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The Activation Sequence-1 Cognate Promoter
Elements Play Critical Roles in the Activation of
Defense-Related Genes in Higher Plants
Chengbin Xiang
Iowa State University, Ames, Iowa
527I. INTRODUCTION
Sessile plants rely on environmental cues to program their development and growth. Meanwhile, they are
unable to escape from unfavorable, sometimes hostile, environmental conditions. In order to survive,
higher plants have evolved and deployed sophisticated systems that sense the changes in their living en-
vironment and activate corresponding defense mechanisms. These adaptive responses are crucial for plant
survival and crop productivity. The genetic basis for the plasticity and capacity of these defense mecha-
nisms has shaped the current geographic distribution of higher plants on the earth. One central process of
adaptive responses is the activation of defense-related genes in response to biotic and abiotic stresses in
their living environment. Increasing evidence has shown that the activation sequence-1 (as-1) cognate
promoter elements were recruited by the promoters of plant defense-related genes as well as by the gene
promoters of plant pathogens. A number of studies indicated that elements of this type play a critical role
in the activation of defense genes. This review chapter focuses on the general features of this class of cis-
acting promoter elements and discusses how diverse stress signals converge on this type of element to ac-
tivate defense genes against biotic and abiotic stresses.
II. THE as-1–TYPECIS-ACTING ELEMENTS ARE EXPLOITED BY
PLANT PATHOGENSTheas-1DNA element was first identified in the 75 region of the 35S promoter of cauliflower mosaic
virus [1]. The as-1element is composed of two tandem repeats of a TGACG motif that resembles the
mammalian activator protein-1 (AP-1) binding site (Figure 1). Similar TGACG motif–containing pro-
moter elements were also found in the gene promoters of plant pathogens. These include the nos-1ele-
ment (131 to 111) of the nopaline synthase gene promoter [2], the mas-1element of the mannopine
synthase [9], and the ocselement (193 to 173) of the octopine synthase gene promoter of Agrobac-
terium tumefaciensT-DNA [3].
A tobacco nuclear protein factor, activation sequence factor-1 (ASF-1) was found to interact specif-
ically with the as-1element [1]. This factor can also bind to a number of cis-acting elements containing
the TGACG motif such as nos-1andocs-1. A single as-1element is sufficient to confer root expression
when fused to a leaf-specific promoter [1] and is responsive to multiple stress-related stimuli.