Functional diversity of phytochrome family in the control of light and gibberellin‐mediated germination in Arabidopsis

Abstract

In several species, seed germination is regulated by light in a way that restricts seedling emergence to the environmental conditions that are likely to be favourable for the success of the new individual, and therefore, this behaviour is recognized to have adaptive value. The phytochromes are one of the most relevant photoreceptors involved in light perception by plants. We explored the redundancy and diversity functions of the phytochrome family in the control of seed responsiveness to light and gibberellins (GA) by using a set of phytochrome mutants of Arabidopsis. Our data show that, in addition to the well-known role of phyB in the promotion of germination in response to high red to far-red ratios (R/FR), phyE and phyD stimulate germination at very low R/FR ratios, probably by promoting the action of phyA. Further, we show that phyC regulates negatively the seed responsiveness to light, unravelling unexpected functions for phyC in seed germination. Finally, we find that seed responsiveness to GA is mainly controlled by phyB, with phyC, phyD and phyE having relevant roles when acting in a phyB-deficient background. Our results indicate that phytochromes have multiple and complex roles during germination depending on the active photoreceptor background.

Bottom-up Assembly of the Phytochrome Network

“…Despite its negligible role as an inducer of germination when present alone, phyE acted synergistically with phyC and with phyD to induce germination after exposure to continuous R or to a single pulse of R (Fig 1). Further, phyE acted synergistically with phyB at low R:FR ratios (i.e., during FR treatments), consistent with previous reports [6, 19]. phyC, phyD, and phyE interacted synergistically with phyA under continuous FR, a treatment that specifically activates phyA [18].…”

“…phyB and phyA have emerged as the most important phytochromes [1, 2, 18, 29], whereas the roles of phyD, phyE, and phyC have been assumed to be minor and redundant with phyB [6, 11, 12, 14, 15, 22, 24, 40, 41]. However, our knowledge about the capacity of individual phytochromes to elicit specific responses and the interactions among them is still incomplete.…”

“…Hence, phyD and phyE, which were previously believed to work mostly in a redundant fashion [6, 11, 12, 14, 15, 22, 24, 40, 41], have distinct roles. At this point it is unclear how this may occur.…”

“…Phytochromes regulate plant development throughout the life cycle, from germination to flowering. For instance, phytochromes promote germination by stimulating gibberellin (GA) synthesis and sensitivity [6, 7], promote deetiolation during early seedling development, inhibit hypocotyl and stem elongation by altering auxin levels [8], entrain the circadian clock, and regulate flowering time [2]. …”

Photo-biotechnology as a tool to improve agronomic traits in crops

“…The significance of individual phytochromes in seed germination was first established in a comparative analysis of mutants deficient in PhyA, PhyB, and PhyA/PhyB in Arabidopsis. These studies indicated that PhyB is an important regulator of seed germination in red light via the low fluence response (LFR) mode, while PhyA mediates very low fluence responses (VLFR) in red and far-red light (Shinkle and Briggs, 1984;Footitt et al, 2013;Arana et al, 2014). Heschel et al (2007) demonstrated that temperature-responsive phytochrome-mediated germination pathways affect seed germination, with PhyA and PhyE promoting seed germination at warmer and colder temperatures, respectively, and PhyB promoting seed germination across a range of temperatures.…”