RAGHOTHAMA, K. G.

VASCONCELOS, M. J.

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Gaspe flint maize as a model to study the effect of phosphorus on growth and development ofmaize from seed to seedVasconcelos, M.J.!; Raghothama K. G. 2'Embrapa Maize and Sorghum, PO boxl51, Sete Lagoas, MG, Brazil, 35701-9702 Department of Horticulture and Landscape Architecture, Purdue University, West Lafayette-IN47907IntroductionToday Arabidopsis is the main model system for laboratory studies in basic plant biology.Understanding Arabidopsis nutrient transport will assist in understanding and improving nutrientacquisition efficiency in crop plants which could eventually lead to the development ofenvironmentally sound agricultural practices with low inputs of inorganic fertilizers. However, ithas to be noted that some biological phenomena, such as species-specific plant pathogen interactions,mycorrhizal symbiosis or nitrogen fixation, cannot be studied in this model plant (Poirier and Bucher,2002). It is also difficult to translate this data on role of Pi deficiency on reproductive organs ofArabidopsis to crop plants like maize. In view of these shortcomings, specificalIy for phosphatenutrition, gaspe flint was evaluated to see if it was suitable as a model to study the effect of phosphoruson growth and development from seed to seed in a hydroponics system. The following characteristicsof gaspe flint make it a model for phosphate nutrition studies. Gaspe flint is a dwarf maize genotypewith a short life cycle. Interestingly the gaspe flint life cycle can be completed within eight weeks,similar to that of Arabidopsis. This short life cycle provides a uni que opportunity to examine theeffect ofPi deficiency on all aspects of plant growth under controlled conditions. Pollination occurs28-30 days after emergence and results in 50-150 mature seed within four weeks ofpolIination.Furthermore, it is well adapted to different environmental conditions. Gaspe flint grows well underlong day photoperiod conditions (14 hours light, 10 hours dark) and under varying temperature(between 18°C to 32 °C). The seedlings normalIy emerge within five-six days after sowing. Plantsattain microspore meiosis by about 14-16 days after emergence. Tillers begin to form 14 days afteremergence and they contain cobs while tassels are clearly visible in about three weeks after emergence.PolIen shedding begins four weeks after emergence. The ear is generally harvested four weeks afterpollination even though the plants are still green. Therefore, this unique maize line was used as amodel to study the effect of phosphate on growth and development of maize from seed to seed.Gaspe flint became useful as a maize model plant for phosphorus studies.MethodologyGaspe flint seeds were germinated and one-week-old seedlings were removed from the soilmedium. The roots were washed free ofmedia and the plants were transferred to a hydroponics set-up containing half-strength modified Hoagland's solution (Liu et al., 1998). One week aftertransferring the seedlings to hydroponics the experimental treatments were initiated. Sets ofplantswere transferred to half strength nutrient solution with different concentration of Pi (O, 5, 10, 25,50, 100 and 250 micromolar). The solutions were changed every two days during the treatmentperiod to maintain the concentration of nutrients and pH. Furthermore, after 15 days of growthunder P+ and P- conditions, roots, stem, young leaves and old leaves were harvested separately forphysiological, biochemical and molecular evaluation. Some plants were maintained until maturityto evaluate the effect of varying concentrations of phosphorus on plant growth and reproductiveorgan development.Proceedings 013"/ International Symposium on Phosphorus Dynamics in the Soil-Plant Continuum 109Results and DiscussionThe results presented here clearly show that all the phosphate starvation responses arereplicated in gaspe flint. This plant has shown a profound effect on development of femalereproductive organ (cobs) correlated with the phosphate availability in the nutrient solution. Thisobservation supports the notion that phosphorus deficiency results in a delay in flower developmentand reduction in fruit set, in addition to a range ofvegetative phenotypes (Marschner, 1995). Thelack of synchrony between male and female gametophytes resulting in lack of pollination is a majorfactor affecting com yield in phosphate deficient soils. This feature of maize reproduction could beused to select Pi efficient genotypes in the field. This is one example where significant newinformation on plant response was obtained using a model planto Anthocyanin content and acidphosphatase activity increased with decreasing concentration ofPi in the medium. Interestingly themaize plants accumulated significant quantities of anthocyanins in the roots. This was a clearcorrelation between anthocyanin accumulation in gaspe flint plants and phosphorus concentrationin the nutrient solution. Accumulation of anthocyanins in photosynthetic tissues, which is one ofthe most characteristic visible symptoms of phosphate deficiency, appears to serve a photoprotectiverole by optical masking of chlorophyll and by facilitating the conversion of excess absorbed lightenergy to heat (Niyogi et al., 1997; Field et al., 2001).Use of gaspe flint as a crop model plant enhanced our ability to analyze the tissue specificexpression of phosphate transporters. The results showed expression of ZmPT in all parts of theplant while older leaves showed strong expression of most of the transporters. The root specificexpression correlates well with those functions in phosphate uptake. There were striking differencesin the expression pattem of ZmPTs in response to Pi resupply. When plants starved for Pi werereplenished with phosphate, the rapidity with which transcript levels decreased was markedly differentfor the four transporters. The replenishment experiments further confirm the innate ability of plantsto regulate Pi uptake based on availability and demand for PioThe results presented in this study clearly support our choice of using Gaspe flint as a cropmodel to study physiological, morphological, biochemical and molecular aspects of phosphatestarvation response. Furthermore, Gaspe flint can serve as an efficient model plant for analyzingmany nutrient deficiencies from seed to seed.ReferencesLiu C, Muchhal US, Uthappa M, Konowicz AK, Raghothama KG (1998). Plant Physiol 116: 91-99.Marschner H (1995) Mineral nutrition in plants 2nd ed. Academic Press, San DiegoMeinke DW, Cherry JM, Dean C, Rounsley SD, KoomneefM (1998). Sci 282: 662-682Niyogi KK, Bjorkman 0, Grossman AR (1997). Plant Cell9: 1369-1380Poirier Y, Bucher M (2002). The Arabidopsis Book. American Society ofPlant Biologists11 O Proceedings 013rr1 International Symposium on Phosphorus Dynamics in the Soil-Plant Continuum

Gaspe flint maize as a model to study the effect of phosphorus on growth and development of maize from seed to seed.

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