SummaryAt harvest, sunflower (Helianthus annuus L.) seeds are dormant and unable to germinate at temperatures below 15°C. Seed storage in the dry state, known as after-ripening, is associated with an alleviation of embryonic dormancy allowing subsequent germination at suboptimal temperatures. To identify the process by which dormancy is broken during after-ripening, we focused on the role of reactive oxygen species (ROS) in this phenomenon. After-ripening entailed a progressive accumulation of ROS, namely superoxide anions and hydrogen peroxide, in cells of embryonic axes. This accumulation, which was investigated at the cellular level by electron microscopy, occurred concomitantly with lipid peroxidation and oxidation (carbonylation) of specific embryo proteins. Incubation of dormant seeds for 3 h in the presence of hydrogen cyanide (a compound that breaks dormancy) or methylviologen (a ROS-generating compound) also released dormancy and caused the oxidation of a specific set of embryo proteins. From these observations, we propose a novel mechanism for seed dormancy alleviation. This mechanism involves ROS production and targeted changes in protein carbonylation patterns.

Main conclusionProvides a first comprehensive review of integrated physiological and molecular aspects of desiccation toleranceXerophyta viscosa. A synopsis of biotechnological studies being undertaken to improve drought tolerance in maize is given.Xerophyta viscosa (Baker) is a monocotyledonous resurrection plant from the family Vellociacea that occurs in summer-rainfall areas of South Africa, Lesotho and Swaziland. It inhabits rocky terrain in exposed grasslands and frequently experiences periods of water deficit. Being a resurrection plant it tolerates the loss of 95 % of total cellular water, regaining full metabolic competency within 3 days of rehydration. In this paper, we review some of the molecular and physiological adaptations that occur during various stages of dehydration of X. viscosa, these being functionally grouped into early and late responses, which might be relevant to the attainment of desiccation tolerance. During early drying (to 55 % RWC) photosynthesis is shut down, there is increased presence and activity of housekeeping antioxidants and a redirection of metabolism to the increased formation of sucrose and raffinose family oligosaccharides. Other metabolic shifts suggest water replacement in vacuoles proposed to facilitate mechanical stabilization. Some regulatory processes observed include increased presence of a linker histone H1 variant, a Type 2C protein phosphatase, a calmodulin- and an ERD15-like protein. During the late stages of drying (to 10 % RWC) there was increased expression of several proteins involved in signal transduction, and retroelements speculated to be instrumental in gene silencing. There was induction of antioxidants not typically found in desiccation-sensitive systems, classical stress-associated proteins (HSP and LEAs), proteins involved in structural stabilization and those associated with changes in various metabolite pools during drying. Metabolites accumulated in this stage are proposed, inter alia, to facilitate subcellular stabilization by vitrification process which can include glass- and ionic liquid formation.

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Craterostigma wilmsii Engl. (homoiochlorophyllous) is a resurrection species that is thought to rely primarily on the protection of cellular components during drying to survive desiccation. The time taken for this protection to be instituted is thought to preclude recovery after rapid drying. Thus the response of C. wilmsii plants to rapid dehydration was investigated. The effect of rapid drying on sucrose accumulation was determined and the cellular ultrastructure was investigated during natural (slow) and rapid dehydration and on subsequent rehydration. The dependence of naturally and rapidly dried C. wilmsii on de novo transcription and translation during and after rehydration was determined by examining quantum efficiency, changes in photosynthetic pigments and subcellular organization of excised leaves with rehydration in water and using the metabolic inhibitors, distamycin A and cycloheximide. Slowly dried C. wilmsii required no new transcription or translation during rehydration in order to recover. With rapid dehydration, cells showed ultrastructural damage, which indicated that at least some protective mechanisms were affected (as evidenced by a reduced accumulation of sucrose). C. wilmsii was able to limit the damage and recover upon rehydration in water, but rapidly dried plants did not survive if mRNA or protein synthesis was inhibited by distamycin A or cycloheximide, respectively. This demonstrates an induction of repair mechanisms during rehydration, which enables recovery from rapid drying. Thus, although C. wilmsii does rely almost entirely on protection during natural drying, it apparently has the ability to repair if protection is inadequate and damage is incurred.

De-etiolation of dark-grown seedlings is a commonly used experimental system to study the mechanisms of chloroplast biogenesis, including the stacking of thylakoid membranes into grana, the response of the nuclear-chloroplast transcriptome to light, and the ordered synthesis and assembly of photosystem II (PSII). Here, we present the xeroplast to chloroplast transition during rehydration of the resurrection plant Xerophyta humilis as a novel system for studying chloroplast biogenesis, and investigate the role of light in this process. Xeroplasts are characterized by the presence of numerous large and small membrane-bound vesicles and the complete absence of thylakoid membranes. While the initial assembly of stromal thylakoid membranes occurs independently of light, the formation of grana is light dependent. Recovery of photosynthetic activity is rapid in plants rehydrated in the light and correlates with the lightdependent synthesis of the D1 protein, but does not require de novo chlorophyll biosynthesis. Light-dependent synthesis of the chlorophyll-binding protein Lhcb2 and digalactosyldiacylglycerol synthase 1 correlated with the formation of grana and with the increased PSII activity. Our results suggest that the molecular mechanisms underlying photomorphogenic development may also function in desiccation tolerance in poikilochlorophyllous resurrection plants.