In a wide variety of plants the floral transition relies on the length of the daylight period, also known as photoperiod. CBGP reserchers have characterised the role of the HOS1 protein in the degradation of the main component of the photoperiod pathway , which regulates flowering time in the model plant Arabidopsis thaliana.
The most reliable environmental cue used by plants to detect the season of the year is the number of light hours in a 24 h cycle (photoperiod). Floral transition is regulated by changes in photoperiod in a wide range of species, and plants that show defects in the mechanisms involved in daylenght perception do not flower properly in nature. CBGP researchers have demonstrated that two proteins present in plants (HOS1 and COP1) interact synergistically in the photoperiodic control of flowering.
Among varieties that respond to photoperiod, there are plants in which flowering is induced by short days and others in which flowering is induced by long days. In short days the dark period is longer than the light period, and this is characteristic of late autumn and winter; whereas in long days the light period is longer than the dark period, and this is common in spring and early summer. In contrast, other plant species are photoperiod insensitive.
Plants have developed a sophisticated molecular mechanism to measure daylength based on the coincidence of an internal rhythm, set by the circadian clock, with an external cue, such as light. The ability to distinguish long days from short days is largely the result of the complex regulation of CONSTANS (CO) gene, which in turn, regulates the expression levels of the floral integrator FLOWERING LOCUS T (FT). The FT protein is part of the florigen, the signal that induces flowering. Arabidopsis thaliana, the model organism in plant research, is a facultative long day plant because it flowers earlier in long days than in short days.
INIA-CBGP researchers have isolated the early in short days6 (esd6) mutant in a screen for mutations that accelerate flowering time in Arabidopsis. The esd6 phenotype is caused by a lesion in the HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENES1 (HOS1) locus, which encodes an E3 ubiquitin ligase involved in selective targeted protein degradation. The esd6 mutation requires CO protein for its early flowering phenotype under long days. Moreover, CO and HOS1 physically interact in vitro and in planta, and HOS1 regulates CO abundance, particularly during the daylight period. Accordingly, hos1 causes a shift in the regular long-day pattern of expression of FT transcript, starting to rise 4 h after dawn in the mutant. In addition, they have demonstrated that HOS1 interacts synergistically with CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), another regulator of CO protein stability, in the regulation of flowering time. Taken together, these results indicate that HOS1 is involved in the control of CO abundance, ensuring that CO activation of FT occurs only when the light period reaches a certain length and preventing precocious flowering in Arabidopsis. Getting deeper insight into the molecular mechanisms involved in the photoperiodic regulation of flowering time has many biotechnological applications. Manipulation of these mechanisms could contribute to reproductive success in plants considering that floral transition relies on favourable environmental conditions.