  Plant or soil water status are required in many scientific fields to
understand plant responses to drought. Because the transcriptomic response to
abiotic conditions, such as water deficit, reflects plant water status, genomic
tools could be used to develop a new type of molecular biomarker. Using the
sunflower (Helianthus annuus L.) as a model species to study the transcriptomic
response to water deficit both in greenhouse and field conditions, we
specifically identified three genes that showed an expression pattern highly
correlated to plant water status as estimated by the pre-dawn leaf water
potential, fraction of transpirable soil water, soil water content or fraction
of total soil water in controlled conditions. We developed a generalized linear
model to estimate these classical water status indicators from the expression
levels of the three selected genes under controlled conditions. This estimation
was independent of the four tested genotypes and the stage (pre- or
post-flowering) of the plant. We further validated this gene expression
biomarker under field conditions for four genotypes in three different trials,
over a large range of water status, and we were able to correct their
expression values for a large diurnal sampling period.
