bHLH transcription factors LP1 and LP2 regulate longitudinal cell elongation.

Basic helix-loop-helix/helix-loop-helix (bHLH/HLH) transcription factors play substantial roles in plant cell elongation. In this study, two bHLH/HLH homologous proteins leaf related protein 1 and leaf-related protein 2 (AtLP1 and AtLP2) were identified in Arabidopsis thaliana. LP1 and LP2 play similar positive roles in longitudinal cell elongation. Both LP1 and LP2 overexpression plants exhibited long hypocotyls, elongated cotyledons, and particularly long leaf blades. The elongated leaves resulted from increased longitudinal cell elongation. lp1 and lp2 loss-of-function single mutants did not display distinct phenotypes, but the lp1lp2 double mutant showed decreased leaf length associated with less longitudinal polar cell elongation. Furthermore, the phenotype of lp1lp2 could be rescued by the expression of LP1 or LP2. Expression of genes related to cell elongation was upregulated in LP1 and LP2 overexpression plants but downregulated in lp1lp2 double mutant plants compared with that of wild type. LP1 and LP2 proteins could directly bind to the promoters of Longifolia1 (LNG1) and LNG2 to activate the expression of these cell elongation related genes. Both LP1 and LP2 could interact with two other bHLH/HLH proteins, IBH1 (ILI1 binding BHLH Protein1) and IBL1 (IBH1-like1), thereby suppressing the transcriptional activation of LP1 and LP2 to the target genes LNG1 and LNG2. Thus, our data suggested that LP1 and LP2 act as positive regulators to promote longitudinal cell elongation by activating the expression of LNG1 and LNG2 genes in Arabidopsis. Moreover, homodimerization of LP1 and LP2 may be essential for their function, and interaction between LP1/LP2 and other bHLH/HLH proteins may obstruct transcriptional regulation of target genes by LP1 and LP2.

Lu R ，Zhang J ，Wu YW ，Wang Y ，Zhang J ，Zheng Y ，Li Y ，Li XB ... -  《-》

Antagonistic HLH/bHLH transcription factors mediate brassinosteroid regulation of cell elongation and plant development in rice and Arabidopsis.

Zhang LY ，Bai MY ，Wu J ，Zhu JY ，Wang H ，Zhang Z ，Wang W ，Sun Y ，Zhao J ，Sun X ，Yang H ，Xu Y ，Kim SH ，Fujioka S ，Lin WH ，Chong K ，Lu T ，Wang ZY ... -  《-》

LONGIFOLIA1 and LONGIFOLIA2, two homologous genes, regulate longitudinal cell elongation in Arabidopsis.

Lee YK ，Kim GT ，Kim IJ ，Park J ，Kwak SS ，Choi G ，Chung WI ... -  《-》

A triantagonistic basic helix-loop-helix system regulates cell elongation in Arabidopsis.

Ikeda M ，Fujiwara S ，Mitsuda N ，Ohme-Takagi M ... -  《-》

The bHLH/HLH transcription factors GhFP2 and GhACE1 antagonistically regulate fiber elongation in cotton.

Basic helix-loop-helix/helix-loop-helix (bHLH/HLH) transcription factors play important roles in cell elongation in plants. However, little is known about how bHLH/HLH transcription factors antagonistically regulate fiber elongation in cotton (Gossypium hirsutum). In this study, we report that two bHLH/HLH transcription factors, fiber-related protein 2 (GhFP2) and ACTIVATOR FOR CELL ELONGATION 1 (GhACE1), function in fiber development of cotton. GhFP2 is an atypical bHLH protein without the basic region, and its expression is regulated by brassinosteroid (BR)-related BRASSINAZOLE RESISTANT 1 (GhBZR1). Overexpression of GhFP2 in cotton hindered fiber elongation, resulting in shortened fiber length. In contrast, suppression of GhFP2 expression in cotton promoted fiber development, leading to longer fibers compared with the wild-type. GhFP2 neither contains a DNA-binding domain nor has transcriptional activation activity. Furthermore, we identified GhACE1, a bHLH protein that interacts with GhFP2 and positively regulates fiber elongation. GhACE1 could bind to promoters of plasma membrane intrinsic protein 2;7 (GhPIP2;7) and expansions 8 (GhEXP8) for directly activating their expression, but the interaction between GhFP2 and GhACE1 suppressed transcriptional activation of these target genes by GhACE1. Taken together, our results indicate that GhACE1 promotes fiber elongation by activating expressions of GhPIP2;7 and GhEXP8, but its transcription activation on downstream genes may be obstructed by BR-modulated GhFP2. Thus, our data reveal a key mechanism for fiber cell elongation through a pair of antagonizing HLH/bHLH transcription factors in cotton.

Lu R ，Li Y ，Zhang J ，Wang Y ，Zhang J ，Li Y ，Zheng Y ，Li XB ... -  《-》