Molecular mechanism of sensing moisture by rice coleorhiza during germination

Despite extensive research in the past several decades, how plants sense soil drying is still a fundamental question in our understanding of plant drought tolerance. Simply, we do not know the initial molecular mechanism of sensing soil moisture and the activation of downstream adaptive traits. Classically, hydrotropism is coined for the differential growth of root towards high moisture gradient but the genetic basis of hydrotropism is largely unknown, although a couple of genes have been reported in the hydrotropic responses in Arabidopsis. Hydrotropic response is difficult to be dissected as other tropisms, particularly gravitropism, strongly interact with hydrotropism.

We have observed an intriguing response that rice seed can sense moisture gradient (air humidity) even before root emergence during seed germination. Profuse hairs are developed from the surface of coleorhiza at near saturated humidity before root emergence, while low air humidity restricts the development of such hairs as well as root emergence and ultimately seed germination, even though the seeds are almost fully imbibed. Coleorhiza hairs are known to anchor germinating seeds in seedling establishment but our finding suggests that they function to sense appropriate moisture for the seed germination.

This adaptive phenotype suggests that specific cell types may sense moisture around seed surface and develop hairs originated from epidermal cells of coleorhiza. Similarly, root hairs are initiated on the root surface, although coleorhiza hairs are initiated quicker, longer and denser in nature than root hairs. But are they the same? We, therefore, propose first of all to investigate if the root hair specific genes are also working in coleorhiza hairs too. Overexpression and knock-out/down lines of several root hair specific genes will be analyzed. This may confirm the specific identities of coleorhiza hair, as their developmental regulations may suggest specific genes and therefore, specific functions.

For deep exploration of this adaptive phenotype and its genetic basis, we propose to screen for its rice mutants. Mutant screening can start from available mutant resources or induced by EMS mutagenesis. In addition to search for genes from mutants, we also propose to examine the spatio-temporal gene expressions of coleorhiza tissue using RNA sequencing to provide clues of the specific genes of this phenotype.

Outcome of this research will reveal the key genes in coleorhiza hair induction and increase our understanding of how plants sense moisture gradient and activate downstream adaptive responses to soil drying.