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The activity of SHOOT MERISTEMLESS (STM) is required for the functioning of the shoot apical meristem (SAM). STM is expressed in the SAM but is down-regulated at the site of leaf initiation. STM is also required for the formation of compound leaves. However, how the activity of STM is regulated at the transcriptional, posttranscriptional, and posttranslational levels is poorly understood. We previously found two conserved noncoding sequences in the promoters of STM-like genes across angiosperms, the K-box and the RB-box. Here, we characterize the function of the RB-box in Arabidopsis (Arabidopsis thaliana). The RB-box, along with the K-box, regulates the expression of STM in leaf sinuses, which are areas on the leaf blade with meristematic potential. The RB-box also contributes to restrict STM expression to the SAM. We identified FAR1-RELATED SEQUENCES-RELATED FACTOR1 (FRF1) as a binding factor to the RB-box region. FRF1 is an uncharacterized member of a subfamily of four truncated proteins related to the FAR1-RELATED SEQUENCES factors. Internal deletion analysis of the STM promoter identified a region required to repress the expression of STM in hypocotyls. Expression of STM in leaf primordia under the control of the JAGGED promoter produced plants with partially undifferentiated leaves. We further found that the ELK domain has a role in the posttranslational regulation of STM by affecting the nuclear localization of STM.

The activity ofSHOOT MERISTEMLESS (STM)is required for the functioning of the shoot apical meristem (SAM).STMis expressed n the SAM but is down-regulated at the site of leaf initiation.STMis also required for the formation of compound leaves. However, how the activity ofSTMis regulated at the transcriptional, posttranscriptional, and posttranslational levels is poorly understood.We previously found two conserved noncoding sequences in the promoters ofSTM-like genes across angiosperms, the K-box and the RB-box. Here, we characterize the function of the RB-box in Arabidopsis (Arabidopsis thaliana). The RB-box, along with the K-box, regulates the expression ofSTMin leaf sinuses, which are areas on the leaf blade with meristematic potential. The RB-box also contributes to restrictSTMexpression to the SAM. We identified FAR1-RELATED SEQUENCES-RELATED FACTOR1 (FRF1) as a binding factor to the RB-box region. FRF1 is an uncharacterized member of a subfamily of four truncated proteins related to the FAR1-RELATED SEQUENCES factors. Internal deletion analysis of theSTMpromoter identified a region required to repress the expression ofSTMin hypocotyls. Expression ofSTMin leaf primordia under the control of theJAGGEDpromoter produced plants with partially undifferentiated leaves.We further found that the ELK domain has a role in the posttranslational regulation of STM by affecting the nuclear localization of STM.

Cherry leaf roll virus (CLRV) is thought to cause a hypersensitive resistance response (HR) in black walnut trees in California orchards (Figure 1b), likely due to a single resistance (R) gene in the black walnut genome. Evidence has led to the belief that the HR causes a black line of necrotic cells to form at the graft union of English walnut scions grafted onto black walnut rootstocks as a result of a programmed cell death (PCD) response to the viral infection. Blackline disease inevitably results in girdling and death of the tree. If the CLRV resistance gene is identified and cloned, it could be used as a more efficient marker for marker-assisted breeding; it also could be used as a target for RNA interference (RNAi) to silence the resistance response in rootstocks. Very little is known about the walnut genome; therefore, finding and silencing the R gene in black walnut to control blackline disease may prove to be a difficult and lengthy process. Another possible approach to managing blackline involves targeting CLRV with RNAi via a modified interstock grafted between the scion and rootstock. To do this, a hairpin, stem-loop RNAi construct was created to target the 3' UTR of RNA 1 and RNA 2, with the intention of using it to prevent the CLRV infection from reaching the rootstock and subsequently triggering blackline (Figure 4a). It is possible that by introducing a transformed interstock, carrying the RNAi hairpin construct between the scion and rootstock, the infection could be stopped before blackline disease has a chance to develop, thus protecting the yield of nuts and preventing the death of the tree. We have little understanding of the molecular interactions between CLRV and walnut rootstocks that subsequently lead to cell death and blackline disease. Therefore, one critical starting point involves determining the sequence of the complete genome of the walnut strain of CLRV (W8-CLRV) to gain insight into the molecular players involved. This knowledge will not only lead to a better understanding of blackline disease but also could reveal unique and pertinent interactions between CLRV and walnut that could be used in managing the disease. In this effort, double-stranded RNA (dsRNA) extracted from W8-CLRV-inoculated Chenopodium quinoa plants (Figure 5) was used to make a cDNA library that then was amplified and sequenced, using Illumina sequencing technology (Eureka Genomics, Hercules, CA); and the contigs were assembled to obtain the complete genome sequence of W8-CLRV. Viral pathogens possess the ability to develop genomic sequences that mimic those of their host when the sequence proves to be beneficial to the virulence and/or fitness of the virus. MicroRNAs (miRNAs) are an excellent example of a sequence that a virus would want to mimic because they can be useful in many aspects of virus proliferation. Some plant viruses have been shown to possess miRNA sequence mimics to hijack the gene-silencing capabilities of these sequences. Here, we searched the walnut CLRV genome for similarities to known plant miRNAs, especially those known to be involved with disease resistance. Our hypothesis is that CLRV may have acquired a miRNA mimic sequence with the potential to directly or indirectly silence particular plant genes that play a role in fighting off the virus, once it has inoculated the plant host.