Plant responses to red and far-red lights, applications in horticulture

Gravitropic orientation and the elongation of etiolated hypocotyls are both regulated by red light through the phytochrome family of photoreceptors. The importance of phytochromes A and B (phyA and phyB) in these red light responses has been established through studies using phy mutants. To identify the roles that phytochromes play in gravitropism and elongation of roots, we studied the effects of red light on root elongation and then compared the gravitropic curvature from roots of phytochrome mutants of Arabidopsis (phyA, phyB, phyD and phyAB) with wild type. We found that red light inhibits root elongation approximately 35% in etiolated seedlings and that this response is controlled by phytochromes. Roots from dark- and light-grown double mutants (phyAB) and light-grown phyB seedlings have reduced elongation rates compared with wild type. In addition, roots from these seedlings (dark/light-grown phyAB and light-grown phyB) have reduced rates of gravitropic curvature compared with wild type. These results demonstrate roles for phytochromes in regulating both the elongation and gravitropic curvature of roots.

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The Roles of Phytochromes in Elongation and Gravitropism of Roots

Phytochrome B (phyB) can adjust morphological and physiological responses according to changes in the red:far-red (R:FR) ratio. phyB-driven acclimation of plants to open environments (high R:FR ratio) increases carbon gain at the expense of increased water loss. This behaviour alleviates stressful conditions generated by an excess of light, but increases the chances of desiccation. Here we evaluated how phyB modulates this drought-tolerance response by comparing wild-type Arabidopsis thaliana adult plants to the null phyB in response to water shortage. phyB wilted before the wild type, and this was due to phyB maintaining open stomata under a reduction in soil water availability. Although phyB presented enhanced ABA levels under well-watered conditions, this mutant was less sensitive than the wild type in diminishing stomatal conductance in response to exogenous ABA application. Reduced sensitivity to ABA in phyB correlated with a lower expression of ABCG22, which encodes a putative ABA influx transporter, and PYL5, which encodes a soluble ABA receptor. Furthermore, the expression of RAB18 and RD29A, both typical ABA-induced genes, was lower in phyB than the wild type after ABA treatment. We propose that phyB contributes to the acclimation of plants to open environments by enhancing ABA sensitivity when soil water becomes limiting.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2012.02529.x

Phytochrome B increases drought tolerance by enhancing ABA sensitivity in Arabidopsis thaliana

To increase their fitness, plants sense ambient light conditions and modulate their developmental processes by utilizing multiple photoreceptors such as phytochrome, cryptochrome and phototropin. Even roots, which are normally not exposed to light, express photoreceptors and can respond to light by developing chloroplasts. In the present study, root greening was observed in Arabidopsis thaliana. Seedlings were grown under monochromatic light and chlorophyll levels in the roots were determined. It was found that blue light was far more effective at inducing chloroplast development in Arabidopsis roots than was red light, and this response was under the control of a strong synergistic interaction between phytochromes and cryptochromes. As expected, the cry1 mutant was deficient in this response. Interestingly, the phyAphyB double mutant failed to respond to blue light under these conditions. This strongly suggests that either phytochrome A or phytochrome B, in addition to cryptochrome, was required for this blue light response. It was further demonstrated that the expression of photosynthetic genes was regulated in the same way. Dichromatic irradiation experiments indicated that this interaction depends on the level of phyB P(FR). Analysis of the cop1, det1 and hy5 mutants indicated that the corresponding factors were involved in the response.

