<i>DFR1</i>-mediated Inhibition of Proline Degradation Pathway Regulates Drought and Freezing Tolerance in <i>Arabidopsis</i>

Abstract

As sessile organisms, plants frequently face unfavourable environmental stresses such as drought and freezing. Proline accumulation is one of the most important adaptation mechanisms in response to these stresses in many plant species. Although proline metabolism has been well documented, the molecular regulation mechanism of proline homeostasis in response to these stresses is largely unknown. Here, we identified a novel gene DFR1 (Drought and Freezing Responsive gene 1) encoding an unkown mitochondria protein that is involved in regulation of proline degradation in Arabidopsis. Two alternatively spliced isoforms (DFR1.1 and DFR1.2) were identified and their expressions were strongly induced by drought and cold stresses. The DFR1 knockdown mutants showed hypersensitivity to drought and freezing stresses, whereas the transgenic plants over-expressing DFR1.1 or DFR1.2 exhibited enhanced tolerance. Importantly, absolute absence of DFR1 was lethal, suggesting that DFR1 is vital for plant growth and development. DFR1-mediated drought and freezing tolerance was positively correlated with the proline accumulation level. Moreover, we found that both DFR1.1 and DFR1.2 interact with PDH1, PDH2 and P5CDH, three enzymes conferring proline degradation, and suppress their activities. Genetic epistasis analysis showed that DFR1 acts upstream of PDH1/2 and P5CDH to positively regulate drought and freezing stress tolerance. Taken together, our results demonstrate a novel regulatory pathway that, in response to drought and freezing stresses, DFR1 interacts with PDH1/2 and P5CDH to abrogate their activities to maintain the proline homeostasis, thereby conferring drought and freezing tolerance.