Findings

Most paddy soils have a natural population of Cyanobacteria. Some representatives of these prokaryotic photosynthethic microorganisms are a potential source of combined nitrogen because they are capable of fixing atmospheric nitrogen [1]. An additional benefit of cyanobacteria is their capacity to synthesize and liberate bioactive substances such as auxins, gibberellins, cytokinins, vitamins, polypeptides, aminoacids, which promote plant growth and development [2–4].

Coastal salinity and accumulation of salts in irrigated land are primary factors depressing yield in rice crop. Salinity can affect germination, metabolism, the size of plants, branching, leaf size and overall plant anatomy. Salt also affects photosynthetic components such as enzymes, chlorophyll and carotenoid contents [5] as well as the activity of ALA-D [6], second enzyme of porphyrin biosynthetic pathway that produce heme group substances and chlorophyll in plants. The inhibitory effect of salt stress on plant growth is exhibited at several levels and involves an array of cellular processes such as cell division and expansion. These cellular processes are regulated by hormones for which homeostasis may be altered by salt. Several reports have indicated that application on crops of growth regulators, such as GA3 and cytokinin, produced some benefit in alleviating the adverse effects of salt stress [5, 16]. However, in other model systems, such as salt tolerant green algae, chlorophyll:carotenoids ratio tend to decrease as a function of salinity [17]. There are very few data available on the increase of chlorophyll content by salt. Pervaiz et al. demonstrated that chlorophyll content was progressively increased with the salinity in wheat [18].

GA3 and EP increased ALA-D activity alike, surpassing the control value by about 50% (Fig. 1E). Total free porphyrin was reduced by NaCl+GA3 by 70% comparing with the control without salt, so this parameter resulted in 55% reduction compared to NaCl treatment (Fig. 1F). GA3 did not counteract the effect of salt on chlorophyll a (Fig. 1G), but it did on chlorophyll b (Fig. 1H).

Compared to salt, EP increased total free porphyrin by 18% (Fig. 1F) and compared to the control without salt increased chlorophyll a content (Fig. 1G). With respect to chlorophyll b, EP did not produced a significant difference compared with salt (Fig. 1H). So, GA3 and EP produced similar effects on ALA-D activity and carotenoid content. EP partially reverted salt effect on total free porphyrin and increased chlorophyll a content; GA3 totally reverted the salt effect on the chlorophyll b content taking it to the control value. The decrease of carotenoid content produced by salt was totally counteracted by EP as well as by GA3.

In Lupinus termis, Haroun and Hussein [19] established that seed priming in cyanobacterial cultures filtrates increased a and b chlorophyll and reduced carotenoids content. Regarding plant growth regulators, this treatment also increased auxin, gibberellic acid and cytokinin content and decreased ABA content. There is a general agreement on the induction of the endogenous hormone levels by exogenous application of different growth regulators [20]. In our experiment the improvement of rice seedling salt tolerance was probably due to the presence of hormones in the EP produced by the cyanobacterium. Treatment with GA3 may have counteracted the excess of ABA produced as a response to the salt stress, reducing alterations in the growth phytoregulators ratios.

S. hofmanni EP reverted completely or partially many of the NaCl-induced effects on growth and also the biochemical alterations of Yerua PA rice seedlings. EP partially coincided with GA3 in the amelioration of salt affected parameters. The cyanobacterial EP would contain gibberellin-like substances which may be responsible of the alleviation of salt stress adverse effect on hormone homeostasis.

However, it would be important to confirm the presence of gibberellins in S. hofmanni extracellular products as well as the alteration of the hormonal homeostasis in this rice cultivar under salt stress.