The understorey origin of coffee trees and the strong plasticity of Coffea arabica leaves in relation to contrasting light environments have been largely shown. The adaptability of coffee leaves to changes in light was tested under controlled conditions by increasing the illumination rate on C. arabica var. Naryelis seedlings acclimated to low light conditions and observing leaf responses at three different developmental stages (juvenile, growing and mature). Only mature leaves proved capable of adapting to new light conditions. In these leaves, different major mechanisms were found to contribute to maintaining a good photosynthetic level. With increased illumination, a high photosynthetic response was conserved thanks to fast nitrogen remobilization, as indicated by SPAD values and the photorespiration rate. Efficient photoprotection was accompanied by a great ability to export sucrose, which prevented excessive inhibition of the Calvin cycle by hexose accumulation. In contrast, in younger leaves, increased illumination caused photodamage, observable even after 9 days of treatment. One major finding was that young coffee leaves rely on the accumulation of chlorogenic acids, powerful antioxidant phenolic compounds, to deal with the accumulation of reactive oxygen species rather than on antioxidant enzymes. Due to a lack of efficient photoprotection, a poor ability to export sucrose and inadequate antioxidant protection, younger leaves seemed to be unable to cope with increased illumination. In these leaves, an absence of induced antioxidant enzyme activity was accompanied, in growing leaves, by an absence of antioxidant synthesis or, in juvenile leaves, inefficient synthesis of flavonoids because located in some epidermis cells. These observations showed that coffee leaves, at the beginning of their development, are not equipped to withstand quick switches to higher light levels. Our results confirm that coffee trees, even selected for full sunlight conditions, remain shade plants possessing leaves able to adapt to higher light levels only when mature.

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Immunofluorescence and cytohistochemical studies have been performed to understand the host-parasite relationships in the pathosystem: peach-Xanthomonas arboricola pv. pruni (Xap). Using a commercial immunodetection kit, Xap cells were specifically identified in tissues from infected leaves and fruits. Sections from infected leaves showed that the pathogen penetrates the mesophyll via stomata and develops in the intercellular spaces where it degrades the cell wall components. This leads to cell collapse and consequently to the formation of necrotic lesions. The same events have been noted in sections from infected fruits. However, the contaminated zones of mesocarp parenchyma exhibited cell dedifferentiation and generated somatic embryo-like structures. Sections from midrib samples collected at different distances from infected lamina revealed the presence of Xap cells in the sieve tubes and xylem suggesting a systemic trafficking of the pathogen. The results are discussed in terms of cytological effects and epidemiology of Xap.

Pepper is known to be a recalcitrant species to genetic transformation via Agrobacterium tumefaciens. A. rhizogenes-mediated transformation offers an alternative and rapid possibility to study gene functions in roots. In our study, we developed a new and efficient system for A. rhizogenes transformation of the cultivated species Capsicum annuum. Hypocotyls and foliar organs (true leaves and cotyledons) of Yolo Wonder (YW) and Criollo de Morelos 334 (CM334) pepper cultivars were inoculated with the two constructs pBIN-gus and pHKN29-gfp of A. rhizogenes strain A4RS. Foliar explants of both pepper genotypes infected by A4RS-pBIN-gus or A4RS-pHKN29-gfp produced transformed roots. Optimal results were obtained using the combination of the foliar explants with A4RS-pHKN29-gfp. 20.5% of YW foliar explants and 14.6% of CM334 foliar explants inoculated with A4RS-pHKN29-gfp produced at least one root expressing uniform green fluorescent protein. We confirmed by polymerase chain reaction the presence of the rolB and gfp genes in the co-transformed roots ensuring that they integrated both the T-DNA from the Ri plasmid and the reporter gene. We also demonstrated that co-transformed roots of YW and CM334 displayed the same resistance response to Phytophthora capsici than the corresponding untransformed roots. Our novel procedure to produce C. annuum hairy roots will thus support the functional analysis of potential resistance genes involved in pepper P. capsici interaction.