Interplay between Proline Metabolism and ROS in the Fine Tuning of Root-Meristem Size in Arabidopsis

We previously reported that proline modulates root meristem size in Arabidopsis by controlling the ratio between cell division and cell differentiation. Here, we show that proline metabolism affects the levels of superoxide anion (O₂^•-) and hydrogen peroxide (H₂O₂), which, in turn, modulate root meristem size and root elongation. We found that hydrogen peroxide plays a major role in proline-mediated root elongation, and its effects largely overlap those induced by proline, influencing root meristem size, root elongation, and cell cycle. Though a combination of genetic and pharmacological evidence, we showed that the short-root phenotype of the proline-deficient p5cs1 p5cs2/P5CS2, an Arabidopsis mutant homozygous for p5cs1 and heterozygous for p5cs2, is caused by H₂O₂ accumulation and is fully rescued by an effective H₂O₂ scavenger. Furthermore, by studying Arabidopsis mutants devoid of ProDH activity, we disclosed the essential role of this enzyme in the modulation of root meristem size as the main enzyme responsible for H₂O₂ production during proline degradation. Proline itself, on the contrary, may not be able to directly control the levels of H₂O₂, although it seems able to enhance the enzymatic activity of catalase (CAT) and ascorbate peroxidase (APX), the two most effective scavengers of H₂O₂ in plant cells. We propose a model in which proline metabolism participates in a delicate antioxidant network to balance H₂O₂ formation and degradation and fine-tune root meristem size in Arabidopsis.