Spermine Regulates Pollen Tube Growth by Modulating Ca2+-Dependent Actin Organization and Cell Wall Structure

Proper growth of the pollen tube depends on an elaborate mechanism that integrates severalmolecularandcytologicalsub-processesandensuresacellshapeadaptedtothe transport of gametes. This growth mechanism is controlled by several molecules among which cytoplasmic and apoplastic polyamines. Spermine (Spm) has been correlated with various physiological processes in pollen, including structuring of the cell wall and modulation of protein (mainly cytoskeletal) assembly. In this work, the effects of Spm on the growth of pear pollen tubes were analyzed. When exogenous Spm (100 µM) was supplied to germinating pollen, it temporarily blocked tube growth, followed by the induction of apical swelling. This reshaping of the pollen tube was maintained also after growth recovery, leading to a 30–40% increase of tube diameter. Apical swelling was also accompanied by a transient increase in cytosolic calcium concentration and alteration of pH values, which were the likely cause for major reorganization of actin filaments and cytoplasmic organelle movement. Morphological alterations of the apical and subapical region also involved changes in the deposition of pectin, cellulose, and callose in the cell wall. Thus, results point to the involvement of Spm in cell wall construction as well as cytoskeleton organization during pear pollen tube growth

Transglutaminases: Widespread Cross-linking Enzymes in Plants

\u2022 Background Transglutaminases have been studied in plants since \u201987 within investigation aimed at interpreting some of the molecular mechanisms of growth and differentiation exerted by polyamines. Transglutaminases are a widely distributed enzyme family having a myriad of biological reactions in animals. In plants the catalysis of the post-translational modification of proteins by polyamines forming inter- or intra-molecular cross-links has been mainly studied. \u2022Characteristics of plant transglutaminases The few plant transglutaminases sequenced so far have a scarce sequence homology with the best known animal enzymes, except for the catalytic triad, however they share a possible structural homology; proofs of their catalytic activity are: i) their ability to produce glutamyl-polyamine derivatives, ii) their recognition by animal transglutaminase antibodies, iii) biochemical features, such as calcium dependency, etc. However many of their fundamental biochemical and physiological properties still remain elusive. \u2022 Transglutaminase is ubiquitous Its activity has been detected in algae and in Angiosperms in different organs and sub-cellular compartments, chloroplasts being the most studied organelle. \u2022 Possible roles concern the structure modification of specific protein substrates. In chloroplasts transglutaminases appear to stabilise the photosynthetic complexes and Rubisco, being regulated by light and other factors, possibly exerting a positive effect on photosynthesis and photoprotection. In the cytoplasm, they modify cytoskeletal proteins. Preliminary reports suggest an involvement in the cell wall construction/organization. Other roles appear to be related to plant fertilisation, abiotic and biotic stresses, senescence and programmed cell death, including hypersensitive reaction. \u2022 Conclusions The widespread location of transglutaminases in all organs and cell compartments studied until now suggests the relevance of their yet to be fully defined physiological roles. At present, it is not possible to classify this enzyme family in plants, due to the scarce information of their codifying genes

Regulation of Pollen Tube Growth by Transglutaminase

In pollen tubes, cytoskeleton proteins are involved in many aspects of pollen germination and growth, from the transport of sperm cells to the asymmetrical distribution of organelles to the deposition of cell wall material. These activities are based on the dynamics of the cytoskeleton. Changes to both actin filaments and microtubules are triggered by specific proteins, resulting in different organization levels suitable for the different functions of the cytoskeleton. Transglutaminases are enzymes ubiquitous in all plant organs and cell compartments. They catalyze the post-translational conjugation of polyamines to different protein targets, such as the cytoskeleton. Transglutaminases are suggested to have a general role in the interaction between pollen tubes and the extracellular matrix during fertilization and a specific role during the self-incompatibility response. In such processes, the activity of transglutaminases is enhanced, leading to the formation of cross-linked products (including aggregates of tubulin and actin). Consequently, transglutaminases are suggested to act as regulators of cytoskeleton dynamics. The distribution of transglutaminases in pollen tubes is affected by both membrane dynamics and the cytoskeleton. Transglutaminases are also secreted in the extracellular matrix, where they may take part in the assembly and/or strengthening of the pollen tube cell wall