Salt-induced delay in cotyledonary globulin mobilization is abolished by induction of proteases and leaf growth sink strength at late seedling establishment in cashew.

Seedling establishment in saline conditions is crucial for plant survival and productivity. This study was performed to elucidate the biochemical and physiological mechanisms involved with the recovery and establishment of cashew seedlings subjected to salinity. The changes in the Na+ levels and K/Na ratios, associated with relative water content, indicated that osmotic effects were more important than salt toxicity in the inhibition of seedling growth and cotyledonary protein mobilization. Salinity (50mM NaCl) induced a strong delay in protein breakdown and amino acid accumulation in cotyledons, and this effect was closely related to azocaseinolytic and protease activities. In parallel, proline and free amino acids accumulated in the leaves whereas the protein content decreased. Assays with specific inhibitors indicated that the most important proteases in cotyledons were of serine, cysteine and aspartic types. Proteomic analysis revealed that most of the cashew reserve proteins are 11S globulin-type and that these proteins were similarly degraded under salinity. In the late establishment phase, the salt-treated seedlings displayed an unexpected recovery in terms of leaf growth and N mobilization from cotyledon to leaves. This recovery coordinately involved a great leaf expansion, decreased amino acid content and increased protein synthesis in leaves. This response occurred in parallel with a prominent induction in the cotyledon proteolytic activity. Altogether, these data suggest that a source-sink mechanism involving leaf growth and protein synthesis may have acted as an important sink for reserve mobilization contributing to the seedling establishment under salinity. The amino acids that accumulated in the leaves may have exerted negative feedback to act as a signal for the induction of protease activity in the cotyledon. Overall, these mechanisms employed by cashew seedlings may be part of an adaptive process for the efficient rescue of cotyledonary proteins, as the cashew species originates from an environment with N-poor soil and high salinity.

Source-sink regulation of cotyledonary reserve mobilization during cashew (Anacardium occidentale) seedling establishment under NaCl salinity.

Seedling establishment is a critical process to crop productivity, especially under saline conditions. This work was carried out to investigate the hypothesis that reserve mobilization is coordinated with salt-induced inhibition of seedling growth due to changes in source-sink relations. To test this hypothesis, cashew nuts (Anacardium occidentale) were sown in vermiculite irrigated daily with distilled water (control) or 50mM NaCl and they were evaluated at discrete developmental stages from the seed germination until the whole seedling establishment. The salt treatment coordinately delayed the seedling growth and the cotyledonary reserve mobilization. However, these effects were more pronounced at late seedling establishment than in earlier stages. The storage protein mobilization was affected by salt stress before the lipid and starch breakdown. The globulin fraction represented the most important storage proteins of cashew cotyledons, and its mobilization was markedly delayed by NaCl along the seedling establishment. Free amino acids were mostly retained in the cotyledons of salt-treated seedlings when the mobilization of storage proteins, lipids and starch was strongly delayed. Proline was not considerably accumulated in the cotyledons of cashew seedlings as a response to NaCl salinity. According to these results it is noteworthy that the salt-induced inhibition of seedling growth is narrowly coordinated with the delay of reserve mobilization and the accumulation of hydrolysis products in cotyledons. Also, it was evidenced that free amino acids, especially those related to nitrogen transport, are potential signals involved in the regulation of storage protein hydrolysis during cashew seedling establishment under NaCl salinity.

Somatic embryogenesis in cassava genotypes from the northeast of Brazil

A method for the induction of somatic embryogenesis in eight cassava genotypes from northeastern Brazil is described. The explants used were shoot apexes isolated both from in vitro grown plants and from shoots that sprouted from stem cuttings. Somatic embryogenesis was achieved in high frequencies by the addition in the induction medium of the auxin picloram over a wide range of concentrations. Green cotyledons of primary somatic embryos were used as explants to induce somatic (cyclic) secondary embryogenesis in an inducing medium supplemented with picloram at 12 mg/L. The method could be used not only for the mass production of plants of the cassava genotypes, but also to generate explants (green cotyledons of somatic embryos) as themselves excellent targets for genetic transformation.

Um método para a indução de embriogênese somática em oito genótipos de mandioca cultivados no Nordeste brasileiro foi desenvolvido. A indução de embriogênese somática foi feita utilizando como explantes ápices caulinares isolados de plantas cultivadas in vitro e ápices caulinares isolados a partir de brotações induzidas em casa-de-vegetação em manivas de plantas adultas. Em todos os genótipos a auxina picloram, em uma ampla faixa de concentrações, foi capaz de induzir embriogênese somática em altas freqüências e com um grande número de embriões por explante. Foi mostrado também, que é possível induzir embriogênese somática secundária (cíclica) a partir de cotilédones verdes de embriões somáticos maduros, utilizando picloram no meio de indução. O método aqui apresentado poderá ser utilizado para a produção em massa de plantas dos genótipos utilizados. A alta freqüência de embriogênese somática secundária obtida quando cotilédones verdes de embriões somáticos são utilizados como explantes, mostra que tais cotilédones podem se constituir em excelentes alvos para a transformação genética e posterior obtenção de plantas transgênicas de mandioca.