RNAseq Reveals Genes Underlying Early Season Yellowing in ALS Resistant Sorghums.

Acetolactate synthase (ALS) herbicides are among widely marketed herbicide chemistries that act both against grass and broad-leaved weeds. ALS resistant sorghums were developed as a viable post-emergence weed control option in sorghum. However, many of the lines resistant to ALS herbicides tend to show reduced vigor and inter-veinal chlorosis at seedling stage. Though affected plants eventually green-up at advanced stage, the unfamiliar phenotype may undermine adoption of ALS resistant sorghums by farmers. The objective of this study was to identify genetic causes and plant mechanisms underlying such bizarre phenotype. Two selected ALS resistant sister lines expressing yellow and normal phenotypes were grown and tissues harvested at four different times with the last sampling made after the genotypes have re-greened. RNA was extracted from all samples and profiled using RNAseq. Differential gene expression analysis was performed using DESeq2 followed by Gene Ontology (GO) enrichment. A closer look at SorghumCyc metabolic pathways based upon GO analysis revealed significant up-regulation of Pheophorbide a Oxygenase (PAO) and Red Chlorophyll Catabolite Reductase (RCCR) in yellow genotypes only during first three samplings. Both enzymes are involved in chlorophyll degradation pathway and are highly up-regulated when plants enter senescence, while stay-green plants are considered to carry a defective PAO or a factor regulating PAO activity. It appears that chlorosis in some ALS lines may comprise an altered PAO/RCCR regulation which may direct a developmental shift in chloroplasts forming gerontoplasts found in senescing leaves. However, owing to up-regulated toxin catabolic processes and feedback regulation of chlorophyll degradation has lead to re-differentiation of gerontoplasts to chloroplasts. This was reflected through gradual re-greening of yellow plants. However, proteins involved in speculated altered PAO/RCCR regulation are yet to be investigated.