The Module WmABF1-1-WmMYB111 Manipulates the Leaf Senescence of Welwitschia by Integrative Regulation of Abscisic Acid Biosynthesis, Chlorophyll Degradation and Nitrogen Transportation.

Welwitschia mirabilis has been well noted for its two continuously grown leaves which could survive for thousands of years in the desert. Compared to great efforts on elucidation of Welwitschia's leaf adaptivity to external stresses, the regulatory mechanism of its internal leaf aging was largely unknown. Here, we revealed the module WmABF1-1-WmMYB111 has manipulated the proactive leaf senescence of Welwitschia by integrative regulation of abscisic acid (ABA) biosynthesis, chlorophyll degradation and nitrogen (N) transportation. We identified WmMYB111 as an ABA-inducible transcription factor which could directly bind to and activate the promoters of WmNCED6 and WmNCED9, creating a feedback loop to enhance ABA biosynthesis. Meanwhile, WmMYB111 positively modulated the senescence-associated genes (i.e., WmSEN1, WmSEN2), chlorophyll catabolism genes (i.e., WmPAO1-1, WmPAO2-1) and N transporter genes (i.e., WmNRT1.7a, WmNRT2.5). Moreover, WmABF1-1, a core ABA-responsive element binding factor, has directly bound to and interacted with WmMYB111, shaping a transcriptional cascade as WmABF1-1-WmMYB111-WmNCEDs/WmSENs/WmPAOs/WmNRTs. The WmABF1-1 independently activated the above downstream target genes. Our findings underscored the sole module governing multidimensional regulatory mechanisms during leaf senescence, which bridged the ABA accumulation to N utilisation efficiency. To our knowledge, this is the first time transcriptional regulation was detailed in Welwitschia which will provide insightful clues on understanding its complex survival strategies in multi-stressed conditions.