Phototropism is an adaptive response allowing plants to optimize photosynthetic light capture. This is achieved by asymmetric growth between the shaded and lit sides of the stimulated organ. In grass seedlings, the site of phototropin-mediated light perception is distinct from the site of bending; however, in dicotyledonous plants (e.g., Arabidopsis), spatial aspects of perception remain debatable. We use morphological studies and genetics to show that phototropism can occur in the absence of the root, lower hypocotyl, hypocotyl apex, and cotyledons. Tissue-specific expression of the phototropin1 (phot1) photoreceptor demonstrates that light sensing occurs in the upper hypocotyl and that expression of phot1 in the hypocotyl elongation zone is sufficient to enable a normal phototropic response. Moreover, we show that efficient phototropism occurs when phot1 is expressed from endodermal, cortical, or epidermal cells and that its local activation rapidly leads to a global response throughout the seedling. We propose that spatial aspects in the steps leading from light perception to growth reorientation during phototropism differ between grasses and dicots. These results are important to properly interpret genetic experiments and establish a model connecting light perception to the growth response, including cellular and morphological aspects.

This paper reveals the surprising result that a single-parent non-elitist evolution strategy (ES) can be locally faster than the (1+1)-ES. The result is brought by mirrored sampling and sequential selection. With mirrored sampling, two offspring are generated symmetrically or mirrored with respect to their parent. In sequential selection, the offspring are evaluated sequentially and the iteration is concluded as soon as one offspring is better than the current parent. Both concepts complement each other well. We derive exact convergence rates of the (1, λ)-ES with mirrored sampling and/or sequential selection on the sphere model. The log-linear convergence of the ES is preserved. Both methods lead to an improvement and in combination they can sometimes even double the convergence rate. Naively implemented into the CMA-ES with recombination, mirrored sampling leads to a bias on the step-size. However, the (1,4)-CMA-ES with mirrored sampling and sequential selection is unbiased and appears to be faster, more robust, and as local as the (1+1)-CMA-ES.

Phototropism allows plants to align their photosynthetic tissues with incoming light. The direction of incident light is sensed by the phototropin family of blue light photoreceptors (phot1 and phot2 in Arabidopsis), which are light-activated protein kinases. The kinase activity of phototropins and phosphorylation of residues in the activation loop of their kinase domains are essential for the phototropic response. These initial steps trigger the formation of the auxin gradient across the hypocotyl that leads to asymmetric growth. The molecular events between photoreceptor activation and the growth response are only starting to be elucidated. In this review, we discuss the major steps leading from light perception to directional growth concentrating on Arabidopsis. In addition, we highlight links that connect these different steps enabling the phototropic response.