Arabidopsis AUXIN RESPONSE FACTOR6 and 8 Regulate Jasmonic Acid Biosynthesis and Floral Organ Development via Repression of Class 1 KNOX Genes

Abstract

Two mutations in Arabidopsis thaliana, auxin response factor6 (arf6) and arf8, concomitantly delayed the elongation of floral organs and subsequently delayed the opening of flower buds. This phenotype is shared with the jasmonic acid (JA)-deficient mutant dad1, and, indeed, the JA level of arf6 arf8 flower buds was decreased. Among JA biosynthetic genes, the expression level of DAD1 (DEFECTIVE IN ANTHER DEHISCENCE1) was markedly decreased in the double mutant, suggesting that ARF6 and ARF8 are required for activation of DAD1 expression. The double mutant arf6 arf8 also showed other developmental defects in flowers, such as aberrant vascular patterning and lack of epidermal cell differentiation in petals. We found that class 1 KNOX genes were expressed ectopically in the developing floral organs of arf6 arf8, and mutations in any of the class 1 KNOX genes (knat2, knat6, bp and hemizygous stm) partially suppressed the defects in the double mutant. Furthermore, ectopic expression of the STM gene caused a phenotype similar to that of arf6 arf8, including the down-regulation of DAD1 expression. These results suggested that most defects in arf6 arf8 are attributable to abnormal expression of class 1 KNOX genes. The expression of AS1 and AS2 was not affected in arf6 arf8 flowers, and as1 and arf6 arf8 additively increased the expression of class 1 KNOX genes. We concluded that ARF6 and ARF8, in parallel with AS1 and AS2, repress the class 1 KNOX genes in developing floral organs to allow progression of the development of these organs.

KNOX genes: versatile regulators of plant development and diversity

“…(C) Mini-KNOX proteins interact with BELL proteins through a MEINOX domain. three KNOX genes rescues many of these defects in the as1 mutant (Ikezaki et al, 2010). ARP proteins are, therefore, required to repress KNOX expression during leaf development.…”

“…Thus, it will be important to understand whether repression of KNOX gene expression in leaf founder cells is a direct read-out of auxin signaling. One possibility is that AUXIN RESPONSE FACTORs (ARFs) repress KNOX transcription because ectopic KNOX expression causes floral organ defects in arf6 arf8 double mutants (Tabata et al, 2010). Despite mounting evidence for antagonism between KNOX and auxin activities, the proportion of auxin action in lateral organ formation that depends on KNOX repression at the SAM boundary is unclear.…”

A bHLH-Type Transcription Factor, ABA-INDUCIBLE BHLH-TYPE TRANSCRIPTION FACTOR/JA-ASSOCIATED MYC2-LIKE1, Acts as a Repressor to Negatively Regulate Jasmonate Signaling inArabidopsis

“…During stamen development, auxin and GA stimulate the expression of the JA biosynthetic gene DAD1 (Yu et al, 2004;Nagpal et al, 2005;Ito et al, 2007;Cheng et al, 2009;Tabata et al, 2010;Reeves et al, 2012). Elevated biosynthesis of JA in anthers leads to expression of MYB21, MYB24, and MYB57, which are involved in filament elongation, pollen maturation, and anther dehiscence (Mandaokar et al, 2006;Mandaokar and Browse, 2009;Reeves et al, 2012).…”

Physical Interaction of Floral Organs Controls Petal Morphogenesis in Arabidopsis

“…Final petal size is determined by the balance of cell proliferation and expansion (Mizukami and Fischer, 2000;Szécsi et al, 2006). The coordinated growth of petals and other floral organs leads to flower opening, which is regulated by auxin, jasmonic acids, and transcription factors such as MYB21, MYB24, AUXIN RESPONSE FACTOR6 (ARF6), ARF8, and BIGPETALp (Brioudes et al, 2009;Tabata et al, 2010;Varaud et al, 2011;Reeves et al, 2012). Most of the known genes involved in petal development encode transcription factors and act in the early developmental processes; however, regulators involved in later stages of the petal elongation process remain unidentified.…”