Laying the pipes and fast: The HD-AGP gene is upregulated in vascular tissue during rapid growth, and down-regulated by factors…

Histidine-domain arabinogalactan proteins (HD-AGPs) are extracellular glycoproteins that are abundant in vascular tissue, which transport food and water in plant organs. We hypothesized that the proteins enable rapid growth of the elongated cells that eventually develop into fluid-conducting tubes or strengthening fibers. To test the hypothesis, we used immunoblots and quantitative PCR (qPCR) to compare the levels of the protein, and mRNA that encode it, in a variety of organs and in response to factors known to affect the growth of plant organs. We found that HD-AGPs are more abundant in rapidly expanding young leaves than in mature non-expanding leaves. They are more abundant in dark-grown seedlings (which grow exceptionally fast) compared to light-grown seedlings. The mRNA encoding the protein display the same pattern of abundance. With the sensitivity of qPCR, we were also able to demonstrate that the apex of a young leaf has about half as much of the mRNA encoding HD-AGPs as the basal part, which is known to exhibit higher growth rates. Seedlings treated with gibberellin, a growth-promoting hormone, have higher levels of HD-AGP mRNA. In contrast, treatment with methyl jasmonate, a stress-induced growth inhibitor, reduced transcript abundance in seedlings. The immunoblot and qPCR data support the hypothesis that the protein is involved in the rapid growth of vascular tissue in seedlings and leaves. Our study paves the way for a better understanding of growth processes in food and fiber crops, and also in wood accumulation in forest species.