Arabidopsis FHY3 defines a key phytochrome A signaling component directly interacting with its homologous partner FAR1

In Arabidopsis, phytochrome A (phyA) is the primary photoreceptor mediating various plant responses to far-red (FR) light. Here we show that phyA signaling involves a combinatorial action of downstream intermediates, which controls overlapping yet distinctive sets of FR responses. FHY3 is a prominent phyA signaling intermediate sharing structural similarity to FAR1, a previously identified phyA signaling component. The fhy3 and far1 mutants display similar yet distinctive defects in phyA signaling; however, overexpression of either FHY3 or FAR1 suppresses the mutant phenotype of both genes. Moreover, overexpression of partial fragments of FHY3 can cause a dominant-negative interference phenotype on phyA signaling that is stronger than those of the fhy3 or far1 null mutants. Further, we demonstrate that FHY3 and FAR1 are capable of homo- and hetero-interaction. Our data indicate that FHY3, together with FAR1, defines a key module in a signaling network underlying phyA-mediated FR light responses.     

fhy1 defines a branch point in phytochrome A signal transduction pathways for gene expression

Physiological analysis of the fhy1 mutant of Arabidopsis has led to the proposal that the mutant is deficient in a downstream component of the phytochrome A signal transduction pathway. To define this lesion at the molecular level, we have examined the expression of a range of phytochrome-regulated genes in fhy1. In far-red light, the regulation of genes such as CHS and CHI is blocked in fhy1, whereas the induction of CAB and NR genes is affected minimally. In contrast, the induction of all genes tested is blocked in a phytochrome A-deficient mutant, confirming that gene expression in far-red light is regulated solely by phytochrome A. Thus, fhy1 defines a branch point in phytochrome A signal transduction pathways for gene expression. Contrary to the general opinion that responses to continuous red light are mediated by phytochrome B and other photostable phytochromes, we have shown also that red light-induction of CHS is mediated almost entirely by phytochrome A. Furthermore, phytochrome A-mediated induction of CHS by red light is blocked in fhy1. The induction of CHS by blue light, however, is normal in fhy1, suggesting that although FHY1 is a component of the phytochrome A signaling pathway, it is not a component of the blue-light signaling pathway for CHS expression.     

Nuclear accumulation of the phytochrome A photoreceptor requires FHY1

The phytochrome family of red/far-red (R/FR)-responsive photoreceptors plays a key role throughout the life cycle of plants . Arabidopsis has five phytochromes, phyA-phyE, among which phyA and phyB play the most predominant functions . Light-regulated nuclear accumulation of the phytochromes is an important regulatory step of this pathway, but to this date no factor specifically required for this event has been identified . Among all phyA signaling mutants, fhy1 and fhy3 (far-red elongated hypocotyl 1 and 3) have the most severe hyposensitive phenotype, indicating that they play particularly important roles . FHY1 is a small plant-specific protein of unknown function localized both in the nucleus and the cytoplasm . Here we show that FHY1 is specifically required for the light-regulated nuclear accumulation of phyA but not phyB. Moreover, phyA accumulation is only slightly affected in fhy3, indicating that the diminished nuclear accumulation of phyA observed in fhy1 seedlings is not simply a general consequence of reduced phyA signaling. By in vitro pull-down and yeast two-hybrid analyses, we demonstrate that FHY1 physically interacts with phyA, preferentially in its active Pfr form. Furthermore, FHY1 and phyA colocalize in planta. We therefore identify the first component required for light-regulated phytochrome nuclear accumulation.     

LTP3 contributes to disease susceptibility in Arabidopsis by enhancing abscisic acid (ABA) biosynthesis

Several plant lipid transfer proteins (LTPs) act positively in plant disease resistance. Here, we show that LTP3 (At5g59320), a pathogen and abscisic acid (ABA)‐induced gene, negatively regulates plant immunity in Arabidopsis. The overexpression of LTP3 (LTP3‐OX) led to an enhanced susceptibility to virulent bacteria and compromised resistance to avirulent bacteria. On infection of LTP3‐OX plants with Pseudomonas syringae pv. tomato, genes involved in ABA biosynthesis, NCED3 and AAO3, were highly induced, whereas salicylic acid (SA)‐related genes, ICS1 and PR1, were down‐regulated. Accordingly, in LTP3‐OX plants, we observed increased ABA levels and decreased SA levels relative to the wild‐type. We also showed that the LTP3 overexpression‐mediated enhanced susceptibility was partially dependent on AAO3. Interestingly, loss of function of LTP3 (ltp3‐1) did not affect ABA pathways, but resulted in PR1 gene induction and elevated SA levels, suggesting that LTP3 can negatively regulate SA in an ABA‐independent manner. However, a double mutant consisting of ltp3‐1 and silent LTP4 (ltp3/ltp4) showed reduced susceptibility to Pseudomonas and down‐regulation of ABA biosynthesis genes, suggesting that LTP3 acts in a redundant manner with its closest homologue LTP4 by modulating the ABA pathway. Taken together, our data show that LTP3 is a novel negative regulator of plant immunity which acts through the manipulation of the ABA–SA balance.     

