Identification and functional characterization of the Marshallia (Asteraceae) Clade III Cytokinin Response Factor (CRF)

Cytokinin Response Factor (CRF) genes are a subgroup of AP2/ERF domain-containing transcription factors that are defined by the CRF domain, from which five clades of CRF genes have been identified. Clade III CRFs are strongly induced by cytokinin, as well as other abiotic stress factors, such as oxidative stress. While this appears well studied for the Clade III CRFs in Arabidopsis and tomato, there have been almost no studies done outside of these model systems. This study expands upon that and represents the first CRF research in the Sunflower family, Asteraceae. Fifty Asterid Clade III CRF protein sequences were examined, and novel Clade III CRF C-terminus motifs were identified. Clade III CRF genes of Marshallia mohrii and M. caespitosa were assembled from genome-skimming and transcriptomic data. Expression experiments were conducted on M. caespitosa to test responsiveness to both cytokinin and oxidative stress. Low levels of basal expression for the McCRF1 were found to be strongly induced in both treatment groups. These are the first experiments to show regulation of a nuclear gene in a Marshallia species, and these results suggest there is broad conservation in the sequence, form, and regulation of Clade III CRF genes and proteins.