Fig. 9

Schematic models for the roles of HAKs based on gene expression profiles in sugarcane. In the maturing and mature zones of the leaves and stems, HAK7/9/18 were the main expressed genes. Moreover, these genes also presented a diurnal expression pattern. HAK25 was mainly expressed in the maturing and mature zone of leaf tissues, while HAK2 was mainly expressed in the stem. Low-K⁺ stress induced the upregulation of the expression of HAK1 and HAK21. Transcription factors such as DDF2 and JLO may directly bind to the promoters of HAK1/21 to induce gene expression and subsequently promote HAK transporters, such as HAK1 and HAK 21, to acquire K⁺ in roots. HAK1 may be phosphorylated and activated by the CBL1-CIPK23 complex or receptor-like kinase, RUPO (ruptured pollen tube). The K⁺ concentration in the vacuole is highly varied to maintain cellular K⁺ homeostasis. Some HAK transporters, such as HAK10, located in the tonoplast of vacuoles may play a role in regulating the K⁺ concentration in vacuoles