Viroids and most viral satellites have small, noncoding, and highly structured RNA genomes. How they cause disease symptoms without encoding proteins and why they have characteristic secondary structures are two longstanding questions. Recent studies have shown that both viroids and satellites are capable of inducing RNA silencing, suggesting a possible role of this mechanism in the pathology and evolution of these subviral RNAs. Here we show that preventing RNA silencing in tobacco, using a silencing suppressor, greatly reduces the symptoms caused by the Y satellite of cucumber mosaic virus. Furthermore, tomato plants expressing hairpin RNA, derived from potato spindle tuber viroid, developed symptoms similar to those of potato spindle tuber viroid infection. These results provide evidence suggesting that viroids and satellites cause disease symptoms by directing RNA silencing against physiologically important host genes. We also show that viroid and satellite RNAs are significantly resistant to RNA silencing-mediated degradation, suggesting that RNA silencing is an important selection pressure shaping the evolution of the secondary structures of these pathogens.V iroids and most viral satellites, which are the smallest known infectious agents in plants, have single-stranded RNA genomes of 200-400 nt and do not encode proteins (1-3). Whereas viroids replicate autonomously by using host-encoded RNA polymerase, satellite RNAs multiply only in the presence of a helper virus that provides the appropriate RNA-dependent RNA polymerase (2, 4). Intriguingly, some viroids and satellites can induce unique, highly host species-specific disease symptoms despite their exceedingly small size and lack of mRNA activity. Previous studies have shown that one, or a few, nucleotide changes in their RNA genomes can dramatically alter the virulence of these subviral RNAs or the host-plant specificity of the disease symptoms (5-7). Despite intensive investigation, major questions remain as to how these minor sequence variations modulate viroid and satellite pathology and how host plants develop symptoms in response to specific sequences. A striking similarity among viroids and small satellites is that they tend to form characteristic secondary structures due to intramolecular base-pairing. These structures are clearly important, because the evolution of these small RNAs appears to be constrained by the need to preserve their distinct structural features. However, the host factor(s) that imposes this evolutionary pressure has yet to be identified.RNA silencing is a sequence-specific RNA degradation process directed by double-stranded RNA (dsRNA) or selfcomplementary hairpin RNA (hpRNA). This dsRNA or hpRNA is cleaved by an RNase III-like enzyme known as Dicer to generate small (21-to 25-nt) RNAs, termed small interfering RNAs (siRNAs), which are used to guide siRNAribonuclease complexes [known as RNA-induced silencing complexes (RISC)] to degrade cognate single-stranded RNA (8). Recent studies have shown that plants infected with pota...
Botrytis grey mould (BGM), caused by Botrytis cinerea Pers. ex. Fr., is an economically important disease of chickpea (Cicer arietinum L.), especially in areas where cool, cloudy, and humid weather persists. Several epidemics of BGM causing complete crop loss in the major chickpea-producing countries have been reported. The pathogen B. cinerea mainly survives between seasons on infected crop debris and seeds. Despite extensive investigations on pathological, physiological, and molecular characteristics of B. cinerea causing grey mould type diseases on chickpea and several other hosts, the nature of infection processes and genetic basis of pathogen variability have not been clearly established. This lack of information coupled with the need for repeated application of chemical fungicides forced the deployment of host plant resistance (HPR) as a major option for BGM management. Effective and repeatable controlled-environment and field-screening techniques have been developed for identification of HPR. Of the selected portion of chickpea germplasm evaluated for BGM resistance, only few accessions belonging to both cultivated and wild Cicer spp. were tolerant to BGM, and the search for higher levels of disease resistance continues. Fungicide application based on disease predictive models is helpful in precision-based fungicide application. Integrated disease management (IDM) of BGM has proved more effective than any of the individual disease management components in large-scale, on-farm studies conducted in India, Nepal, and Bangladesh. Further information on the biology of B. cinerea and epidemiology of the disease is needed to strengthen the IDM programs. In this paper the biology of B. cinerea including its variability, epidemiology of BGM, identified sources of resistance, and other management options, and available information on biochemical and genetic basis of disease resistance have been reviewed with a mention of future research priorities.
Delaying leaf senescence in plants, especially under water stress conditions, can help to maintain the remobilization of stored nutrients in source–sink relationships, thus leading to improved crop yields. Leaf senescence can be delayed by plant hormones such as cytokinin. Here, the Isopentenyl transferase (IPT) gene, encoding a cytokinin biosynthesis enzyme, driven by a modified AtMYB32xs promoter was transformed into wheat. Transgenic wheat plants exhibited delayed leaf senescence, retaining chlorophyll for longer under controlled environment conditions. Selected independent transgenic events and their corresponding nulls were grown under field conditions for two consecutive years under well-watered and water stress treatments using automated rainout shelters. Three independent transgenic events had improved canopy green cover, lower canopy temperatures, and higher leaf water potential than their respective non-transgenic nulls, with no abnormality in morphology and phenology. Increased grain yield was observed in transgenic events under both water treatments, with the yield increase more pronounced under water stress (26–42%). These results have shown that delayed leaf senescence using the chimeric transgene AtMYB32xs-p::IPT can be a useful strategy to achieve grain yield gains in wheat and potentially other crops for sustainable food production.
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