Gene-environment interactions in plants are often similar to those in animals, but there are also distinct differences. Small RNAs, transcribed from the DNA of a plant’s genome, are involved in plant development, reproduction, and genome reprogramming –– as described in the attached (2o15) review. The large variety of small‑RNA pathways in plants likely contributes to their phenotypic plasticity (i.e. ability to adapt to any changing environment). Most likely, these pathways evolved as a cellular defense mechanism against RNA viruses and transposable elements, again emphasizing the constant drive of survival of the species. Later in evolution, these pathways most likely then became involved in regulating the expression of endogenous genes having other functions.
The abundance and diversity of small‑RNA classes varies among plant species, suggesting co-evolution between environmental adaptations and gene‑silencing mechanisms. Authors [first paper attached] review the field of plant small RNAs (e.g. microRNAs, secondary small-interfering RNAs (siRNAs) and heterochromatin-associated siRNAs) (heterochromatin plays a role in the expression of genes. Although this chromosomal structure is tightly packed, much of this DNA gets transcribed, but it is continuously turned over via RNA-induced transcriptional silencing). These small RNAs exhibit diverse cellular and developmental functions –– including plant reproductive function, genomic imprinting, and paramutation (in epigenetics, paramutation is an interaction between two alleles at a single locus –– whereby one allele induces a heritable change in the other allele; these changes often involve DNA-methylation or histone modifications).
Authors [second paper attached] studied the regulation of parental genome dosage in Arabidopsis thaliana (a flowering plant in the mustard family, its small genome size, rapid life cycle, large production of seeds, and ability to grow under laboratory conditions –– have made it a model organism for studying plant genetics, physiology, biochemistry and development). Regulation of parental genome dosage is of fundamental importance in animals and plants (examples include X-chromosome inactivation and dosage compensation). The “triploid block” (i.e. three sets of chromosomes instead of the normal two) is a classic example of dosage regulation in plants –– establishes a reproductive barrier between species that differ in chromosome number. This barrier is active in the (embryo-nourishing) endosperm tissue and is able to abort hybrid seeds through as-yet-unknown mechanism. Authors [2nd paper attached] show that depletion of paternal epigenetically-activated small-interfering RNAs (easiRNAs) bypasses the triploid block in response to increased paternal ploidy (number of chromosome sets).
Paternal loss of a particular RNA polymerase enzyme is able to suppress easiRNA formation and rescue triploid seeds by restoring small-RNA-directed DNA-methylation at transposable elements (“jumping genes”), and this restoration is correlated with decreased expression of paternally expressed imprinted genes (PEGs). These data suggest that easiRNAs represent a quantitative signal for paternal chromosome number, and that a balanced chromosomal dosage is required for post-fertilization genome stability and seed viability.
Nat Rev Mol Cell Biol Dec 2o15; 16: 727–741 & Nature Genet Feb 2o18; 50: 193–198