We recently showed that proline affects root meristem size, and in turn, root growth, in Arabidopsis (Biancucci et al., 2015). The effect of this amino acid on meristem size turned out to be uncoupled from the expression of genes controlling cell differentiation at the transition zone, and independent from the action of auxin, cytokinin and gibberellic acid. On the contrary, proline was found able to affect cell division activity in early stages of postembryonic root development, as shown by the proline-dependent expression of the G2/M-specific CYCLINB1;1 (CYCB1;1) gene (Biancucci et al., 2015). Overall, proline control over cell division altered the ratio between cell division and cell differentiation leading, in turn, to root elongation.
A similar effect on root meristem size has been ascribed to some reactve oxygen species (ROS), as reported by Dunand et al. (2007) and Tsukagoshi et al. (2010), who showed that the ratio between superoxide anion (O2-) and hydrogen peroxide can affect root meristem growth in a hormone-independent manner. Moreover, proline has been long proposed as an effective ROS scavenger (Smirnoff and Curves, 1998), although the molecular mechanism underlying this function is not yet clearly understood and the question whether proline exerts a direct or indirect effect on ROS detoxification is still an open question. This proposal aims at investigating a possible relationship between proline and ROS in the control of root meristem size.
The multifunctional amino acid proline has been attracting strong interest in the scientific community ever since it became clear that it was not just a component of the protein synthesis machinery. A number of different functions have been assigned to proline since then, including adaptation to stress conditions, redox buffering, ROS scavenging, as well as signaling functions, capable to transduce endogenous and environmental clues to modulate plant development.
Regardless of, or perhaps because of this panoply of functions, no clear-cut mechanism of action has been convincingly proposed so far, and the effects of proline have been ascribed to different putative properties of this amino acid, such as its capability to act as compatible osmolyte and/or redox buffer and/or ROS scavenger, and/or energy source, and/or cellular pH buffer. However, the long-standing question of how proline can manage all these functions remains essentially unanswered.
In the present proposal we correlate, for the first time, the capacity of proline to modulate root meristem size and root growth with its capability to remove H2O2 from the cytosol and to produce O2¿- in the mitochondrion, in support of a novel general model of proline function.
The general model, of which this proposal is a part, hypothesizes that the effect of proline on root meristem size (and more broadly on plant development) is exerted by controlling the ratio between superoxide and hydrogen peroxide through a regulatory circuitry involving proline synthesis in the transition and differentiation zone and proline catabolism in the division zone of the root meristem.
Because of the importance of proline in plant development and stress tolerance, the amount of preliminary information already acquired on the subject, and the network of international collaborations linked to this project, we are confident that this project has the potential to bring significant advances over the state of the art with important theoretical and practical implications.