Biosentinels for biotic or abiotic stresses?

Signals for biotic stresses

    Soil-borne pathogens of food, fiber, and ornamental crops such as take-all disease, potato scab decline, Pythium, Rhizoctonia root rot, phytophtera, and rice leaf blast

1.    does the best approach include engineering  local bacterial populations to produce higher levels of PCA or 2,4-DAPG antibiotics known to suppress the disease?
2.    sentinel plants that sense the level of antibiotic produced and produces a signal?
3.    sentinel plant that recognizes the  pathogen and induces the production of antibiotic(s)?
4.    markers to distinguish the various pseudomonas spp that are producers of PCA or 2,4 DAPG?
5.    differentiate between suppressive soils and conducive ones?

Signals for abiotic stresses

    Regulation of invertase (s) expression in cereal anthers and their potential use as biosensors of water deficit in high water-use-efficient cereal crops breeding programs in rainfed ecosystems

In a real cropping system, yield and water do not have a linear relationship because yield may also be constrained by several other factors such as weeds, diseases, frost, inadequate nutrients, acid soil, etc..  If and when water is limiting, yield then becomes a function of 1) the amount of water used by the crop; 2) how efficiently the crop uses this water for biomass growth (above ground); and 3) the harvest index (ratio of grain yield /aboveground biomass) (Passioura, 1977).  These three components are relatively independent and thus understanding and manipulating anyone of these would translate into a yield increase.

The amount of water used by the crop depends on the level of initial underground water, rainfall or irrigation schedules, temperature, and the efficiency of the water uptake, use, and distribution by the plant.   Since attempting genetic increases in yield under rainfed conditions is complicated by the challenging genotype x season x location interactions, a focus on maximizing water use efficiency within a plant is important. 

In the life of a cereal crop, seed germination and reproductive development are the most water-stress-sensitive phases.  Within the reproductive phase, the sensitivity of male organs increases dramatically from the start of meiosis to the break-up of tetrad, events that last 24 h in a single anther (Koonjul et al., 2005).  By contrast, the female tissue remains insensitive to water stress during the same period (Saini and Aspinall, 1981).  Water deficit during meiosis induces pollen sterility and lead to a failure in fertilization and hence grain set (Saini and westgate, 2000).  Such an impact on cereal yield under rainfed conditions can be detrimental to farming communities.

In wheat, pollen development fails when a brief episode of moderately severe water deficit coincides with meiosis (Saini, 1997).  The failure appears to be due to some cellular lesion that is triggered by unidentified signal from the vegetatitve organs (Koonjul et al., 2005).  During their final stages of development, normal pollen grains of wheat and other cereals accumulate large quantities of starch that is used later to support pollen germination and growth of pollen tube.  Water-stressed-affected wheat pollen grains do not accumulate starch (Saini et al., 1984).  Upon studying the mechanisms that regulate this deficiency, Dorion et al. (1996) found and later Koonjul et al., ascertained  that water stress during meiosis irreversibly impairs invertase activity in anthers.  This effect precedes any visible developmental lesion and it is specific because other enzymes in the starch biosynthesis pathway are not affected.  The decline in invertase activity is followed by an accumulation of sucrose, a change in the profile of other sugars, and some spatial redistribution of starch within the anther (Dorion et al., 1996; Lalonde et al., 1997a).  A similar pattern of events was also reported in rice (Sheoran and Saini, 1996).  

Since invertase is the dominant sucrolytic enzyme in wheat anthers, these results suggest that the inhibition of invertase-mediated sucrose utilization in anthers may be the signal for pollen development failure under stress.  So, Koonjul et al. (2005) selected three invertase cDNAs cloned from a wheat anther cDNA library and showed that the effect of water stress is at the transcriptional level and is highly gene and cell specific.

We can propose developing biosentinels for the differential expression of these invertases in the anther and check their utility for breeding water efficient rice, wheat, or barley varieties under rainfed conditions.  By the way, one homologue invertase from rice was characterized and it shares 57% aa identity with that in wheat.  No reports yet for anther invertases in barley.

All these are just thoughts based on my readings, if you have any suggestions or you would like to add/modify the information, please email me (osmat@cambia.org).

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