Link Found Between GRF genes in Sorghum and Aphid Resistance

Sorghum, the fifth most important cereal crop, provides food for more than a half billion people. It is also used as livestock feed and as a key component in biofuel. Biotic and abiotic stress resistant, sorghum is an ideal crop to be grown under adverse circumstances like poor soil or water stress conditions. Growth Regulating Factors (GRFs), which encode transcription factors regulating plant growth and development, also respond to changes in the environment. The study identified 8 Sorghum GRF genes containing QLQ (glutamine, leucine, glutamine) and WRC (tryptophan, arginine, cysteine) domains, which evolved in 4 clades. Understanding the role of the GRF gene family in pest resistance could contribute to the development of aphid resistant sorghum, which is of great interest to scientists, growers and breeders alike.

In an effort to better understand this role, Shi et al. from Shanghai Jiao Tong University, Hebei Academy of Agriculture & Forestry Sciences, Hebei Branch of China National Sorghum Improvement Center, Hebei Nijiao Brewing Technology Innovation Center, and Hebei Seed Management Station, identified eight GRF genes in the sorghum genome, evaluating their functional domain architecture and expression patterns under both normal and stressed  (heat, salt, drought, aphid) conditions. These SbGRF genes were found to be expressed in most tissues and over half of them (including SbGRF1, 2, 3, 6, 7) expressed at the highest level in inflorescence. The expression level in inflorescence (flower clusters) is indicative of the role of GRFs in growth regulation. The authors hypothesize that flower development may be repressed by down-regulated SbGRFs and ectopic expression of the genes may increase the yield and biomass of sorghum plants. 

The researchers performed a transcriptional analysis and discovered that more than half of the SbGRFs responded to abiotic stressors (heat, salinity and drought). This confirms the findings of other studies (Casadevall et al., 2013; Fina et al., 2017, Cao et al., 2020, Kim et al., 2012). However, the effects of biotic stressors, such as insects, on GRF expression has not been the focus of much study. The researchers found that the expression of SbGRF1, 2, 4 and 7 was greatly enhanced when sorghum plants were exposed to aphid stress. This indicates that GRFs help sorghum plants to defend against attacks from pests. The SbGRF response to aphids was validated by qRT-PCR. The results of this study could be used to target certain sorghum GRF genes for selective breeding in an effort to create aphid-resistant cultivars.

SorghumBase examples:

SorghumBase offers powerful comparative pangenomic views of extended gene families.

Figure 1: Centered on SbGRF1 (Sobic.001G104500, SORBI_3001G104500, the above image shows the phylogenetic relationships among the GRF gene family members described in this article. The SorghumBase tree (release #4, July 2022), originally built with 26 species including 18 different varieties of sorghum, was pruned to only display sorghum reference BTx623 genes.
Figure 2: The same SorghumBase tree showing all the species (including maize and rice) that were used to build it. Note that in addition to all the GRF genes mentioned by Shi and colleagues, two additional sorghum genes, SORBI_3010G013500 and SORBI_3004G282601, were included in this tree, suggesting additional members of the SbGRF family.
Figure 3: SbGRF2 maps to a region associated with QTLs conferring resistance to biotic stressors such as shoot fly, head bug and striga, a parasitic weed. The image was taken from the SorghumBase genome browser and includes a track for genetic variation (see above Variant inset), which enables researchers to rapidly locate color-coded variants such as chemically induced point mutations in splicing areas (orange vertical marks).


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