Auxin promotes susceptibility to Pseudomonas syringae via a mechanism independent of suppression of salicylic acid‐mediated defenses

Auxin is a key plant growth regulator that also impacts plant–pathogen interactions. Several lines of evidence suggest that the bacterial plant pathogen Pseudomonas syringae manipulates auxin physiology in Arabidopsis thaliana to promote pathogenesis. Pseudomonas syringae strategies to alter host auxin biology include synthesis of the auxin indole‐3‐acetic acid (IAA) and production of virulence factors that alter auxin responses in host cells. The application of exogenous auxin enhances disease caused by P. syringae strain DC3000. This is hypothesized to result from antagonism between auxin and salicylic acid (SA), a major regulator of plant defenses, but this hypothesis has not been tested in the context of infected plants. We further investigated the role of auxin during pathogenesis by examining the interaction of auxin and SA in the context of infection in plants with elevated endogenous levels of auxin. We demonstrated that elevated IAA biosynthesis in transgenic plants overexpressing the YUCCA 1 (YUC1) auxin biosynthesis gene led to enhanced susceptibility to DC3000. Elevated IAA levels did not interfere significantly with host defenses, as effector‐triggered immunity was active in YUC1‐overexpressing plants, and we observed only minor effects on SA levels and SA‐mediated responses. Furthermore, a plant line carrying both the YUC1‐overexpression transgene and the salicylic acid induction deficient 2 (sid2) mutation, which impairs SA synthesis, exhibited additive effects of enhanced susceptibility from both elevated auxin levels and impaired SA‐mediated defenses. Thus, in IAA overproducing plants, the promotion of pathogen growth occurs independently of suppression of SA‐mediated defenses.     

Characterization of plant immunity-activating mechanism by a pyrazole derivative

ABSTRACT

A newly identified chemical, 4-{3-[(3,5-dichloro-2-hydroxybenzylidene)amino]propyl}-4,5-dihydro-1H-pyrazol-5-one (BAPP) was characterized as a plant immunity activator. BAPP enhanced disease resistance in rice against rice blast disease and expression of a defense-related gene without growth inhibition. Moreover, BAPP was able to enhance disease resistance in dicotyledonous tomato and Arabidopsis plants against bacterial pathogen without growth inhibition, suggesting that BAPP could be a candidate as an effective plant activator. Analysis using Arabidopsis sid2-1 and npr1-2 mutants suggested that BAPP induced systemic acquired resistance (SAR) by stimulating between salicylic acid biosynthesis and NPR1, the SA receptor protein, in the SAR signaling pathway.     

Arabidopsis FHY3 specifically gates phytochrome signaling to the circadian clock

Circadian gating of light signaling limits the timing of maximum responsiveness to light to specific times of day. The fhy3 (for far-red elongated hypocotyl3) mutant of Arabidopsis thaliana is involved in independently gating signaling from a group of photoreceptors to an individual response. fhy3 shows an enhanced response to red light during seedling deetiolation. Analysis of two independent fhy3 alleles links enhanced inhibition of hypocotyl elongation in response to red light with an arrhythmic pattern of hypocotyl elongation. Both alleles also show disrupted rhythmicity of central-clock and clock-output gene expression in constant red light. fhy3 exhibits aberrant phase advances under red light pulses during the subjective day. Release-from-light experiments demonstrate clock disruption in fhy3 during the early part of the subjective day in constant red light, suggesting that FHY3 is important in gating red light signaling for clock resetting. The FHY3 gating function appears crucial in the early part of the day for the maintenance of rhythmicity under these conditions. However, unlike previously described Arabidopsis gating mutants that gate all light signaling, gating of direct red light-induced gene expression in fhy3 is unaffected. FHY3 appears to be a novel gating factor, specifically in gating red light signaling to the clock during daytime.     

An incoherent feed‐forward loop mediates robustness and tunability in a plant immune network

Immune signaling networks must be tunable to alleviate fitness costs associated with immunity and, at the same time, robust against pathogen interferences. How these properties mechanistically emerge in plant immune signaling networks is poorly understood. Here, we discovered a molecular mechanism by which the model plant species Arabidopsis thaliana achieves robust and tunable immunity triggered by the microbe‐associated molecular pattern, flg22. Salicylic acid (SA) is a major plant immune signal molecule. Another signal molecule jasmonate (JA) induced expression of a gene essential for SA accumulation, EDS5. Paradoxically, JA inhibited expression of PAD4, a positive regulator of EDS5 expression. This incoherent type‐4 feed‐forward loop (I4‐FFL) enabled JA to mitigate SA accumulation in the intact network but to support it under perturbation of PAD4, thereby minimizing the negative impact of SA on fitness as well as conferring robust SA‐mediated immunity. We also present evidence for evolutionary conservation of these gene regulations in the family Brassicaceae. Our results highlight an I4‐FFL that simultaneously provides the immune network with robustness and tunability in A. thaliana and possibly in its relatives.