Living under a 'dormant' canopy: a molecular acclimation mechanism of the desert plant Retama raetam

Desert plants are exposed to a combination of environmental stress conditio ns, including low water availability, extreme temperature fluctuations, hig h irradiance and nutrient deprivation. Studying desert plants within their natural habitat may therefore reveal novel mechanisms and strategies that e nable plants to resist stressful conditions. We studied the acclimation of Retama raetam, an evergreen stem-assimilating desert plant, to growth withi n an arid dune ecosystem. Retama raetam contained two different populations of stems: those of the upper canopy, exposed to direct sunlight, and those of the lower canopy, protected from direct sunlight. During the dry season , stems of the upper canopy contained a very low level of a number of essen tial proteins, including the large and small subunits of rubisco, ascorbate peroxidase and the D1 subunit of the reaction centre of photosystem II. Ho wever, RNA encoding these proteins was present; cytosolic transcripts were associated with polysomes, while chloroplastic transcripts were not. Upon w ater application, as well as following the first rainfall of the season, th ese 'photosynthetically suppressed' stems recovered and accumulated essenti al proteins within 6-24 h. In contrast, stems of the lower canopy contained the essential proteins throughout the dry season. We suggest that R. raeta m uses an acclimation strategy of 'partial plant dormancy' in order to surv ive the dry season. 'Dormancy', as evident by the post-transcriptional supp ression of gene expression, as well as the suppression of photosynthesis, w as induced specifically in stems of the upper canopy which protect the lowe r canopy by shading.