synthesis of specific proteins in maize tissues.