Metabolic Enzyme MeHNL11 Regulates MeCAS1b Transcription for Cyanide Reutilization in Response to Nitrate Deficiency...

Cassava (Manihot esculenta Crantz) exhibits exceptional tolerance to infertile soils and contains abundant cyanogenic glucosides (CGs). Previous research has indicated that CGs can serve as a significant reservoir of organic nitrogen in plants. However, the extent to which its high-CG content contributes to efficient nitrogen utilisation and adaptation to low nitrogen (N) in cassava remains to be further elucidated. This study represents the first identification of MeHNL11 as a bifunctional protein. In response to N deficiency, the hydroxynitrile lyase activity of MeHNL11 promotes the generation and accumulation of cyanide and the Cys245 residue of MeHNL11 is critical for its nuclear oligomerization, in which the protein functions as a transcription factor. Following the cyanide transmission into the nucleus, the oligomeric form of MeHNL11 dissociates into monomers, leading to a dramatic upregulation of MeCAS1b transcription. This regulatory mechanism helps sustain intracellular cyanide homeostasis within cassava and facilitates the synthesis of primary N metabolites, thereby alleviating N deficiency. The exogenous application of the cyanide antidote hydroxocobalamin (COB) inhibited cyanide assimilation by MeCAS1b, leading to exacerbated N deficiency symptoms, such as leaf yellowing and a significant reduction in the contents of NH₄ ⁺ and free amino acids (AA) in cassava seedlings under low-N conditions (LN). Our research demonstrates that the MeHNL11-MeCAS1b module plays a pivotal role in CG recycling, offering new insights into the underlying mechanisms governing cassava's exceptional tolerance to low N stress.