TABLE 1.
Major Vascular Development Genes in the Arabidopsis Root
| Gene | Mutant Phenotype | Ectopic or overexpression phenotype | Identity | Functions | References |
|---|---|---|---|---|---|
| ALTERED PHLOEM DEVELOPMENT (APL) | Defective phloem differentiation; xylem TE- like cells in phloem pole position; seedling lethal | Expression throughout the stele does not result in ectopic phloem, but represses TE differentiation | MYB transcription factor (TF) | Promotes phloem differentiation; represses stages of xylem differentiation | 60, 61 |
| WOODEN LEG (WOL)/CRE1/AHK4 (ARABIDOPSIS HISTIDINE KINASE 4) | Short, determinate root with few vascular initials. Vascular tissue differentiates only as protoxylem. | Cytokinin receptor and histidine kinase, CRE1/AHK4. The wol allele of CRE1 has no cytokinin binding activity and cannot phosphorylate downstream partners in the presence of cytokinin. | Controls asymmetric cell division of root vascular initials. Phosphorylates AHP proteins in the presence of cytokinin; acts as a phosphatase in the absence of cytokinin | 65–69 | |
| LONESOME HIGHWAY (LH) | Diarch symmetry lost, only one strand of provascular tissue (protoxylem and protophloem); lateral roots produced only on one side of the root; only half the wt number of cells in the stele; columella stem cells differentiate at 13 days | Plant specific protein with some similarity to bHLH Tfs. Can activate transcription in the Y2H system. | Required to establish and maintain normal vascular cell numbers and for meristem maintenance | 75 | |
| VASCULAR RELATED NAC DOMAIN 6 (VND6) | No phenotype (T-DNA insertion lines) | Induces metaxylem formation in vascular and nonvascular cells of root | NAC domain TF | Promotes metaxylem cell fate; master regulator of metaxylem development? | 76 |
| VASCULAR RELATED NAC DOMAIN 7 (VND7) | No phenotype (T-DNA insertion lines) | Induces protoxylem formation in vascular and nonvascular cells of root | NAC domain TF | Promotes protoxylem cell fate; master regulator of protoxylem cell fate? | 76 |
| ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6 (AHP6) | Sporadic protoxylem differentiation, disrupted cytokinin signaling, identified in a screen for wol suppressor mutations | 1 of 6 AHPs in Arabidopsis. In response to cytokinin, AHPs are phosphorylated by AHKs, and in turn, phosphorylate B-type ARRs. AHP6 is likely not a true AHP as it inhibits phosphotransfer from AHPs to ARRs | Promotes protoxylem development by restricting cytokinin signaling to specific domains within the stele. | 64 |