Identification and functional analysis of proteins in response to light intensity, temperature and water potential in Brassica rapa hypocotyl

The combined effects of light intensity, temperature, and water potential on wall deposition in regulating hypocotyl elongation of Brassica rapa

Hypocotyl elongation is a critical sign of seed germination and seedling growth, and it is regulated by multi-environmental factors. Light, temperature, and water potential are the major environmental stimuli, and their regulatory mechanism on hypocotyl growth has been extensively studied at molecular level. However, the converged point in signaling process of light, temperature, and water potential on modulating hypocotyl elongation is still unclear. In the present study, we found cell wall was the co-target of the three environmental factors in regulating hypocotyl elongation by analyzing the extension kinetics of hypocotyl and the changes in hypocotyl cell wall of Brassica rapa under the combined effects of light intensity, temperature, and water potential. The three environmental factors regulated hypocotyl cell elongation both in isolation and in combination. Cell walls thickened, maintained, or thinned depending on growth conditions and developmental stages during hypocotyl elongation. Further analysis revealed that the imbalance in wall deposition and hypocotyl elongation led to dynamic changes in wall thickness. Low light repressed wall deposition by influencing the accumulation of cellulose, hemicellulose, and pectin; high temperature and high water potential had significant effects on pectin accumulation overall. It was concluded that wall deposition was tightly controlled during hypocotyl elongation, and low light, high temperature, and high water potential promoted hypocotyl elongation by repressing wall deposition, especially the deposition of pectin.

Piriformospora indica enhances freezing tolerance and post-thaw recovery in Arabidopsis by stimulating the expression of CBF genes

The root endophytic fungus Piriformospora indica plays an important role in increasing abiotic stress tolerance of its host plants. To explore the impact of P. indica on freezing tolerance, Arabidopsis seedlings were co-cultivated with P. indica exposed to -6°C for 6 h. Freezing stress decreased the survival rate, electrolyte leakage, leaf temperature, water potential and chlorophyll fluorescence of Arabidopsis plants in comparison to the controls. P. indica colonizion reduced the negative effects of freezing, and the plants contained also higher amounts of soluble proteins, proline and ascorbic acid during the post-thaw recovery period (4°C; 12 h). In contrast, the H₂O₂ and malondialdehyde levels were reduced in seedlings colonized by the fungus. The brassinolide (BR) and abscisic acid (ABA) levels dramatically increased and the transcript levels of several crucial freezing-stress related genes (CBFs, CORs, BZR1, SAG1 and PYL6) were higher in inoculated plants during the post-thaw recovery period. Finally, inocculated mutants impaired in the freezing tolerance response (such as ice1 for INDUCER OF CBF EXPRESSION1, a crucial basic helix-loop-helix transcription factor for the cold-response pathway in Arabidopsis, cbf1, -2, -3 for C-REPEAT-Binding Factor, cor47 and -15 for COLD-REGULATED and siz1 encoding the SUMO E3 LIGASE) showed better survival rates and higher expression levels of freezing-related target genes after freezing compared to the inocculated controls. Our results demonstrate that P. indica confers freezing tolerance and better post-thaw recovery in Arabidopsis, and stimulates the expression of several genes involved in the CBF-dependent pathway.