Plant proximity perception dynamically modulates hormone levels and sensitivity in Arabidopsis

Abstract

The shade avoidance syndrome (SAS) refers to a set of plant responses initiated after perception by the phytochromes of light enriched in far-red colour reflected from or filtered by neighbouring plants. These varied responses are aimed at anticipating eventual shading from potential competitor vegetation. In Arabidopsis thaliana, the most obvious SAS response at the seedling stage is the increase in hypocotyl elongation. Here, we describe how plant proximity perception rapidly and temporally alters the levels of not only auxins but also active brassinosteroids and gibberellins. At the same time, shade alters the seedling sensitivity to hormones. Plant proximity perception also involves dramatic changes in gene expression that rapidly result in a new balance between positive and negative factors in a network of interacting basic helix-loop-helix proteins, such as HFR1, PAR1, and BIM and BEE factors. Here, it was shown that several of these factors act as auxin- and BR-responsiveness modulators, which ultimately control the intensity or degree of hypocotyl elongation. It was deduced that, as a consequence of the plant proximity-dependent new, dynamic, and local balance between hormone synthesis and sensitivity (mechanistically resulting from a restructured network of SAS regulators), SAS responses are unleashed and hypocotyls elongate.

Keywords: Arabidopsis; auxins; brassinosteroids; gibberellins; hypocotyl elongation; plant proximity; shade avoidance syndrome..

Fig. 1.

Analysis of hormone levels in wild-type seedlings treated with simulated shade. Seedlings were germinated and grown for 7 d under W and then either kept in W or transferred to W+FR for 4h (A) or 24h (B). Values are means ±standard error (SE) of three experiments; FW, fresh weight. Different letters denote significant differences (P<0.01 for IAA levels; P<0.05 for CS levels; and P<0.1 for GA₄ levels) among means.

Fig. 2.

Effect of simulated shade on the hypocotyl response to hormone application. Wild-type seedlings were germinated and grown for 2 d under W and then either kept in W or transferred to W+FR for 5 more days. (A) Medium was supplemented with different concentrations of PIC, EBL, or GA₃. (B) Hypocotyl length of seedlings grown as depicted in (A) was measured for each treatment. (C) Medium was supplemented with 0.5 µM of PCZ or 1 µM PAC with different concentrations of EBL or GA₃. (D) Hypocotyl length of seedlings grown as depicted in (C) was measured for each treatment. In (B) and (D), values are means ±SE. Black asterisks indicate significant differences (Student’s t-test) relative to the corresponding W-grown controls; red asterisks indicate significant differences (two-way ANOVA) between W- and W+FR-grown seedlings in response to the corresponding hormone applied (*P<0.05, **P<0.01). (This figure is available in colour at JXB online.)

Fig. 3.

Merging of microarray data from shade-regulated and IAA-regulated genes. (A) Venn diagrams illustrating the overlap between the rapidly downregulated (left) and upregulated (right) group of genes in wild-type and sav3 mutant seedlings in response to 1h of simulated shade (Tao et al., 2008), as listed in Table S2. The total number of genes in each group is indicated in parentheses. Comparisons between genotypes defined two classes of robust shade-regulated genes: shade downregulated (class SD) and shade upregulated (class SU), as listed in Table S3. The numbers of genes in each sector is indicated. (B) Venn diagrams illustrating the overlap between the class SD (left), class SU (right), and class of IAA upregulated (top) and downregulated (bottom) genes. The comparison between the different groups of genes defines the class S+A (Table S3).

Fig. 4.

Hypocotyl response of seedlings with altered levels of HFR1 to hormone application. Hypocotyl length of wild-type (wt), hfr1-5 (A) or truncated HFR1 overexpressor (B) seedlings germinated and grown under W for 7 d on medium supplemented with increasing concentrations of PIC (upper panels) or EBL (lower panels). Hypocotyl length was measured for each line and treatment. Values are means ±SE. Black asterisks indicate significant differences (Student’s t-test) relative to the corresponding wild-type plants; red asterisks indicate significant differences (two-way ANOVA) between genotypes in response to the corresponding hormone applied (*P<0.05, **P<0.01). (This figure is available in colour at JXB online.)

Fig. 5.

Hypocotyl response of seedlings with altered levels of PAR1 or PAR2 to hormone application. Hypocotyl length of wild-type (wt), PAR1-RNAi (A), par2-1 (B), or PAR1 and PAR2 overexpressor (C) seedlings germinated and grown under W for 7 d on medium supplemented with increasing concentrations of PIC (upper panels) or EBL (lower panels). Hypocotyl length was measured for each line and treatment. Values are means ±SE. Black asterisks indicate significant differences (Student’s t-test) relative to the corresponding wild-type plants; red asterisks indicate significant differences (two-way ANOVA) between genotypes in response to the corresponding hormone applied (*P<0.05, **P<0.01). (This figure is available in colour at JXB online.)

Fig. 6.

Hypocotyl response of bee123 and bim123 mutant seedlings to hormone application. Hypocotyl length of wild-type (wt), bee123 (A), or bim123 (B) seedlings germinated and grown under W for 7 d on medium supplemented with increasing concentrations of PIC (upper panels) or EBL (lower panels). Hypocotyl length was measured for each line and treatment. Values are means ±SE. Black asterisks indicate significant differences (Student’s t-test) relative to the corresponding wild-type plants; red asterisks indicate significant differences (two-way ANOVA) between genotypes in response to the corresponding hormone applied (*P<0.05, **P<0.01). (This figure is available in colour at JXB online.)