Increased Light Quantity Promotes and Adjusts the Growth of Leaves

One final aspect worth considering, the control of the final morphology and internal anatomy of the light-capturing organ, the leaf, by light itself, is being discussed separately (Ferjani et al., this volume). However, we would like to consider here the multiplicity of light sensors in relation to separate aspects of growth control.

Under high fluence rate light, it is well known that the leaf cellular anatomy adapts by forming a thick palisade, made of multiple cell layers, with cells which are anticlinally elongated (perpendicular to the leaf surface). This, therefore, involves both the control of cell divisions and of cell expansion. The ultimate outcome is the generation of a leaf anatomy in which internal shading is high, in which many chloroplasts can position themselves parallel to the direction of the light (minimising its absorption), and in which a number of chloroplasts are exposed to light of a non-photodamaging intensity (Walters 2005). In contrast, under low light, the existing biomass is organised as a thin leaf blade so as to maximise its efficiency in capturing the available irradiance.

These palisade cell responses are under the control of the irradiance of light itself. Interestingly, it has been known for some time that the establishment of a palisade cell morphology has two prerequisites: the presence of blue light (Schuerger et al. 1997, see Fig. 3a) and the presence of functional chloroplasts (Chatterjee et al. 1996, see Fig. 3b). The specificity for blue wavelengths argues for a photomorphogenic light sensor. The nature of that sensor remains, however, elusive, as neither cryptochromes (Weston et al. 2000) nor phototropins alone (Lopez-Juez et al. 2007) appear to be essential. Meanwhile the examination of adjacent green and albino sectors of a variegated mutant has revealed that, while palisade cell elongation was still observable, albeit reduced, in albino sectors, palisade cellular proliferation absolutely requires the presence of green chloroplasts (Tan et al. 2007, see Fig. 3b). In other words, plastids themselves are the photoreceptors for the cell proliferation response of the palisade. Interestingly, the signal for such behaviour is systemic (Yano and Terashima 2001). A candidate signal is photosynthate itself, in the form of mobile sugars. Sophisticated sugar sensory mechanisms are be-

Fig. 3 Multiple light signals control palisade cell division and expansion. A Top row: Ara-bidopsis wild-type plants grown under elevated irradiance of blue or red light. Middle row: sections through leaves from plants corresponding to those shown on the top row (at a fluence rate of 200 |imolm-2 s-1). Bottom row: sections through leaves grown as shown above, but at a reduced fluence rate (15 |imolm-2 s-1). Reproduced from Lopez-Juez et al. (2007), with permission. B Top: Arabidopsis chm1 variegated mutant plant grown under elevated (600 |imolm-2 s-1) fluence rate of white light. Middle: section through leaf from a plant identical to the one above. A green sector is on the right and a white sector on the left. Bottom: section through a chm1 leaf grown under reduced fluence rate (60 iimolm-2 s-1) of white light. Green sectors are on the left, white sectors on the right. Note the increased cell number under high light only in the green sector. Reproduced from Tan et al. (2007), with permission. Scale bar: 100 |im

Fig. 3 Multiple light signals control palisade cell division and expansion. A Top row: Ara-bidopsis wild-type plants grown under elevated irradiance of blue or red light. Middle row: sections through leaves from plants corresponding to those shown on the top row (at a fluence rate of 200 |imolm-2 s-1). Bottom row: sections through leaves grown as shown above, but at a reduced fluence rate (15 |imolm-2 s-1). Reproduced from Lopez-Juez et al. (2007), with permission. B Top: Arabidopsis chm1 variegated mutant plant grown under elevated (600 |imolm-2 s-1) fluence rate of white light. Middle: section through leaf from a plant identical to the one above. A green sector is on the right and a white sector on the left. Bottom: section through a chm1 leaf grown under reduced fluence rate (60 iimolm-2 s-1) of white light. Green sectors are on the left, white sectors on the right. Note the increased cell number under high light only in the green sector. Reproduced from Tan et al. (2007), with permission. Scale bar: 100 |im ing unravelled (Moore et al. 2003) and, as discussed above (see also Ferjani et al., this volume), they could mediate important aspects of light-regulated growth.

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