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Cover Illustration: One of the fundamental characteristics of plants is their polar organization and polar growth. An auxin gradient has been implicated in directing all polar patterns of development and differentiation. Auxin effl ux carriers, PINs, present only in specifi c cell membranes, drive polar auxin transport and the PID protein kinase in Arabidopsis regulates transport by targeting PIN transporters. If this gene plays the same role in all plant species, it must be among the most important genes that control morphological diversity. The PsPK2 gene, the PID homolog in pea, is expressed in all growing parts of pea and is positively regulated by auxin, gibberellin, and cytokinin. To compare regulatory characteristics of PsPK2 and PID, we constructed a reporter gene and transformed Arabidopsis with it. GUS activity is illustrated in transgenic PsPK2::GUS (upper 3 rows) and DR5::GUS (lower 3 rows) Arabidopsis seedlings in response to treatment with water (control) and four auxins: 4-chloro-indole acetic acid (4-Cl-IAA), indole-3- acetic acid (IAA), indole-3-butyric acid (IBA), indole-3- acetic acid methyl ester (MeIAA), or ɑ-naphthaleneacetic acid (NAA), respectively. The blue color in seedlings (rows 1 and 4), cotyledons (rows 2 and 5), and fi rst pair of leaves (rows 3 and 6) indicates that DR5 and PsPK2 promoter responses are driving the expression of GUS. PsPK2::GUS responds to different auxins like DR5::GUS does, but PID::GUS's auxin responses are complex (not shown). See Bai and DeMason—Hormone interactions and regulation of PsPK2::GUS compared to DR5::GUS and PID::GUS in Arabidopsis thaliana, pp. 133–145 in this issue. Transcriptional responses to cytokinin and GA and to treatments with all pairwise hormone combinations are also described. Comparisons to DR5::GUS are necessary to isolate strict auxin responses. Photo credit: Fang Bai.