Regulation of Arabidopsis floral development and transcriptome during drought stress by NAC transcription factors

As sessile organisms, plants are exposed to adverse environmental factors that negatively affect their growth and development. Many plants are particularly sensitive and susceptible to drought conditions at their reproductive stage. There are a number of studies about drought effects on crop reproductive development, but the molecular mechanism underlying drought response during reproduction is still unclear. A previous study on the inflorescence of wild-type (WT) Arabidopsis thaliana (hereafter, Arabidopsis) found that many genes are induced by drought, including NAC019, which encodes a putative transcription factor that belongs to the NAC family. However, the function of NAC019 in drought response during floral development is not clear. This study aimed to investigate NAC019 function in the floral response to drought conditions, with a focus on its role in the stress regulatory network. To achieve this objective, we subjected the nac019 mutant to drought and characterized its phenotype. The stamens and pistils of nac019-1 were smaller than WT, but their length ratio was not significantly different. Despite the smaller size, the pollen produced under drought conditions in the mutant was viable and able to generate a pollen tube, indicating that pollen fertility in nac019-1 was not severely affected by drought. Moreover, when we examined the emergence of new open flowers from a newly recovered inflorescence, we found that, after drought-induced arrest of floral development, nac019-1 took several days longer than WT to produce a new open flower. This indicates that recovery of nac019-1 inflorescence from drought was delayed compared to WT. Additionally, nac019-1 could still produce siliques with normal number of seeds, but the silique number was reduced relative to WT. It also contained more undeveloped siliques than WT. These results suggest that nac019-1floral meristem development was severely affected by drought, but not its floral morphology. iv The observation that the nac019-1 phenotype was relatively weak suggested that NAC019 and its related genes might share redundant functions. Therefore, a triple mutant with additional mutations from two closely related genes was generated and subjected to drought. Our preliminary data indicated that the vegetative tissue of the triple mutant had a more severe phenotype than nac019-1. Throughout drought stress, relative water content of this triple mutant was lower, and recovered more slowly than the single mutant. To understand the regulatory role NAC019 plays, microarray analysis was performed on the inflorescence of nac019 during early drought treatment. In nac019-1, fewer genes were induced by drought, indicating that the mutant might have a slower response to drought at the onset of this stress. We compared this transcriptomic profile to WT to identify putative NAC019 target genes. We found that many genes were associated with stress and hormone response as well as floral development. Some of the genes are DREB2B, ARF2, MYB21 and MYB24. We also examined the expression of a few genes over a time course of drought stress. Interestingly, NAC019 was highly expressed throughout drought stress, indicating that it is essential for the inflorescence response to drought. Based on our results, we propose a model that includes NAC019 function in a regulatory network for floral development under drought conditions. Future studies are required to test the hypotheses in this model, including the interaction between NAC019 and its target genes. Towards this goal, a GR-inducible system has been generated for NAC019, providing a functional assay for NAC019 function in regulating the drought-responsive floral transcriptome.