LinearLayout Beneath LinearLayout full width?

I have an ImageView and a TextView that are the only items in my Android UI. Currently the TextView is in the same LinearLayout as the ImageView, so the text sometimes runs to 2 lines. I want to get the text on its own line, with the full width of the parent element so it will be the whole width of the screen. So far everything I've tried moves the ImageView outside the window or makes it very small.

<LinearLayout xmlns:android="http://ift.tt/nIICcg"
    xmlns:tools="http://ift.tt/LrGmb4"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:weightSum="4"
    android:gravity="center"
    tools:context=".MainActivity"
    android:baselineAligned="false">

    <LinearLayout
        android:layout_width="0dp"
        android:orientation="vertical"
        android:layout_weight="2"
        android:gravity="center_vertical"
        android:layout_height="wrap_content">

        <ImageView
            android:id="@+id/activity_main_imageview_gototags"
            android:src="@drawable/img_main_gototags"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:scaleType="centerCrop"
            android:onClick="imageViewGoToTagsOnClickOnClick"
            android:adjustViewBounds="true"/>

        <TextView
            android:id="@+id/activity_main_textview_tagline"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/activity_main_textview_tagline"
            android:textColor="@color/dark_blue"
            android:gravity="center"
            android:textSize="30sp"
            android:layout_gravity="center_horizontal" />
    </LinearLayout>
</LinearLayout>