Rapid assessment of the effect of reduced levels of gene products is often a
bottleneck in determining how to proceed with an interesting gene candidate. Additionally,
gene families with closely related members can confound determination of the role of even a
single one of the group. We describe here an in vivo method to rapidly determine gene
function using transient expression of artificial microRNAs (amiRNAs) in Arabidopsis
thaliana mesophyll protoplasts. We use a luciferase-based reporter of circadian clock activity
to optimize and validate this system. Protoplasts transiently co-transfected with promoter-
luciferase and gene-specific amiRNA plasmids sustain free-running rhythms of
bioluminescence for more than six days. Using both amiRNA plasmids available through the
ABRC, as well as custom-design of constructs using the Weigel amiRNA design algorithm,
we show that transient knockdown of known clock genes recapitulates the same circadian
phenotypes reported in the literature for loss-of-function mutant plants. We additionally show
that amiRNA designed to knockdown expression of the casein kinase 2 beta subunit (CKB) gene family lengthens period, consistent with previous reports of a short period in CKB
overexpressors. Our results demonstrate that this system can facilitate a much more rapid
analysis of gene function by obviating the need to initially establish stably transformed
transgenics to assess the phenotype of gene knockdowns. This approach will be useful in a
wide range of plant disciplines when an endogenous cell-based phenotype is observable or
can be devised, as done here using a luciferase reporter.