Triticeae Genomics and Genetics, 2025, Vol.16, No.3, 101-109 http://cropscipublisher.com/index.php/tgg 104 others were discarded. Over time, the PAV models among different varieties have also become distinct. Large-scale genotyping techniques and whole-genome studies have demonstrated that modern cultivated barley often lacks PAV, which is only present in wild relatives. However, this does not mean that modern varieties are "lacking something", because through breeding and adaptive accumulation, they have gradually acquired a batch of new PAVs (Gao et al., 2020). These seemingly opposite processes actually jointly contributed to the formation of the genetic differences in barley among different groups that we see today. 3.3 Evolutionary consequences of PAVs for genome plasticity and adaptation With the change in climate and the increase in pests and diseases, how should crops respond? At this point, PAV might come in handy. They can enable crops to acquire or lose certain gene functions in a short period of time, thereby adjusting their own state to adapt to environmental changes. This "dynamic" response is actually a manifestation of the plasticity of the barley genome. Some PAVs are particularly focused on genes related to DNA integration, transposition activity and stress response, suggesting that they may have played an important role in species evolution and adaptation to new environments. More importantly, PAVs are not as easily screened out as other sequence variations. They are often "latent" genetic resources that can be rapidly "activated" once external pressure emerges. From this perspective, PAV does indeed make barley, as a crop, more resilient to wind and rain and have a brighter future. 4 Functional Impact of PAVs on Barley Agronomic Traits 4.1 Influence on disease resistance genes and immune responses Not every barley plant can resist viral infection. The key lies in whether it has those "defensive" resistance genes. Genes like Bdv2 and RYd2 are present in some barley while in others they are completely absent. This is the difference brought about by PAV. Varieties with these genes, even if infected with pathogens like the barley yellow dwarf virus (BYDV-PAV), have less impact on the growth of their root systems and plants, and are less likely to experience a significant reduction in dry matter. Not only that, these resistance genes may also promote the synthesis of phenolic compounds and impede the transmission path of the virus in the phloem tissue, indicating that their roles in the defense system are not limited to one (Choudhury et al., 2018). But if these genes happen to be missing due to PAV, then the crops will be "completely defensed" against the virus, and it is not surprising that this will eventually lead to more severe tissue damage and a decline in yield. 4.2 Associations with abiotic stress tolerance (e.g., drought, salinity) Some traits are not as conspicuous as diseases, such as the stress brought by drought or saline-alkali land. In such cases, whether barley can "withstand" may also be related to PAV. In the PAV region, some genes related to stress responses can often be found, such as functional genes involved in REDOX reactions, transmembrane transport, or DNA integration. Moreover, these expression variations of PAV usually do not have a direct correlation with the surrounding sequences, that is to say, they themselves can bring additional genetic diversity. This characteristic makes them particularly useful in predicting performance under abiotic stress. If PAV data is combined with other genomic information, the prediction of drought tolerance or salt stress will be more accurate and more conducive to breeding screening. 4.3 Contribution to yield-related traits (e.g., tiller number, grain size) A decline in output is not necessarily due to poor management; sometimes it is also closely related to genes. For instance, after some barley is infected with BYDV-PAV, both the grain count and the harvest index will drop significantly, and the overall yield can even be reduced by more than half (Nancy Arrow et al., 2021). This is actually directly related to the deletion of certain key genes, and these genes have precisely been "deleted" by PAV. Tillering number, grain size, biomass-these yield-related traits may all be affected by PAV. Sometimes, it is the existence of a certain type of PAV that enables certain varieties to maintain stable yields even in adverse conditions. In addition, if PAV is combined with other sequence variations, the prediction of yield performance will be more accurate, and it can even directly guide precision breeding work. Simply put, PAV itself is a very useful "trait marker library".
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