75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

261 The Role of SHB1 in Photomorphogenic Development and Photoperiodic Flowering
Yun Zhou, Xiaodong Sun, Min Ni
Department of Plant Biology University of Minnesota, Twin Cities
The red, far-red, and blue light perception and signaling regulate photomorphogenic development, photoperiodic
flowering, and circadian rhythm. We have identified an Arabidopsis mutant shb1 for short hypocotyl under blue 1, a
knockout allele. However, shb1-D, a dominant overexpression allele, exhibits a long hypocotyl phenotype under red,
far-red, and blue light. Therefore, SHB1 is involved in cry-mediated blue light signaling, and overexpression of SHB1
may expand its signaling activity to red and far-red light. We have since undertaken deletion analysis to define the
functional domains of SHB1 in transgenic Arabidopsis. The transgenic plants that overexpress its N-terminal 520 amino
acids phenocopied shb1-D with a long hypocotyl phenotype under red, far-red, and blue light. The N-terminal truncation
has a SPX motif homologous with SYG1 protein family members, and the SPX domain is apparently critical for SHB1
function. In contrast, the transgenic plants that overexpress three C-terminal truncations resembled shb1 with a short
hypocotyl under blue light. The phenotypes may be created through a dominant negative mechanism, and all three C-
terminal truncations contain a putative EXS domain found in the SYG1 protein family. We have further conducted genetic
screens for suppressors and enhancers of shb1-D and worked toward the identification of genes that genetically interacts
with SHB1. shb1-D also flowered early under long days but the same as wild type or slightly early under short days.
SHB1 may mediate the photoreceptor regulation on the expression of several key flowering genes, and may define a new
signaling step in the regulation of both photomorphogenic development and photoperiodic flowering. Future studies on
SHB1 will reveal the critical crosstalk and integration steps of blue light with red and far-red light signaling branches.
262 ADP-Ribosylation in Plant Disease and Plant Disease Resistance
Lori Adams-Phillips, Andrew Bent
University of Wisconsin-Madison
We have discovered that ADP-ribosylation processes are part of the plant defense response. ADP-ribosylation is
the transfer of ADP-ribose from NAD + onto specific sites on target proteins, causing activation or inactivation of those
proteins. This is a new, largely unstudied area of plant pathogenesis. However, ADP-ribosylation is very well known in
the mammalian pathogenesis field, both from the causal role of bacterial ADP -ribosyltransferases in cholera, pertussis
and diptheria diseases, and from host regulation of cellular stress, inflammation and cell death through modulation of
ADP-ribosylation. A number of independent lines of evidence reveal the involvement of ADP-ribosylation during plant/
host interactions. We have discovered that genes encoding a plant ADP-ribose pyrophosphatase and a plant poly(ADPribose)
glycohydrolase are among the 39 most reliably up-regulated genes during the Arabidopsis defense response. In
addition, we have determined that levels of poly(ADP-ribose) increase substantially during compatible interactions with
Pseudomonas syringae and before the onset of HR cell death during the RPS2-mediated defense response. Furthermore,
plants mutated in the upregulated ADP-ribose pyrophosphatase gene exhibit a reduced HR and increased resistance to
Pseudomonas syringae. Plant lines have been constructed that carry either constitutive or inducible over-expression
constructs, or gene silencing constructs for genes involved in ADP-ribosylation. Pharmacological inhibitors of ADPribosylation
are also being utilized to identify the aspects of plant disease resistance where the observed shifts in ADPribosylation
activities exert a significant impact.