Maize is better when it’s hairy

How can you increase the surface area of your roots, without investing heavily in biomass? Maize uses root hairs, single-cell extensions of the epidermis to contact more of the soil. These hairs allow them to increase the surface area for taking up water and soluble nutrients, or so it is thought. So found exactly how much the hairs contribute, Florian Klamer and colleagues compared normal maize with a mutant that had short hairs. “This study confirms the importance of root hairs for water and P uptake in dry conditions and when P bioavailability is low in maize, similar to results in other species,” write the authors in their paper.

The scientists compared wild-type maize with the rth2 maize mutant. This mutant has very short root hairs, so a comparison should show what contribution the root hairs make.

The scientists looked into root hairs as some botanists have been puzzled about their role, Klamer and colleagues said. “The importance of root hairs has been questioned in some rice varieties recently, where root hairs improved P efficiency only in some genotypes. In a population of native Arabidopsis, root hair density and length responded surprisingly heterogeneously to differential P supply and some genotypes reduced hair length or density when locally lacking P in agar plates. However, predictions of root hair behaviour in real soils from root hair density and length on agar plates must be made with great care, as enormous variability in root hair traits was found in single genotypes between synthetic growth substrates and real soils. Surprisingly, the mutant performed better than the wild type in well-supplied hydroponics, which may reflect a substantial energetic cost of building root hairs in conditions where they are not required.”

The experiments compared control samples with plants grown under reduced water (-W) and reduced phosphorus (-P). The drought treatments reduced the wild-type shoot biomass by a little under a third, and the mutant by a little over a third. For the low P treatment shoot biomass plummeted by 75% in the wild-type and over 80% in the mutant.

“Under P limitation, as well as under drought stress in the carbonate-rich soil, the rth2 mutant performed worse than the wild type. Thus, growth of rth2 was most drastically impaired under combined stress with low P and drought, in agreement with the substantial importance of root hairs for P and water uptake in maize under low availability,” write the authors in their paper.

However, while the experiment shows that P uptake is improved by root hairs, they had little effect on internal P concentrations. Whether this is a limitation of the root hairs, or a feature of the altered root architecture of the mutant, it is not possible to say, say the authors.