Characterization of Phytochrome Interacting Factors from the Moss Physcomitrella patens Illustrates Conservation of Phytochrome Signaling Modules in Land Plants.

Across the plant kingdom, phytochrome (PHY) photoreceptors play an important role during adaptive and developmental responses to light. In Arabidopsis thaliana, light-activated PHYs accumulate in the nucleus, where they regulate downstream signaling components, such as phytochrome interacting factors (PIFs). PIFs are transcription factors that act as repressors of photomorphogenesis; their inhibition by PHYs leads to substantial changes in gene expression. The nuclear function of PHYs, however, has so far been investigated in only a few non-seed plants. Here, we identified putative target genes of PHY signaling in the moss Physcomitrella patens and found light-regulated genes that are putative orthologs of PIF-controlled genes in Arabidopsis. Phylogenetic analyses revealed that an ancestral PIF-like gene was already present in streptophyte algae, i.e., before the water-to-land transition of plants. The PIF homologs in the genome of P. patens resemble Arabidopsis PIFs in their protein domain structure, molecular properties, and physiological effects, albeit with notable differences in the motif-dependent PHY interaction. Our results suggest that P. patens PIFs are involved in PHY signaling. The PHY-PIF signaling node that relays light signals to target genes has been largely conserved during land plant evolution, with evidence of lineage-specific diversification.

Shedding (far-red) light on phytochrome mechanisms and responses in land plants.

In order to monitor ambient light conditions, plants rely on functionally diversified photoreceptors. Among these, phytochromes perceive red (R) and far-red (FR) light. FR light does not constitute a photosynthetic energy source; it however influences adaptive and developmental processes. In seed plants, phytochrome A (phyA) acts as FR receptor and mediates FR high irradiance responses (FR-HIRs). It exerts a dual role by promoting e.g. germination and seedling de-etiolation in canopy shade and by antagonising shade avoidance growth. Even though cryptogam plants such as mosses and ferns do not have phyA, they show FR-induced responses. In the present review we discuss the mechanistic basis of phyA-dependent FR-HIRs as well as their dual role in seed plants. We compare FR responses in seed plants and cryptogam plants and conclude on different potential concepts for the detection of canopy shade. Scenarios for the evolution of FR perception and responses are discussed.

An evolutionarily conserved signaling mechanism mediates far-red light responses in land plants.

Phytochromes are plant photoreceptors important for development and adaptation to the environment. Phytochrome A (PHYA) is essential for the far-red (FR) high-irradiance responses (HIRs), which are of particular ecological relevance as they enable plants to establish under shade conditions. PHYA and HIRs have been considered unique to seed plants because the divergence of seed plants and cryptogams (e.g., ferns and mosses) preceded the evolution of PHYA. Seed plant phytochromes translocate into the nucleus and regulate gene expression. By contrast, there has been little evidence of a nuclear localization and function of cryptogam phytochromes. Here, we identified responses to FR light in cryptogams, which are highly reminiscent of PHYA signaling in seed plants. In the moss Physcomitrella patens and the fern Adiantum capillus-veneris, phytochromes accumulate in the nucleus in response to light. Although P. patens phytochromes evolved independently of PHYA, we have found that one clade of P. patens phytochromes exhibits the molecular properties of PHYA. We suggest that HIR-like responses had evolved in the last common ancestor of modern seed plants and cryptogams and that HIR signaling is more ancient than PHYA. Thus, other phytochromes in seed plants may have lost the capacity to mediate HIRs during evolution, rather than that PHYA acquired it.

Pollen tube tip growth depends on plasma membrane polarization mediated by tobacco PLC3 activity and endocytic membrane recycling.

Phosphatidyl inositol 4,5-bisphosphate (PI 4,5-P2) accumulates in a Rac/Rop-dependent manner in the pollen tube tip plasma membrane, where it may control actin organization and membrane traffic. PI 4,5-P2 is hydrolyzed by phospholipase C (PLC) activity to the signaling molecules inositol 1,4,5-trisphosphate and diacyl glycerol (DAG). To investigate PLC activity during tip growth, we cloned Nt PLC3, specifically expressed in tobacco (Nicotiana tabacum) pollen tubes. Recombinant Nt PLC3 displayed Ca2+-dependent PI 4,5-P2-hydrolyzing activity sensitive to U-73122 and to mutations in the active site. Nt PLC3 overexpression, but not that of inactive mutants, inhibited pollen tube growth. Yellow fluorescent protein (YFP) fused to Nt PLC3, or to its EF and C2 domains, accumulated laterally at the pollen tube tip plasma membrane in a pattern complementary to the distribution of PI 4,5-P2. The DAG marker Cys1:YFP displayed a similar intracellular localization as PI 4,5-P2. Blocking endocytic membrane recycling affected the intracellular distribution of DAG but not of PI 4,5-P2. U-73122 at low micromolar concentrations inhibited and partially depolarized pollen tube growth, caused PI 4,5-P2 spreading at the apex, and abolished DAG membrane accumulation. We show that Nt PLC3 is targeted by its EF and C2 domains to the plasma membrane laterally at the pollen tube tip and that it maintains, together with endocytic membrane recycling, an apical domain enriched in PI 4,5-P2 and DAG required for polar cell growth.