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The plant hormone (phytohormone), auxin plays a crucial role in a wide variety of growth and developmental processes involving cell elongation, division and differentiation. The cellular responses to auxin involve not only electrophysiological changes at the plasma membrane, but also fast alterations of gene expression. Currently, the involvement of auxin in the regulation of gene expression is well-recognized. Using differential screening approaches, a number of auxin-regulated genes have been identified, mainly in elongating tissues and dividing cells. mRNA levels of these genes were altered within minutes after auxin application and were unaffected by treatment with protein synthesis inhibitor, cycloheximide. It means that protein synthesis is not required for their activation, suggesting that the hormonal signal is transmitted to the nucleus via preexisting components. These genes are referred to as early or primary auxin response genes and classified into three major classes known as the Aux/IAA, GH3 and SAUR gene families. Members of the Aux/IAA gene family are involved in light regulation of auxin responses. Several GH3 genes encode acyladenylate-forming enzymes that catalyze conjugation of indole-3-acetic acid, jasmonic acid and salicylic acid to amino acids. The GH3 enzymes regulate auxin homeostasis by conjugating excess hormone to amino acids. Analysis of GH3 mutants indicated the involvement of these genes in photomorphogenesis, root and hypocotyl elongation and both biotic and abiotic stress adaptation responses. GH3 genes are also regulated by light suggesting a role of GH3 proteins in light-auxin interactions. SAUR are small short-lived basic nuclear proteins that physiological functions remain unknown. Some members of SAUR family have been implicated in calcium/calmodulin-mediated auxin responses. The conserved sequences TGTCTC named the auxin response elements (AuxREs) within the promoters of early auxin response genes have been identified and a family of auxin response factors (ARFs) binding to AuxRE has also been characterized. ARF proteins either promote or inhibit target gene expression. Aux/IAA genes encode short lived nuclear proteins that themselves do not directly bind DNA, but bind to ARF proteins resulting in repression of their transcriptional activity. Auxin promotes the interaction between Aux/IAA and TIR1/AFB proteins and increases the degradation rate of Aux/IAA proteins in ubiquitin/proteasom 26S pathway, such that ARF activity is derepressed and numerous auxin-mediated transcriptional changes occur. ARF proteins released from their repressor counterparts regulate the transcription of auxin response genes. This review describes recent advances in studies on early auxin response genes and physiological functions of the proteins encoded.
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The Editorial Board
Andrzej Łukaszyk - przewodniczący, Zofia Bielańska-Osuchowska, Szczepan Biliński, Mieczysław Chorąży, Aleksander Koj, Włodzimierz Korochoda, Leszek Kuźnicki, Aleksandra Stojałowska, Lech Wojtczak

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