CONICET | Buscador de Institutos y Recursos Humanos

Microalgae are considered as a renewable energy source due to their high photosynthetic activity and rapid growth comparedto higher plants and their ability to accumulate high content of carbon reserves (starch and neutral lipids). It is known thatnutrients limitation and/or other triggers stimulate the accumulation of carbon reserves but at the expense of a growth slowdown. Therefore, the study of the signal transduction pathways connecting growth and C-reserves synthesis is of greatinterest. The phytohormone auxin promotes plant cell growth, but its effect and the molecular mechanisms behind are poorlyunderstood in green algae. In this study, we characterized the effect of indole-3 acetic acid (IAA) in the green algaeChlamydomonas reinhardtii (cc125). C. reinhardtii cells were cultivated at different concentrations of IAA (1, 3, 10 and100 μg/ml) in complete TAP media, under nitrogen deficiency or without acetate in order to analyze the effect of IAA ondifferent physiological conditions. Samples were taken at different time points to determine growth parameters (OD, numberof cells, dry weight), cell volume and starch/total lipids/protein content. The results showed that AIA at 1μg/ml and 3 μg/mlconcentration did not increase or slightly induced growth, respectively, regardless of the availability of nitrogen or acetatein the culture medium. Besides, no significant differences were found between control and auxin treatment on theaccumulation of C-reserves. However, higher concentrations of IAA (10-100μg/ml) caused a decrease in growth and anincrease in cell volume. High concentration of IAA also produced an increase in lipid and starch content as a per-cell-basisthroughout the whole growth curve and after 24 h, respectively. Taken together, data suggest that high concentration of IAAat the beginning of the growth curve causes an arrest of cell division. On the other hand, if the auxin was added in an earlystationary growth phase, the inhibitory growth effect did not occur. We also investigated the putative effect of IAA asinductor of protective mechanisms against different abiotic stresses, and preliminary experiments showed greater growthunder oxidative stress in cultures pretreated with the auxin. In conclusion, the results of this study indicate that in C.reinhardtii, IAA acts as a negative growth regulator at high levels, possibly by decreasing cell division, but it does not havegrowth-promoting effect as in plants. It is important to highlight that the inhibition of growth triggered by high IAA appearsto be general for green algae but not the growth-enhancing effect, response that may be species-specific. Future studies onIAA signal transduction, including synthesis and transport, the components of the transduction cascade itself and theexpression of responsive genes, will be necessary for a more complete understanding of microalgal regulation of biomassproduction, as a feedstock for bioenergy and other applications.