/pdf/cryptochromes-and-phytochromes-synergistically-regulate-33r43ogvwu.pdf

Cryptochromes and Phytochromes Synergistically Regulate Arabidopsis Root Greening under Blue Light

Manganese (Mn) is the second most prevalent transition metal in the Earth's crust but its availability is often limited due to rapid oxidation and low mobility of the oxidized forms. Acclimation to low Mn availability was studied in Arabidopsis seedlings subjected to Mn deficiency. As reported here, Mn deficiency caused a thorough change in the arrangement and characteristics of the root epidermal cells. A proportion of the extra hairs formed upon Mn deficiency were located in atrichoblast positions, indicative of a post-embryonic reprogramming of the cell fate acquired during embryogenesis. When plants were grown under a light intensity of >50 μmol m−2 s−1 in the presence of manganese root hair elongation was substantially inhibited, whereas Mn-deficient seedlings displayed stimulated root hair development. GeneChip analysis revealed several candidate genes with potential roles in the reprogramming of rhizodermal cells. None of the genes that function in epidermal cell fate specification were affected by Mn deficiency, indicating that the patterning mechanism which controls the differentiation of rhizodermal cells during embryogenesis have been bypassed under Mn-deficient conditions. This assumption is supported by the partial rescue of the hairless cpc mutant by Mn deficiency. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) analysis revealed that, besides the anticipated reduction in Mn concentration, Mn deficiency caused an increase in iron concentration. This increase was associated with a decreased transcript level of the iron transporter IRT1, indicative of a more efficient transport of iron in the absence of Mn.

/pdf/manganese-deficiency-alters-the-patterning-and-development-16ofy5ps1v.pdf

Manganese deficiency alters the patterning and development of root hairs in Arabidopsis

lacking phytochrome A, phytochrome B or both (double mutant) were analyzed by comparing their photoresponse with that of the wild type. Results indicate that root hair formation in Arabidopsis was strongly stimulated by light irradiation. Both phytochrome A and phytochrome B are responsible for photoinduction by continuous red light irradiation, while only phytochrome A mediates the response under continuous far-red light. The fluence response relationships to a red light pulse in the wild type displayed a biphasic trend similar to that previously observed in lettuce seedlings, with the first phase showing a sharp maximum at 78.3 Jm−2, and the second one operating over a wider fluence range (3,100–9,400 Jm−2) two orders of magnitude higher than the first one. Analysis of the fluence response curves for red light induction in the phytochrome mutants revealed that phytochrome A is responsible for the first phase in the wild type, while the second is the result of the combined action of both phytochrome A and phytochrome B.

Effect of light on root hair formation in Arabidopsis thaliana phytochrome-deficient mutants.

. The inductive effect of 100 Jm−2 red light could be partially reversed by subsequent far-red light only one time. On the other hand, the inductive effect of 1,000 Jm−2 red light was partially reversed by subsequent far-red light irradiation at least twice. These results indicate the involvement of phytochrome in this response. The inductive effect of blue light was repeatedly reversed by subsequent far-red light irradiation, suggesting that the blue-light induction was mainly mediated by phytochrome.

Evidence of Phytochrome Mediation in the Low-pH-Induced Root Hair Formation Process in Lettuce (Lactuca sativa L. cv. Grand Rapids) Seedlings

L. cv. Grand Rapids) seedlings under acidic conditions by a phytochrome-mediated response. Microbeam irradiation of 1 mm root segments with the first (100 Jm−2) and second (1,000 Jm−2) maxima of the fluence response curve for red light induction of root hair initiation indicated that the ca. 5 mm apical portion of 12 mm long roots was the site of photoperception. The root hair-forming portion of the root was situated at a distance of 1.7 mm from the root tip at the time of red light irradiation and extended (at most 1.5 mm) towards the basal end at a later stage of development, irrespective of which portion of the root was irradiated with red light.

Silvia De Simone

Papers

Effect of light on root hair formation in Arabidopsis thaliana phytochrome-deficient mutants.

Evidence of Phytochrome Mediation in the Low-pH-Induced Root Hair Formation Process in Lettuce (Lactuca sativa L. cv. Grand Rapids) Seedlings

Photoperceptive Site of the Photoinduction of Root Hairs in Lettuce (Lactuca sativa L. cv. Grand Rapids) Seedlings Under Low pH Conditions