| Selective protein degradation in multicellular organisms is directed by the ubiquitin (Ub) 26S proteasome system (UPS). Through an ATP-dependent enzymatic cascade involving Ub activating enzymes (E1s), Ub conjugation enzymes (E2s) and Ub protein ligases (E3s), Ub becomes covalently attached to various short-lived proteins. The ubiquitylated species are recognized and degraded by the 26S proteasome.;One type of E3s, the SKP1/CULLIN1/F-Box (SCF) ligase, is a four-protein complex which utilizes a F-box protein to determine its substrate specificity. In Arabidopsis thaliana, almost 700 loci have been identified that encode F-box proteins, making this family one of the largest in plants. While a number of F-Box proteins have been shown to control important developmental and physiological processes in plants through forward genetics, the functions of most of them are unknown.;To help define function(s) of the poorly-understood members, I generated a large collection of T-DNA insertion mutants in individual Arabidopsis F-box genes that have not been studied previously. In total, I screened 280 Salk T-DNA insertion alleles and obtained useful mutants for 123 different loci.;Among these F-Box T-DNA insertion mutants, I primarily focused on a set affecting a pair of Auxin-up-Regulated F-Box (AUF) genes, AUF1 and AUF2. AUF1 expression is dramatically induced in the root upon auxin treatment, yet auf1 and auf2 mutants have no significant phenotypic changes in response to auxin. Intriguingly, roots of auf1 and auf1/auf2 plants exhibit increased sensitivity to auxin transport inhibitors. Auxin transport assay reveals that auf1 plants have significant defects in acropetal and basipetal transport in the roots, but the gravitropism response in the auf1 seedlings is normal. In addition, study on the auxin transporter PIN-formed (PIN) proteins revealed that AUF1 regulates polar auxin transport by modulating transcription of multiple PIN genes, which in turn, at least for PIN2, affects protein abundance on the plasma membrane. Roots of auf1 plants also have increased sensitivity to exogenous cytokinin. Root growth kinematic analyses showed that AUF1 does not affect cell division but regulates root cell expansion in the distal elongation zone. Collectively, I conclude that AUF1 is involved in an auxin-cytokinin cross-talk circuit that controls Arabidopsis root development. |
