Today's Tears-Tomorrow's Friend
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Scientific Article ARANYA; Vol I (I); January - June 2011 Today’s Tears-Tomorrow’s Friend Dr. Amal Kumar Mondal1 and Dr. Sanjukta Mondal (Parui)2 1. Associate Professor and Former Head of the Department of Botany and Forestry, Plant Taxonomy, Biosystematics and Molecular Taxonomy Laboratory, VIDYASAGAR UNIVERSITY, Midnapore-721 102, West Bengal, India Email: [email protected] 2. Associate Professor, Department of Zoology, Lady Brabourne College, Kolkata-700 017, Wst Bengal, India Email: [email protected] Abstract The Guttation, a process of exuding water from plants is not just restricted to leaf margins and tips but to entire leaf surface, and can now serve as a medium for production of recombinant proteins Key words Guttation, hydathodes, phyllosecretion We are very familiar with the dew drops found on the grasses and the leaves of other plants, in the early morning hours, giving the plants the fresh young look, soothing our eyes during our morning walks. But these drops of water are not always dewdrops, but are instead, water exuded by the leaves of the plants - a process called guttation. It is the secretion of water on to the leaf surface, particularly along the leaf margin, through specialized pores called hydathodes. This process occurs most frequently during conditions of high humidity, when the rate of transpiration is low. It has been observed during warm humid nights. However it occurs maximum during day and minimum during nights. The term “guttation” was proposed by Burgerstien for the exudation of water from plants in the form of liquid. The leaf blade (lamina) as we know consists of an upper and lower epidermis, which is a thin, usually transparent, colorless layer of cells, and is generally referred to as the skin of the leaf blade. Sandwiched in between these two layers of epidermis is the spongy layer of tissue, called the mesophyll, in which runs a branching system of veins. The epidermis is most often covered by a layer of cutin called cuticle, which is a waxy substance secreted by the epidermal cells. The epidermis together with the cuticle prevents excessive loss of water from the leaves and also protects the leaves from injury. The plant generally transpires through pores called stomata, which are scattered throughout the epidermis. However, the numbers of stomata are more on the undersurface of the leaf, than on the upper surface, which prevents excessive loss of water from evaporating from the upper surface of the leaf, which is exposed to the sun. The stomata however does not always remain open and the opening and closing of each stomata is regulated by a pair of bean-shaped cells called guard cells, in response to heat and light. The stomata usually close at night, which further helps in water conservation. Hydathodes in plants are of two types – Epithem hydathodes and Active hydathodes. In case of Epithem hydathode, water is forced out by the root pressure whereas in case of Active hydathode, water is secreted by the force developed within the cells themselves. However root pressure is generally the main cause of guttation and the guttation rate is reduced by conditions reducing root pressure such as cold and dry aerated soil and also mineral deficiency. The force that drives the water through the root is based on differences in the water potential of the soil i.e. the root’s surrounding and the xylem sap of the root. The ions that pass through the endodermis of the root along with the water is trapped and cannot leave the stele any more. Due to osmosis, a pressure called endosmotic root pressure develops in the xylem that presses or forces the water along with the dissolved ions upwards. This root pressure acts most efficiently at night, but its efficiency decreases during the day when it’s rate becomes much smaller than the rate of transpiration. The guttated water contains various kinds of sugars, amino acids, enzymes, organic acids, vitamins, other organic compounds and minerals such as calcium. This leaves a white crust on the leaf surface when the guttated water dries. A typical example is the lime secretion of Saxifraga sp. and the salt glands of halophytes. Scientific Article Scientific Article ARANYA; Vol I (I); January - June 2011 ARANYA; Vol I (I); January - June 2011 Guttation has however not been observed universally. This process has been reported to occur in 333 Proteins of interest targetted for the outside are synthesized by ribosomes which are attached to the rough endoplasmic genera belonging to 115 families. It is restricted to certain genera of mostly herbaceous plants. Heavy guttation has reticulum. As they are synthesized, these proteins are translocated into the lumen of the endoplasmic been observed in Colocasia