Meta-analysis reveals key features of the improved drought tolerance of plants overexpressing NAC transcription factors

Overexpression of various transcriptional regulators belonging to the NAC (NAM/ATAF/CUC) family was shown to enhance plant tolerance to drought, a most common environmental challenge. However, inconsistencies among the type of outcomes evaluated and their magnitudes complicate the comparison of results across studies and the interpretation of the underlying mechanisms by which NAC gene products increase drought tolerance. We report herein a meta-analysis of ninety-six peer-reviewed publications examining the effects of NAC overexpression on drought tolerance. We found that NAC expression impacted on the magnitude of 18 out of the 19 most frequently reported plant characteristics, increasing positive responses by at least 40 %, while decreasing negative ones by at least 23 %. Survival, water retention and antioxidant metabolism topped the list of measured parameters, whereas strongest NAC effects were determined on survival, biomass accumulation, protection of photosynthesis and proline synthesis. We also explored how several experimental moderators influenced the extent of such responses. They showed negative impacts on plant development under normal growth conditions when using Arabidopsis as receiver plant and the strong promoter CaMV35S to drive NAC expression, although improvements in drought tolerance were significantly higher with CaMV35S compared to other promoters. Benefits from NAC overexpression were more evident in soil-grown plants and with water withholding as the source of stress. NAC-encoding genes from monocots performed slightly better than those obtained from dicots. Water retention and antioxidant metabolism were protected at early stages of water deficit, whereas differential preservation of photosynthesis, root development and survival was maximal after long treatments. Our findings could aid future research on the impact of NAC overexpression in plant tolerance to drought stress.