Temperature ( T ) and water potential ( y ) are two primary environmental regulators of seed germination. Seeds exhibit a base or minimum T for germination ( T b ), an optimum T at which germination is most rapid ( T o ), and a maximum or ceiling T at which germination is prevented ( T c ). Germination at suboptimal T can be characterized on the basis of thermal time, or the T in excess of T b multiplied by the time to a given germination percentage ( t g ). Similarly, germination at reduced y can be characterized on a hydrotime basis, or t g multiplied by the y in excess of a base or threshold y that just prevents germination ( y b ). Within a seed population, the variation in thermal times to germination among different seed fractions ( g ) is based on a normal distribution of y b values among seeds ( y b ( g )). Germination responses across a range of suboptimal T and y can be described by a general hydrothermal time model that combines the T and y components, but this model does not account for the decrease in germination rates and percentages when T exceeds T o . We report here that supra-optimal temperatures shift the ψ ψ ψ ψ b ( g ) distribution of a potato ( Solanum tuberosum L.) seed population to more positive values, explaining why both germination rates and percentages are reduced as T increases above T o . A modified hydrothermal time model incorporating changes in ψ ψ ψ ψ b ( g ) at T > T o describes germination timing and percentage across all T and ψ ψ ψ ψ at which germination can occur and provides physiologically relevant indices of seed behaviour.

.)Raffinose family oligosaccharides (RFOs) have been implicated in mitigating the effects of environmental stresses on plants. In seeds, proposed roles for RFOs include protecting cellular integrity during desiccation and/or imbibition, extending longevity in the dehydrated state, and providing substrates for energy generation during germination. A gene encoding galactinol synthase (GOLS), the first committed enzyme in the biosynthesis of RFOs, was cloned from tomato (Lycopersicon esculentum Mill. cv Moneymaker) seeds, and its expression was characterized in tomato seeds and seedlings. GOLS (LeGOLS-1) mRNA accumulated in developing tomato seeds concomitant with maximum dry weight deposition and the acquisition of desiccation tolerance. LeGOLS-1 mRNA was present in mature, desiccated seeds but declined within 8 h of imbibition in wild-type seeds. However, LeGOLS-1 mRNA accumulated again in imbibed seeds prevented from completing germination by dormancy or water deficit. Gibberellin-deficient (gib-1) seeds maintained LeGOLS-1 mRNA amounts after imbibition unless supplied with gibberellin, whereas abscisic acid (ABA) did not prevent the loss of LeGOLS-1 mRNA from wild-type seeds. The presence of LeGOLS-1 mRNA in ABA-deficient (sitiens) tomato seeds indicated that wild-type amounts of ABA are not necessary for its accumulation during seed development. In all cases, LeGOLS-1 mRNA was most prevalent in the radicle tip. LeGOLS-1 mRNA accumulation was induced by dehydration but not by cold in germinating seeds, whereas both stresses induced LeGOLS-1 mRNA accumulation in seedling leaves. The physiological implications of LeGOLS-1 expression patterns in seeds and leaves are discussed in light of the hypothesized role of RFOs in plant stress tolerance.The raffinose family oligosaccharides (RFOs) are soluble galactosyl-Suc carbohydrates that constitute a significant component of phloem-transported sugars in some plants (Haritatos et al., 2000). Their accumulation in plants is associated with stressful environmental conditions such as cold, heat, or dehydration (Santarius, 1973; Santarius and Milde, 1977; Hinesley et al., 1992; Ashworth et al., 1993; Wiemken and Ineichen, 1993; Bachmann et al., 1994; Taji et al., 2002). The potential role of RFOs in stress tolerance has been intensively studied in seeds, particularly with respect to desiccation tolerance and longevity in the dehydrated state. RFOs are abundant in most mature desiccation-tolerant ("orthodox") seeds and are often rare or absent in "recalcitrant" seeds that cannot withstand desiccation (Lin and Huang, 1994; Sun et al., 1994). In seeds of many species, RFO accumulation coincides with the development of desiccation tolerance during seed maturation (Koster and Leopold, 1988; Leprince et al., 1993; Bewley and Black, 1994; Horbowicz and Obendorf, 1994; Black et al., 1996; Brenac et al., 1997a Brenac et al., , 1997b, and RFO content has been positively correlated with seed longevity in storage (Bernal-Lugo and Leopold, 1992; Horbowicz and Obendorf, 1994; Lin ...

ARGONAUTE proteins (AGOs) are known to be key components of the RNA silencing mechanism in eukaryotes that, among other functions, serves to protect against viral invaders. Higher plants encode at least 10 individual AGOs yet the role played by many in RNA silencing-related antiviral defense is largely unknown, except for reports that AGO1, AGO2, and AGO7 play an antiviral role in Arabidopsis (Arabidopsis thaliana). In the plant virus model host Nicotiana benthamiana, Tomato bushy stunt virus (TBSV) P19 suppressor mutants are very susceptible to RNA silencing. Here, we report that a N. benthamiana AGO (NbAGO) with similarity to Arabidopsis AGO2, is involved in antiviral defense against TBSV. The activity of this NbAGO2 is shown to be directly associated with anti-TBSV RNA silencing, while its inactivation does not influence silencing of transiently expressed transgenes. Thus, the role of NbAGO2 might be primarily for antiviral defense.