Light Promotes Leaf Initiation and Cotyledon and Leaf Blade Expansion

One of the most remarkable aspects of the growth control by light is that it can simultaneously operate in opposite directions in adjacent organs. The transition from skotomorphogenesis to photomorphogenesis, in particular, brings about the expansion of the embryonic cotyledons and the reactivation of the shoot apical meristem (SAM). For example, a combined deficiency in the two major phytochromes, phyA and phyB, and both cryptochromes leads to extraordinarily long hypocotyls in the light, but also to a dramatic delay in leaf initiation (Mazzella et al. 2001). In spite of their importance, among the many light responses, we know little about how photoreceptors control cotyledon expansion and particularly the activity of the SAM.

From the little data available, we do know that the light signal involves, besides the COP/DET group of repressors, the HY5 transcription factor, since its mutant phenotype affects both hypocotyls and cotyledons in same the way that light does. In contrast, the majority of other genes identified as potential primary light targets, through their rapidly regulated expression, appear to have unidirectional effects, i.e. their deletion either causes no phenotype or leads to increased or decreased size of both hypocotyls and cotyledons (Khanna et al. 2006).

An unexpected, interesting observation is the fact that making sucrose directly available to the shoot apex, either by growth in liquid medium or by contact on vertical tissue-culture plates, causes reactivation of the SAM (Araki and Komeda 1993; Roldan et al. 1999). The meristem then gives rise to fully formed leaves, albeit with a very exaggerated petiole and extremely reduced leaf blade. How this sugar response relates to the "natural" light response is unknown, but it is known that loss of a sugar sensor, a specific hexokinase, leads to diminished, rather than enhanced, leaf blade expansion at increased light intensities (Moore et al. 2003). Clearly the potential role of sugar sensing in controlling light responses deserves further attention.

Other information on how light promotes leaf initiation is extremely limited. GAs are potential players in the response, since both GAs and GRFs play roles in leaf blade expansion and in conferring determinacy to the expanding tissue (Hay et al. 2002; Kim et al. 2003). Genetic interference with auxin and brassinosteroid responses also causes some (Tian et al. 2002) or much (Li et al. 1996) reactivation of the shoot apex, but a clear picture is far from emerging. In contrast, downstream agents of the growth response are starting to emerge from transcriptomics data. Earlier gene expression studies analysing responses to light used whole seedlings, this complicating the interpretation of any result from the growth perspective (see Jiao et al. 2007). One study (Ma et al. 2005) for the first time used a spotted oligo microarray to establish the responses to light of dissected cotyledons, hypocotyls and roots. While no key regulators emerged immediately, it was evident that a number of cell wall expansion proteins, including members of the expansin and the xyloglucan endotransglucosylase families, were differentially, sometimes oppositely, regulated by light in cotyledons and hypocotyls. We have recently embarked on a similar analysis of light responses in specific seedling organs, concentrating on the shoot apex and the cotyledons (Lopez-Juez, Dillon, Bog-re and Shanahan, unpublished data). Our observations suggest that, while many previously known light-responsive genes follow similar regulation in both organs, changes specific to the shoot apex can also be observed, including rapid down-regulation of RING-finger ubiquitin ligases, and a rapid, transient loss of auxin and ethylene and a gain of cytokinin and GA responses in shoot meristems, preceding or accompanying the time of leaf primor-dia initiation. Our data also show a large, coordinated promotion of cellular cytoplasmic growth and of cell cycle genes under the control of this environmental cue.

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