The Conserved PFT1 Tandem Repeat Is Crucial for Proper Flowering in Arabidopsis thaliana

It is widely appreciated that short tandem repeat (STR) variation underlies substantial phenotypic variation in organisms. Some propose that the high mutation rates of STRs in functional genomic regions facilitate evolutionary adaptation. Despite their high mutation rate, some STRs show little to no variation in populations. One such STR occurs in the Arabidopsis thaliana gene PFT1 (MED25), where it encodes an interrupted polyglutamine tract. Although the PFT1 STR is large (∼270 bp), and thus expected to be extremely variable, it shows only minuscule variation across A. thaliana strains. We hypothesized that the PFT1 STR is under selective constraint, due to previously undescribed roles in PFT1 function. We investigated this hypothesis using plants expressing transgenic PFT1 constructs with either an endogenous STR or synthetic STRs of varying length. Transgenic plants carrying the endogenous PFT1 STR generally performed best in complementing a pft1 null mutant across adult PFT1-dependent traits. In stark contrast, transgenic plants carrying a PFT1 transgene lacking the STR phenocopied a pft1 loss-of-function mutant for flowering time phenotypes and were generally hypomorphic for other traits, establishing the functional importance of this domain. Transgenic plants carrying various synthetic constructs occupied the phenotypic space between wild-type and pft1 loss-of-function mutants. By varying PFT1 STR length, we discovered that PFT1 can act as either an activator or repressor of flowering in a photoperiod-dependent manner. We conclude that the PFT1 STR is constrained to its approximate wild-type length by its various functional requirements. Our study implies that there is strong selection on STRs not only to generate allelic diversity, but also to maintain certain lengths pursuant to optimal molecular function.     

The power of mutants for investigating jasmonate biosynthesis and signaling

Mutant analysis includes approaches that range from traditional screening of mutant populations (forward genetics), to identifying mutations in known genes (reverse genetics), to examining the effects of site-specific mutations that encode modified proteins. All these methodologies have been applied to study jasmonate synthesis and signaling, and their use has led to important discoveries. The fad3 fad7 fad8 mutant of Arabidopsis, and other mutants defective in jasmonate synthesis, revealed the roles of jasmonate in flower development and plant defense against necrotrophic fungal pathogens. The coi1 mutant identified the F-box protein that is now known to be the receptor for jasmonoyl-isoleucine, the active form of jasmonate hormone. Investigations of how JASMONATE-ZIM DOMAIN (JAZ) proteins bind to COI1 and facilitate jasmonate perception have relied on the jai3 mutant, on JAZΔJas constructs, and on site-specific mutations in the Jas and ZIM domains of these proteins.