Karrikins trigger degradation of SMAX1 through an F-box protein–mediated mechanism that is highly similar to strigolactone signaling.

The class IV homeodomain leucine zipper transcription factor GLABRA2 (GL2) acts in a complex regulatory circuit that regulates the differentiation of trichomes in Arabidopsis thaliana. We describe a genetic interaction with HOMEODOMAIN GLABROUS11 (HDG11), previously identified as a negative regulator of trichome branching. gl2 hdg11 double mutants display enhanced trichome cell-type differentiation defects. Transgenic expression of HDG11 using the GL2 promoter partially suppresses gl2 trichome phenotypes. Vice versa, expression of GL2 under the control of its native promoter partially complements hdg11 ectopic branching. Since gl2 hdg11 and gl2 myb23 double mutants and the triple mutant display similar trichome differentiation defects, we investigated a connection to the R2R3-MYB transcription factor MYB23. We show that MYB23 transcript levels are significantly reduced in shoots from gl2 mutants and that GL2 can drive the expression of a MYB23-promoter fusion to green fluorescent protein. Yeast one-hybrid, chromatin immunoprecipitation, and in planta reporter gene experiments indicate that an L1-box in the MYB23 promoter acts as a GL2 binding site. Taken together, our findings reveal a functional redundancy between GL2 and HDG11, two homeodomain leucine zipper transcription factors previously thought to mediate opposing functions in trichome morphogenesis. A model is proposed in which GL2 transcript levels are maintained through a positive feedback loop involving GL2 activation of MYB23.

Renal cell carcinoma (RCC) represents one of the most resistant tumors to radiation and chemotherapy. Current therapies for RCC patients are inefficient due to the lack of diagnostic and therapeutic markers. Our recent studies have suggested an association of sperm-associated antigen 9 (SPAG9) with ovarian carcinomas. In the present study, we investigated the clinical relevance of SPAG9 in RCC patients. RT-PCR analysis showed expression of SPAG9 transcript in RCC tissues and RCC cell lines. In situ RNA hybridization and immunohistochemistry analyses confirmed the expression of SPAG9 in 88% of cancer patients, suggesting that SPAG9 participates in renal cancer. In addition, immunoblotting and ELISA analyses revealed a humoral immune response against SPAG9 in the sera of RCC patients but not in healthy individuals. Consistent with the clinical findings, knockdown of SPAG9 expression in RCC cells with specific siRNA significantly reduced cell growth and colony formation. Using in vitro wound healing and Matrigel invasion assays, we found that cell migration and invasive ability were also significantly inhibited. Furthermore, in vivo xenograft studies in nude mice revealed that administration of a SPAG9 siRNA plasmid significantly inhibited tumor growth. In conclusion, SPAG9 expression is associated with clinicopathologic features of tumors, suggesting that SPAG9 could contribute to the early spread of cancer. These results indicate that SPAG9 may have a role in tumor development and metastasis and thus could serve as a novel target for early detection and treatment of RCC. [Cancer Res 2008;68(20):8240–8]

Significance: Karrikins are chemicals in smoke that stimulate regrowth of many plants after fire. However, karrikin responses are not limited to species from fire-prone environments and can affect growth after germination. Putatively, this is because karrikins mimic an unknown signal in plants, KAI2 ligand (KL). Karrikins likely require modification in plants to become bioactive. We identify a gene, KUF1 , that appears to negatively regulate biosynthesis of KL and metabolism of a specific karrikin. KUF1 expression increases in response to karrikin or KL signaling, thus forming a negative feedback loop that limits further activation of the signaling pathway. This discovery will advance understanding of how karrikins are perceived and how smoke-activated germination evolved. It will also aid identification of the elusive KL.