Fig. 3.

Signaling functions of NPCs under stress conditions. The black arrow indicates established links while the green arrow represents putative links. BL may induce NPC activity through transcriptional activation of cell expansion genes, TCH4 and LRX2. The cell cycle events and cell expansion might be controlled by NPC3/NPC4-produced DAG through BL signaling. In response to humidity and light, NPC generated-DAG probably activates H + -ATPase pump through an unknown intracellular signal leading to malate synthesis. Salt stress modulates NPC4 and it is speculated that NPC4's product DAG may undergo lipid-protein interaction to mediate the signaling events. In addition, NPC4 also accomplishes phosphosphingolipid hydrolysis under phosphate deficit condition to regulate root growth. Phospholipid to galactolipid conversion is also accomplished through NPC4/NPC5 activity under phosphate deficit conditions.