Cytochrome c and the transcription factor ABI4 establish a molecular link between mitochondria and ABA-dependent seed germination.

During germination, seed reserves are mobilised to sustain the metabolic and energetic demands of plant growth. Mitochondrial respiration is presumably required to drive germination in several species, but only recently its role in this process has begun to be elucidated. Using Arabidopsis thaliana lines with changes in the levels of the respiratory chain component cytochrome c (CYTc), we investigated the role of this protein in germination and its relationship with hormonal pathways. Cytochrome c deficiency causes delayed seed germination, which correlates with decreased cyanide-sensitive respiration and ATP production at the onset of germination. In addition, CYTc affects the sensitivity of germination to abscisic acid (ABA), which negatively regulates the expression of CYTC-2, one of two CYTc-encoding genes in Arabidopsis. CYTC-2 acts downstream of the transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4), which binds to a region of the CYTC-2 promoter required for repression by ABA and regulates its expression. The results show that CYTc is a main player during seed germination through its role in respiratory metabolism and energy production. In addition, the direct regulation of CYTC-2 by ABI4 and its effect on ABA-responsive germination establishes a link between mitochondrial and hormonal functions during this process.

Class I TCP proteins TCP14 and TCP15 are required for elongation and gene expression responses to auxin.

KEY MESSAGE: Two class I TCP transcription factors are required for an efficient elongation of hypocotyls in response to auxin and for the correct expression of a subset of auxin-inducible genes In this work, we analyzed the response to auxin of plants with altered function of the class I TEOSINTE BRANCHED 1, CYCLOIDEA, PCF (TCP) transcription factors TCP14 and TCP15. Several SMALL AUXIN UP RNA (SAUR) genes showed decreased expression in mutant plants defective in these TCPs after an increase in ambient temperature to 29 °C, a condition that causes an increase in endogenous auxin levels. Overexpression of SAUR63 caused a more pronounced elongation response in the mutant than in the wild-type at 29 °C, suggesting that the decreased expression of SAUR genes is partly responsible for the defective elongation at warm temperature. Notably, several SAUR genes and the auxin response gene IAA19 also showed reduced expression in the mutant after auxin treatment, while the expression of other SAUR genes and of IAA29 was not affected or was even higher. Expression of the auxin reporter DR5::GUS was also higher in a tcp15 mutant than in a wild-type background after auxin treatment. However, the elongation of hypocotyls in response to auxin was impaired in the mutant. Remarkably, a significant proportion of auxin inducible genes and of targets of the AUXIN RESPONSE FACTOR 6 are regulated by TCP15 and often contain putative TCP recognition motifs in their promoters. Furthermore, we demonstrated that several among them are recognized by TCP15 in vivo. Our results indicate that TCP14 and TCP15 are required for an efficient elongation response to auxin, most likely by regulating a subset of auxin inducible genes related to cell expansion.

Class-I TCP Transcription Factors Activate the SAUR63 Gene Subfamily in Gibberellin-Dependent Stamen Filament Elongation.

In autogamous plants like Arabidopsis (Arabidopsis thaliana), stamen filament elongation must be finely regulated to ensure that anthers reach the pistil at the correct developmental stage. In this work, we studied the roles of Arabidopsis TEOSINTE BRANCHED1, CYCLOIDEA, PCF15 (TCP15), and related class-I TCP transcription factors in stamen filament elongation. Plants with decreased expression of class-I TCPs and plants that express a fusion of TCP15 to a repressor domain (pTCP15::TCP15-EAR) had shorter stamens, indicating that class-I TCPs stimulate filament growth. These plants also showed reduced expression of several SMALL AUXIN UP RNA (SAUR)63 subfamily genes, which contain TCP target motifs in their promoters. Mutational analysis indicated that the TCP target motif in the SAUR63 promoter is required for expression of SAUR63 in stamen filaments. Moreover, TCP15 directly binds to the SAUR63 promoter region that contains the TCP target motif in vivo, highlighting the role of the TCPs in this process. Class-I TCPs are also required for the induction of SAUR63 subfamily genes by gibberellins (GAs). In addition, overexpression of SAUR63 restores filament growth in pTCP15::TCP15-EAR plants, whereas overexpression of TCP15 rescues the short stamen phenotype of GA-deficient plants. The results indicate that TCP15 and related class-I TCPs modulate GA-dependent stamen filament elongation by direct activation of SAUR63 subfamily genes through conserved target sites in their promoters. This work provides insight into GA-dependent stamen filament elongation.