Transglutaminase as polyamine mediator in plant growth and differentiation

Transglutaminases (TGases) are ubiquitous enzymes catalyzing many biological reactions. The best-known TGase activity, namely the transamidation of specific proteins by polyamines (PAs), has been studied in plants to verify if TGase is a mediator of PAs mechanism of action to re-interpret some of PAs effects. Usually, the TGase activity is present at basal level in plant cells, but it can be induced by internal or external events or stresses, like rehydration, wounding, light, developmental differentiation and programmed cell death (PCD). Here, two models of induced growth are presented, namely pollen apical growth and dedifferentiation followed by reacquisition of the pluripotency of already differentiated cells. Moreover, PAs and TGase involvement during the differentiation and the activity of organelles and finally during the terminal organ differentiation or self-incompatibility-induced PCD are reported. In all of these models, TGase plays a role. The enzyme was detected in several cell compartments, like cytosol, chloroplasts and possibly mitochondria, microsomal fraction, cell wall and also extracellularly. The products of TGase catalysis, modified with PAs, mainly consist of high molecular mass complexes. Among the protein substrates until now identified we mention the cytoskeletal proteins, actin and tubulin, whose PA modification also affects their interaction with motor proteins and the dynamic of cytoskeleton. The most widely studied substrates are component of chloroplast photosystems, in particular light-harvesting complexes, whose modification is light dependent and whose differentiation and size are affected by TGase, thereby conditioning photosynthetic efficiency and photoprotection. Finally, modification of cell wall substrates affects wall growth and reinforcement.