Journal club: roots perceive light transmitted through a plant

Light greatly influences plant growth and development. For instance, light is needed to generate energy via photosynthesis, and light cues allow plants to respond to neighbors that might overshadow them in the near future. Roots also respond to light, by changing their response to gravity and their amount of branching. However, these effects were mostly shown in experiment conducted in laboratory settings, where plants are grown on petri dishes and the roots are exposed to light. In contrast, in natural settings light often does not reach the roots, since light only penetrates a few millimeters into the soil.

Lee and colleagues were interested in the effect of light on roots in natural settings. First, they want to check whether exposing a plant to light changes gene expression in the root. They show that both direct root exposure to light and exposure of only the aboveground part of the plant, the so-called shoot, induce changes in gene expression in the root. Thus, in roots of plants that were exposed to light either at the root or at the shoot, different parts of the genetic code were ‘read’ compared with roots of plants grown in the dark. The light-induced changes were not the same in the root versus shoot exposed plants. There is a set of genes that always changes after exposure to light, though, independent of the exposure localization. Among these genes is the gene encoding the protein HY5, which is involved in the response of plants to light. The researchers show that a plant mutated in this gene, that is, a plant without a functional HY5 gene, that is grown in soil is impaired in its root’s natural response to gravity.

A clue about the method of activation of HY5 came from plants mutated in light receptors.  Light receptors are proteins that start signaling networks after absorption of light. A plant that is mutated for the gene encoding the light receptor phytochrome B (phyB), i.e. a plant with a non-functional phyB, activates HY5 much less when the shoot is exposed to light than a wild-type plant. In consequence, the mutants have less HY5 protein in their roots. The researchers then asked themselves whether root- or shoot-localized activatin of phyB results in HY5 activation. To test this, they grafted – joined parts of two plants together to form a ‘new’ plant – both a phyB mutant root with a wild-type shoot and the other way around. They then exposed the shoots of these plants to light and monitored induction of HY5 in the root. Interestingly enough, a mutation in the light receptor in the shoot did not affect HY5 expression in the root, but a mutation in the root prevented HY5 expression. This indicates that perception of light in the root is the cause of HY5 induction.

The researchers considered two possible mechanisms for the induction of phyB and HY5 expression. First, compounds produced in response to light in the shoot might travel to the root to induce a response there. Second, light itself might be transported to the root. The researchers first show that the compounds known to travel through plants to control light-mediated responses do not induce the activation of phyB in roots of dark-grown plants. Next, they checked their light transmission hypothesis. Light was shown on plant segments consisting of both root and shoot tissue with an optic fiber. This light could be detected at the root end of the segment. Thus, light can travel through the plant and could thus be responsible for the activation of phyB in the plant root. If this is true, the search for a signal that ‘informs’ the root of the light-status of the shoot can be stopped: light is transported through plants and can thus activate any necessary responses all by itself.

Lee, H. et al. (2016) Stem-piped light activates phytochrome B to trigger light responses in Arabidopsis thaliana roots. Science Signaling 9, ra106. DOI: 10.1126/scisignal.aaf6530

Disclaimer: blog posts in the category ‘journal club’ are not intended to cover the whole paper discussed. Instead, I discuss the parts that I think are most interesting for a general public. I try my utmost to prevent any mistakes in these blogs, I apologize in advance for any mistakes that I make anyway.

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