antiquorum, in which the amount of guttated water ranges from a few drops to 100 ml reticulum, where they are glycosylated and molecular chaperones aid the protein folding. or even more per day. Other common guttating plants include the grasses, Ambrosia trifida, Brassica oleracea, The misfolded proteins are retrotranslocated to the cytosol where they are degraded by a proteasome by the Cleome viscosa, Commelina benghalensis, Hordeum vulgare (barley), Hydrocotyle, Lactuca, Lycopersicon endoplasmic reticulum associated degradation system. The vesicles containing the properly-folded proteins then esculentum (tomata), Physocarpus, Pilea pumila, Potentilla palustris, Spilanthes, Trifolium, Tropoelum sp, Urtica enter the Golgi apparatus. In the Golgi apparatus, the glycosylation of the proteins is modified and further posttranslational dioica, Vitex trifolia, Equisitum (Pteridophyte), etc. Further, the hydathodes are restricted to the tips and margins modificat ions takes place, incl uding cleavage and functionalizat ion. The proteins are then moved of leaves, due to which the droplets are seen deposited on the margin and tips of leaves. However recently guttation into secretory vesicles which travel along the cytoskeleton to the edge of the cell where the vesicle fuses with the has been observed in some unreported genera like Ficus hispida, Ficus cunea and Fleurya interupta more cell membrane in a process called exocytocis, releasing its contents out of the cell’s environment. During this interestingly, the guttated water has been observed on the entire leaf blade and not just restricted to leaf tips and entire sequence, strict biochemical control is maintained by usage of a pH gradient. The pH of the cytosol is 7.4, margins. This study indicates that although hydathodes represent the main points of guttation fluid production, but the endoplasmic reticulum’s pH is 7.0, and the cis-golgi has a pH of 6.5. Secretory vesicles have pHs ranging guttation fluid can also be released through the cuticle or stomata, as has been reported earlier by Lausberg (1935) between 5.0 and 6.0. and Baid (1952) respectively. In the process of phyllosecretion, the proteins destined for export must first be labelled correctly so thet the As mentioned earlier, some proteins are naturally secreted into the plant guttation fluid. Proteins like catalase trans-Golgi network can dispatch the proteins to their correct destination. The labelling may be either in the form and peroxidase enzyme have been reported in the guttation fluid of Zea mays (maize), Avena sativa (oat), several of a signal sequences at the end of the polypeptide chain or signal patches, which are dispersed throughout the peroxidases in Fragaria ananassa (strawberry), Lycopersicon esculentum (tomato) and Cucumis sativus (cucumber) polypeptide chain, but are brought together when in a 3D conformation. All cells must export proteins to the and reductase in Phleum pzatense (timothy). Very recently guttation has been successfully used as a vehicle exterior of the plasma membrane, but specialized secretory cells must also control what domain of the plasma for recombinant protein production in plants by a process termed “phyllosecretion” by a group of scientists of membrane the proteins are released on. Thus, all cells have the constitutive secretory pathway, or default pathway, Biotech Center, Cook College, Rutgers University, New Jersey and Phytomedics, Dayton, New Jersey. They engineered but specialized secretory cells also have a regulated secretory pathway. In the secretory pathway, proteins are the tobacco plant (Nicotiana tabacum L. cv Wisconsin) to secrete human placental secreted alkaline phosphatase packaged into secretory vesicles, which then aggregate next to the plasma membrane where they await an extracellular (SEAP), green fluorescent protein (GFP) from jellyfish (Aequorea victoria) and xylanase from Clostridium signal to fuse with the plasma membrane and release their contents. thermocellum through the plant cell default secretion pathway. They found that recombinant proteins directed to the leaf intercellular space i.e. apoplast are effectively released into the plant guttation fluid, which can be collected Thus by using endoplasmic reticulum signal peptides fused to the recombinant protein sequences, these scientists continuously throughout the plant’s lifetime. have generated transgenic tobacco (Nicotiana tabacum L. cv Wisconsin) plants that secrete three heterologousproteins of different genetic backgrounds (bacterial xylanase, green fluorescent protein of jellyfish [Aequorea Plants serve as suitable bioreactors for the production of many valuable recombinant proteins used as pharmaceuticals, victoria], and human placental alkaline phosphatase) through the leaf intercellular space into tobacco guttation