Few organisms are able to withstand desiccation stress; however, desiccation tolerance is widespread among plant seeds. Survival without water relies on an array of mechanisms, including the accumulation of stress proteins such as the late embryogenesis abundant (LEA) proteins. These hydrophilic proteins are prominent in plant seeds but also found in desiccation-tolerant organisms. In spite of many theories and observations, LEA protein function remains unclear. Here, we show that LEAM, a mitochondrial LEA protein expressed in seeds, is a natively unfolded protein, which reversibly folds into a-helices upon desiccation. Structural modeling revealed an analogy with class A amphipathic helices of apolipoproteins that coat low-density lipoprotein particles in mammals. LEAM appears spontaneously modified by deamidation and oxidation of several residues that contribute to its structural features. LEAM interacts with membranes in the dry state and protects liposomes subjected to drying. The overall results provide strong evidence that LEAM protects the inner mitochondrial membrane during desiccation. According to sequence analyses of several homologous proteins from various desiccationtolerant organisms, a similar protection mechanism likely acts with other types of cellular membranes.

Seeds are in a natural oxidative context leading to protein oxidation. Although inevitable for proper progression from maturation to germination, protein oxidation at high levels is detrimental and associated with seed aging. Oxidation of methionine to methionine sulfoxide is a common form of damage observed during aging in all organisms. This damage is reversible through the action of methionine sulfoxide reductases (MSRs), which play key roles in lifespan control in yeast and animal cells. To investigate the relationship between MSR capacity and longevity in plant seeds, we first used two Medicago truncatula genotypes with contrasting seed quality. After characterizing the MSR family in this species, we analyzed gene expression and enzymatic activity in immature and mature seeds exhibiting distinct quality levels. We found a very strong correlation between the initial MSR capacities in different lots of mature seeds of the two genotypes and the time to a drop in viability to 50% after controlled deterioration. We then analyzed seed longevity in Arabidopsis thaliana lines, in which MSR gene expression has been genetically altered, and observed a positive correlation between MSR capacity and longevity in these seeds as well. Based on our data, we propose that the MSR repair system plays a decisive role in the establishment and preservation of longevity in plant seeds.seed deterioration | seed viability | redox systems | antioxidant enzymes S uccessful crop implantation is crucial for maximal plant yield and relies on seed physiological quality, which is acquired at the end of development, during maturation. Quality is expressed by the capacity of seeds to germinate in a fast and homogenous manner before and after storage (seed longevity) and to provide proper seedling establishment in various environments (seed vigor). Quality is controlled by genetic traits and varies greatly among species and cultivars. The establishment of quality also depends on the prevailing environmental conditions during seed development and storage (1, 2).During the maturation phase, mechanisms are put in place to prevent and repair desiccation damage, as well as to allow survival of seeds in the dry state (3-5). Nonetheless, drying of seeds on mother plants can occur prematurely in drought conditions, leading to low-quality seeds (6-8). Thus, in the context of a warming climate, producing good-quality seeds represents a global challenge to sustain current yield levels (9, 10).The mechanisms underlying seed quality are controlled in part by abscisic acid (ABA), the phytohormone involved in plant responses to dehydration that is produced during seed maturation. Of note, ABA triggers the accumulation of late-embryogenesis abundant proteins, heat-shock proteins (HSPs), and storage proteins (11). Late-embryogenesis abundant proteins and HSPs are thought to primarily protect cell structures when water is removed. Reserve proteins, through their sensitivity to reactive oxygen species (ROS), play a determinant role in limiting oxidative damage...